DG3J 35
Electronic Fault Finding
March 2008
© SQA
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Acknowledgements
SQA gratefully acknowledges the contributions made by Scotland’s colleges in the
authoring, editing and publishing of this material.
No extract from any source held under copyright by any individual or organisation has
been included in this publication.
© Scottish Qualifications Authority – Material developed by Robert Keddie.
This publication is licensed by SQA to COLEG for use by Scotland’s colleges as commissioned
materials under the terms and conditions of COLEG’s Intellectual Property Rights document,
September 2004.
No part of this publication may be reproduced without the prior written consent of COLEG and
SQA.
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Contents
Acknowledgements
2
Contents
3
Introduction to the unit
5
What this unit is about
5
Outcomes
5
Unit structure
5
How to use these learning materials
5
Symbols used in this unit
6
Other resources required
8
Assessment information
9
How you will be assessed
9
When and where you will be assessed
9
What you have to achieve
9
Section 1: Techniques of fault diagnosis
11
Introduction to this section
13
Assessment information for this section
14
Introduction
15
Logical fault-finding methods
16
Sequential and non-sequential methods
17
Block diagrams
21
Exceptional faults
33
Summary of this section
37
Answers to SAQs
39
Section 2: Implementing a fault location strategy
41
Introduction to this section
43
Assessment information for this section
44
Introduction
45
Safety
46
Safe working practices
48
Fault location strategy
49
Worked example
50
Summary of this section
55
Answers to SAQs
57
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Section 3: Locating faults to component level
59
Introduction to this section
61
Assessment information for this section
62
Introduction
63
Analogue example
64
Digital circuits
71
Digital example
72
Summary of this section
75
Answers to SAQs
77
Glossary
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Introduction to the unit
What this unit is about
This unit is designed to enable candidates to understand the concept of electronic fault
finding and enable them to be proficient in designing and implementing a fault location
strategy. This unit is particularly suited for candidates who expect to work as electronic
technicians (especially in a maintenance role) but is also relevant to all those on an
electronic study programme who require a practical understanding of electronic fault
finding.
Outcomes
On completion of this unit, the candidate should be able to:
1.
Explain the techniques of fault diagnosis in electronic circuits and systems.
2.
Implement a fault location strategy in an electronic system.
3.
Locate faults to component level in digital and analogue circuits.
Unit structure
This unit contains the following study sections:
Section number and title
Approx.
study time
1
Techniques of fault diagnosis
5 hours
2
Implementing a fault location strategy
10 hours
3
Locating faults to component level
25 hours
How to use these learning materials
These learning materials have been written with the emphasis on fault-finding theory.
However, practical experience is vital. You will spend more time performing practical
fault-finding exercises than studying the theoretical principles. A good approach is to
read through the notes first, then attempt to put the principles into practice, before rereading the material. That way, you should gain a clearer understanding, and be able
to put the theory into context. Then you can transfer that knowledge to other systems
and become a more effective engineer.
Your tutor will also need to supply you with information about the systems and circuits
on which you are to perform fault finding. This information cannot be included here as it
depends on the equipment that is available at the centre. Understanding the
manufacturer’s documentation is a key part of the fault-finding process.
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Symbols used in this unit
These learning materials allow you to work on your own with tutor support. As you work
through the course, you will encounter a series of symbols which indicate that
something follows that you are expected to do. You will notice that as you work through
the study sections you will be asked to undertake a series of self assessed questions,
activities and tutor assignments. An explanation of the symbols used to identify these is
given below.
Self assessed question
1
This symbol is used to indicate a self assessed question (SAQ). Most commonly, SAQs
are used to check your understanding of the material that has already been covered in
the sections.
This type of assessment is self contained; everything is provided within the section to
enable you to check your understanding of the materials.
The process is simple:
•
you are set SAQs throughout the study section
•
you respond to these by writing either in the space provided in the assessment
itself or in your notebook
•
on completion of the SAQ you turn to the back of the section to compare the
model SAQ answers to your own
•
if you are not satisfied after checking your responses, turn to the appropriate part
of the study section and go over the topic again.
Remember – the answers to SAQs are contained within the study materials. You are
not expected to guess at these answers.
Activity
1
This symbol indicates an activity, which is normally a task you will be asked to do that
should improve or consolidate your understanding of the subject in general or a
particular feature of it.
The suggested responses to activities are given at the end of each section.
Remember that the SAQs and activities contained within your package are intended to
allow you to check your understanding and monitor your own progress throughout the
course. It goes without saying that the answers to these should only be checked after
the SAQ or activity has been completed. If you refer to these answers before
completing the SAQs or activities, you cannot expect to get maximum benefit from your
course.
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Tutor assignment – formative assessment
1
This symbol means that a tutor assignment follows. These are found at the end of each
study section. The aim of the tutor assignment is to cover and/or incorporate the main
topics of the section and prepare you for unit (summative) outcome assessment.
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Other resources required
A selection of analogue and digital systems is necessary to give practical experience.
The unit specification states that various electronic systems may be used for fault
finding. A commercial/industrial electronic system would be particularly appropriate but
it is recognised that this will not always be possible. Other suitable systems could
include:
•
a programmable logic controller operating external equipment
•
a desk-top computer driving a multiapplications board
•
a microprocessor driving a multiapplications board
•
an alarm system
•
a multistage AF or RF amplifier
•
a static inverter.
For practical work in Section 2 it would be best to have a complex system which can be
subdivided into clearly separate stages. For Section 3, it is necessary to have access
to an individual PCB or unit so that fault finding to component level can be performed.
In order for the candidate to practice fault finding, several systems need to be available
to test. A wide variety of systems would be helpful.
