This manual will give you a solid understanding in electronic terminology and symbols, as well as the construction and operation of common electronic components and the testing and repairing of printed circuit boards.
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First published 2009.
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1 Introduction to Troubleshooting 1
1.1 Troubleshooting Basics 1
1.2 Common Troubleshooting Techniques 5
1.3 Gaining Circuit Familiarity 8
1.4 Getting Prepared for Troubleshooting 20
1.5 Summary 21
2 Failure Analysis and Prevention in Electronic Circuits 23
2.1 Failure Symptoms 23
2.2 Failure Causes 25
2.3 Failure Types 26
2.4 Some Useful Terms in Failure 28
2.5 Summary 29
3 Device Troubleshooting – I 31
3.1 Tools for Servicing 31
3.2 Test and Measuring Instruments 36
3.3 Safety Issues – Test and Tagging of Portable Electrical Equipment 54
3.4 Summary 55
4 Device Troubleshooting – II 57
4.1 Testing of Passive Components 57
4.2 Testing of Semiconductor Devices 62
4.3 Testing Bipolar Transistors 63
4.4 Testing Other Active Components 65
4.5 Testing Diodes, Transistors and In-circuit Semiconductors Using Oscilloscopes 78
4.6 Switches 80
4.7 Safety Issues for Plugs, Sockets and Portable Appliances 82
4.8 Summary 83
5 Troubleshooting Digital Systems 85
5.1 Moving from Analog to Digital 85
5.2 Moving into the Digital Circuits 87
5.3 Typical Faults in Digital Systems 97
5.4 Digital Circuit Troubleshooters 99
5.5 Digital Integrated Circuits 106
5.6 Programmable Logic Device (PLD) and Memory Definitions 108
5.7 Practical Tips 112
5.8 Precautions 112
5.9 Summary 112
6 Power Supply and Subsystems Troubleshooting 115
6.1 Power Supply 115
6.2 Regulators 119
6.3 Switched Mode Power Supplies (SMPS) 123
6.4 Oscillators 125
6.5 Amplifiers 129
6.6 Troubleshooting of RS-232 Serial Data Standard 131
6.7 Troubleshooting Microprocessor Based Systems 134
6.8 Summary 138
7 Temperature as a Parameter for Testing, Signal Injection and Signal Tracing 139
7.1 Effect of Temperature on Electronic Circuits 139
7.2 Testing 142
7.3 Actual Troubleshooting 143
7.4 Signal Injection 144
7.5 Signal Tracing 147
7.6 Summary 149
8 Phenomenal Troubleshooting 151
8.1 Noise 151
8.2 Intermittent 158
8.3 Sources of Interference 159
8.4 Static Discharge 160
8.5 EMI/EMC and its Sources 160
8.6 Cross-Talk 164
8.7 Summary 167
9 PCB Testing and Soldering Techniques 169
9.1 What is soldering? 169
9.2 Process of Soldering 170
9.3 Soldering Tools 171
9.4 Solder and Flux 175
9.5 Component Forming 176
9.6 Temperature Range in Soldering 177
9.7 Component Replacement 178
9.8 Inspecting Solder Joint 179
9.9 Unsoldering Connections 180
9.10 Additional Soldering Tips 183
9.11 Additional De-soldering Tips 183
9.12 First Aid Steps 184
9.13 Printed Circuit Board 184
9.14 Troubleshooting of Surface Mounted PCBs 189
9.15 Testing and Troubleshooting with ATE 191
9.16 Summary 198
10 Maintenance and Safety Aspects 201
10.1 Do we need Maintenance? 201
10.2 Types of Maintenance 202
10.3 Aims of Maintenance 204
10.4 Advantages of Preventive Maintenance 204
10.5 Importance of Sound Maintenance Management 204
10.6 Maintenance Policy 205
10.7 Maintenance Organization 206
10.8 Maintenance Manuals 207
10.9 Safety Aspects 207
10.10 Summary 211
Appendix A – Question and Answers 213
Appendix B – Questions and Answers 241
Appendix C – Troubleshooting of Variable Regulated Power Supply 269
Electronic equipment can develop a wide variety of problems. The act of troubleshooting arises in order to make the problems disappear so that the equipment works as per the expectation. This introductory chapter provides an overview of troubleshooting processes and various troubleshooting techniques. It also emphasizes how to prepare and read a circuit diagram, as a first step for troubleshooting.
In general, for any application, equipment is designed and manufactured to work trouble free within its specified limits during its useful time. However, sometimes there is a conflict between the expectations of the user and the performance of the instrument. Thus develops the need for troubleshooting and maintenance.
Troubleshooting is the process of isolating and correcting a problem in malfunctioning equipment so that it returns to its expected performance level. The process of troubleshooting requires a systematic fault finding approach. Whenever a fault occurs, two things can happen:
The second type of fault can be further sub-divided into:
The basics which can be applied to troubleshooting are given below:
The following table shows a list of component failure in their order of probable occurrence:
Order of occurrence of failure | Component |
1 | Mechanical and electromechanical devices such as relays, switches, plugs and sockets. |
2 | Components that get hot in their normal operation, like power amplifiers and rectifiers. |
3 | Electrolytic capacitors of small versions and those subjected to high voltage. |
4 | Active devices like transistors and SCRs. |
5 | Passive devices like resistors and capacitors. |
The process of troubleshooting comprises the following steps:
Let’s address these steps in brief:
It is important to establish the presence of a fault in equipment before taking any other action. In some cases a system may be reported faulty, but it could be a case of faulty operation or a system failure may be reported with either very little or misleading information. It is essential that a functional test, checking the system’s actual performance against its specification must be made and all fault systems must be noted.
It is also important to check the history of the equipment and repair and servicing work carried out earlier by any other person.
This involves pin-pointing the cause of the fault by studying the literature relevant to servicing, maintenance and repairs. The fault is located first in subsystem and then in a single component in the sub system.
Fault correction consists in replacing or repairing the faulty component. This is followed by a thorough functional check on the whole system.
Troubleshooting aids help in quickly analyzing a malfunction and taking corrective action. The following points are discussed in this aspect:
A basic set of tools and test equipment like multimeter and oscilloscopes are necessary. Sometimes specialized equipment are required, such as a high speed scope. The maintenance technician is required to have all this knowledge.
A complete set of documentation is a must. Most manufacturers supply the following documents with their manuals:
A good list of data manuals is essential. There are data books from all major component manufacturers which can be collected.
The various troubleshooting techniques given below are used in the majority of electronic systems. The type of system being handled will decide which technique should be adopted.
An electronic system comprises several functional parts such as power supplies, amplifier, signal converters, etc. When the system fails to give the expected performance, the problem could be in any of these functional areas. Therefore, it is essential to troubleshoot the system in order to isolate the fault to the failing functional area and then to the failing component. The logical approach of isolating a fault is through a process of elimination of the functional areas that are performing properly. Once a failure is isolated, further analysis of the circuitry within this area is carried out to isolate the malfunction to the faulty component. This functional area approach is also called the Block-Diagram approach to troubleshooting.
In this technique, as the name suggests, the circuit is split in half and the output is checked at the half-way point in case of an absence of an output. This helps to isolate the failing circuit in the first or second part. When the faulty half is determined, the ageing circuit is split into half for further isolation of failure. This splitting is continued until the failure is isolated to one function or component.
The Half-split method is extremely useful when the system is made up of a large number of blocks in the series:
Many electronic systems do not involve only series connected blocks. They may have feedback loops or parallel branches in a part of the circuit. Hence use of this method is rather restricted.
Here the output from one block is fed to two or more blocks. In such systems, it is best to start by checking the common feed point. Alternatively if output is normal (at A or B in fig. 1.3), check after the divergence point. Conversely, if one output is abnormal, check before the common point. The most common example is that of the power supply circuit which supplies dc power to various subsystems in equipment.
In a convergent path two or more input lines feed a circuit block:
In order to check such a scheme, all inputs at or before the point of convergence must be checked one by one. If any of the inputs is incorrect (at C or D in fig. 1.4), then the fault lies in that particular input circuit. If all are found to be correct, the fault lies beyond the convergent point. For example, if C and D are correct and there is no output at E, the fault lies in unit 3. But if input at C is faulty, the fault lies in block 1 or before that.
The feedback loop usually corrects the output of some block with the input of an earlier block via some network called feedback network. Since the circuit behaves as a closed loop, any fault within the loop will appear as if all the output blocks within the system are at fault:
Before starting the troubleshooting of a system having feedback loop, the type of the feedback and its use should be well understood. Feedback paths are basically provided for the following functions:
Having identified the type of feedback circuit, one can proceed as follows regarding locating the fault.
For the first type, i.e. modifying feedback, it may be possible to break the feedback loop and convert the system to a straight linear data flow. Each block can then be tested separately without the fault signal to be fed around the loop. In some cases instead of completely breaking the loop, the feedback can be modified at or near the point where it rejoins the main forward path. If the output appears normal, check the feedback path, otherwise, check the forward path.
For the second type, i.e. sustaining type, feedback is disconnected from the output and a suitable test signal is injected to check the performance of various circuit blocks.
If a system has switch-able parts and if the circuit function is found faulty in one position of the switch then throw the switch to another position. If the problem persists, check the switch in common circuitry. If the problem disappears with this action, check that the circuitry switched out.
A circuit diagram is a graphical representation of the interconnections of various components constituting the equipment. It is the most important document for the maintenance technician. Usually every assembly in electronic equipment is assigned an assembly number which appears on the circuit board and on the diagram. Commonly used symbols in electronic circuits are shown below in Figure 1.7.
The maintenance technician should be well versed with the circuit of the system before actually starting troubleshooting. A circuit diagram is the most important document for the technician. Many-a-time the circuit diagram of the system or equipment is not ready or not provided by the manufacturer. In that case, the technician has to prepare the circuit diagram. The circuit diagram makes the fault finding process easy.
The technician should be experienced enough to draw a circuit diagram. Usually, it is not recommended for larger systems. A larger system is broken into parts (subsystems) and then circuit diagrams for the smaller, suspected systems is drawn to trace the fault. The following points must be noted when preparing a circuit diagram:
A circuit diagram is a graphical representation of interconnections of various components constituting the equipment. It is the most important document for the maintenance technician. Usually every assembly in electronic equipment is assigned an assembly number which appears on the circuit board and on the diagram. Commonly used symbols in electronic circuits are shown below:
(The following comparison of circuit symbols is based upon the following international/national specifications:
-IEC 60617 graphic symbol database (DIN EN 60617-2 to DIN EN 60617-12
-NEMA ICS 19-2002, ANSI Y32.2/IEEE 315/315 A, CSA Z99)
A circuit diagram illustrating some symbols is also shown below:
An electronic circuit makes use of both active and passive components. These components are physically interconnected with each other to form any electronic circuit. There are three major techniques to interconnect the components. Let us have a brief overlook of these methods:
This method makes use of a solder and a wire to interconnect electronic components. It is a very slow method and is very cumbersome if a large number of devices are to be connected.
This technique tightly winds a small gauge wire around a wire-wrap metal post or terminal. There are special wire-wrap metal post sockets for the ICs that have longer posts for wire-wrapping the wire. Also, special tools are needed for wrapping and un-wrapping the wire.
This technique includes interconnections between points printed in metal on the non-conductive board. The circuit is printed on the board by a series of photographic and chemical procedures. Most of the equipment in practice make use of PCB. They are generally made for completely checked out and working boards, as it is difficult to make wiring changes on the PCB.
There can be one or more circuit boards inside of electronic equipment. They are mounted inside a wooden or metallic cabinet with some arrangement of interconnecting the circuit boards. This arrangement is called Edge Connectors.
The purpose of edge connectors is to bring signals and powers to and from the circuit boards without having to connect a wire to the circuit board itself. This arrangement provides easy installation and removal of the circuit board in equipment:
Also inside of electronic equipment there is an assembly called card rack for the compact placement of the PCBs. But it is difficult to put a test probe on the circuit board for making any type of measurement or for troubleshooting. To eliminate this problem, special circuit boards called ‘extender cards’ are inserted into the card rack and the circuit board is extended into the extender card.
An extender card is just a wiring extension to make the circuit board accessible for testing.
In the literature, terms such as component, equipment or system have been used. The following table distinguishes between these terms:
Serial Number | Nomenclature | Description | Example |
1 | System | Collection of equipment arranged to perform a function. | Television, Missile. |
2 | Equipment | Collection of components arranged to operate without the need for other components. | Radio Transmitter, the central part of a missile. |
3 | Assembly | Collection of components in a prescribed order not all of which have, as yet, been so arranged. | Terminal board with components parts attached. |
4 | Component | Collection of elements arranged in a prescribed order. | Resistor. |
5 | Element | A simple object which can not be further sub-divided. | Filament, a relay contact. |
A close visual inspection is a good and quick start for troubleshooting. It gives a clue for problems such as burned spots and places where high voltage arc has occurred. A quick look to the circuit also gives an idea of the condition of fuses and circuit breakers.
The troubleshooting technician should collect the history of the system. He/she should know whether the problem had occurred before and what is the frequency of the occurrence of the problem.
On the basis of the knowledge of how the system works, the kind of failure can be detected. This would lead you to select the troubleshooting technique.
If the system is not producing the desired end result, look for what is doing it correctly. You can locate where the problem is not present so that you can then focus on another location for troubleshooting.
If the system has been having problems immediately after some kind of maintenance or other change, the problems could be linked to those changes.
After all, the choice of techniques and strategies for troubleshooting totally depends upon the technician. The following points would be helpful for effective troubleshooting:
For the process of troubleshooting, preparing the circuit diagram is the initial and basic process performed by the maintenance technician.
The components can be physically interconnected to each other using solder, wire-wrap and printed circuit board methods.
Any electronic system consists of an element, component, assembly and equipment. All these parts together make a complete electronic system.
The troubleshooting process consists of fault establishment, fault location and fault correction.
Functional area approach, Split half method, Divergent path, Convergent path, Feedback path, Switching path are the common troubleshooting techniques. Which technique has to be applied totally depends on the type of system.
To start the process of troubleshooting, first the technician should have a close visual inspection of the system. He should understand the basic functionality of the system. Then he can proceed to analyze the cause of the trouble.
A well designed, well engineered, thoroughly tested, and properly maintained system should, ideally, never fail in operation. However, in practice it is observed that even the best design, manufacturing and maintenance efforts do not completely eliminate the occurrence of failures. There can be various causes and types of failures.
For the enhancement of system reliability, it is necessary that the design engineer understands the causes of failure, so as to trace the deficiency in the system. The primary concern is to identify the correct causes of failure and to decide on appropriate corrective action to assure higher system reliability.
The reliability of equipment improves considerably when it is operated under certain favorable conditions (which may vary from equipment to equipment) such as operating the components well below the maximum ratings, subjecting the components to minimum vibrations and shocks, etc. In spite of all the favorable operating conditions, failures are seen to occur.
The frequency at which the failure occurs is termed as reliability of the system. The less is the occurrence of failure, the more is the reliability of the system.
Any equipment or a system may breakdown due to a faulty component. For each component or item, the properties that it must possess in the course of its use are listed.
A deviation in the properties of the component or item from prescribed condition is considered as a fault. A state of fault is denoted by the term failure.
Failure of equipment refers to its inability to perform its required function, such as when characteristics change to such a degree that it can not perform its specified level of performance.
The fundamental sources of failure include many aspects of design, material selection, material imperfections, fabrication and processing, assembly, inspection, testing, quality control, storage and shipment, service conditions, mechanical and chemical damage to system.
The traditional bathtub curve in figure 2.1 indicates component life in three stages.
During the first stage, failure rate begins high and decreases rapidly with time. This stage is known as Infant Mortality Period and it has a Decreasing Failure Rate (DFR). The infant mortality period is followed by a steady state failure rate period, which is usually long. This period is known as random failure period or useful life of the equipment and this part of the curve being identified as normal operating life curve.
In the third stage where the curve ends, i.e. beyond the useful life period, there is a gradual increase in failure rate. This is a period of ageing and wears out with increasing failure rate.
Failures may be partial failures resulting from deviation in characteristics or parameters beyond the specified limits but not such as to cause complete lack of required function. If the characteristics deviate beyond the specified limits such as to cause complete breakdown of the required function, it is called as complete failure.
Failures can be anticipated by prior examination, whereas in some cases, it can be a sudden failure which could not be predicted on routine examination. A sudden and complete change in equipment performance is called as catastrophic failure.
These may be caused by an open circuit or short circuit failure which is irreversible.
