Electronic fault diagnosis.

146 Pages·1988·29.688 MB·English

by  George Loveday

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Electronic tau!¼
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Electronic
Fault
Diagnosis
George Loveday
CED~, 'MlE_R~,.
Senior Lecturer in Electronic ·en91ilaering
Bromley College of Techr.i o1ogy ·
.,
Third edition
........ Longman
...... Scientific &
...... Technical
Longman Scientific & Technical,
Longman Group UK Limited,
Longman House, Burnt Mill, Harlow,
Essex CM20 2JE, England
and Associated Companies throughout the world.
© G. C. Loveday 1982
This edition © Longman Group UK Limited 1988
All rights reserved; no part of this publication
may be reproduced, stored in a retrieval system,
or transmitted in any form or by any means, electronic,
mechanical, photocopying, recording, or otherwise
without either the prior written permission of the Publishers or a
licence permitting restricted copying in the United Kingdom issued by
the Copyright Licensing Agency Ltd, 33-34 Alfred Place, London, WC1 E 7DP.
First published in Great Britain by Pitman Publishing Limited 1977
Reprinted 1979, 1981, 1982
Second edition 1982
Reprinted 1983 (twice), 1984, 1985
Reprinted by Longman Scientific & Technical 1986, 1988
British Library Cataloguing in Publication Data
Loveday, George
Electronic fault diagnosis. - 3rd ed.
1. Electronic equipment. Faults. Detection -
Manuals
I. Title
621.381
ISBN 0- 582- 02358- 2
Produced by Longman Group (FE) Limited
Printed in Hong Kong
Contents ·
Preface vi 6 Pulse and Waveform Shaping Circuits 74
1 Basics of Fault Diagnosis 1 7 Thyristor and Trlac Circuits 88
2 Single Stage Transistor Amplifier 16 8 Circuits using Analogue and Digital Integrated
3 Power Supply Circuits 24 Circuits 102
4 Amplifier Circuits 39
5 Oscillator and Time Base Circuits 57 Answers to Exercises 125
·,
•
Preface
The ability to rapidly diagnose the causes of faults conditions, and it is intended that the student
in electronic equipment and circuits is one of the should construct or breadboard the circuits as
important skills that can be acquired by the elec practical project work. For this reason, readily
tronic technician or mechanic. This book is available components, wherever possible, have
intended to serve as an introduction to the subject. been chosen.
Naturally, fault diagnosis skill is not achieved Primarily the book is intended for students
easily, since it combines a good understanding of studying City and Guilds 224 Electronics Servic
component and circuit operation together with ing, and the BTEC Certificate and Diploma
knowledge on testing methods and on how compo courses. However, it is hoped that the text and
nents fail. The exercises throughout the book are exercises will prove interesting and helpful to a
designed to assist the student in improving his fault much wider readership.
diagnosis technique. The text concentrates mainly
on component faults occurring in particular types
of circuit rather than on the fault finding tech Note to second edition
niques used for localising faults in complete elec
tronic instruments or systems. There is, however, The revision has involved the correction of a
a section that deals briefly with system fault number of remaining errors, some minor improve
finding methods. ments throughout, and, chiefly, an almost complete
The majority of the circuits have been built, rewrite of Chapter 8 into an expanded version
tested, and then measurements made under fault containing four new exercises.
Preface to the third edition
The demand for staff with fault finding skills con available chips, designed around an 8-bit
tinues to rise and now the Service Engineer is also microprocessor.
expected to diagnose and repair faults in a wide
This expanded version of the book also reflects
range of systems that are microprocessor based. In other important trends in Electronics and includes
this edition I have therefore included a basic
notes and exercises on opto-isolators, timer ICs,
introduction to fault finding on microprocessor SMPU designs and powerFETs.
based systems with an example, using readily
1 Basics of Fault Diagnosis
1.1. Circuits and Test Readings ...-------- -----4>----<> + 24 V
An electronic circuit is a collection of components
R, Rz
connected together to perform a particular electronic
47 k 5k6 01
function. Each component has its part to play in the
®
operation of the circuit. If any component should
@
fail, then the operation will be drastically changed. 0
For an example, consider the simple relay amplifier Tr Tr2
1
circuit of Fig. 1.1. If R were to go open circuit,
1
there would be no forward bias current for Tr The
1 .
0------ - ----------oov
collector voltage ofTr would rise, Tr would then
1 2
conduct, and the relay coil would be permanently Fig. I. I Relay amplifier
energized.
was faulty. Component testing is described later in
A faulty component produces a certain set of
this chapter.
symptoms, which can be used to indicate the com· For more complex circuits, especially those which
ponent and its type of fault. Such symptoms are, use direct coupling, the effects of one component
for example, the voltage levets at various points in fault can be extensive. However the fault symptom
the circuit. invariably indicates which component is at fault, and
The voltages,* measured with a standard multi the exercises in the following chapters are intended
range meter at the test points of Fig. 1.1, when the to give the reader experience in diagnosing faulty