Clearly, each system should have a fault on it somewhere, preferably of a type which
does not make the source of the fault obvious. This might mean shorting a component
out with a solder bridge, deliberately damaging a capacitor, cutting a track where it will
not be noticed, or replacing a resistor with one of a much higher value.
Alternatively, a set of switches could be used to implement shorts or open circuits.
They could be wired to the track side of the PCB, then the wiring covered up to prevent
the candidate seeing what faults can be switched in. This would not be appropriate if it
makes it unreasonably difficult to access the board with test equipment or if surfacemounted components are used so that they are on the same side as the PCB tracks.
Section 3 contains a couple of examples of simple circuits. It may be practical to build
multiple versions of each design, with different faults on them. A simulation package
could also be used to simulate a working circuit, then various shorts or open circuits
implemented to confirm the effect of these faults. Simulations are not recommended for
the practical assessments.
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Assessment information
How you will be assessed
Assessment for this unit should take the following form:
•
Section 1 – short written test lasting 30 minutes.
•
Section 2 – practical exercise on an electronic system
•
Section 3 – one practical exercise on an analogue system, one practical exercise
on a digital system.
The written test for Section 1 may be composed of a balance of short answer,
restricted-response and structured questions. Sections 2 and 3 are practical and it is
recommended that candidates be assessed by the use of checklists and a report
written by the candidate which includes a fault-finding log. Assessment for both the
written test and the practical exercises will be conducted under controlled, supervised
conditions.
When and where you will be assessed
Assessments will normally be at the end of each section. For Sections 2 and 3 the
centre will inform you of the deadlines for the submission of reports.
What you have to achieve
Each section lists the knowledge and skills covered. You should check these lists
before attempting the assessments.
Evidence for the knowledge in Section 1 may be provided on a sample basis. This
means that only a selection of the material will be covered in the assessment, but your
responses should satisfactorily demonstrate your competence.
Centres are recommended to use appropriate checklists to monitor the candidate’s
fault-finding activities during the assessment of Sections 2 and 3. Written reports will
need to contain relevant information. A list of what is required will be issued with each
assessment and is included at the start of Sections 2 and 3 in these notes.
Opportunities for reassessment
Normally, you will be given one attempt to pass an assessment with one reassessment
opportunity.
Your centre will also have a policy covering ‘exceptional’ circumstances, for example if
you have been ill for an extended period of time. Each case will be considered on an
individual basis and is at your centre’s discretion (usually via written application), and
they will decide whether or not to allow a third attempt. Please contact your tutor for
details regarding how to apply.
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Section 1: Techniques of fault diagnosis
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Introduction to this section
What this section is about
This section covers a variety of techniques for fault diagnosis in electronic circuits and
systems. These are presented in general terms so that they can be applied to all
electronic devices.
Outcomes, aims and objectives
At the end of this section you will be able to understand:
•
sequential and non-sequential fault location methods
•
systematic fault location methods, e.g. input to output, output to input, half-split
•
fault location methods in complex systems, e.g. divergence, convergence,
alternative path
•
exceptional faults, e.g. manufacturing faults, multiple faults, catastrophic failure.
Approximate study time
5 hours.
Other resources required
None.
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Assessment information for this section
How you will be assessed
Evidence for the knowledge in this section may be provided on a sample basis. The
evidence may be presented in responses to specific questions. Each candidate will
need to demonstrate that she/he can answer correctly questions based on a sample of
the items shown above under ‘Outcomes, aims and objectives’. In any assessment of
this section, one item from each of the four parts should be sampled.
Candidates must provide a satisfactory response to all four items.
When and where you will be assessed
Evidence should be generated through an assessment paper lasting 30 minutes
undertaken in supervised conditions. Candidates may not bring to the assessment any
notes, textbooks, handouts or other material.
What you have to achieve
At the end of this section you will be able to:
•
explain the difference between sequential and non-sequential fault location
methods
•
explain the use of input to output and output to input methods
•
explain the use of half-split methods
•
explain the significance of diverging paths in fault finding
•
explain the significance of converging paths in fault finding
•
explain the significance of alternative paths in fault finding
•
describe typical manufacturing faults
•
explain the added difficulties involved when trying to locate multiple faults
•
explain the significance of catastrophic failure.
Opportunities for reassessment
In order to ensure that candidates will not be able to foresee what items they will be
questioned on, a different sample from each of the four knowledge items is required
each time the section is assessed.
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Introduction
Fault finding is an important activity for most engineers. Whether it is a matter of testing
and repairing items as they come off the production line, or maintaining equipment on
the production line itself, or debugging a new design as it is developed, almost every
engineer will have some experience of fault finding.
It is vital that fault finding is done in an effective and efficient manner. It has been
estimated that repairing faulty goods costs around six times as much if done after the
product has left the factory. Making an inadequate repair will not only cost more than
fixing the item properly, it will also harm the manufacturer’s reputation.
If equipment on the production line fails in use, then it can incur huge costs for the
company. Consider the wages of workers who are unable to do productive work while
machinery sits idle – the company management will want the fault fixed as quickly as
possible. However, they will not want a poor repair which will fail later, possibly causing
additional damage. Neither will they want to risk injury to their employees.
In today’s marketplace companies need to innovate to survive. They need to get new
products on sale faster than those of their competitors. When new designs are under
development, it is often delays fixing problems which cause deadlines to be missed.
This is especially likely when new technology is involved – unexpected problems can
arise, causing the launch of the product to be postponed.
It is generally accepted that fault finding needs to be done in a logical manner.
Familiarity with a particular piece of equipment can mean that problems can be fixed
without resorting to a methodical approach, but this level of expertise only comes after
years of experience. Also, it lacks ‘transferability’ – a good engineer should be able to
fault find on unfamiliar equipment, and become competent quickly.