Failures may also occur gradually and in a partial manner. Such a type of failure is referred to as degradation failure. These failures are encountered mostly with analog systems when some noise or disturbance is present.
Symptoms are useful to locate the general area of problem. The symptoms are sometimes described by the equipment owner or user. In an industrial electronic plant, it may be the foreman of the division who uses the equipment. In consumer electronic equipment, the description is often from the customer who owns the equipment.
The most reliable symptom analysis is given by the technician after energizing the equipment. Technicians know that certain symptoms in a system usually mean that a certain component has failed.
Symptoms are indeed a valuable guide for troubleshooting. However, one should avoid basing a complete troubleshooting procedure on the knowledge of symptoms alone. For example, distortion in radio’s output sound can be caused by different reasons such as low terminal voltage of aging battery, overuse of transistor, a tear on a speaker cone and so on.
An item is considered to have failed because of one of the following three conditions:
For the enhancement of system reliability, it is necessary that the design engineer understands the causes of failures, so as to trace the deficiency. Equipment failures take place due to many reasons. The primary concern is to recognize the causes of failure and to take corrective action to achieve higher reliability. The causes can be classified as follows:
During the complete life cycle of the operation of a system, failures are broadly classified into three categories as follows:
The failure rate is expressed in terms of failures per unit of time, such as failures per hour or failure per 10 or 100 hours. The failure rate of a component can be calculated by operating large numbers of the component for a known period and noting the number of failures that take place during that period.
It is computed as a simple ratio of number of failures, f, during a specified test interval, to the total test time of the items undergoing test. Thus:
Failure Rate = f / T,
Where: f = number of failures during the test interval
T = total test time
For example, suppose 1000 transistors are put on test, out of which 50 fail over a 1000-hour period, and then by definition, the failure rate is given by following equation:
Failure Rate = 50 / 1000
= 0.050 per 1000 hours
= 0.050 / 1000 per hour
= 5 * 10 -5 per hour
Often the failure rate is expressed as a percentage.
Failure Rate = 50 / 1000 * 100 % per 1000 hours
= 5 % per 1000 hours
The smaller the value of the failure rate, the higher is the reliability of the system.
From the failure rate data, it is possible to calculate the ‘mean time to failure’ (MTTF). If one transistor is used in the system, then:
MTTF = 1 / 5 * 10 -5
= 20,000 hours
It is also calculated on the basis of the results of life testing of components. For example, if there are 3 transistors which are tested until failure, and the time to failure were 300, 600 and 400 hours.
Total test time = 300 + 600 + 400
= 1300 hours
MTTF = Total test time / number of components
= 1300 / 3
= 433.33 hours
MTTF is normally applied on items which can not be repaired, such as resistors, capacitors, diodes, transistors, etc.
The Mean Time between Failures (MTBF) is the reciprocal of the constant failure rate or the ratio of the test time to the number of failures. It is measured by testing it for a time period (T), during which faults may occur. The equipment is tested after every repair of fault:
MTBF = 1 / Failure rate
= T / f
Where: f = number of failures during the test interval
T = total test time
The MTBF of a system is estimated by first determining the failure rate of each component and then summing them all up to obtain the system failure rate. If a small system has for components with individual failure rates FR1, FR2, FR3, and FR4 respectively, the total failure rate of the system is:
Total Failure rate = FR1 + FR2 + FR3 + FR4
MTBF = 1 / Total failure rate
Mean Time to Repair is an important consideration for selecting a system. It is the average time required to bring a system from a failed state to an operational state. It is defined as the total corrective maintenance time divided by the total number of corrective maintenance actions during a given period of time. MTTR includes the time taken to diagnose, locate and repair the fault.
A deviation in the properties of the component or item from prescribed condition is considered as a fault. A state of fault is denoted by the term failure.
The component life is divided into three stages. The first stage is Infant Mortality Period, the second stage being Random Failure Period and the third stage is beyond the useful life period which is the Wear out Stage.
Symptoms are usually the very first information about the failure. Symptoms are generally provided by the user of the equipment. However, the maintenance engineer should not decide the path of troubleshooting on the basis of symptoms alone.
There can be various reasons for failures: Production deficiency, Processing deficiency, Assembly errors, inadequate storage and transport conditions are some of the major causes of failure.
Failure Rate, Mean Time to Failure, Mean Time between Failure and Mean Time to Repair are some important mathematical terms in failure.
Device Troubleshooting is divided into two sections. The first section consists of a study of various hand tools and testing and measuring instruments which are used in troubleshooting of electronic circuits (analog). Digital troubleshooters are explained in chapter 5. The second section consists of actual testing and troubleshooting of various components and devices (chapter4).
This chapter provides a brief overview of tools such as Spanners, Wrenches, Screw Drivers, and Files. Analog multimeter, digital multimeter, oscilloscopes are the basic test and measuring instruments required in troubleshooting to test and measure three basic quantities: current, voltage and resistance. This chapter shows how to use these instruments.
In the absence of proper tools, the servicing of equipment, even by the best technician, is incomplete. Availability of proper hand tools and their prior knowledge is essential for the best troubleshooting results.
A maintenance technician is expected to handle a wide variety of tools in the proper way as per the needs. Some of the tools which are often used are listed below.
They are used for tightening nuts and bolts. A torque is exerted which is applied to the head of the bolt or nut. Below are some types of spanners.
They are used primarily on large hexagonal nuts. The opposite ends of the spanner have successive jaw sizes.
Here both the ends have closed rings whose inner sides are serrated to have six point grip or more. Both the heads are offset relative to the handle.
They can be placed on the nut only from the top and can be used with one hand without possibility of slippage.
As the name suggests the jaw size of the spanner is adjustable.
When no correctly fitting spanner is available, adjustable spanners are used.
Allen Wrenches are also known as Hexagonal Socket Bar Spanner (Allen Key). They are most useful for opening control knobs. These are right angled rods of hexagonal cross section with one short arm and one long arm.
The short arm is used for assembly and the long arm is used for tightening. They also come as a set in metric or inch sizes. Allen Wrenches are most useful for opening many control knobs.
These are the most common tools for securing screws. Following are the two types of screw drivers:
They are used in practice for slotted screws.
The above figure shows various parts of screw driver. The handle is usually made of tough, transparent colored plastic and shaped to provide a firm, comfortable grip. The handle has a smooth, semi-rounded heel which fits the palm comfortably.
The blade or shank is made of steel which is heat treated and tempered to apply torque to the screw head. The blade and tip are chrome plated.
For a flat blade screw driver the width of the blade and the thickness of the blade should be correct.
These screw drivers have star-shaped holes in their heads as opposed to straight slots. There are four (No. 1 to No, 4) standard sizes of Philips screw drivers. No. 1 and No. 2 are usually needed.
The star-shaped hole in a Philip’s screw driver and the tip of a Philip’s screw driver must fit together properly so that the walls of the screw head or the tip of the driver, or both, will not be damaged.
In addition, there are some other special types of screw drivers which are used occasionally. For small and delicate work Jeweler’s screw driver sets are available. The barrel of the handle is knurled with a top finger rest.
Ratchet’s Screw Drivers have a selector level that will allow the screw driver to rotate freely in either the clockwise or anticlockwise direction and obtain the ratchet driving action in the other direction.
Nut drivers are used as screw drivers but are designed to accommodate hexagonal types of machine nuts instead of screw heads.
They are very useful in mounting a nut on a threaded stud and in holding a nut while its screw is being tightened.
Files are made of hardened high carbon steel. Filing is the most important skill to be acquired in electronic fabrication. Files are classified according to their length, cut of teeth and cross-section. Following are files of different cross section:
During repair work, if there is a need to use a different sized replacement component, like a potentiometer or a fuse carrier. In those cases files are very useful to enlarge holes. They are also used to smooth the scratch marks on surfaces.
There are many types of testing and measuring instruments available for electronic troubleshooting. A certain amount of personal opinion is involved in troubleshooting methods. One may prefer to use a voltmeter for troubleshooting problems, another may use oscilloscope leads. Although, a personal choice is always there, the technician should be familiar with all the methods, advantages and disadvantages, limitations, and types of troubleshooting instruments.
Analog and digital multimeter [volt-ohm-multimeter (VOMs)] is available for troubleshooting analog circuits.
The multimeter is the most useful instrument for the troubleshooting technicians. This instrument facilitates the measurement of DC voltage, AC voltage, DC current and resistance values. With proper accessories it can also measure other parameters like high frequency signals, high voltages and so on.
Voltmeters and ammeters both AC and DC and ohm-meters are available in various ranges and configurations. A multimeter is a combination of all these meters which makes it very useful in the field.
An analog multimeter is used when merely the presence of a value near one specified is required rather than a measured value that is exactly as expected. An analog indication of approximate voltage value is more quickly observed as compared to digital reading. They are less susceptible to extraneous noise.
When a high accuracy is required, especially when very small changes in a level need to be detected, a digital multimeter is preferred.
Analog multimeter is the most widely used test and measuring instrument. It operates with a permanent magnet moving coil, which can become a DC voltmeter, an AC voltmeter, and DC milli-ammeter or an ohm meter. Sometimes an AC current measuring facility is also present.
It has a coil of fine wire wound on a rectangular aluminum frame. It is mounted in the air space between the poles of a permanent horse-shoe magnet. Refer to the following figure:
When electric current flows through the coil, a magnetic field is developed that interacts with the magnetic field of the permanent magnet to force the coil to rotate. The direction of rotation depends upon the direction of electron flow in the coil. The magnitude of the pointer deflection is proportional to the current. In usual meters, the full scale deflection (FSD) is about 90 degrees.
The multimeter operates without any error if some preliminary adjustments are undertaken while using the multimeter. A scale plate of a standard multimeter is shown in the following figure:
Following are the settings of the multimeter:
The moving coil meter is basically sensitive to current and is therefore an ammeter. For the direct current measurement, place the meter (ammeter to measure current) in series with the circuit. When the ammeter is included in the circuit, its internal resistance adds up, thereby reducing the current in the measuring branch. Usually, this resistance is small and can be ignored.
For alternating current measurement, rectifier type meters are used which will respond to the average value of the rectified alternating current. The meter has to be calibrated in amperes rms (root mean square) for the measurement of sine waves.
The current meter can be used to measure voltage. The moving coil meter has a constant resistance. So, the current through the meter is proportional to the voltage.
To measure the potential difference between two points, connect the two voltmeter leads to these points. So, in contrast with the ammeter, the voltmeter is connected in parallel with the circuit whose potential has to be measured.
To measure AC voltage, rectification is required. As in the AC current meters, AC voltmeters respond to the average value of the rectified voltage but are calibrated in volts rms for a sine wave.
The moving coil meter can be used to measure unknown resistance. Test probes are short circuited and the ohms adjust control is turned so that the current through the total circuit resistance has a full scale deflection.
An ohm meter is never used while the circuit is in operation. Sometimes the resistances depend upon the circuit conditions, in that case measure the voltage across the resistance, the current through it and calculate the resistance.
In the analog type of multimeter the value of the parameter being measured is estimated from the position of a pointer along a calibrated scale. Even when using a high grade meter of this type it is difficult to take readings with a precision which is better than about 1 percent of the full scale value.
This limitation is largely imposed by the physical arrangement of the scale and the pointer scheme. For more precise measurements it would be better if the actual value of the voltage or current could be displayed directly as a numerical value.
The digital meter displays the measurements as discrete numerical instead of a pointer deflection on scale. They have high input impedance and the user has to only set the function switch and read the measurement.
The basic function performed is an analog to digital conversion. The analog signal input might be a DC voltage, an AC voltage, a resistance or an AC or DC current. Thus a digital value is converted to a proportional time duration which in turn starts or stops an accurate oscillator. The oscillator output is fed to a counter which drives a digital readout arrangement in terms of voltage values.
DMM is classified according to the number of full digit displayed. An over range digit is an extra digit to allow the user to read values beyond full scale. An over range digit is sometimes known as ‘one half’ digit. For example if a signal changes from 9.999 to 10.012 a four digit display will require a change in range and the second measurement will read 10.01V. The 0.0002 will not be read. On a Four and Half digit display this problem will not occur.
Apart from reading the values of voltage, current and resistance, DMM can also be used to measure temperature, frequency, duty cycle, capacitance, and other parameters with the help of optional accessories. They are used to perform Diode Checks and Continuity Checks in a circuit.
The diode is a semiconductor device, which conducts direct current in one direction only. In other words, the diode exhibits a very low resistance when it is forward-biased and an extremely high resistance, when it is reverse-biased. An ohmmeter applies a known voltage from an internal source (batteries) to the measured resistor. Theoretically, this voltage can reach 1.5 V or 3 V. The diode requires a voltage of 0.7 V to become forward-biased. Therefore, if the positive test lead of the ohmmeter is connected to the anode and the negative test lead of the ohmmeter is connected to the cathode, the diode becomes forward-biased. In this case, the ohmmeter reads a very low resistance. If the test leads are reversed with respect to the anode and the cathode, the diode becomes reverse-biased. Then, the ohmmeter reads a very high resistance. Thus an ordinary ohmmeter can be used to test a diode.
Most digital multimeters (DMMs) have a diode test function. It is marked on the select switch with a small diode symbol. When the DMM is set to diode test mode, it provides a sufficient internal voltage to test the diode in both directions. The positive test lead of the DMM (in red color) is connected to the anode, and the negative test lead of the DMM (in black color) is connected to the cathode. If the diode is in good working order, the multimeter should display a value in the range between 0.5 V and 0.9 V (typically 0.7 V). Then the test leads of the DMM are reversed with respect to the anode and the cathode. As the diode in this case appears as an open circuit to the multimeter, practically all of the internal DMM voltage will appear across the diode. The value on the display depends on the meter’s internal voltage source and it is typically in the range between 2.5 V and 3.5 V.
A defective diode appears either as an open circuit or as a closed circuit in both directions. The first case is more common and it is mainly caused by internal damage of the pn-junction due to overheating. Such a diode exhibits a very high resistance when it is both forward-biased and reverse-biased. On the other hand, the multimeter reads 0 V in both directions if the diode is shorted. Sometimes a failed diode may not exhibit a complete short circuit (0 V) but may appear as a resistive diode, in which case the meter reads the same resistance in both directions (for example 1.5 V). This is illustrated in Figure 3.16.
As was mentioned earlier, if a special diode-test function is not provided in a particular multimeter, the diode still can be checked, by measuring its resistance in both directions. The selector switch is set to OHMs. When the diode is forward-biased, the meter reads from a few hundred to a few thousands ohms. The actual resistance of the diode normally does not exceed 100 Ω, but the internal voltage of many meters is relatively low in the OHMs range and it is not sufficient to forward-bias the pn junction of the diode completely. For this reason, the displayed value is higher. When the diode is reverse-biased, the meter usually displays some type of out-of-range indication, such as “OL”, because the resistance of the diode in this case is too high and cannot be measured from the meter.
The actual values of the measured resistances are unimportant. What is important, though, is to make sure that there is a great difference in the readings, when the diode is forward-biased and when it is reverse-biased. In fact, that is all you need to know. This indicates that the diode is working properly.
So far we have looked at meters which give a picture of the static levels of voltage or current. For more compete tests on the operation of a circuit, we need to be able to examine the way in which the signal varies with time. This involves displaying a graph of the signal being examined against a base of time, and the instrument employed for this is the Oscilloscope.
It gives a visual indication of what a circuit is doing and shows what is going wrong more quickly than any other instrument. Multimeter can detect the presence of signals and if the shape of the signal is known the average, peak, rms or peak to peak can be calculated. However, if the waveform is not known, then this is not possible. Noise may be superimposed on the signal and the multimeter will not be able to give the proper information. The oscilloscope gives a true and clear picture of the waveforms.
The following figure shows all the essential controls on the front panel. The controls can be present in some different form than shown, but they have to be present in the oscilloscope.
The controls are as follows:
Sometimes the ON / OFF control can be combined with Intensity / Brilliance control.
The instrument is directly plugged in the mains supply. After switching on the instrument, wait for a while until the CRT heater warms up. Turn the Brilliance control to clockwise direction until you observe a horizontal line of the trace on the screen.
If the trace does not appear on the screen then turn the Brilliance control right up to the fully clockwise direction. Turn Time/cm control to the slowest speed, but not to the off position. With these settings, a light spot should appear on the screen moving slowly from left to right.
Still if nothing is seen, adjust the Trig/Level control in clockwise direction and observe if something appears. Adjust the vertical and horizontal position controls until the trace appears.
If all the above steps do not result in showing a trace on the screen, the instrument is faulty. Unplug the mains and check the fuses.