circuit is working correctly, and with no input components from a given set of symptoms.
applied, are Ski! ful fault diagnosis requires both theoretical
-knowled'ge and practical experience. Before attemp
Test point I 2 3
ting the diagnosis of faulty components the technician
Voltage +0-7 +0·1 +24
will need to understand the purpose of the circuit
and its operation. This clearly presupposes that he
However, with R open circuit the readings would
1
also understands the principle of operation of the
change to
various electronic components used. A review of the
Test point I 2 3 common components follows in the next section.
Voltage 0 +0•7 +0-15
1.2 Components and Common Faults
These readings indicate that Tr is non-conducting
1
Before considering the individual types of component
and, since the base voltage of Tr is at zero volts,
1
let us look at the ways in which a component can
that possibly R is open circuit and cannot supply
1 fail. A component can be said to have failed if any
base current to Tr It is worth noting here that a
1• one of its constants is out of its specified limit.
base-emitter short circuit on Tr would also cause the
1 For example, if a 5k6 ohm ± 5% resistor actually
same symptoms. A resistance check would be
has a resistance value of 6 kD., or if the leakage
necessary to discover which of the two components
current of a 64 µF 12 V electrolytic capacitor is
15 0 µA when it is specified as a maximum of 10 µA,
•Throughout the book, the voltage readings in these test
then both components have failed.
point tables are given in volts.
l
2 ELECTRONIC FAULT DIAGNOSIS
•
Both these examples can be described.as partial It is perhaps easy to understand failures caused by
failures, since they do not necessarily lead to a com defects and overloads, but why should a component
plete loss of performance, rather to a small change. fail in normal use? Basically the component is ageing
Partial failures are especially important when the because of the stresses that are acting continuously
component is used in a critical circuit position. upon it. These stresses are of two kinds, operating
The faults we are more concerned with are called and environmental. The operating stress is due to
catastrophic failures, when the failure of the com the design conditions, and the life of a component
ponent is both sudden and complete. For example, can be prolonged by operating it well within its
a resistor goes very high in value, or becomes open rated maximum value of current, voltage, and power.
circuited, or a diode develops a short circuit This is called derating. Environmental stresses are
between anode and cathode. Such failures lead to a those caused by the surrounding conditions. High
complete loss of performance and are usually temperature, high humidity, mechanical shock and
accompanied by a drastic change in d.c. bias levels. vibration, high or low pressure, and corrosive chemi
As a general rule certain types of component fail cals or dust in the air, are the major adverse conditions.
in a particular way. When resistors, especially the All of these stresses affect the component and cause
film type, fail they often go open circuit, since a some deviation from the specification, and finally
small break in the resistance spiral is a much more
the component will fail. For example, consider a
likely event than a short circuit across the whole
component subjected to continual cycles of heating
resistor. Electrolytic capacitors on the other hand
and cooling; this may cause the materials from which
a.re more prone to fail short circuit. Here we are
the component is made to become brittle, and any
discussing the way in which components fail; this
mechanical shock may then cause the coml?onent to
should not be confused with the rate at which they
fail open circuit.
fail. The reliability of present-day components is
The effects of adverse environmental conditions
extremely high. In other words, the failure rate is
can usually be minimized by careful design, and this
low. Resistors, in particular, are very reliable.
is increasingly important when an electronic instru
ment forms an integral part of some industrial manu
Table I. I indicates the more probable types of
facturing process where high temperatures, vibration
failure for various types of electronic component.
and other hazards are present.
Another cause of component failure is high
voltage pulses or "spikes", generated from switched
inductive loads, being transmitted along the mains
and appearing on internal supply wires. These
TABLE 1.1
"spikes" can easily lead to the breakdown of junc
Component Common type of fault
tions in semiconductor devices.
Resistors High in value or open circuit.
Variable resistors Open circuit or intermittent
contact resulting from mechanical 1.3 Operating Principles of Common Active
wear. Components
Capacitors Open or short circuit.
The following is intended only as a review; other
Inductors Open circuit. Shorted turns. Short texts should be consulted.
(including circuit coil to frame (iron cored
transformers) types).
( 1) Semiconductor diodes
Thermionic valves Filament open circuit. Short
These devices (Fig. 1.2) have a low slope resistance
circuited electrodes (i.e. grid to
cathode). Low emission. when the anode is positive with respect to the
n
Semiconductor Open or short circuit at any cathode, a typical value being 25 at a forward
devices junction. current of I mA. When the anode is negative with
Diodes, Transistors,
respect to the cathode, the resistance is very high,
FETs, SCRs etc.
greater than l 00 Mn for a silicon diode.