This unit has been written to suit readers with a wide range of circumstances. As a
result, it cannot go into great detail on particular systems. Instead, a variety of
examples are quoted. Some of these involve equipment such as cars and domestic
appliances, with which most people are familiar. Others are electronic systems, of
which you should have technical understanding.
Later sections involve practical exercises, but to begin with we shall cover the general
principles of fault finding.
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Logical fault-finding methods
In everyday life you will occasionally encounter problems with equipment. Familiarity
with its use means that you will often be able to fix the problem immediately without
giving it much thought.
For example, from time to time you may need to use a head-cleaning disc on a CD
player. When listening to music you may find the CD jumping or repeating sections.
Using the head cleaner makes the problem go away, and you conclude that the issue
has been resolved.
Dampness can penetrate the electrics of car engines. The symptoms are that the car
can be hard to start, or may suffer intermittent losses of power until the engine bay
heats up and the dampness has been expelled. A temporary solution is to spray the
ignition leads with water-repellent spray. In the long term it may be necessary to
replace the high-tension leads.
In both of these cases we are using knowledge of the system to attempt a repair.
However, these approaches rely more on statistics than logic. If you are sure that a CD
is not jumping because the disc is scratched (easily checked), then dirt on the laser
lens is the next most likely cause of the problem. If you own a car for a few years you
become familiar with its faults, and it is easy to assume that a problem is due to the
same cause as before.
A moment’s thought will show that there are a number of faults which could cause
similar symptoms. Erratic starting and running could be the result of fuel contamination,
a blocked fuel filter, blocked jets, faulty sensors, or a problem with the engine
management system. Some modern cars deliberately limit the engine power if they
detect a fault – this is intended to let the owner drive to a garage without causing
further damage.
It is not logical to assume that fault symptoms are the result of a particular failure, just
because this was the case on previous occasions. It may be statistically likely, but by
no means guaranteed. There is the risk of ‘premature diagnosis’, where one jumps to
conclusions about what the source of the problem is. You need to check that your
hypothesis is correct, and take care when interpreting symptoms – it is easy to delude
oneself that they support your fault-finding diagnosis.
If you are lucky, there may exist a fault-finding procedure which can guide you.
Instructions for domestic appliances usually contain these, but are of limited help to
anyone with engineering competence. For example, the manual for a TV satellite
receiver suggests that if you cannot find a channel you have previously watched, it may
be because the channel only broadcasts for part of the day. The manual for the
computer on which these notes were typed suggests that if the monitor is blank, the
user should check that the cable is fully connected.
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Manuals for industrial equipment are usually much more helpful. They typically contain
block diagrams, schematics, a brief explanation of how the system works, a list of
suitable test points, and the voltages and signals which should be found at them.
Complex equipment can have such a wide variety of possible failures that it may be
impossible to list all possible faults and the procedures for correcting them. This is
where you need to apply a logical approach.
Finally, remember than the system may not actually be faulty. It is possible that the
user is operating the equipment incorrectly, or perhaps has encountered unusual
behaviour and assumed it is the result of a fault. For example, if an oscilloscope input
is switched into ac-coupling mode, then a steady dc voltage appears as 0 V. If one
were not paying attention, it might seem that either the CRO was broken, or the signal
being measured was faulty.
Also, if unreliable equipment is prone to giving incorrect signals, one is liable to ignore
them even when they are correct. There is the case of an airliner which crashed after
the pilots ignored a warning message from safety equipment which frequently triggered
false warnings. Perhaps the moral of this case is that unreliable equipment is worse
than equipment which is broken.
Sequential and non-sequential methods
Fault finding can be described as a six-step process, namely:
•
collect evidence
•
analyse evidence
•
locate fault
•
determine and remove cause
•
rectify fault
•
check system.
This may seem little more than stating the obvious in formal language, but there are
important points to be made here.
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In many cases evidence of the fault will already be available – the equipment will be
handed in for repair with a description of the problem. However, the user may not have
reported the symptoms accurately, or might have included extra information which
confuses the issue. It is to be expected that users will see the problem from their
perspective rather than that of the person attempting a repair. For example, a
complaint might be ‘the photocopies of my report are unreadable’ which is of obvious
concern to the author. A better description would be "copies of text on coloured paper
have the background printed too dark". This is more helpful in terms of identifying the
fault.
If possible, it is best to see the fault for yourself. This may be impractical in the case of
a unit removed from a larger system. You may need to have specialist equipment to
simulate inputs, or a variable power supply and signal generator may be enough. For
example, you could test an audio amplifier for distortion by connecting a pure 1 kHz
signal. Alternatively, other audio equipment such as a CD player or a video recorder
might be suitable.
In debugging it is said that a good test is one with a high probability of finding an error,
and a satisfactory test is one which uncovered a previously unknown problem. This
applies to fault finding as well. We can increase our efficiency by carefully choosing
what to test.
‘Milking the front panel’ is a term given to a technique for collecting evidence. Use
switches and selectors on the equipment to determine which functions are working and
which are not. This may help to determine which part of the equipment is at fault. This
is the second step in the process – analysing the evidence.
A block diagram is useful at this stage. If the system can be split into separate parts
with their own functions, the fault symptoms may indicate that one unit has failed. This
block may be suspected as the culprit because all system functions using it are faulty,
while functions not requiring that unit are operational. This is not a guaranteed method
– a faulty selector switch might give the same symptoms. To get confirmation, we need
to locate the fault more accurately. This may mean using test gear to check signals and
power going into the unit, and output signals. If the inputs are correct but the outputs
are wrong, the unit is clearly inoperative.
One issue we need to be aware of is that the original cause of the fault may be entirely
separate from the final result. If the output from a voltage regulator is shorted to the
input, then components running off that voltage supply may fail. The result might be
that all the logic devices on a board overheat. The operator may then switch off the
equipment after only one or two components have burnt out. If these components are
then replaced without fixing the original short, the fault will then recur.