After getting a trace on the screen use vertical and horizontal position controls to start the trace at the left hand side of the screen and lie along the centre line. Focus control is used to get the line as thin as possible. Reduce the Brilliance setting to a comfortable viewing level.
When making oscilloscope measurements, a pair of probes is very valuable and this facilitates making a contact on the point of measurement in a convenient manner. Probes connect the measurement points in the device under test to the inputs of the oscilloscope.
When the signals being examined have relatively low frequencies, such as the waveforms expected of an audio amplifier, the capacitance of the test leads usually poses no problem and has little effect on either the waveform of the signal being displayed or the circuit being tested.
When high frequency signals or fast pulses are being examined the capacitance between the core and screen of the input cable can affect the waveforms that are displayed and may upset the circuit being tested.
The capacitance between the core and screen of a typical 1 meter long input cable could be about 50pF which, when added to 50pF input capacitance of the amplifier will give a total shunt capacitance of 100pF across the circuit being tested.
Suppose the circuit being examined is a video amplifier with a load impedance of 1K and the signal being examined is a 10Mz square wave. The displayed waveform on the oscillator will become triangular in shape because the capacitor is unable to charge and discharge fast enough through the amplifier load resistor to be able to follow the 10Mz square wave.
One way of overcoming this problem is a special probe at the input end of the test lead. This probe is usually arranged to act as a divide by ten attenuator and the circuit arrangement is as shown in the figure below:
The dc component of the signal is attenuated by a pair of resistance, forming a simple potential divider. To balance up the capacitive reactance a small series capacitor is connected across R1. The value of this capacitor is adjusted so that it has a capacitive value which is 1/9 of that of shunt capacitance of the lead and the oscilloscope amplifier input.
For example where the oscilloscope has a shunt capacitance of the order of 50pF, the series connection capacitor becomes approximately 5pF. Now, when the probe is used to examine the video amplifier circuit, it presents an effective reactance of around 3K at 10Mz and will therefore have much less effect on the signal being examined.
When a probe is included in the input line, it is important to match the probe to the oscilloscope input. This is usually achieved by adjusting the small compensation capacitor in the probe to produce the correct results on a square wave input. Most oscilloscopes provide a square wave test signal for setting up input probes. This signal is applied to the probe input and the probe capacitor is then adjusted to give a correct square on the screen.
If the compensation capacitor in the probe is too large, it will not produce the correct attenuation ratio for high frequency signals. In a square wave input this will give rise to overshoot on the edges of the square wave as shown in following figure:
When the compensation capacitor is too small the higher frequencies are attenuated too much and this produces rounded corners on the square wave as shown in the figure (b).
With the correct setting of the compensation capacitor there could be no overshoot or rounding off on the edges of the square wave and the waveform is displayed correctly.
When using an oscilloscope, it is very easy to plug the oscilloscope probe in and start to make measurements. Unfortunately oscilloscope probes need to be calibrated before they are sued to ensure that their response is flat. There is a built in calibrator on virtually every oscilloscope for this purpose. It provides a square wave output, and there is a small preset adjustor on the probe. With the oscilloscope probe connected to the output of the calibrator the shape of the waveform displayed on the screen should be adjusted until it is perfectly square. If the high frequency response of the probe is down then the edges of the square wave will be rounded. If it is up then the square wave edges will show overshoot.
Although a simple adjustment, it is essential that it is undertaken to ensure that the performance of the probe is correct.
Oscilloscope’s greatly and effectively help in finding out the amplitude of voltage.
The number of centimeters on the vertical scale from the negative peak to the positive peak is counted. This count is multiplied by the setting of the volts per centimeter switch.
For example: if 5 V/cm is the volts/cm setting and the waveform measures 4.8V from peak to peak then the waveform voltage is 4.8 * 5 = 24V Peak to Peak.
For frequency measurement the time period of one complete cycle is measured. This is merely the horizontal distance between the two identical points on the neighboring waves.
This distance is then multiplied by the setting of the Time/cm switch and the period of one cycle is calculated. The reciprocal of this time is the frequency of the wave.
For example if the peaks of the waveform are 5 cm apart, and the Time / cm switch is set to 200 μ s / cm, the time of one complete cycle is 5* 200 = 1000 μ s = 1 ms and the frequency is 1 / 1000 = 1 KHz.
If we have two signals of the same frequency and wish to measure the phase difference between them, we can do it by using a dual trace oscilloscope. One signal is fed to CHANNEL1 input and the other to CHANNEL2 input.
The VH1 position is adjusted to place the CH1 Trace so that it is centered about the horizontal axis of the screen. The CH2 trace is then moved to place it over the CH1 trace. The X position control is then adjusted to move the point where the CH1 trace crosses horizontal axis to line up with the left hand vertical line.
The distance between the crossing point of the CH1 trace and the corresponding point of CH2 trace is then measured along the horizontal axis as shown in the following figure. The total period of one cycle of CH1 waveform is also measured:
The phase shift will be the difference in position between the two traces divided by the total wave period and the result is multiplied by 360 to get phase in degrees.
If we have to compare the phase relationship between two AC signals, then apply one signal to X plate of the tube and the other signal to Y plate of the tube. This produces a display which is generally referred to as Lissajous figure.
On dual trace oscilloscope there is usually a position of TIME / DIV switch which selects the CH2 signal. With this mode selected one signal is applied to the CH1 input and the other to the CH2 input.
When the two signals applied have the same frequency and are exactly in phase, the result will be a diagonal line on the cathode ray tube which will run from the bottom left of the screen to the top right as shown in the following figure(a):
If one of the signals is now reversed in polarity, so that it is 180 degrees out of phase with another signal, the result is still a straight diagonal line but now it will run from the top left to bottom right of the screen as shown in figure (b).
When two signals are not quite in phase with one another, the diagonal line changes to an ellipse running diagonally from bottom left to top right of the screen as shown in figure (c).
As the phase difference is increased, the thickness of the ellipse will increase until it becomes a circle when the signals are 90 degrees out of phase as shown in figure (d).
The above results assume that the signals being compared are sine waves which are of the same amplitude. It is also assumed that the deflection sensitivities of X and Y circuits of the oscilloscope are the same. If the signal amplitudes or deflection sensitivities are not identical then the resultant image will be stretched in a direction with higher sensitivity.
When the waveforms being examined are not sine waves the Lissajous display becomes distorted but generally follows a similar type of pattern.
An oscilloscope is an excellent tool to see what is happening in the circuit and with experience much can be gained from the correct interpretation of what is displayed.
If a sine wave is given to an amplifier and the oscilloscope displays a flat topped waveform when connected at its output, it means that clipping is taking place in the amplifier.
Oscilloscopes have always been an important measurement tool for the engineer. The design of oscilloscopes has evolved slowly from early instruments which were used to simply view a waveform, to oscilloscopes with calibrated ranges and graticules (grid) on the display to enable measurements to be made, up to the modern digital storage oscilloscope (DSO) which have many advanced measurement functions built in as standard. The latest designs now use digital LCD displays instead of the tradition CRT (cathode ray tube) and are putting even more measurement power in the hands of the engineer in ever more portable instruments. The oscilloscope is still evolving, the latest step is the scope meter which combines the functions of an oscilloscope with those of the DMM in one instrument. Each evolutionary step has added to the measurement capability of the oscilloscope, making the calibration of these instruments even more important.
All types of oscilloscopes require calibration of these main functions.
The oscilloscope amplitude is calibrated by applying a low frequency square wave and adjusting its gain to meet the height specified for different voltage levels (shown by the graticule line divisions on the oscilloscope). The voltages that are used for calibration are selected using the corresponding setting as per the amplitude ranges on the oscilloscope. Using this output the waveforms should be aligned with the graticule markings on the oscilloscope display. When calibrating the oscilloscope’s amplitude gain, it isl needed to set different voltages and check that the gain matches the graticule height lines on the display of the oscilloscope within the specifications as supplied by the oscilloscope’s manufacturer.
The time base of an oscilloscope is calibrated to ensure the horizontal deflection meets the manufacturers specifications. A time marker signal is generated from the calibrator of which the peaks are aligned with the graticule scale on the oscilloscope display.
Calibration of bandwidth requires a constant amplitude sine wave of variable frequency up to and above that of the oscilloscopes specification. Many calibration procedures also call for a 50kHz reference level to set the start amplitude.
Trigger level can be tested by using a sinusoidal signal at 6 divisions high and adjusting the trigger level control to produce a stable trace starting at any point on either the positive or negative slope depending on scope selection. Sensitivity is tested by applying a much smaller signal (typically 10% of FS) and checking a stable trace can be obtained even when the position controls are used to move the trace to the top or bottom of the display. Bandwidth of the triggering and operation of the HF noise filters on some scopes can be tested by using the leveled sweep output and increasing the frequency or until stable triggering is lost.
Make the following settings before switching on the oscilloscope or after completion of its use:
Use fully screened probes at high frequencies to avoid possibility of signal degradation. The use of a compensated probe unit reduces the effect due to amplitude attenuation and phase distortion in a coaxial cable.
Keep the beam intensity down to the minimum required for a particular setting.
Make sure that the vertical gain control is set above the voltage of the signal to be measured. Start with the highest voltage setting and minimum sensitivity, then work down the range until the correct setting is reached.
Avoid displaying a stationary bright dot for a long time. It may burn the phosphor on the screen.
The simplest form of resistance measurement is a continuity test which merely checks to see if there is a conducting path between two points in a circuit. This test simply indicates whether the resistance between two points is high or low and is convenient for tracing individual wires through a multi-wire cable or for tracing out track connections on a printed circuit board. One popular circuit for a continuity tester is shown in the following figure:
Here a buzzer is connected in series with a battery and the two test leads. One test probe is connected to one end of the wire or circuit that is to be checked and the second probe is applied to the other end of the circuit. If the resistance between the two test points is low, the buzzer sounds to indicate continuity.
As an alternative to the buzzer the continuity tester might use a filament lamp or a light emitting diode as continuity indicator as shown in the following figures. The lamp or LED lights up when continuity is detected between the points to which the test probes are applied:
Most modern audio signal sources provide not only a sine wave but also square and triangular wave signals as well. These instruments are generally referred to as waveform generators to distinguish them from ordinary signal generators which produce only a sine wave output.
In this instrument the basic triangular waveform is generated by using a capacitor charged and discharged at constant current as the timing device. The basic block diagram of such a device is shown below:
The triangular signal is generated by using the voltage produced across a capacitor which is charged and discharged alternately by being switched to current source I1 and sink I2. The voltage the capacitor is fed to a pair of level comparators which detect when the capacitor voltage reaches two preset voltage levels. The output of the comparators drive a flip-flop which in turn switches the constant current sources I1 and I2 via switch S1.
For the rising slope of the triangular wave the capacitor is switched so that it charges linearly with time from current source I1. When the capacitor voltage reaches the reference level of comparator A1, the output of A1 triggers the flip-flop circuit and this in turn operates switch S1. The capacitor is now discharged by the current source I2 and falls linearly with time until it reaches the reference level of comparator A2.
The output of A2 is used to reset the flip-flop and this operates switch S1 so that the capacitor again discharges from I1 to start the new oscillation cycle. The result is that the voltage across the capacitor rises and falls linearly between the two reference levels to produce the triangular output waveform.
The amplitude of the waveform is determined by the voltage reference levels applied to the two comparators and the frequency by the value of the capacitor and the levels of current from generators I1 and I2.
Since the flip-flop switches state each time the triangular reverses its direction, the output from the flip-flop is a square wave whose frequency is the same as that of triangular wave.
The square wave produced will be 90 degrees out of phase with the triangular wave since the flip-flop is switched at the peaks and troughs of the triangular wave.
For experimental troubleshooting, a switched resistance box is a useful accessory. The ideal arrangement is a true decade resistance box giving perhaps three decades of selectable resistance. The basic circuit arrangement of this type of resistance box is shown in the following figure:
For simplicity the diagram shows only two decades. In this arrangement the box provides a range of resistance from 0 to 9.9KΩ in 100Ω steps. A typical box might have four banks with the lowest, giving 10Ω steps and the highest giving 10KΩ steps which would allow resistance values from 0 to 99.99KΩ to be selected in 10Ω steps.
Thus in a 10kΩ bank each resistor has the value 10kΩ. At the zero position the bank is shorted out but as the switch rotor is moved 10k, resistors are added in series between the rotor and the input terminal.
The output of a 10k bank switch feeds the top end of the 1k resistor bank and here the switch adds a selected number of 1k resistors in series. The 100Ω and 10Ω banks are wired in the same way and finally the wiper of the 10Ω selector switch comes out to the other input terminal of the resistance box.
The switches can be decimal type thumbwheel switches and the resistors in this type of box need to be at least 1 percent tolerance metal oxide types to give useful results.
For a home constructed unit using 1 percent components only the two most significant digits of the reading on the selector switches should be considered valid when assessing the value of resistance. In a commercial resistance box, the resistors are usually 1 percent tolerance components which have been measured and selected to give the correct values to within 0.1 percent or better.
It is possible to have a switched capacitor box which operates in a similar way to the resistor box. In this case the capacitors in each decade are connected successively in parallel to produce the desired value of the capacitor and the total capacitance of each decade is connected in parallel with that of the other decades.
Because of stray capacitance effects, about the lowest increment of capacitance that is practical would be 100pF. Thus a box could be built with the first decade going up to 1nF and successive decades to 10nF, 100nF and 1µF respectively.
For the lower decades polystyrene or silver mica capacitors with 2 percent tolerance might be used to give reasonable accuracy and good stability. For the higher ranges metallised polyester film capacitors of 5 percent tolerance might be used.
Testing and tagging is the process of visually inspecting and electrically testing in-service electrical equipment.
The aim of test and tagging is to determine if the appliance is electrically safe for personal use. The appliance undergoes a visual inspection for defects such as damage or missing components and a number of electrical tests to measure earth continuity, insulation resistance and polarity.
Test and Tagging is sometimes referred to as PAT Testing (UK). This terminology was first used in the United Kingdom. A Portable Appliance Test or PAT, is a process by which electrical appliances are routinely checked to see whether they are safe. The term PAT – PAT Testing – more accurately describes the actual test equipment used by technicians (not the appliance being tested) as it is generally hand held and/or portable.
In Australia and New Zealand, this is done using the Standard; AS/NZS 3760:2010 “In-service inspection and testing of electrical equipment” as a reference document.
PAT tests include:
-earth resistance test
-earth continuity test
-insulation resistance tests
-polarity checks etc.
In addition to above the regulations require that wherever any plug, socket or adapter is supplied, that it complies with the appropriate current standard. This means that they must conform to the relevant British Standards (i.e. BS1363) or approved alternatives. British Standard BS1363 covers 13 Amp fused plugs, switched and unswitched sockets. The standard now requires that the live and neutral pins on plugs are part insulated so as to prevent shocks when removing plugs from sockets.
The multimeter facilitates the measurement of DC voltage, AC voltage, DC current and resistance values.
The analog multimeter can become a DC voltmeter, an AC voltmeter, and DC milli-ammeter or an ohm meter. The digital meter displays the measurements as discrete numerical instead of a pointer deflection on scale.
An oscilloscope gives a visual indication of what a circuit is doing and shows what is going wrong more quickly than any other instrument. Amplitude and frequency can be measured with an oscilloscope.
The Continuity tester checks to see if there is a conducting path between two points in a circuit. In a Waveform Generator, the basic triangular waveform is generated by using a capacitor charged and discharged at constant current as the timing device.
In the last chapter we have seen various tools, testing and measuring instruments. In this chapter we are going to study troubleshooting of passive and active components. The chapter also emphasizes testing of some other active components such as JFET, MOSFET, UJT, SCR, DIAC, TRIAC, LED, Photo detectors, Op-amp and Timer IC555. The equipment required for testing are Digital Multimeter / Analog Multimeter, Scientific Oscilloscope and test jigs.
Passive components include resistors, capacitors and inductors. Passive components can not amplify or oscillate a signal. Let us now go through the testing procedure of these one by one.
Resistance is the opposition to the flow of current offered by a conductor, device or circuit. According to Ohm’s Law:
R = V / I in ohm, Where:
R is Resistance (ohm)
V is Voltage (volts)
I is Current (amperes)
Basically resistors are categorized into two types:
They come in following forms:
They can be divided into three categories depending upon resistive material used:
Depending upon the number of resistance and control arrangement they are divided into:
The CT is switched on by depressing the CT push-button. A shortened horizontal trace will be observed. For the component connection, two simple test leads with 4 mm banana plugs are required. The test leads are connected to the insulated CT socket and the adjustment ground socket in Y-section.