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We could say that the key issue here is that we need to be sure we are fixing the fault,
not just fixing the symptoms. It is important to check the system after the supposed
repair to ensure that it really is fixed. If there is an underlying problem which is still to
be addressed, then the equipment will fail a second time. It is better that this happens
while the equipment is in the workshop, than later when it has been returned to the
customer with an invoice.
One point often forgotten when considering the six-step method is that it is often an
‘iterative’ process – parts of it have to be repeated as necessary. Fixing a fault may
only deal with part of the problem – part of the equipment may still not be fully
functional. This can happen not only with power supply faults, but also in many other
circumstances.
The user may continue to use equipment when it is operating below standard, then ask
for a repair once more serious faults have arisen. You may discover a fault which has
gone unnoticed.
A ‘common mode failure’ is one where apparently separate systems fail simultaneously
in the same way because of an underlying problem. Four-engined aircraft are believed
to be particularly safe because the chances of more than one breaking down on a flight
are minute. In fact, there are a variety of reasons this could happen, such as fuel
contamination, bird strikes, software problems and so on.
Critical computer systems, such as those on an aircraft, usually have multiple parallel
processors, sometimes sourced from separate manufacturers, in an attempt to ensure
they cannot all fail simultaneously. In practice, this is not guaranteed either. For a start,
such a system requires ‘arbitration’ – if the computers are giving different outputs, a
choice has to be made over which signals to ignore. This arbitration system could itself
fail. Also, the engineers writing the software could all make the same faulty
assumption, leading to all the computers generating an incorrect signal. Finally, there is
the possibility that the specification is wrong.
Formal methods of fault finding can be classified as either sequential or non-sequential.
Non-sequential methods are those used by automated test equipment (ATE). ATE
performs a large number of tests on the system under investigation and uses a
database of possible fault symptoms to determine the problem. As far as the operator
is concerned, the tests are all done simultaneously. Also, the sequence of tests may
always be the same, independent of any errors detected. In this sense, ATE is said to
be using a non-sequential method.
Sequential testing is where tests are done in a sequence, one after the other. The
result of each determines what test is done next.
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Manual testing done to a proper strategy is systematic testing. Tests are performed
according to a plan. The strategy can be based on reliability data, where the system
under test has been in use long enough to give helpful information on where faults are
likely to occur. Whether this is possible depends on individual circumstances. A factory
building thousands of mobile telephones may quickly acquire such statistics. A small
workshop repairing a variety of devices may find that every fault is different, and has to
use a different approach.
It is only worthwhile to perform extensive research into fault modes if either large
numbers of the unit are produced, without design changes, or if testing is hazardous or
expensive.
This section is largely concerned with fault finding based on block diagrams. Three
kinds of systematic test will be covered, as shown in Figure 1.1. For completeness, the
diagram also includes random testing, which is non-systematic. This method is rarely
used – it is difficult to conceive of a circumstance in which it would be appropriate.
Perhaps if the symptoms gave no indication of where the fault was located, and no
block diagram for the system was available, this might be worthwhile. The larger the
system, the more efficient it is to have a logical approach.
Fault Finding
Methods
Sequential
Non-sequential
(used by ATE)
Systematic
Non-systematic
(random testing)
Based on
reliability
statistics
Based on
block
diagram
(input to output,
output to input,
half-split method)
Figure 1.1 Classification of fault-finding methods.
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Block diagrams
Block diagrams are a useful way of describing how a system is constructed. They
present an intermediate level between the instruction manual or specification, and the
schematic diagram. The instructions state what functions the equipment performs and
how to operate it, while the schematics show the components and the electrical
connections between them. These are both necessary for fault finding, but for a system
of any size a block diagram is an aid to comprehension.
Figures 1.2 and 1.3 show two typical block diagrams.
Large systems may need two or more layers of block diagram, breaking it down into
units then subdividing them. It might also be helpful to use a block diagram to show
how the equipment is connected to other devices, as in Figure 1.2.
DVD player
Aerial
HDD
recorder
Television
Satellite
dish
Satellite
receiver
Figure 1.2 Block diagram for connections to domestic television.
ASCII
Data
Interface
Crystal
Character
Generator
ROM
Character
Store
RAM
÷ 512
÷8
Shift
Register
÷ 32
Serial
Video out
Vertical
Sync
Horizontal
Sync
Figure 1.3 Block diagram for monitor circuit.
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Block diagrams can often appear complex. One would like to think that they have been
designed to make them easy to follow – sadly this is not always the case. If you are
drawing a block diagram yourself, try to position the blocks so that the connections
between them are as simple as possible. Try to ensure that signal paths cross no more
than necessary – apart from that, signals flow from left to right by convention. (Power
rails are usually shown separately on schematic diagrams to reduce the complexity.)
In some cases it is possible to have a clear relationship between the units in the block
diagram, the schematic, and the circuitry. Analogue circuits are often easier to lay out
than digital ones in this respect. An integrated circuit with four gates on it may have
three of these used in widely different parts of the circuit, making it impossible to lay
these out separately on the printed circuit board (PCB). Also, space constraints may
make it impossible to design so that units shown in the block diagram are separate on
the PCB.
A
B
C
D
E
F
G
Figure 1.4 Diagram for system with linear connections between blocks.
The easiest block diagrams to follow are ‘linear’ – signals flow from one unit to the next,
without splitting or joining. Figure 1.4 is an example. We shall consider three
systematic methods of fault finding on such a system:
•
input to output
•
output to input
•
the half-split method.
Input to output
This is perhaps the most obvious approach. With a suitable input connected, the first
block in the chain is tested. If the system is connected as in Figure 1.4, the output from
unit A would be compared with the correct signal. If A’s output shows no error, it is
assumed to be working correctly. Unit B would be tested next, followed by C, D and so
on. The process is repeated until a faulty signal is found. The broken unit is then the
most recently checked one. For example, if testing proceeds with all signals correct
until the output of E is faulty, then E is the source of the problem.
Input-to-output testing has the advantages of being simple and direct. Once the faulty
unit has been repaired or replaced, the system can then be checked. If it is still not
functioning correctly, there may be a second fault. In the example, one would check
that the output of unit E is now correct, then proceed through F onwards.
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Remember that it is possible that the fault lies in the wiring – all the blocks could be
working normally. To check for this problem, test the signal at either end of the
connecting cables.
Output to input
This method operates in the reverse direction. If a system is producing a faulty output,
one could start by testing the input to the final stage, G in Figure 1.4. If G has a correct
input but faulty output then that is the source of the problem. Otherwise, we would
check the input to F. The process would be repeated until a unit with a correct input
signal is found; it should be the source of the problem.
Of course, there is always the chance that more than one block is damaged. With
output-to-input testing, additional faults may turn up in blocks which have already been
checked. It could be argued that this makes it an inferior method to the previous
approach. Fault finding by looking for the first signal which is not faulty might also
cause confusion and lead to more mistakes.
The half-split method
This is also known as the binary split method, and is a more sophisticated strategy.
The first signal to be tested is in the middle of the system. If it is correct, the second
half must be faulty. If the signal is wrong, the first half of the system has an error. The
next step is to narrow down the search by looking at a signal in the middle of the faulty
half, and repeating until the damaged unit has been identified.
In Figure 1.4, if unit E is faulty we would expect to track it down as follows:
•
This example has an odd number of blocks so there is no suitable point half-way
through. Suppose we check the output from C first. It will be correct.
•
Next, check half-way between C and the final output; this is the signal connecting
E to F. It will be corrupted.
•
Finally, check the input to E; this is in the middle between the last two test points.
It should be correct, indicating that E is faulty.
If the last test had given a faulty signal then D would be identified as the source of the
problem.
Figure 1.5 illustrates this process. Note how each test halves the size of the area
containing the fault.
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suspect area
A
B
C
D
E
F
G
D
E
F
G
F
G
faulty
output
test here
A
B
C
test here
A
B
C
D
E
test here
Figure 1.5 Identifying a fault with the half-split method.
Now, this may seem an overly complex method compared to the other two. Its
advantage is that it is theoretically the most efficient strategy, assuming the fault could
be in any block with equal probability, and all the signals are equally easy to test.
In the example, three tests are enough to identify the faulty unit, whichever one it is. To
see how this is efficient, consider a much more complex system, with 50 units.
Three tests would divide the system into halves, quarters and eighths, which would be
enough if there are no more than eight blocks. To check a system of 50 blocks needs
six tests – five tests could only check 32 blocks (25 = 32) while six tests are needed for
a system of 33–64 blocks (26 = 64). In contrast, using input-to-output or output-to-input
testing would require an average of 25 tests, assuming there is a single fault at a
random location.
A sensible block diagram would not have 50 blocks in it; this would be unreasonably
complex. However, it is possible to have a system with 50 similar units, such as a
network of computers or a delay line with multiple filters connected in series.
In a practical situation, we would want to modify the half-split method to take account of
a number of realities. Firstly, we usually cannot assume that the fault is equally likely to
be in any block. Some blocks will be more complex than others, or less reliable, and in
any case the fault symptoms will allow us to narrow down the search, even if only by
eliminating some units as the source of the problem.
Secondly, not all tests are equally expensive, in the sense of the time and effort taken
to perform them. It may be easy to access test points at the output of some circuits,
while others may require much dismantling. Testing may involve complicated
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equipment such as a logic analyser, which will take a while to set up and give results
that are time-consuming to analyse.
One should also consider possible hazards involved in testing. You could concentrate
on low-voltage systems first and avoid live circuits where possible. There are other
risks to take into account, such as possible damage to the equipment under test, either
through dismantling and reassembling it, or as a result of testing it, or running it while a
fault is present.
In short, a more sophisticated approach to the half-split method would be to weight the
testing to take account of the likely fault location and the cost or risk of testing. Aim to
minimise the expected effort required to find the fault. Instead of picking a point halfway through the faulty section, try to find a node with a 50% chance of showing a faulty
signal, with a bias towards tests that are easy to do.
1.1
Referring to the system in Figure 1.4, list the test points you would use with the halfsplit method, if the faulty unit is:
1.
F
2.
B
3.
G
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Non-linear structures
Block diagrams are rarely as simple as Figure 1.4. It is usually impossible to lay out the
structure in a straight line, as some units have more than one input or output. If one
part of the equipment has multiple outputs, feeding several other blocks, then the
signal flow is said to diverge. A unit with multiple inputs is said to be convergent. It
takes signals from more than one source.
Complex structures with converging and diverging paths can make fault finding more
complex, but they can actually help track down the problem. For example, consider the
system in Figure 1.6. This is a divergent system, centred around block B, which has
three outputs. There are three final outputs from the system, arising from D, E and H. If
the output from D is faulty, while E and H are correct, then we can narrow down the
search immediately before even testing internal signals. Blocks C and D are obvious
candidates.
E
A
B
C
D
F
G
H
Figure 1.6 System with diverging signal paths.
A lot depends on exactly how B’s outputs are connected – Figure 1.6 shows them as
three different signals, but it could be one signal split three ways. For example,
consider Figure 1.7. In (a), a signal generator is shown as a single block, with three
separate outputs. It is conceivable that one of these is damaged, while the other two
are functioning normally.
Signal
generator
square wave
sine wave
TTL
(a) Block with three outputs
open
circuit
+12V
regulator
+5V
regulator
E
Motor
driver
B
RF
data link
(b) Block with one output to three
destinations
C
F
(c) Alternative
block diagram
Figure 1.7 Alternative way of representing diverging signal paths.