Semiconductor diodes are broadly classified into two areas:
Bipolar: where the device action depends upon the flow of the type of charge carriers across forward or reverse biased p-n junction. In an n-p-n transistor, the current is controlled by a forward biased base-emitter junction. The commonly used bipolar devices are transistors, diodes, UJT, Thyristor, Logic ICs (TTL) and Linear ICs.
Unipolar: where the devices use only majority carriers for current flow which is controlled by an electrostatic field. FET, MOSFET, CMOS logic and some linear ICs are some good examples of unipolar devices.
Please refer Chapter 3 for diode testing using DMM.
The field effect transistor is a voltage operated device. Unlike a bipolar transistor, a FET requires virtually no input (signal or bias) current. This gives it an extremely high input resistance, which is its most important advantage over a bipolar transistor. There are two major categories of FET: junction FET and MOSFET (metal oxide semiconductor FET). These are further sub-divided into p-channel and n-channel.
With zero gate-to-source bias, these devices are off, and are increasingly turned on by the application of increasing gate-to-source bias. This bias is positive for n-channel type and negative for p-channel type.
FET has three terminals named source, drain and gate (which correspond to emitter, collector and base of junction transistor). Source and drain leads are attached to the same block (channel of n or p semiconductor material). A band of oppositely doped material around the channel between the source and drain leads is connected to gate lead.
Because of the infinite input impedance, no current can flow between the input terminals of the amplifier. The voltage across the input terminals of op-amp must be zero or negligibly small.
Op-amp has infinite voltage gain; it means that we get a very large voltage output from a very small voltage input. Even with a small voltage at input, the amplifier is driven into positive or negative saturation very easily.
Op-amps can be tested with the help of a test jig as shown in the following figure:
IC 555 can be tested with a test jig as shown in figure 4.21:
In electronic systems, switches are used to make a particular function (like power) either be ON or OFF. There are only two active states of a switch: ON and OFF. These two states are achieved with the help of two pieces of metal which are known as contacts. These contacts touch to make a circuit and separate to break a circuit. The common types of switches used in electronic equipment are as follows:
Let us address these one by one:
This is the most popularly used switch in electronic systems. The meaning of toggle switch is that it can be turned ON when pressed and it springs back to OFF when released. The various versions of toggle switch are:
Here the two terminals are either connected together or not connected to anything.
It is a simple changeover switch where the contacts can be made with either of the two terminals.
It is equivalent to two SPST switches together, which are controlled by a single mechanism.
It is equivalent to two SPDT switches together, which are controlled by a single mechanism:
These switches are used where a reset or preset pulse is required. A pole is a set of contacts that belong to a single circuit. A throw is one of two or more positions that the switch can adopt.
It is similar in action to toggle switch. Many times it incorporates a lamp indicator under the pushing surface.
In this type, there is a holding coil which latches the switching ON position when depressed. When the switch is pressed for the second time, it releases the voltage from the holding coil and returns the switch to its OFF condition.
It is spring loaded switch and is often placed in a drive unit so that closing its contacts causes its cycle to repeat or stop. In other words, when the switch is operated, it causes change in operation.
Usually a defective switch is not repairable. It is recommended to replace the broken or defective switch. The defects in a switch can be loss of continuity between the contacts, improper spring loading function because of a defective spring, loose toggle, broken or burnt switch body, and so forth.
In order to make an in-circuit test on a switch a VOM can be employed. The following figure shows how to use a VOM for this purpose:
The meter prods are connected or touched to both the sides of switch. The meter shows infinite resistance when the switch is in OFF position.
When the switch is closed, i.e. ON, the meter shows zero resistance indicating that the two sides of the switch are electrically connected.
If the switches are found to be faulty, replace the switch. Carefully switch off the power while replacing it. If there are any connections made to the switches, label all the wires connected to it before removing. When the new switch is replaced, connect all the wires in their proper positions. Turn on the power and check the working of the switch by the procedure described above.
Portable Appliance Testing (commonly known as PAT or PAT Inspection or PAT Testing) is a process in the United Kingdom, New Zealand and Australia by which electrical appliances are routinely checked for safety. The correct term for the whole process is In-service Inspection & Testing of Electrical Equipment.Test and tagging is a generic name given to the process of visually inspecting and electrically testing in-service electrical equipment for personal use and/or safety The aim of test and tagging is to determine if the appliance is electrically safe for personal use. The appliance undergoes a visual inspection for defects such as damage or missing components and a number of electrical tests to measure earth continuity, insulation resistance and polarity.
In Australia and New Zealand, this is done using the Standard; AS/NZS 3760:2010 “In-service inspection and testing of electrical equipment” as a reference document.
The tests an appliance is required to undergo will depend on the type of appliance, it’s electrical Class and subject to a risk assessment by the technician. The tests normally done under PAT testing are:
Passive components like resistors, capacitors and inductors can not amplify or oscillate a signal.
A diode can be conveniently checked with an OHM METER by measuring its forward and reverse resistance. A signal diode shows a low resistance (a few hundred ohms) in the forward direction and a high resistance (nearly infinity) in the reverse direction.
In a bipolar transistor, shorting base to emitter turns off transistor, while forward biasing base-emitter junction turns on transistor. Shorting collector to emitter simulates saturation, as the transistor behaves like a closed switch.
JFET and MOSFET are the two types of FET which are again divided into n-channel and p-channel. The basic functionality of the field effect transistors can be checked by assuring the flow of current from source to drain.
Special care has to be taken while dealing with MOSFET. The SiO2 layer which is very thin may rupture even because of the static charges.
In today’s electronic world Digital Integrated Circuits are used extensively. There is hardly any area in which digital circuits are not used. The digital circuits operate from defined voltage levels which give a certain defined output. The systems which employ mainly digital circuits have predictable behavior of the circuit.
The basic elements of all digital circuits are logic gates that perform logical operations on their inputs.
Information carrying signals are divided into two broad classes:
Analog Signals are continuous where digital signals are discrete. Analog signals are continuously varying where digital signals are based on 0’s and 1’s.
Analog signals are continuous electrical signals that vary in time. The variations follow the original signal, showing that the two are analogous. Hence the name is analog.
In the above analog or continuous signal, voltage is defined for all time instances.
Digital signals consist of pulses or digits with discrete levels or values. The value of each pulse is constant, but there can be an abrupt change from one digit to the next. A digital signal uses some physical property, such as voltage, to transmit a single bit of information.
Suppose we want to transmit the number 6. In binary, that number is 110. We first decide that, say, a “high” means a 1 and “low” means a 0. Thus, 6 might look like:
In the above digital signal, voltage is defined at integer time instances.
Examples:
The information can be represented in terms of some form of analog or digital signal. The digital data stored on a CD will normally have been produced using analog to digital converters.
In fact, Analog and Digital signals are no more than mathematical representations of a signal, which is useful when we want to process information.
Digital circuits cover a wide range from high current industrial motors to microprocessors. However, the basic elements of all digital circuits are logic gates that perform logical operations on their inputs. Therefore, it is essential to understand the basics of logic gates before discussing troubleshooting techniques.
The truth table lists all the possible things that can happen at the input and output terminals. Any logic device can be explained with the help of a truth table. It is a tabular explanation of the logic device which represents its output side for any set of inputs. Following is an example of truth table:
Input States | Output States | |
A | B | |
0 | 0 | 0 |
0 | 1 | 0 |
1 | 0 | 0 |
1 | 1 | 1 |
So there is an input stage and output stage. The above logic circuit has two inputs A and B. Depending upon the combination of these two inputs, the output changes. It is not necessary that these inputs will always be zeroes and ones. The following table shows the other possible things which they can represent:
Motor | Not running = 0 | Running = 1 |
Lamp | Off = 0 | ON = 1 |
Tape Recorder | Not recording = 0 | Recording = 1 |
TV set | Off = 0 | ON = 1 |
Relay | Not energized = 0 | Energized = 1 |
Transistor | Cut off = 0 | Saturated = 1 |
Voltage | 0V = 0 | 5V = 1 |
Door | Open = 0 | Closed = 1 |
There can be a number of logic states. The following table shows some examples:
0 | 1 |
Open | Closed |
Positive | Negative |
True | False |
ON | OFF |
Positive | Negative |
Two-state operation is widespread in digital electronics because it is the most reliable way to operate transistors and other switching devices. With two state operations all the signals are easily recognizable as either high or low.
In the following figure there are only two inputs A and B and one output X. So it is known as a two input AND gate. It is possible for the AND gate to have more inputs, but there is only one output terminal:
Referring to the equivalent circuit, only when, A and B switches are closed simultaneously, there can be an output, i.e. when the power is applied both the switches must be closed before the lamp X will light:
A | B | X |
0 | 0 | 0 |
0 | 1 | 0 |
1 | 0 | 0 |
1 | 1 | 1 |
The truth table shows that there is only one way to get logic 1 at output. That is all the inputs must be at logic 1 level to get logic 1 output.
In digital electronics, the AND gate is used as a control element with one input regulating the traffic through others. When the presence of two or more factors is necessary to produce the desired result, the AND gate is employed.
The equivalent circuit shows that the switches A and B (inputs) are connected in parallel. There will be an output X, whenever there is logic 1 at either or both the inputs. If either or both the switches is closed, power will be applied to the lamp and then it will glow:
A | B | X |
0 | 0 | 0 |
0 | 1 | 1 |
1 | 0 | 1 |
1 | 1 | 1 |
The truth table shows that there is only one way to get logic 0 at output. That is all the inputs must be at logic 0 level to get logic 0 output. In order to get logic 1 output, either or both the inputs must be at logic 1.
In digital electronics, the OR gate provides the means of achieving a desired result with a choice of two or more inputs.
With this type of gate the output is always opposite of input. In the relay circuit shown below, the lamp X is ON whenever the circuit is not energized. That is, whenever the switch A is open, the lamp glows. When the switch A is closed, the relay is energized and the lamp is OFF:
A | X |
1 | 0 |
0 | 1 |
The truth table shows that output is always opposite of input, irrespective of the logic state of input. It is also known as an ‘inverter’ gate. It is applied if it is necessary to change the state of information before it is used.
Most of the times, the inverter is used in conjunction with another logic element. In that case it is represented by a small circle directly connected to the other logic element.
The gate is negated AND gate. The NOT gate and the AND gate can be combined together to get a NAND gate.
In the following figure there are only two inputs A and B and one output X. So it is known as a two input AND gate. It is possible for the AND gate to have more inputs, but there is only one output terminal:
Referring to the equivalent circuit, the switches A and B (inputs) are connected in series and this series combination is connected in parallel to a lamp X (output). In order to make the lamp ON, at least one switch must be open:
A | B | X |
0 | 0 | 1 |
0 | 1 | 1 |
1 | 0 | 1 |
1 | 1 | 0 |
The truth table shows that if any one of the inputs is logic 0, the output is logic 1. When both the inputs are logic 1, the output is logic 0.
The NOR gate can be obtained by inverting the output of an OR gate, which produces a ‘NOT OR’ gate.
In the equivalent circuit, the two switches A and B (inputs) are connected in parallel with the lamp X (output), as shown. The lamp glows, only if all the switches are left open:
A | B | X |
0 | 0 | 1 |
0 | 1 | 0 |
1 | 0 | 0 |
1 | 1 | 0 |
The truth table shows that when both the inputs are at logic 0 levels, then only the output is logic 1, otherwise for all the other combinations the output is logic 0.
It is known as Exclusive OR gate. It can be regarded as a combination of AND and OR gate. The equivalent circuit is as shown in the following figure. The two switches A and B (inputs) are connected as shown. The lamp X glows only when one switch is ON and the other is OFF. The lamp does not glow when both the switches are in same condition, such as either ON or OFF:
A | B | X |
0 | 0 | 0 |
0 | 1 | 1 |
1 | 0 | 1 |
1 | 1 | 0 |
The truth table shows that when both the inputs are at logic 0 levels or logic 1 levels, then the output is logic 0, otherwise the output is logic 1.The EX-OR gate can be used to compare the inputs. It gives zero output whenever the inputs are matched. When the inputs are the same, it gives positive output.
It is known as Comparator gate. It can be regarded as a combination of AND and OR gate. The two input comparator gate is a combination of one AND gate and two OR gates as shown in the configuration:
The output of comparator will stand at defined ‘1’ state only if all the inputs stand at their defined ‘1’ states or if none of the inputs stand at its defined ‘1’ state.
A | B | X |
0 | 0 | 1 |
0 | 1 | 0 |
1 | 0 | 0 |
1 | 1 | 1 |
The truth table shows that when both the inputs are at logic 0 levels or logic 1 levels, then the output is logic 1, otherwise the output is logic 0. The EX-NOR gate can be used to compare the inputs. It gives positive output whenever the inputs are matched. When the inputs are not same, it gives negative output.
It is an OR gate with an inhibiting input. Here, the output is at its ‘1’ state if and only if the inhibit input stands at its defined ‘0’ state AND one or more of the normal OR inputs stand at their defined ‘1’state:
A | B | C | X |
0 | 0 | 0 | 0 |
1 | 0 | 0 | 1 |
0 | 1 | 0 | 1 |
1 | 1 | 0 | 1 |
0 | 0 | 1 | 0 |
1 | 0 | 1 | 0 |
0 | 1 | 1 | 0 |
1 | 1 | 1 | 0 |
The truth table and the configuration shows that when C input (inhibit) is zero AND any one of the normal OR inputs A and B is 1 , then only the output X is at logic 1 state, otherwise it is at logic 0 state.
This gate is very useful for controlling inputs (A and B) by means of the inhibiting signal C. When the inhibiting signal is present (C = 1), the output is always OFF (X=0), but when the inhibiting signal is absent (C = 0), the signals (A and B) can pass through the output.
The digital circuits are invariably constructed with ICs. Digital IC gates are classified not only by their logical operation but also by their specific logic circuit family to which they belong. Each logic family has its own basic electronic circuit upon which more complex digital circuits and functions are developed. The basic circuit in each family is either a NAND or a NOR gate. The electronic components employed in the construction of the basic circuit are usually used to name the logic family. There are many families of digital ICs. The popular logic families are discussed below:
TTL – Transistor-transistor logic
ECL – Emitter-coupled logic
MOS – Metal-oxide Semiconductor
CMOS – Complementary Metal-oxide Semiconductor
These can be some of the reasons for the faults to get induced in any digital circuit:
The typical failures include inputs or outputs shorted to ground, pins shorted to Vcc supply, pins shorted together, open pins and connections with intermittent defects.
There are hundreds and thousands of semiconductor devices assembled on one small chip. The critical problem for the chip manufacturers can be to get the voltages and signals in and out of such a tiny chip.
The wires which are used as inputs and outputs to the chips are very thin. Any type of thermal stress can affect these tiny wires and the bond may break away from the pad to the chip, causing an open connection.
The faults in a gate output manifest themselves as follows:
In the above figure the output is stuck at ‘1’ whereas with logic ‘1’ on inputs the output should be less than 0.8 V. The possible faults in this circuit could be internal transistor open circuit or 0 V line open circuits internally.
Here the output is stuck at ‘0’ whereas it should be logic ‘1’. The possible faults in this gate could be a short-circuited internal transistor, or Vcc line internally open.
The causes of the above faults are possibly due to internal IC failure. The internal failures can be of the following type:
In the above figure a solder bridge between U1 and U2 causes both nodes to indicate functional logic failures. Tracing current flow in the circuit quickly shows the location and cause of the fault.
Modern electronic equipment makes use of digital integrated circuits which are in very high demand. Usually the digital integrated circuits come in dual-in-line packages. An IC can be identified from the information given on the IC itself. On its surface an IC carries the following markings:
There is a white dot or a notch on the IC which indicate the orientation. Pin number 1 is always the upper left hand pin on the end of the IC that includes the notch. The pin numbers up down the left side of the IC and up the right side.
The following types of common faults are observed in an IC:
If the output if an IC is open, it is floating. Let us consider that this output is supposed to be given as an input to another IC. In TTL circuits a floating input rises to approximately 1.5V and usually has the same effect on the circuit operation as a high logic level. This means that an open output bond in an IC will cause all inputs driven by that output to an incorrect level which are usually treated as logic high level by the inputs.
If there is an open input bond inside an IC, the digital signal that drives the signal will be unaffected and will be detectable at input pin. It will be as though the input were at the static high level.
When there is a short circuit between input/output and VCC or ground, all signal lines connected to that input/output are held either high (in case of short to VCC )or low (in case of short to ground ). Thus, a fault usually causes normal signal activity at points beyond the short circuit to disappear and can be detected easily.