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However, the system in Figure1.7(b) has a single output to three blocks. An open
circuit could cause one circuit to fail while the others function normally. It might be
better to draw the block diagram as in Figure 1.7(c) to make this explicit. B, the +12 V
regulator, must be working if at least one of the devices running off it is functioning
normally.
Returning to the example of Figure 1.6, if D and E give correct outputs but H does not,
then the signal paths from B to F and G to H need to be checked. Use one of the
earlier strategies, such as input-to-output testing.
There is a further possibility. All three outputs may be erroneous. In this case, we
would suspect that the fault lies before the point at which the signal paths diverge. In
the example, we would suspect units A and B.
The basic principle is to check any unit where all the components which its output
passes to give a faulty output in turn.
Of course, it may be that there are multiple faults, but this will become clear as testing
and repair proceed.
Converging paths also need to be taken into account. If a unit needs multiple inputs to
work, then if its output is correct all preceding units can be assumed to be functioning.
Figure 1.8 shows an example. A good place to start when testing this system would be
the output of block R. If this is correct, then only S and T need to be investigated.
If R shows an incorrect output, its three inputs would be tested next. If these are all
correct, then R is the source of the problem. Otherwise, it is likely that the search can
be narrowed down to only two blocks.
J
K
L
M
N
P
R
S
T
Figure 1.8 System with converging paths.
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Alternative structures
Figure 1.9 shows a more complex system. It has both diverging and converging paths.
Note how blocks D and E both have two outputs, so they count as divergent. Also,
there are three parallel paths between C and J. It is possible to suggest a logical
strategy for fault finding such a system dependant on details of the fault symptoms.
For example, if the external outputs from D, E and K are all erroneous the fault is likely
to be in A, B and C. A good place to start would be to check the output from C first,
then work backwards. This is applying the output-to-input method to sections A–C.
If D is producing a correct output but E is not, then E would be the likely candidate. K
would probably generate an error also.
If D and E are correct but K is not, then this is now similar to the converging system
problem covered earlier. J may be faulty or the problem may lie with the two paths F/G
and H. E may also be damaged, depending on whether its two outputs are separate.
A
B
C
D
E
F
G
J
K
H
Figure 1.9 System with diverging and converging paths.
Figure 1.10 shows a system with alternative paths. It is a computerised control system,
with four computers running in parallel so that the system will still work if one fails. (This
is known as a quadruplex redundant system.) One of the computers is sourced from a
different manufacturer to reduce the chance of common mode failure.
A side-effect of such a system is that if one unit fails it needs to give a prominent
warning so it can be fixed. If this did not happen, the operator would continue using the
system until it failed completely. This would probably be disastrous, as redundant
systems are only worth the extra expense and complexity for safety critical situations.
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A domestic example is the ring mains, where power to the sockets comes from a loop
of cable wired to the distribution at both ends. This will still work if there is a single
break in the wires. In electronics, it is sometimes necessary to wire several
components in parallel to achieve the necessary properties. For example, several
power transistors would be connected in parallel if the current being handled exceeds
the limit for a single transistor. Power supplies tend to need a large smoothing
capacitance, often achieved by connecting two or more capacitors in parallel.
In both of these cases, if one unit fails the system will keep working, but may overheat
and be damaged.
When fault finding a system with alternative paths, it may be necessary to test each of
the subsystems connected in parallel. There are also maintenance issues, with
equipment being tested on a regular basis to ensure no failure has occurred
undetected.
Computer
A1
Pilot
Controls
Computer
A2
Arbitration
Computer
A3
Control
Surfaces
Computer
B
Figure 1.10 System with alternative paths.
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1.2
In the three systems below, the shaded block is faulty. List which units will give a
correct output and which will not.
1.
D
A
B
C
E
F
H
G
2.
S
T
U
V
W
Y
Z
X
3.
G
A
C
D
E
F
B
H
J
Feedback
A further type of system is one with feedback paths. These can be tricky to test as
faults effectively circulate round the system, meaning that most of the signals are
incorrect, even for units working properly. The feedback may be used to improve the
performance of the system, perhaps to compensate for changes in the environment. In
this case, it may be possible to disconnect the feedback, possibly replacing it with a
substitute signal.
However, if the system relies on feedback to work properly it may not be possible to
break the loop.
Closed loop systems require more thorough knowledge of how the device is supposed
to work. It is often necessary to understand how each part of the system will work if
given an incorrect input. Each unit is liable to have a faulty input and output – you need
to identify a device where the output is inconsistent with its input, or at least eliminate
those which are working consistently.
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For example, consider the temperature controller in a piece of industrial plant. Figure
1.11 shows a block diagram. The fault symptoms are that heat is applied continuously,
leading to an over-temperature condition, triggering an alarm and an emergency cutoff.
Set
point
0-12V
+
±12V
Σ
Controller
4-20mA
0-100%
0-40kW
Actuator
Heater
Plant
0-400°C
0-12V
Signal
processing
0-0.5V
Thermocouple
Figure 1.11 Temperature control system with feedback.
The way this equipment is supposed to work is that the operator sets the desired
temperature on a dial, while the actual temperature is sensed by a thermocouple. A
controller works a heating element, applying energy proportional to the difference
between the required and actual temperature.
The block diagram in Figure 1.11 shows the signal types and their ranges. For
example, the unit shown as ‘set point’ has an output in the form of a dc voltage. It has a
dial calibrated as 0–400°C, and will be linear so that if the operator turns it to 200°C,
the output is at half of the range, or 6 V.
Now, if the temperature is seen to rise continuously, past 200–300°C and beyond, we
would expect most of the signals to be different from normal. For example, suppose we
find an output of 12 mA from the controller. This causes the heater to run at half power,
even though the temperature is now above the set point.
However, checking the controller input shows it is receiving a voltage of 6 V. It is likely
that the controller is working normally; it is operating the heater at half power because
its input is wrong.
Figure 1.12 shows some measurements of signals in the system. The thermostat is the
likely cause of the problem as its output is 0 V even though the temperature is 300 C
and rising. As a rough guess the output should be three-quarters of the maximum,
0.375 V in this case. The manual should allow a more accurate calculation.
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Set
point
6V
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6V
+
Σ
12mA
Controller
50%
Actuator
20kW
Heater
Plant
300°C
0V
Signal
processing
0.0V
Thermocouple
Figure 1.12 Temperature control system with faulty signals.
Other blocks seem to be working normally, with outputs consistent with their inputs.
One way to operate this equipment without feedback would be to connect a variable
power supply in its place. Set this to 0 V to simulate the system being cold – the heater
should come on full. Turning the voltage up to 12 V should represent over-temperature
and cause the heater to switch off. Slowly turning up the voltage should simulate the
temperature rising to the set point, and should cause the heater to throttle back.
The temperature control example is assumed to use proportional control – the amount
of heat applied is proportional to the difference between desired and actual
temperature. Control systems which incorporate integral action are harder to fault find.
Connecting a DC input instead of the feedback will not help as any error signal will
cause the controller output to ramp up or down.
Other systems may be much harder to test with feedback disconnected. A control
system for a missile, for example, has to handle signals which change in a small
fraction of a second. In such circumstances, one would hope that the manufacturer has
devised a suitable test procedure and documented it in the manual.
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Exceptional faults
Other types of faults we need to consider are multiple faults, catastrophic failure and
manufacturing faults.
Multiple faults
So far we have tended to assume that there is only one fault in the system under test.
This may not be the case. The obvious approach when dealing with multiple faults is to
repeat the process, fixing one problem at a time until they are all repaired.
The methods we have covered are appropriate for dealing with multiple failures,
although with diverging paths the symptoms can be misleading. If two units which take
inputs from a common source both fail, we would tend to suspect the wrong block.
If multiple faults are present, it is unlikely to be a coincidence. One failure may have
triggered further damage.
A good place to start is to check the power supply. If that has failed, then none of the
units requiring power will work. If the power rails have gone over voltage, for example,
allowing unregulated voltage to reach the rest of the system, then there is likely to be
further damage. Fix this first, then set about repairing the consequential damage.
If several blocks in the system are faulty, it would be helpful if they could all be tested
individually. This can be difficult if suitable signals need to be present for a unit to be
tested properly. In these circumstances, it can be worthwhile to build a circuit to
generate suitable test signals to permit a unit to be tested on its own. There is also the
possibility of ‘board swapping’, transferring a circuit board from a working system to a
faulty one to see if this cures the fault. This technique needs to be used with care. It
could endanger working boards, and increases the amount of dismantling involved.
Also, it is not appropriate to use units from equipment that is to be sold as ‘new’.
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Catastrophic failure
A catastrophic failure is defined as one which is sudden and complete. There is no
intermediate stage where the equipment operates with reduced performance, and no
warning signals that it is about to fail. A gradual failure would allow us to make repairs
before the equipment fails completely, and would prevent failure in service, with
possibly expensive consequences.
One advantage of catastrophic failure is that it is better than an intermittent fault. Most
of us have experience of a recurring problem which disappears when the equipment is
taken for repair. It can be difficult to track down a fault when there are no symptoms
present. The normal approach is to identify a likely candidate for the source, then
stress it in the hope of triggering the fault. For example, a loose connector may work
most of the time. Wiggling it while the equipment is running may make the problem
reappear. There exist sprays which can be used to cool down components, which
sometimes helps identify those which have worked loose in their sockets. An
alternative is to heat them up with a hair dryer. Flexing cables can help track down
loose wires.
Manufacturing faults
These should be identified and corrected by the manufacturer before the unit leaves
the factory. Failing that, they should be covered by the warranty. However, there is
always a chance that a fault may not lead to a problem until after many years of
service. Also, the manufacturer still has the problem of fault finding and repairing the
equipment at the factory.
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Table 1.1 Typical manufacturing faults.
Fault
Comment
Loose cables, integrated
circuits and connectors
Connectors are often designed with locking levers to
prevent them coming loose. Also, ICs are usually soldered
direct to the PCB instead of using sockets. Heating and
cooling can cause them to work loose over time otherwise.
Faulty soldering
Automatic soldering should eliminate this. However, it is
sometimes necessary to solder manually, especially during
a repair, and poor solder joints can work to begin with, only
to fail later.
Shorts and open circuits
on circuit boards
Loose drops of solder and strands of wire can float around
inside an enclosure, occasionally causing a short. Bad
soldering can also short IC pins, or cause open circuits.
Damage to a PCB can cause cracking. Gaps can appear in
very narrow tracks during etching, especially at sharp
corners.
Stressed joins and wires
If the mechanical design of the equipment is poor it can
mean that joins are under continuous stress. A circuit board
may be seated at a slight angle in its slot, or wires may be
not quite long enough so that they bend at a sharp angle
right next to the connector. The slightest knock to the unit
may cause something to break, or it may eventually fail
through metal fatigue.
Under-rated components
These will overheat. If this is bad enough for them to smoke
or catch fire the problem will be fixed quickly. However, a
less dramatic failure will be a gradual deterioration in
performance over time. The component may show
discolouration or swelling or leaking.
Incorrect polarity
Diodes, transistors and integrated circuits all need to be
inserted the correct way round. IC sockets have the correct
polarity marked on them, but there is always the possibility
that the socket has been soldered in the wrong way.
Applying a reverse voltage to an electrolytic capacitor
eliminates the dielectric, turning it into a short.