The following figure shows some common faults in a digital IC:
The faults are listed as follows:
Troubleshooting of the logic ICs can be performed in three different types of tests. Functional test, AC test and DC test. All these tests can be made using a pulse generator with DC offset facility.
Analog and Digital signals are mathematical representation of a signal, which is useful when some information is required to be processed.
The basic element of all digital circuits are logic gates that perform logical operations (AND, OR etc) on their inputs.
A logic circuit can have three basic signals: Logic 1, Logic 0 and a pulsating voltage signal that alternates between 0 and 1. A logic probe is used to measure these signals. It is always preferred over a voltmeter in digital troubleshooting.
The logic probe detects voltage levels or pulses at a given point in a digital circuit.
A Logic Pulser is used to stimulate digital circuits and supplemented by a logic probe it aids in testing for circuit response to easily check gates, lines, buses and nodes. The Logic Current Tracer detects whether current is flowing or not and where the current is flowing.
A logic comparator clips onto powered TTL or DTL ICs and detects functional failures by comparing the in-circuit test IC with a known good reference IC. The Logic Analyzer includes an oscilloscope for displaying various digital states.
Functional, AC and DC are the tests used for troubleshooting of ICs.
Electronic systems can be sub-divided into smaller sub-systems that are easily recognizable units like oscillators, power supplies, regulators, digital circuits and so forth. The technician must have a fair knowledge of troubleshooting of these sub-systems in order to troubleshoot an electronic system successfully.
In troubleshooting power supplies, the following steps are performed:
For the selection of power supply the following questions must be answered:
What is the voltage range over which the system must perform?
How much current does the system require?
How much ripple voltage can be tolerated?
How much voltage drift with temperature is acceptable?
Do we use one larger regulator or many individual regulators, to match the various load impedances?
Which type of regulator is most suited?
Having answered all the above questions, one can select a perfect power supply. However, cost and size of the power supply and consequently the system is equally important from the selection point of view.
Depending upon the technique which is used to provide regulated DC voltage, regulators are divided into the following three types:
Several features need to be incorporated in the series regulator circuits to protect them from overload currents. The two schemes that are usually employed for this purpose are discussed below.
This makes the power supply switch, to give almost zero output voltage if the value of the load current is exceeded beyond its specified range:
An SCR is the device which is used to limit the current.
A current monitoring resistor is placed in the power supply return line.
The voltage developed across this resistor is used to switch on SCR.
In case of an overload, SCR is switched on and the voltage across it falls to approximately 0.9V, which is insufficient to forward bias the series transistor.
The result of this is an appearance of zero voltage across the output.
If SCR is triggered once, it remains on until the fault is removed, after switching off the power supply, the output will remain zero.
Over voltage protection is required for all the digital circuits. If their Vcc terminal receives more voltage than required, then the ICs get damaged.
A zener diode is used to achieve the over voltage protection:
The zener senses the voltage across the power supply’s output terminal.
When the DC voltage rises, the zener starts conducting. This turns on SCR Q2.
The voltage at Q1 collector falls rapidly to zero and the fuse blows.
Above circuit can be called as Crowbar Circuit, which limits the output to the pre-set value.
Following are the troubleshooting techniques against High frequency oscillations, Low frequency oscillations, Ground loops and supply bypassing.
Many a times the problem occurs when connecting a control system to a device. A control system can be a PC and a controlled device can be any RS-232 based controlled device for example a telephone modem. The reason can be improper wire connection. When the connection of a control system to the controlled device is being done, the Transmit and Ground pins of the control system are connected to Receive and Ground pins on the controlled device respectively:
In the above figure, the short forms are used as follows:
XMT —– Transmit pin,
RCV —– Receive pins, and
GND —– Ground pins
The XMT pin from controlled device is connected to RCV pin of control system. This is done because the control system can receive an acknowledgement from the controlled device after the completion of data transfer.
Unless all the pins are labeled, it is difficult to make the proper wiring between the control system port and controlled device port. If the controlled device uses a terminal block type connector, it is quite easy to test the voltage using a voltmeter to ensure that the connection has been made correctly. The following figure shows the incorrect connection. Here the Receive and Transmit lines are reversed:
Set the voltmeter to ‘DC’, and test the voltage between the RCV and the XMT pin. The voltage on the Receive line stays at 0V, and the voltage on the Transmit line can be any value between -6V to -12V.
In order to ensure the correct interconnections, as shown below, test the voltage between the RCV pin and GND pin on the terminal block connector. The reading should be between -6V to -12V. The XMT line should also show the same reading:
Even after overcoming the problems due to wrong hardware connections, the problems appear, then you should confirm the communications software properties settings for both the control system and controlled device. The baud rate of both the devices should be identical. If the baud rate of the control system is 4800, the same should be the baud rate of the controlled device.
Data bit, stop bit and parity bit should also be defined properly. The data bit indicates how many bits are there in a single character to be transmitted; the parity bit defines the number of 1s during one byte transfer as even or odd; the stop defines the end of the data to be transmitted. Standard settings for these parameters for most A/V devices are 8, none, and 1 respectively.
Microprocessor is a clock driven semiconductor device consisting of electronic logic circuits. It is capable of performing various computing functions and making decisions. It is also known as a CPU (Central Processing Unit) that is fabricated on a single chip. All the logic circuitry, control units are fabricated on a single chip which is a thin piece of silicon onto which transistors making up the microprocessor have been itched.
A microprocessor is divided into three major parts:
Using a ALU microprocessor can perform mathematical operations like addition subtraction, multiplication and division. It also performs logical operations like AND, OR, EX-OR, etc.
The register array consists of various registers like B, C, D, E, H, L. The registers are primarily used to store data temporarily during the execution of a program. The user can access the registers for temporary data storage and transfer.
The control unit provides the necessary timing and control signals to all the operations in a microcomputer. It controls the flow of data between microprocessors and memory and peripherals.
The microprocessor has:
In the above block diagram of microprocessor registers A, B and C are simply latches made out of flip-flops.
The program counter is a latch used to hold memory address of the next instruction to be executed. It is incremented by 1, or reset to zero when given proper instruction. It keeps the track of memory address of the instruction in a program during the execution of instruction.
The instruction register and instruction decoder are special purpose registers which are responsible for controlling all the other components.
The tri-state buffers can pass a 1, a 0 or it can disconnect its output. It allows multiple outputs to connect to a wire, but only one of them to actually drive a 1 or a 0 onto the line.
The ALU performs all arithmetic and logical operations as said above. The Test register attached to ALU holds some values and can compare two numbers. The instruction decoder uses the value stored by the test register to make a decision.
As said earlier, the microprocessor is divided internally into three major parts: ALU (Arithmetic Logic Unit), Register Array, Control Unit. It is shown in the above figure. The microprocessor has the following sets of lines attached to it:
The microprocessor has several address lines over which it transmits an address to the off-chip memory or to the I/O devices. Collectively they are known as Address Bus. A typical 8-bit microprocessor would have an address bus of 16-bits to transmit the address, for example the address is 16-bits wide. So the microprocessor can access to 216 means 65,536 memory locations.
Data Lines are a set of lines to transmit and receive data. Collectively they are known as Data Bus. A typical 8-bit microprocessor would have 8-bits data bus (for example 8085); a 16-bit microprocessor has 16-bits data bus (for example 8086).
These are the lines to control the signals from input and output devices. The devices could be an electric motor, seven segment display, etc. They also generate control signals for input and output devices.
Multiplexing is a useful technique employed with microprocessors for obtaining additional address and data lines. By multiplexing it is possible to handle 16-bit data signals, through an 8-line data bus.
The address and data lines are also multiplexed and we have address and data available on the same physical lines at the different instants of time.
In a microprocessor based system, if the microprocessor fails to operate as per the requirement, the signals that should be first tested are clock signal, reset signal, address and data lines.
The clock signal is tested first, because it is one of the inputs to the microprocessor. If the clock signal is at fault, the whole working of the system may break down. It may stop working or may show intermittent behavior. The clock pulse is tested with a logic probe or oscilloscope. In modern CPUs the clock generator is built within the chip and it only requires an external crystal. The working of the crystal and the capacitor in the clock circuitry is also checked.
The reset signal makes the microprocessor go back to a known starting point. When the reset key is pressed, the pin should go low and then return to high. A logic probe can be used to check the presence or absence of a reset signal. At the key, if an absence of signal is noticed, then the reset signal line is checked.
The problem which can occur with address and data lines can be a short circuit with one of the data or address lines on the printed circuit board. The lines can be shortened to power supply (+5V DC), or it can be shortened to ground. Again a logic probe is used to check these lines. Each line should show activity which should be observed carefully whether it shows a steady high or steady low indicating a short to power or ground respectively.
The address and data lines are associated with tri-state buffers. If there is any failure in any one of these buffers, the correct signal will not reach the CPU.
If the suspected faulty line does not show short or break and tri-state buffers are also working fine, then there is a possibility of a problem in the other ICs associated with the line.
The purpose of the power supply is to convert the 230 V, 50 Hz AC mains supply into a form necessary for operating the internal circuitry of the equipment, which is usually a regulated DC voltage.
Voltage conversion, rectification, filtering, regulation and isolation are the main functions of power supply.
For a bad power supply, the problem is most likely caused by a faulty transistor, IC or a diode. The component is probably shorted.
Voltage regulator circuits are employed, to render the voltage more constant and to attenuate the ripple. Voltage regulator circuits are also available in IC form which is very popular.
Linear Regulator, IC regulators, Switched Mode Power Supply are the three types of voltage regulator.
Oscillators are used to generate periodic waveforms without any input signal. Lack of oscillations is the major fault in the oscillator which can be because of a faulty transistor, incorrect dc conditions, inadequate loop gain, and open signal bypass.
This chapter gives an idea of how temperature affects an electronic system and what should be the preventive steps to avoid the malfunctioning of the system due to temperature variations. Slightly deviating from the topic, the chapter also provides an understanding of Signal Injection and Signal tracing methods which are used often in troubleshooting processes.
There are some devices, which, when used in the equipment, protect the equipment against excessive temperature due to either a fault in the device or improper use of the equipment. These devices are Thermal Protection Devices. Thermal Fuse and Thermal Switch are used widely for temperature protection.
The methods of Signal Injection and Signal Tracing are especially useful for troubleshooting systems that have no output. However the methods can be used in all electronic systems.
Most of the time, electronic circuits are sensitive to temperature; the reason for this is that the components used in the circuit have some characteristics which are temperature dependant. Hence temperature plays an important role at the time of construction as well as operation of the electronic circuit.
Thermister and sensistor compensation are used against temperature variations. A thermister is a device with a negative temperature coefficient, and a sensistor is a device with a positive temperature coefficient.
The methods of Signal Injection and Signal Tracing are especially useful for troubleshooting systems that have no output. There is a simple device called signal injector which injects a pulse or a wave (square) into the system being tested.
In a signal tracing technique, a signal is injected at the input and the path of the signal through the system in traced. An oscilloscope is used which gives the visual effect of the signal being traced.
There are many phenomena which can cause a considerable amount of error in electronic systems, for example, noise, intermittent, EMI/EMC and so on. This chapter gives a brief idea of all these types of noises and how they are avoided.
The Boltzmann’s constant is also introduced which is used in a mathematical expression to calculate the amount of Noise Power and Noise Voltage.
Where:k is Boltzmann’s constant
T is absolute temperature in degrees Kelvin, and
B is the bandwidth in Hz
Where:k is Boltzmann’s constant
T is absolute temperature in degrees Kelvin
B is the bandwidth in Hz, and
R is resistance of the resistor
When you are using super coolant, you have to be careful, because it can produce instant frostbite! Furthermore, if inhaled into lungs strongly, they can cause permanent lung damage.
Many a time the unprotected environments fall pray to static voltages. These static voltages ranging from 1kV to 30 kV can be present in such environments. Static discharges can damage some electronic components. Care should be taken in order to avoid the effects due to static discharges.
There are four basic types of EMI as follows:
The following figure shows electromagnetic interference. Here the system itself is the source of EMI:
The source of the interference is outside the system. Refer to the following figure:
The source of the interference is inside the system. Refer to the following figure:
Here the electromagnetic energy can escape from the source conducting along wire and cable leaving equipment. The following figure shows conducted EMI:
For example, the following figure depicts a PC board without filtering or shielding:
There are three major types of crosstalk:
As the name suggests, Near-end crosstalk occurs near the transmitting device. A signal is applied to one pair of wire and cable and the amplitude of the induced signal along the other wires is measured. It is more troublesome than far-end crosstalk.
Far-end crosstalk occurs towards the end of the cable run. Here the distance matters and the signals that are transmitted get weaker with distance. The effects of FEXT are less severe than NEXT.
It comes into picture when more than two pairs of wires are transmitting data. PSNEXT measures the cumulative effects of NEXT from all of the other wire and cable pairs.
Some other types are also discussed below:
In direct crosstalk, the disturbing channel couples to the disturbed channel, and in indirect crosstalk, the coupling path between the disturbing and disturbed channels requires the third or tertiary channel. Interaction crosstalk (IXT) is a term used to describe indirect crosstalk that couples from the disturbing channel to the tertiary channel at one place prorogates along the tertiary channel, and subsequently couples into the disturbed channel at another place. Transverse crosstalk (TXT) is a term that includes all direct and indirect crosstalk that is not interaction crosstalk.
Let us see an example of crosstalk between oscilloscope channels. If there is a crosstalk between channels of an oscilloscope, it may result in a measurement error. The following figure shows inter-channel crosstalk in oscilloscope:
In an electronic system, much of the noise is generated by resistors, diodes, transistors and other semiconductor devices.
SNR (signal-to-noise ratio) is an important term to know the amount of noise. If the (SNR) is high enough, the noise problem is eliminated.
Boltzmann’s constant is used to calculate the amount of noise to be expected from a resistor or other semiconductor device. Using Boltzmann’s constant, noise power and noise voltage can also be calculated.
White noise, Flicker noise, partition noise are the types of noise.
The chapter gives an overview of the soldering techniques for repair and replacement of components. It explains the process of soldering, soldering tools and so forth. Also it flashes common faults in professional soldering, and how to recognize a good and bad solder connection. De-soldering technique is also explained.
Cadmium is a highly poisonous material. Never inhale any smoke from soldering. In fact, a small fan should be part of your bench equipment, to push the smoke away from you while you are soldering.
The process of repair of any electronic circuit involves following these major steps:
A good soldering practice is required for the removal and replacement of electronic components. The soldering process involves:
The soldering process needs an understanding of:
The process of soldering requires the following steps:
Many tools are available to facilitate soldering work. The most essential tools used in soldering practice are discussed below:
In the process of soldering, it is essential to ensure a successful solder joint. The solder should form a firm joint. The technician should have enough practice to know the difference between a good joint and a bad joint. Some characteristics of a good and bad joint are explained below.
Printed circuitry and printed circuit boards have been defined and re-defined within the electronics industry. The following are some of the definitions:
“Printed Circuitry is a circuit in which the interconnecting wires have been replaced by conducting strips printed, etched etc. onto an insulating board. It may also include similarly formed components on the base board.”
“A printed circuit board is also called a card chassis or plate. It is an insulating board onto which a circuit has been printed.”
These definitions assume that the reader has had prior experience with point-point wire soldering. In the design process of PCB, there is a replacement of hand-soldered point-to-point wire connections with thin lines of copper. These copper lines are affixed on one or both sides of flat, rigid, glass-epoxy insulated boards through various processes including photography and etching. The board facilitates the rapid assembly of active, passive, discrete, non-discrete, and hybrid electronic components. The result is a single compact assembly where ease of assembly, maintenance and reliability are an order of magnitude better than ever before possible.
A printed circuit board is shown in the following figure:
It is clearly seen that the PCB itself is a board carrying a pattern consisting of conductors and pads. The electronic components are soldered to the pads and electrically interconnected by means of the conductors. In other words a printed circuit board serves to carry and interconnect all the electronic components.
It can be easily appreciated that the use of printed circuit boards results in a more uniform and error-free product if the board pattern is accurately and consistently reproducible. The basic information required when starting to design a PCB comprises the schematic diagram and the component list together with mechanical dimensions and tolerance of the board.
A schematic is the first step because it displays and identifies the components that make up the equipment.
A schematic diagram consists of a system of graphic symbols that represents electronic, electrical, and electromechanical components. The components are connected electrically by the interconnecting lines.
In the layout stage, a sketch of the component location and the interconnections of the components are worked out. After completion of the layout, an art-master (tape-up) is generated. The taping of the artwork means the preparation of a very precise picture of the pattern, very often made on a large scale, for example, four times full size. The taping is usually carried out using self-adhesive pre-cut symbols for the various constituents of the pattern, for example solder pads, contact fingers of edge connectors, and conductors.
The artwork is subsequently reduced to actual size by photography to provide 1:1 scale negative and positive which then becomes a highly accurate tool for the manufacturing of the PC board. The reduced copy is used by the manufacturers as the master pattern. For this reason it is not possible to indicate mechanical dimensions and other relevant specifications on the artwork itself. Instead, it is necessary to prepare an engineering drawing which contains a complete specification of the printed circuit board including the mechanical dimensions, hole diameters, tolerances, and surface treatment of the boards.
During the assembly process, the components are mounted on the circuit board. To facilitate correct and easy mounting of the components, a notation is created which provides the location of the various components. The artwork for this notation is also prepared by the PCB draftsman.
Other documentation prepared from the layout and the reduced positive include fabrication, assembly, parts list and silk-screen drawing, plus whatever special manufacturing sketches or aids that might be required.
Now, let us see the manufacturing process of the PC boards. The raw material is a laminate consisting of a thin, rigid sheet of insulating material which is clad with a very thin copper foil on one or both sides. All the solder holes are drilled in accordance with the master pattern and mechanical drawing. By selectively etching all unwanted copper areas away, only those parts which form the pattern, i.e. the solder pads and the conductors, remain on the finished board. This is a subtractive process, and thus it can be easily understood that an additive process starts with an unclad base material on which the copper necessary for forming the pattern is deposited.
Whether the pattern is derived by the former or later process, its surface has to be protected to avoid oxidization of copper which would add to a serious reduction of the solderability.
An organic coating of colophony resin is given to single-sided boards. For plated-through boards, an electrolytic deposition of tin or lead/tin alloy is used. In plated-through boards, it is possible to deposit copper and tin/lead on the walls of the holes so that the holes become conductive and solderable. The plated holes serve to make a connection between conductors on opposite sides of the board.
Solder masking is a coat of epoxy resin, covering all areas of the PC board except pads which require soldering. Solder masks are increasingly being used to eliminate bridging between adjacent conductors during wave soldering.
Wave soldering is a process whereby component leads are soldered to a printed circuit board by traveling at a predetermined rate of speed through a wave of molten solder.
Printed circuit boards come in several different types as illustrated below:
The two main types are: Plated-through and non-plated-through boards. Plated-through boards are more expensive as compared to non-plated through. Most of the times plated-through boards are double-sided and non-plated-through are single-sided.
Plated-through boards are chiefly used in professional electronic equipment such as measuring instrument, computers and military equipment.
Non-plated-through boards are mostly used for entertainment applications such as radios, televisions, amplifiers, etc.
The different methods of construction of Printed Circuit Board are given in brief as follows:
In the conventional method, a rigid PCB usually of thickness 1.6mm is used. The components are wire-leaded and they are mounted on only one side of the PCB, with all the leads through holes, soldered and clipped. Conventional circuitry is generally easier to debug and repair than Surface mount.
In Surface Mount Technology, instead of inserting leaded components through holes (as in conventional method), special miniaturized components are directly attached and soldered to the printed circuit board. Holes are still needed on the PCB, but not where the component leads are attached. Surface mount circuitry is generally smaller than conventional. Surface mount is generally more suited to automated assembly than conventional.
This method employs both the conventional and the surface mount technology. In practice, most boards are a mix of surface mount and conventional components. This can have its disadvantages as the two technologies require different methods of insertion and soldering.
In this type of method, a bare PCB laminate has tracks on both sides of the board, normally with plated-through holes connecting circuitry on the two sides together.
As the name suggests, in double sided component assembly, the components are mounted on both sides of the PCB. Normally only surface mounts circuitry would be mounted on both sides of a PCB.
A PCB Laminate may be manufactured with more than two layers of copper tracks by using a sandwich construction. The cost of the laminate reflects the number of layers. The extra layers may be used to route more complicated circuitry, and/or distribute the power supply more effectively.
In this technology, the IC die is attached directly to a PCB and bond out wires from the IC connect directly to PCB lands. The chip is then covered with a black blob of epoxy. A technique used mostly with very high volume, cost sensitive applications, for example musical greeting cards.
In many cases, certain areas on a PCB may be gold plated for use as contact pads. Unless the whole PCB is gold plated before etching, this technique is limited in its application, normally to pads on the edge of a PCB, as an electrolytic plating bar must be attached to the pads and then removed part way through the PCB manufacturing process.
Flexible printed circuit boards are used when the requirement is to fit the board into specially a shaped volume. This technique is used extensively with membrane keyboards, combination connector/circuit boards, and circuit boards to fit in awkward shapes, such as. cameras. Flexible printed circuit boards can be stretched, bent, or folded as per the requirement.
Surface Mount Technology for construction of PCB is very popular and it has superseded the conventional through-hole mounting technology. The basic reasons for the demand of this technology are the market trend for miniaturization in electronic assembly, increased reliability, and lower manufacturing costs, improved electrical and mechanical performance.
As mentioned earlier, in this technology, the components are directly attaché and soldered to the printed circuit board. The surface-mounted components and their packing are particularly suitable for automatic assembly. The following figure shows a board constructed with surface mount technology:
Here the components can be assembled on both sides of the board. A non-conductive glue or solder paste is used to attach the components to the board. For leadless components, a ceramic or glass substrate is used as a base for reflow soldering.
The surface mount components do not employ leads like resistors and capacitors have. Instead they have a metal interface that is used for soldering. The following figure shows a comparison of a conventional resistor with a surface mount resistor:
In this technology, the removed component is never replaced back in the circuit. A new component is used for replacement. In order to handle surface mount components, a vacuum parts holder can be used. Also, there is a special solder cream that can be used for soldering onto circuit boards. 60/ 40 type of solder is avoided. The amount of temperature required to melt this solder is surely enough to destroy the component. Also a solder with acid flux is never used in electronics.
Type of Lead Style | Figure | Components |
---|---|---|
Gull Wing: metal lead that bends down and away |
SOIC (Small Outline Integrated Circuit) QFP (Quad Flat Pack) TSOP | |
J-lead: metal lead that bends down and underneath a component in the shape of letter J |
PLCC (Plastic Leaded Chip Carriers) SOD (Small Outline Diode) | |
Ball: metal lead in shape of a ball |
BGA (Ball Grid Array) CHIP SCALE FLIP CHIP (Bump) | |
Metallised Terminations: metal leads are terminated |
Capacitors Resistors Ferrites |
Troubleshooting of faulty SMT assemblies usually require removal and replacement of components. The need for troubleshooting arises when there are damaged pads and tracks due to inappropriate or careless component removal practices. These damages are mostly due to inadequate training of the repair workers in properly understanding and handling the SMT PCBs.
The simplest method to remove the faulty components is cutting all leads. Each leg is cut carefully and then the component is taken off. Each joint is then melted with a fine tip, temperature controlled soldering iron and remaining IC legs are removed with tweezers. The tweezer uses the squeezing action to remove a variety of parts.
After allowing a cool down period, excess solder is removed with a de-soldering braid. This method does not require any tool and special workstation; hence it is cheap. But it can damage the components while cutting, thus in turn damaging the PCB substrate and copper pads. Also, soldering the replacement component in position using a soldering iron requires processing one lead at a time. Hence it is also time consuming.
In order to rework the SMT PCBs, two basic heating methods are introduced. These methods are conductive and convective methods. Let us understand these methods:
The following steps should be performed for removal of a component using a hot gas machine:
The following procedure is followed for replacement of components:
The main objective for testing and troubleshooting of a printed circuit board is to ensure that the components which might have been damaged during assembly, including mass soldering are identified so that they can be replaced before the board reaches the functional test stage. The use of Automatic Test Equipment (ATE) is very common for the purpose. Automatic test equipment is primarily intended to test assembled boards.
Here the test equipment and the assembled board are connected via a bed of nails, on which the board is placed:
The test contact probes are mounted in the fixed base plate:
Guarding technique is a method used by most ATE systems. In this technique, it is possible to measure a component soldered in place on the board, without having to interrupt conductors or to de-solder the components. The PCB pattern can also be tested for interruption and short circuit, even if all the components have been soldered in place.
The guarding technique implies the use of various methods of connecting the equipment, depending upon the type of the component and the adjacent circuitry.
To use the guarding technique, it is necessary that one should have the knowledge of establishing the necessary test points in the form of special test pads, and various design rules.
The test engineer marks the necessary test points on the schematic diagram, which (test points) in the layout stage are treated in the same way as all other components.
The basic function of the test pad is to serve as contact points for the test probe tips. The use of test pads implies an immediate advantage by defining the various test points very precisely. Figure 9.26 shows a test pad.
The test pads are not drilled; hence they do not increase the manufacturing cost. An important advantage of using special test pads is that changes in the circuitry do not necessarily result in changes to the drilling pattern of the base and guide plate/diaphragm. In many cases, the components are moved a little, but the test pads remain untouched.
All the test pads must be placed on the solder side of the board. Conductors act as connecting lines between the solder pads. The conductors are accessible on the component side through the plated-through holes from the solder side. In case of a double-sided non-plated through board, there will be either a component lead, a piece of wire, or an eyelet soldered to both sides of the board. It is therefore necessary to achieve contact with conductors on the component side by connecting the test pads to the solder pads on the solder side. There is a set of design rules, which will aid the PCB test engineer in deciding where to place the test points physically:
While using Automatic Test Equipment, the test contact probes are used to make contact with the selected test points. In practice, there can be contact problems, particularly in the case of assembled boards. Usually solder pads are commonly used as test points. The dimensions of the solder pad are very small because of which it is difficult for the probe to hit the pad. The common problems occurring while making contact are:
When the two component leads are cut a little too long and bent towards each other, there are chances that the contact probe touches both the leads. Sometimes the tip of the probe is so wide that the tip itself causes short circuit.
A printed circuit board should be designed to permit automatic testing without any contact problems. In order to prevent the contact problems discussed above, first the test points are selected and then special test pads are connected to these test points:
These test pads are used to serve as contact points for the test probe tips.
Soldering is an alloying process between two metals. Most of the solders used today are 60/40 type, i.e., 60 percent tin and 40 percent lead. A better is of 63/37 variety which has 63 percent tin and 37 percent lead.
Soldering iron, solder pencil, solder gun and strippers are the soldering tools. A solder is an alloy of lead and tin which is used for the purpose of joining together two or more metals.
A flux is used to scrub away the microscopic film of oxides on the surfaces of metals to be soldered. Flux is applied before or during soldering.
The component to be soldered must be properly formed and fit into the circuit in which they are to be installed. The component forming procedure ensures some important steps that are to be followed, to make the component ready for soldering.
Wicking and Sniffing are the two techniques used in the de-soldering process.
In general, if any system is maintained properly, the failure rate of the system decreases, as a consequence the reliability of the system increases. In this chapter the basic requirement of maintenance is explained. In any organization, it is considered essential to establish a sound policy of maintenance to ensure continuity of service from the equipment.
This chapter provides information on the various groups in a maintenance organization and how they operate.
Depending upon when the maintenance comes into picture, it can be classified into the following categories:
In order to measure the effectiveness of a preventive maintenance program, a statistical feedback is required. The equipment failure and repair history provides valuable information for analysis to see if PM is effective, and if not what steps are to be taken to make it more effective. All the work done on the equipment should be recorded in the history card.
The history card should maintain the number and duration of each repair, name of the person who serviced the equipment, and any parts replaced. The history cards are generally prepared from the maintenance work order.
The preventive maintenance record should include:
Maintenance efforts must be managed effectively to keep any type of interference to a minimum level. Operating maintenance costs money, so it should be planned and controlled just as any other department should be.
Maintenance activities affect equipment return on investment because they represent an expense and because downtime of maintenance may cause missed deliveries. Maintenance done in an effective way affects the operation of the equipment, which directly affects the quality of the system’s output. Maintenance also influences return on investment because it affects the economic lifetime and salvage value of the equipment. The objectives of the maintenance management include:
In any organization, whenever new equipment is purchased or a replacement of an existing worn out equipment is planned, the maintenance department has an important role to play. However, the right selection of equipment holds the key to forming a good maintenance policy. The maintenance policy must consider the following important points for successful continued maintenance:
Let us address all these points one by one.
Any system or equipment, for ensuring its desired way of operation needs proper maintenance. A plant or facility which depends for its operation on particular equipment needs to have a proper organization for the maintenance of its assets.
A maintenance organization would have many departments in it. These departments have the authority to take care of many activities and provide smooth running of the facility. Generally, the departments in the organization streamline their work. To do this, they can have the following groups:
However, the management of these groups and departments depends on the company. It may vary from company to company depending upon the exact requirement. The functions of each group are given below:
When the equipment is sold, the manufacturers usually supply the maintenance manual which contains the following information:
However, the information carried in the manual may vary from manufacturer to manufacturer depending upon the type of the equipment.
The user manual provides necessary information for operating the equipment, start-up and shut down instructions, general design concepts, specification and installation procedures. Operator level maintenance instructions including preventive maintenance manuals are useful for preliminary diagnostics establishing preventive maintenance schedules.
Safety is the first essential requirement a technician should learn before he starts working on equipment. Safety is an attitude, a form of mind for the technician. Safe working habits can not be brought or manufactured, they are learnt through practice.
The person who is handling equipment, or troubleshooting any system, the most important thing which he has to keep in mind, is safety. Personal safety should be given the highest preference, and when following the safety rules properly, gives the best result for troubleshooting. One should practice safe working procedures at all time while working on systems.
An accident is an unplanned and unexpected event which is likely to cause an injury. Proper diagnosis helps in preventing future accidents. Accidents cause loss of man power, work material and money. The main causes of accident at workstations are:
A fire has a small beginning and builds to leave nothing behind it. Fire prevention is always better than fire fighting. Highly flammable materials should be stored separately in another room. Only a minimum quantity of such materials should be allowed into the workplace at any time.
Loose connections of electric wires
Smoking
Overloading on electric wires
Short circuiting of electrical wires
Careless storing of flammable materials
Fire due to electricity, Fire due to oil
Fire due to gas, Fire due to wood
Fire due to cotton, Fire due to chemicals
Fire due to flammable materials
Fire extinguisher, Fire brigade
Buckets full of water, buckets full of sand
Canvas sheets
Inform higher authority of the workshop
Inform other persons in the workshop and ask them to leave the place
Inform the fire brigade
Use fire extinguisher
Switch off the main line
Immediately take away valuable materials
Don’t throw water on electric wires
The accident may occur at any time during servicing. It is necessary to have first aid facilities at the workplace, so that preliminary treatment may be given to the person before reaching the doctor.
The main duties of the first aider are as follows:
To reach as soon as possible the first aid box at the place where the accident occurs
To give necessary treatment to the patient
To bring the patient to the first aid room for further treatment
To call the doctor as soon as possible or to take the patient to the hospital
To have full sympathy with the patient
Tincher iodine, Tincher Benzone, Spirit Ammonia,
Detol, Burnol, Plasters, Bandages, Cotton,
Knife, Scissors, Measuring glass, Antiseptic cream
Eye washing glass, Dropper, Safety Pin,
Stretcher, and necessary capsules and tablets, etc. are required.
The basic motive of maintenance is to prolong the life of equipment and increase Mean Time Between Failure (MTBF).
Corrective Maintenance, Improvement Maintenance, Preventive Maintenance are the three types of maintenance.
A maintenance organization has many departments in it, such as: Engineering Group, Equipment Repair and Maintenance Group, Central Services Group, Construction or Civil Engineering Group, and General Maintenance Group. All these departments are assigned to their related areas and they work separately under one organization.
Safety aspects involve personal safety. The person handling high power equipment should be cautious of shock hazards and aware of proper safety procedures.
A | B | X |
0 | 0 | 1 |
0 | 1 | 1 |
1 | 0 | 1 |
1 | 1 | 0 |
MTBF = 1 / Failure rate or T / f,
Where: f = number of failures during the test interval
T = total test time
An electronic system comprises several functional parts such as power supplies, amplifier, signal converters, etc. When the system fails to give the expected performance, the trouble could be in any of these functional areas. Therefore, it is essential to troubleshoot the system in order to isolate the fault to the failing functional area and then to the failing component. The logical approach of isolating a fault is through a process of elimination of the functional areas that are performing properly. Once a failure is isolated, further analysis of the circuitry within this area is carried out to isolate the malfunction to the faulty component. This functional area approach is also called Block-Diagram approach to troubleshooting.
In this technique, as the name suggests, the circuit is split in half and the output is checked at the half-way point in case of an absence of an output. This helps to isolate the failing circuit in the first or second part. When the faulty half is determined, the circuit that is ageing is split into half for further isolation of failure. This splitting is continued until the failure is isolated to one function or component.
The Half-split method is extremely useful when the system is made up of a large number of blocks in series:
Many electronic systems do not have only series connected blocks.
They may have feedback loops or parallel branches in a part of the circuit.
Hence use of this method is very restricted.
Here the output from one block is fed to two or more blocks. In such systems, it is best to start by checking the common feed point. Alternatively if the output is normal (at A or B in fig) check the divergence point. Conversely, if one output is abnormal, check before the common point. The most common example is that of the power supply circuit which supplies dc power to various subsystems in equipment.
In the convergent path two or more input lines feed a circuit block:
In order to check such a scheme, all inputs at or before the point of convergence must be checked one by one. If any of the inputs is incorrect (at C or D in fig.), then the fault lies in that particular input circuit. If all are found to be correct, the fault lies beyond the convergent point. For example, if C and D are correct and there is no output at E, the fault lies in unit 3. But if input at C is faulty, the fault lies in block 1 or before that.
The feedback loop usually corrects the output of the block with the input of an earlier block via a network called feedback network. Since the circuit behaves as a closed loop, any fault within the loop will appear as if the output in all blocks within the system is at fault:
Before starting troubleshooting of a system having feedback loop, the type of the feedback and its use should be understood well. Feedback paths are provided basically for the following functions:
Having identified the type of feedback circuit, one can proceed as follows to locate the fault.
For the first type, i.e. modifying feedback, it may be possible to break the feedback loop and convert the system to a straight linear data flow. Each block can then be tested separately without the fault signal to be fed around the loop. In some cases instead of completely breaking the loop, the feedback can be modified at or near the point where it rejoins the main forward path. If the output appears normal, check the feedback path, otherwise, check the forward path.
For the second type, i.e. sustaining type, feedback is disconnected from the output and a suitable test signal is injected to check the performance of various circuit blocks.
If a system has switch-able parts and if the circuit function is found faulty in one position of the switch, then throw the switch to another position. If the problem persists, check after the switch in common circuitry. If the problem disappears with this action, check that the circuitry is switched out.
Oscilloscope is an instrument which gives a visual indication of what a circuit is doing and shows what is going wrong more quickly than any other instrument. Multimeter can detect the presence of signals and if the shape of the signal is known the average, peak, rms or peak to peak can be calculated. However, if the waveform is not known, then this is not possible. Noise may be superimposed on the signal and the multimeter will not be able to give the proper information. The oscilloscope gives the true and clear picture of the waveforms.
The following figure shows all the essential controls on the front panel. The controls can be present in some different form than shown, but they have to be present in the oscilloscope.
The controls are as follows:
Sometimes the ON/OFF control can be combined with Intensity/Brilliance control.
The instrument is directly plugged in to the mains supply. After switching on the instrument, wait for a while until the CRT heater warms up. Turn the Brilliance control in a clockwise direction until you observe a horizontal line on the trace on the screen.
If the trace does not appear on the screen then turn the Brilliance control right up to the fully clockwise direction. Turn Time/cm control to the slowest speed, but not to the off position. With these settings, a light spot should appear on the screen moving slowly from left to right.
Again if nothing is seen, adjust the Trig/Level control in a clockwise direction and observe if something is seen. Adjust the vertical and horizontal position controls until the trace appears.
If all the above steps do not result in showing a trace on the screen, the instrument is faulty. Unplug the mains and check out the fuses.
After getting a trace on the screen use vertical and horizontal position controls to start the trace at the left hand side of the screen and lie along the centre line. Focus control is used to get the line as thin as possible. Reduce the Brilliance setting to a comfortable viewing level.
When making oscilloscope measurements, a pair of probes is very valuable and this facilitates making a contact on the point of measurement in a convenient manner. Probes connect the measurement points in the device under test to the inputs of the oscilloscope. The oscilloscope is used to measure the amplitude, frequency and phase difference as follows:
Oscilloscope greatly and effectively helps in finding out the amplitude of voltage:
The number of centimeters on the vertical scale from the negative peak to the positive peak is counted. This count is multiplied by the setting of the volts per centimeter switch.
For example: if 5 V/cm is the volts/cm setting and the waveform measures 4.8V from peak to peak then the waveform voltage is 4.8 * 5 = 24V Peak to Peak.
For frequency measurement the time period of one complete cycle is measured. This is nothing more than the horizontal distance between the two identical points on the neighboring waves:
This distance is then multiplied by the setting of the Time/cm switch and the period of one cycle is calculated. The reciprocal of this time is nothing more than the frequency of the wave.
For example if the peaks of the waveform are 5 cm apart, and the Time / cm switch is set to 200 μ s / cm, the time of one complete cycle is 5* 200 = 1000 μ s = 1 ms and the frequency is 1 / 1000 = 1 KHz.
If we have two signals of the same frequency and wish to measure the phase difference between them, this can be done using a dual trace oscilloscope. One signal is fed to CHANNEL1 input and the other to CHANNEL2 input.
The VH1 position is adjusted to place the CH1 Trace so that it is centered about the horizontal axis of the screen. The CH2 trace is then moved to place it over the CH1 trace. The X position control is then adjusted to move the point where the CH1 trace crosses horizontal axis to line up with the left hand vertical line.
The distance between the crossing point of the CH1 trace and the corresponding point of CH2 trace is then measured along the horizontal axis as shown in the following figure. The total period of one cycle of CH1 waveform is also measured:
The phase shift will be the difference in position between the two traces divided by the total wave period and the result is multiplied by 360 to get the phase in degrees.
The analog multimeter is the most widely used test and measuring instrument. It operates with a permanent magnet moving coil, which can become a DC voltmeter, an AC voltmeter, and DC milli-ammeter or an ohm meter. Sometimes an AC current measuring facility is also present. It has a coil of fine wire wound on a rectangular aluminum frame. It is mounted in the air space between the poles of a permanent horse-shoe magnet. Refer to the above figure.
When an electric current flows through the coil, a magnetic field is developed that interacts with the magnetic field of the permanent magnet to force the coil to rotate. The direction of rotation depends upon the direction of electron flow in the coil. The magnitude of the pointer deflection is proportional to the current. In usual meters, the full scale deflection (FSD) is about 90 degrees. An analog multi-meter can be used to measure current, voltage and resistance as follows:
The moving coil meter is basically sensitive to current and is therefore an ammeter. For the direct current measurement, place the meter (ammeter to measure current) in series with the circuit. When the ammeter is included in the circuit, its internal resistance adds up, thereby reducing the current in the measuring branch. Usually, this resistance is small and can be ignored.
For alternating current measurement, rectifier type meters are used which will respond to the average value of the rectified alternating current. The meter has to be calibrated in amperes rms (root mean square) for the measurement of sine waves.
The current meter can be used to measure voltage. The moving coil meter has a constant resistance. So the current through the meter is proportional to the voltage.
To measure the potential difference between two points, connect the two voltmeter leads to these points. In contrast with the ammeter, the voltmeter is connected in parallel with the circuit whose potential has to be measured.
To measure AC voltage, rectification is required. As in the AC current meters, AC voltmeters respond to the average value of the rectified voltage but are calibrated in volts rms for a sine wave.
The moving coil meter can be used to measure unknown resistance. Test probes are short circuited and the ohms adjust control is turned so that the current through the total circuit resistance has a full scale deflection.
An ohm meter is never used while the circuit is in operation. Sometimes the resistances depend upon the circuit conditions, in that case measure the voltage across the resistance, current through it and calculate the resistance.
A method of testing FET, whether JFETs or MOSFET, involves checking to see if the appropriate voltage on the gate causes the devices to conduct current from source to drain. A simple ohm meter check may not be used with MOSFET, although a check of PN junction from gate to source or drain will give some indication of correct operation in a JFET.
A go/no-go test that is appropriate for N-channel depletion types (JFETs or MOSFET) and P-channel enhancement type (MOSFET) is shown in the above figure.
For an N-channel depletion type, when the switch is open sufficient drain current should flow through the 270 ohm resistor to forward bias the bipolar transistor, and the LED should glow.
If the switch is now closed, the FET should be biased off, and the light should go off. If the LED lights no matter which position the switch is the FET is shorted. If the LED does not light in either position of the switch, the FET is open.
For a P-channel enhancement type, the reverse indications of the LED would be observed. By changing the polarity of the gate supply, other transistor types (P-channel depletion, N-channel MOSFET) may be tested.
The clock signal is tested first, because it is one of the inputs to the microprocessor. If the clock signal is at fault, the whole working of the system may break down. It may stop working or may show intermittent behavior. The clock pulse is tested with a logic probe or oscilloscope. In modern CPUs the clock generator is built within the chip and it only requires an external crystal. The working of the crystal and the capacitor in the clock circuitry is also checked.
The reset signal makes the microprocessor go back to a known starting point. When the reset key is pressed, the pin should go low and then return to high. A logic probe can be used to check the presence or absence of a reset signal. At the key, if absence of a signal is noticed, then the reset signal line is checked.
A problem which can occur with address and data lines can be a short circuit with one of the data or address lines on the printed circuit board. The lines can be shortened to the power supply (+5V DC), or it can be shortened to ground. Again a logic probe is used to check these lines. Each line should show activity which should be observed carefully whether it shows a steady high or steady low indicating a short to power or ground respectively. The address and data lines are associated with tri-state buffers. If there is any failure in any one of these buffers, the correct signal will not reach the CPU. If the suspected faulty line does not show a short or break and tri-state buffers are also working fine, then there is a possibility of a problem in the other ICs associated with the line.
This makes the power supply to switch, give almost zero output voltage if the value of the load current is exceeded beyond its specified range:
An SCR is the device which is used to limit the current.
A current monitoring resistor is placed in the power supply return line.
The voltage developed across this resistor is used to switch on SCR.
In case of an overload, SCR is switched on and the voltage across it falls to approximately 0.9V, which is insufficient to forward bias the series transistor.
The result of this is an appearance of zero voltage across the output.
If SCR is triggered on once, it remains on and until the fault is removed, after switching off the power supply, the output will remain zero.
Over voltage protection is required for all the digital circuits. If their Vcc terminal receives more voltage than required, then the ICs get damaged.
A zener diode is used to achieve the over voltage protection:
The zener senses the voltage across the power supply’s output terminal.
When the DC voltage rises, the zener starts conducting. This turns on SCR Q2.
The voltage at Q1 collector falls rapidly to zero and the fuse blows.
The above circuit can be called Crowbar Circuit, which limits the output to the pre-set value.
The system troubleshooting is done by starting at one end and working towards the other, most of the times. However you can save time by starting in the middle of the system. In such a case, you can divide the system in half and reduce the time for troubleshooting.
Usually a signal is injected in the system when you are starting in the middle of the system. There is a simple device called signal injector which injects a pulse or a wave (square) into the system being tested. In case of a pulse, its high harmonic contents make it useful over a wide range of frequencies. Signal Injectors can be used in RF, IF and audio stages because of their broad harmonic content.
In many types of equipment we can inject a test signal of suitable frequency into the input of the equipment and utilize the inbuilt detector or indicator to test its presence at output. Should the applied test signal fail to be indicated in this way and if we are sure that the indicator is OK then we will be pretty sure that the failure is somewhere in the stage or stages between the point of injection of the test signal and the indicator.
Some technicians use the blade of a screw driver for signal injection when they are working away from their work bench instruments. Using a screw driver blade needs a certain amount of skill and experience. For example, if you tap the center tap of the volume control of a speaker with a screw driver, then the volume control is set for high volume after performing this test.
Consider the following figure in which signal injection is used to locate a defective amplifier in a dead system:
The idea behind this method of testing is to inject a signal into each amplifier one at a time, and then observe the output of the system. If you start by injecting the signal at 1, then 2, then 3, and so forth, you will not get an output signal until you have gone past the trouble. So, if you inject the signal at 2 and do not get the output, but inject at 3 and do get the output, the trouble must be before 3.
There is an advantage of injecting the signal in the sequence described above. Each time you move the signal source, the output signal will get weaker unless you increase the strength of the signal from the signal generator.
This is better than starting at the output and moving back. Moving the other way requires a relatively strong signal to be injected at the output. When moving back to the previous amplifier, you can overdrive it enough to cause damage.
In this technique, a signal is injected at the input and the path of the signal through the system is traced. In signal injection, the signal is injected at each stage and the output is monitored. However in signal tracing the signal is injected at the input itself.
This method speeds up the fault finding process. It includes equipment called Fault Traer which has its own indictor. This may be as simple as a headphone. Usually the oscilloscope is an ideal signal tracer. The idea is to trace the signal between the input and the output at various points, by sampling the signal.
In order to trace the signal a good oscilloscope is used, and with the help of which, you can look at the signal at various points along the signal path. Refer to the following figure:
If the system is audio system, then instead of an oscilloscope, a speaker or a headset can be used to trace the signal.
In order to understand the concept of resistor noise, let us take an example of a perfect amplifier. A perfect amplifier is the one which increases the amplitude of an input signal, but it does not add any noise to the signal output.
When there is no input signal and if the wattmeter is connected to the output terminal, then it will show no power output in a perfect (ideal) amplifier. However in a practical amplifier, the wattmeter would show some output power without an input signal. What can be the reason?
The reason is the noise which is generated within the amplifier. If a resistor is connected across the input of the amplifier, as shown in the following figure, then the output wattmeter shows a measurement, even though, the theoretical amplifier still does not produce any noise. This output measurement is known as Noise Power:
The noise power is generated because the amplifier increases the amplitude of the noise generated by the resistor.
In the above circuit no current flows through the resistor R. Obviously then the noise is not created by electrons bumping into atoms and other electrons as they move in a current flow. Then, where does the noise come from? The following discussion helps you out!
Consider the model of a resistor as shown:
Let us not apply any voltage to this resistor by an outside source of power. The atoms in the resistor are in continuous motion called “Brownian Motion”, at room temperature and at all temperatures above absolute zero.
At room temperature some electrons escape from the atoms for a short period of time. They continuously flow through the material until they are absorbed by another atom that has already lost an electron.
The current caused by this type of motion of the electrons for a very short duration of time is known as intrinsic current.
The electrons move randomly in the material. But at any specific instance of time there will be more electrons moving from one direction to another. At that instant of time a voltage drop is created across the resistor.
When the temperature of the resistor is increased, there will be more electrons moving in the intrinsic current. A voltage is developed across the resistor, whenever the net amount of intrinsic current flowing in one direction is greater than that flowing in the other direction. The higher the temperature, the higher is the amplitude of the voltage. This is because at higher temperature there is more intrinsic current.
The voltages created by the random electron flow in the resistor result in fluctuations over a period of time. Hence for the above amplifier, there is a random fluctuation of voltage at the input.
The random fluctuation is amplified by the (perfect) noiseless amplifier and creates the output noise power. Most amplifiers have resistors at their input terminals and those resistors will produce noise. This noise is very troublesome in some systems. If the noise is greater in amplitude than the incoming signal, then the SNR (signal-to-noise ratio) is very unsatisfactory.
Troubleshooting of faulty SMT assemblies usually require removal and replacement of components. The need for troubleshooting arises when there are damaged pads and tracks due to inappropriate or careless component removal practices. These damages are mostly due to inadequate training of the repair workers in properly understanding and handling the SMT PCBs.
The simplest method to remove the faulty components is cutting all leads. Each leg is cut carefully and then the component is taken off. Each joint is then melted with a fine tip, temperature controlled soldering iron and remaining IC legs are removed with tweezers. The tweezer uses the squeezing action to remove a variety of parts. After allowing a cool down period, excess solder is removed with a de-soldering braid. This method does not require any tool and special workstation; hence it is cheap. But it can damage the components while cutting, thus in turn damaging the PCB substrate and copper pads. Also, soldering the replacement component in position using a soldering iron requires processing one lead at a time. Hence it is time consuming also.
In order to rework the SMT PCBs, two basic heating methods are introduced. These methods are conductive and convective methods. Let us understand these methods:
This method uses a heated tool that contacts the solder joint to effect reflow. The soldering tools are provided with the tips designed to heat all the component’s leads. Their electrodes come in contact with component legs and hold them flat to the copper pads on the PCB.
Sometimes a controlled pulse of current is also used which passes through the electrode which heats the component leads. This melts the solder on the joints and the built-in vacuum pick-up lifts up the component from the surface.
The technique enables all the leads to cool down rapidly after the soldering operation and so allows the leads to be held in position while the solder solidifies. The conductive does not heat the component body and it is very fast and repeatable.
It is very good for replacement as the electrodes will hold the legs flat to the pads during the solder reflow, while the alignment and positioning is ensured with microscope. The process is a little expensive.
It is known as hot gas soldering where hot gas or hot air is used as the heat transfer medium. The hot gas is swept over the leads until full reflow is achieved, after which a part is lifted with a tweezer.
Small parts such as chips, transistors, SIOC and flat packs can be removed with a single point nozzle. With longer components, a component specific nozzle is fitted to the hand piece and brought around the part to remove the surface mounted devices.
The vacuum pick-up lifts up the component after reflow. Sometimes infra-red radiation is also used for re-flow of the solder joints. The process takes a longer time for removal as compared to conductive method.
The following steps should be performed for removal of a component using a hot gas machine:
The following procedure is followed for replacement of a component:
Preventive maintenance means all actions intended to keep equipment in good operating condition and to avoid failure. A good preventive maintenance program is the heart of effective maintenance. It is an activity designed to prevent wear and tear or sudden failure of equipment. It involves a policy of replacement of components of a system before the component actually fails. Let us see the Advantages of Preventive Maintenance Preventive maintenance implies frequent inspection to detect minor faults and the early correction of them, supplemented by periodic overhauling in accordance with a plan so that a possibility of major breakdown is almost entirely eliminated.
An equipment in good working order subjected to regular inspection and adjustments will continue to produce quality products for a longer period than otherwise. An efficiently managed preventive maintenance system improves working conditions and therefore it has a great importance amongst all types of maintenance. For continued, good service of equipment, the equipment has to be in a good working condition. If this is accepted, it becomes obvious that the preventive maintenance system guarantees the continued good working of equipment.
In fact, preventive maintenance occupies the same position with regard to equipment as preventive medicine does with regard to public health. If preventive maintenance is performed at predetermined periods and coordinated with the production schedules, the quantum of production will automatically increase. A branch of preventive maintenance called “Predictive Maintenance” applies sensors and analysis of technical data to determine when the performance of a piece of equipment is about to degrade or when it is about to break down. It is intended to prevent breakdown. Predictive Maintenance is a program of periodically monitoring equipment and tracking certain measures of its performance. Predictive Maintenance is more feasible today because of the technology that is available for equipment surveillance and diagnosis of problems while the machines are still running. The condition of equipment can be monitored by several means. Critical monitor points on the equipment are identified. Sensors may be installed or periodic readings may be taken with portable units to measure the temperature or vibrations. Vibration sensors and ultrasonic sensors are used to feed data into a computer for analysis.
A program of this type prevents unplanned downtime that disrupts production schedules, creates an idle work force and degrades customer service. Equipment is out of service for a very short duration because much of the diagnostic work has already been done and the necessary parts and people are available.
The process of troubleshooting comprises the following steps:
Let’s see these steps in brief
It is important to establish the presence of a fault in equipment before taking any other action. In some cases a system may be reported faulty, but it may be a case of faulty operation or a system failure may be reported with either very little or misleading information. It is essential that a functional test, checking the system’s actual performance against its specification must be made and all fault systems must be noted.
It is also important to check the history of the equipment and repair and servicing work carried out earlier by any other person.
This involves pin-pointing the cause of the fault by studying the literature relevant to servicing, maintenance and repairs. The fault is located first in the subsystem and then in a single component in the subsystem.
Fault correction consists in replacing or repairing the faulty component. This is followed by a thorough functional check on the whole system:
Symptoms are useful to locate the general area of a problem. The symptoms are sometimes described by the equipment owner or user. In an industrial electronic plant, it may be the foreman of the division who uses the equipment. In consumer electronic equipment, the description is often from the customer who owns the equipment.
The most reliable symptom analysis is given by the technician after energizing the equipment. Technicians know that certain symptoms in a system usually mean that a certain component has failed.
Symptoms are indeed a valuable guide for troubleshooting. However, one should avoid basing a complete troubleshooting procedure on the knowledge of symptoms alone. For example, distortion in a radio’s output sound can be caused by different reasons such as low terminal voltage of aging battery, overuse of transistor, a tear on a speaker cone.
An item is considered to have failed because of one of the following three conditions:
When serious deterioration has made it unreliable for its continuous use, thus necessitating its immediate removal from service for repair or replacement
A diode can be conveniently checked with an OHM METER by measuring its forward and reverse resistance. A signal diode shows a low resistance (a few hundred ohms) in the forward direction and a high resistance (nearly infinity) in the reverse direction:
A tunnel diode (TD) is a p-n junction which exhibits a negative resistance interval. Negative resistance values range from 1 to 200 ohms for various types of tunnel diodes. They are utilized in switching circuits. A TD is evaluated using a saw tooth output waveform from an oscilloscope as a current source.
A 670ohm resistor from the saw tooth out connector in series with TD to ground will give a calibrated current/div horizontally (say 1 mA/div).
The saw tooth voltage goes from 0 to 10 volts. Therefore, the horizontal display becomes current/div. Looking at the voltage drop across the diode will give a vertical display of the low / high voltage state of the diode.
The display does not give an indication of switching time but confirms that the device has the ability to switch at the correct current level and will probably perform normally in the circuit.
Usually a defective switch is not repairable. It is recommended to replace the broken or defective switch. The defects in a switch can be loss of continuity between the contacts, improper spring loading function because of a defective spring, loose toggle, broken or burnt switch body, etc.
In order to make an in-circuit test on a switch a VOM can be employed. The following figure shows how to use a VOM for this purpose:
The meter prods are connected or touched to both the sides of the switch. The meter shows infinite resistance when the switch is in OFF position. When the switch is closed, i.e. ON, the meter shows zero resistance indicating that the two sides of the switch are electrically connected.
If the switches are found faulty, replace the switch. Carefully switch off the power while replacing it. If there are any connections made to the switches, label all the wires connected to it before removing. When the new switch is replaced, connect all the wires in their proper positions. Turn on the power and check the working of the switch by the procedure described above.
A Logic Pulser is used to stimulate digital circuits and supplemented by a logic probe it aids in testing for circuit response to easily check gates, lines, buses and nodes. It does the same job in digital systems as a signal generator does in analog systems. It injects a desired signal for the purpose of testing.
The above figure shows a troubleshooting problem where a combination pulser and probe can save time by preventing a wrong interpretation of a measurement.
In figure (a), the logic probe shows a logic 0 output of the AND gate which should be logic 1.
Figure (b) shows how to test for this problem. The pulser injects the signal and the logic probe is used to look for that signal.
If the problem is with the gate, the probe will show a pulse signal. If the point is grounded, the probe will indicate no signal.
Pulse height or amplitude is derived from the power supply that the pulser is connected to. For this reason the pulser should always be powered from the circuit under test or power supply from the same voltage.
The power supply requirements of the pulser are 3 to 18 V dc for CMOS and 4.5 to 5.5 V dc for TTL.
Current tracing is very effective troubleshooting. It is difficult to isolate a bad element when a given circuit node is stuck in one logic state and several elements are common to that node.
The hand held current tracer has one lamp indicator that glows when it is held over a pulsing current path.
The instrument detects whether current is flowing or not and where the current is flowing. For instance if the node is stuck on LOW state due to a shorted input in one of the devices connected to the node, a very strong current exists between the circuit driving the node and the faulty component.
The use of a current tracer helps to pinpoint the faulty point on a node, even on multi layer boards.
The current tracer senses the magnetic field generated by fast rise time current pulses in the circuit (or, provided by a logic pulser), and display steps, single pulses, and pulse trains using a simple one light indicator:
The above figure shows location of a multiple input fault. A logic pulser is used to provide current pulses. Gate U5A is shorted to ground causing the node to be stuck LOW and sinking virtually all current from U1 and other inputs. A current tracer quickly verifies this fault by a clear single lamp indicator on the node.
A current pulser is also used for the following conditions:
A solder bridge fault can be detected using a current tracer.
In the above figure a solder bridge between U1 and U2 causes both nodes to indicate functional logic failures. Tracing current flow in the circuit quickly shows the location and cause of the fault.
It is also known as switching power supply or sometimes chopper controlled power supply. By using a switch as a series element and controlling the on and off time we can vary the average voltage at the DC output level:
The above figure shows four major blocks of SMPS:
The AC mains are rectified and filtered. The high voltage DC is then fed to the high frequency inverter. The operating frequency range is from 20 KHz to 1 MHz.
The high frequency square wave thus generated is stepped down by the high frequency transformer and then rectified and filtered to produce the required DC output.
The output is compared with a reference and pulse width modulated to get the desired regulation by the control circuit. The regulation of the output voltage is achieved by varying the duty cycle of the square wave.
When the load is removed or input increases, the slight rise in the output voltage will signal the control circuit to deliver shorter pulses to the inverter. Conversely, as the load is increased or input is decreased, wider pulses are fed to the inverter.
The efficiency of an SMPS is higher than a series type regulator. They are physically smaller and lighter than linear regulators.
They are noisy electrically and sometimes audibly. Thus they are unsuitable for powering circuits that are sensitive to electrical noise unless adequate filtering and shielding is provided.
When testing SMPS, each branch of the supply will shut itself off, if its load is disconnected. For this reason, a load resistor must be connected to each branch during testing.
The value of the load resistor to be connected would depend upon the current rating of the supply. An isolation transformer and an auto transformer are connected in the AC line during the test. The auto transformer is useful for varying the AC voltage and the isolation transformer is useful for protection.
If this equipment is not available, the troubleshooting of SMPS should be limited to visual inspection for burnt or damaged parts.
A Thermister is a device with a negative temperature coefficient, i.e., its resistance decreases with the rise in temperature. The following figure shows a circuit with Thermister compensation:
As the temperature increases, value of resistance Rt decreases, and the current in the Thermister increases. This leads to an increase in the current in resistance Re. This increase in current results in an increase in voltage Ve across Re, which increases I2, which in turn reduces the base current Ib. The reduction in base current Ib reduces collector current Ic, thereby compensating for any increase in collector current due to a rise in temperature.
Alternatively, the Thermister can be placed across R2 as shown in the following figure:
When temperature increases, resistance of the Thermister decreases, and the value of parallel combination of R1 and R2 also decreases. This leads to an increase in I2 and a decrease in Ib. This results in a decrease in Ic to compensate for any increase in Ic due to a rise in temperature.
A Sensistor is a device with a positive temperature coefficient, for example its resistance increases with temperature. The following figure shows a circuit with Sensistor compensation:
As temperature increases, value of RC increases and hence value of parallel combination of R1 and RC increases leading to a decrease in I1. Thus Ib decreases resulting in a decrease in IC to compensate for any increase in IC due to a rise in temperature.
Alternatively, the Sensistor can be placed either parallel to Re or in place of Re as shown in the following figure:
Any increase in temperature results in an increase in Rs resulting in a higher voltage drop Ve. This increases the voltage at the base, leading to an increase in I2 which reduces Ib. A decrease in Ib results in a decrease in Ic, compensating for any increase in Ic due to temperature.
Shielding is a process aimed at confining radiated energy to the bounds of a specific region or to prevent radiated energy from entering a specific region. It has been found that the shielding of source is required more as compared to the shielding of receptor, because source is the one which is allowed to radiate (for example, Broadcasting stations).
Shielding is accomplished by two mechanisms: Absorption loss and reflection loss. At low frequencies, shielding is accomplished primarily by absorption loss and at high frequencies it is accomplished by reflection loss. However, the total shielding function is a composite of both reflection loss and absorption loss.
The thickness of the shield can be calculated by simplifying these two phenomena. There is a unit of measurement for shielding called ‘one skin depth’. This is the thickness of the piece of metal which is bombarded with EMI.
A total closed shield isolated from outer world is never possible. The source has to have connections from the outside systems which are power system and load terminals. The shielding can be in a form of partition boxes or cable and connector.
A shield reduces EMI field strength. Shielding is measured and specified in terms of reduction in field strength caused by the shield. A material which is used for sealing seams and joints on shield to prevent EMI energy from passing through it is called EMI gasket.
For example, the following figure depicts a PC board without filtering or shielding:
The electromagnetic energy enters the device along cables and traces by radiation through the air. When filter capacitors are installed, conducted interference is reduced. These filters conduct the desired current but reject the undesirable currents. However, energy can still enter the source by direct radiation.
A shield prevents the radiated EM energy from interfering with the equipment. Thus both filtering and shielding are both necessary to prevent possible EMI problems. With the same purpose as with shielding, filtering is used to solve EMI problems when the circuit is the source of noise. The following figure shows the effect of shielding and filtering:
The discrete capacitors and bypass capacitors should be placed as close to the voltage source as possible and adjacent to the active devices if present. In digital circuits, capacitors filtering low frequency supply voltage noise are generally placed adjacent to the voltage input pins. The value of such capacitors can be 10µF or above.
Capacitors filtering high frequency noise should be placed at the IC being filtered. These capacitors are low value capacitors of the range 1µF and below. The ideal position of the capacitor and the IC would be across the supply voltage and ground pins.
EMI filters can be used as a shunt element to divert electrical currents from a trace or conductor; as a series element to block a trace or conductor current; or they may be used as a combination of these functions. Selection of the filter elements should always be based on the desired frequency range and component characteristics.
To filter a signal and to isolate power source using decoupling, a capacitor is the best signal filter within its high frequency performance characteristics. The bypass capacitor greatly reduces the power and ground circuit noise, if they are located properly in design layout.
EMI problems at high frequencies can be reduced by using a low pass filter. It incorporates a capacitive shunt and series resistance or inductance. However, at high frequency, the capacitor can become inductive and the inductor can become capacitive causing the filter to act more like a band-stop filter.
The basic criteria to design a filter should be based on the overall impedance at the circuit’s point of application for proper match. For most EMI applications, a T-filter design is effective and is ideal for analog and digital I/O ports.
Soldering involves removal and replacement of the electronic components from the printed circuit boards. If a component is soldered, it means that there is a continuity of metal. Soldering is an alloying process between two metals.
It is a popular method of connecting circuits. When it is done properly, soldering is a highly reliable and low cost process. Most of the solders used today are 60/40 type, i.e. 60 percent tin and 40 percent lead. A better is of 63/37 variety which has 63 percent tin and 37 percent lead. This alloy is available in wire form and in several gauges.
Most combinations of tin and lead become plastic before they actually melt. So, they have three states: solid, plastic (mushy) and liquid. 60/40 is the plastic state, but there is a very small difference between the liquid and solid state.
The 63/37 solder is eutectic at a temperature of 370°. This means that it goes directly from the solid to the liquid state without having a plastic intermediate step. This is one of the reasons that 63/37 is often used for replacing parts on newer systems where components are very small and sensitive to heat.
The following figure shows the difference between 60/40 and 63/37 when heated:
Plastic solders are also available. Instead of soldering with a combination of lead and tin, plastic solders can be used which are soft in state and are hardened by heat or by adding a chemical called a catalyst or hardener. In either case, the plastic solder is applied first and then hardened.
The plastic solders which use catalyst for hardening are useful in surface mount configuration, where heat must be used sparingly to protect the component. Plastic solders form a solid, strong bond and they are easy to apply.
There is one more type of solder called silver solder which employs indium. Another type of solder combines indium, tin, lead, cadmium and gallium. These solders melt at very low temperatures, but they do not flow easily like the more conventional solders. Some solders melt at such low temperatures that they can be melted with the flame from a match.
In its molten state, solder dissolves some of the metal with which it comes into contact. The metals to be soldered are more often than not covered with a thin film of oxide that the solder can not dissolve. A flux is used to remove this oxide film from the area to be soldered.
The process of repair of any electronic circuit involves the following major steps:
A good soldering practice is required for the removal and replacement of electronic components. The soldering process involves:
The soldering process needs an understanding of:
The process of soldering ensures the following steps:
In any organization, whenever new equipment is purchased or a replacement of an existing worn out equipment is planned, the maintenance department has an important role to play. However, right selection of equipment holds the key to form a good maintenance policy. The maintenance policy must consider the following important points for successful maintenance:
Let us see all these points one by one.
To troubleshoot a commercially available variable power supply.
All resistors are ¼ W, ± 5% Carbon film unless otherwise specified.
For this exercise, a commercially available power supply is used. The following instructions can be suitably changed, based on the model used:
The variable power supply (30V, 2A) was tested. The faults found during troubleshooting are (specify the faults).
The faults are rectified by (specify the action)
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