Damage to components
CMOS devices are easily damaged by static electricity.
Soldering in components can also cause heat damage. If
the leads on devices such as resistors and capacitors are
bent too close to the component, the casing can crack,
allowing dampness to enter and degrade the performance.
IC pins may be bent or broken during insertion.
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Other possible faults are if the wrong value of component is used, or inserted in the
wrong place. If automated machinery is used, it is conceivable that a whole batch of
boards could be manufactured. Such faults should be picked up on the production line.
A batch of faulty components may have been supplied. This has happened with
batteries and capacitors in recent years. It can prove extremely costly to the
manufacturer if large numbers of the product enter the market place then fail later.
1.1
If you can, give details of three component failures you have encountered yourself. The
author could list a tantalum capacitor which was soldered in with the wrong polarity and
caught fire, soldered battery connectors which broke free when dismantling the unit,
and a power supply with a wire connected to the wrong end of a resistor so that it
overheated.
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Summary of this section
We have covered a number of basic logical approaches to fault finding. Block diagrams
have been used to represent various system configurations, and we saw how to
approach fault finding with more complex layouts, such as those featuring divergence
and convergence.
We also considered possible manufacturing faults and considered some examples in
general terms.
The next section covers practical fault finding to block level.
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1.1
1.
2.
Define:
a.
sequential fault location methods
b.
systematic fault location methods.
If the receiver unit fails in the system below, describe how you would find this
fault using:
a.
the input-to-output method
b.
the output-to-input method
c.
the half-split method.
front
panel
3.
diode
matrix
microcontroller
power
amp
IR
LED
receiver
microcontroller
motor
driver
Define diverging and converging systems with reference to the system below.
State the consequences of failure in:
a.
Unit A
b.
Unit D
c.
Unit G
d.
Units B and E simultaneously.
A
B
C
D
E
G
F
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Answers to SAQs
SAQ 1.1
1.
An acceptable answer is:
•
test the output of C (correct)
•
test the output of E (correct)
•
test the output of F (incorrect)
•
conclude that F is faulty.
Since the number of blocks is not a whole power of two, other alternatives are
just as efficient. For example, an alternative would be:
2.
3.
•
test the output of D (correct)
•
test the output of F (incorrect)
•
test the output of E (correct)
•
conclude that F is faulty.
An acceptable answer is:
•
test the output of D (incorrect)
•
test the output of B (incorrect)
•
test the output of A (correct)
•
conclude that B is faulty.
An acceptable answer is:
•
test the output of C (correct)
•
test the output of E (correct)
•
test the output of F (correct)
•
conclude that G is faulty.
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SAQ 1.2
1.
Correct output – A, B, C, D
Incorrect output – E, F, G, H
2.
Correct output – S, T, U, V, X
Incorrect output – W, Y, Z
3.
Correct output – A, B, C
Incorrect output – D, E, F, G, H, J
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Section 2: Implementing a fault location strategy
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Introduction to this section
What this section is about
This section covers the process of implementing a fault location strategy in an
electronic system.
Outcomes, aims and objectives
At the end of this section you will be able to:
•
identify risks and use safe working practices
•
identify fault symptoms in terms of system operation
•
interpret fault symptoms using test equipment and/or diagnostic aids
•
locate faulty circuits using system documentation and test equipment.
Approximate study time
10 hours.
Other resources required
A selection of analogue and digital systems is necessary to give practical experience.
The unit specification states that various electronic systems may be used for fault
finding. A commercial/industrial electronic system would be particularly appropriate but
it is recognised that this will not always be possible. Other suitable systems could
include:
•
a programmable logic controller operating external equipment
•
a desk-top computer driving a multiapplications board
•
a microprocessor driving a multiapplications board
•
an alarm system
•
a multistage AF or RF amplifier
•
a static inverter.
It would be best to have a complex system that can be subdivided into clearly separate
stages.
In order for the candidate to practice fault finding, several systems need to be available
to test. A wide variety of systems would be helpful.
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Assessment information for this section
How you will be assessed
Candidates will submit evidence to satisfy this section in the form of a report which
documents a practical fault-finding activity.
When and where you will be assessed
The assessment will normally take place at the end of this section.
Centres are recommended to develop and use appropriate checklists to monitor the
candidate’s fault-finding activities and provide a check on the authenticity of the report.
What you have to achieve
The report should include:
•
the identification of risks
•
appropriate safe working practices used
•
a description of the fault location strategy implemented
•
details of tests carried out and the test equipment and/or diagnostic aids used
•
sketches of appropriate block diagrams
•
reference to the documentation used
•
a contemporaneous log of the fault-finding activities.
Opportunities for reassessment
If the report is incomplete or erroneous, it is likely that you will be asked to correct and
resubmit it. It may be necessary to perform further tests in the laboratory, adding the
results to the report as required.
In extreme cases you may be asked to fault find on a different system and submit a
new report.
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Introduction
This section is of a more practical nature than Section 1. You need to demonstrate the
ability to implement a fault-finding strategy. An outline of the process will be presented
here, and later you will need to perform an assessed exercise.
Obviously, experience with repairing equipment will be very helpful. You can learn
more by practical experience than by reading theory in a book, but you need to be
aware of the basic principles. We have an additional problem in that these notes
cannot suit your circumstances exactly. We could cover a particular piece of equipment
in detail but it is likely that you will meet different equipment instead. We will cover the
general principles, with some examples for illustration. This will be more useful in the
long term as you can then apply these principles to a wide variety of fault-finding
problems.
Remember that if you only needed to be familiar with one type of system, there would
be no need to study fault-finding principles as in theory your employer could supply a
repair strategy which covered all likely problems.
If you already have some fault-finding experience, these notes should help you to put
your skills in context. They will allow you to extend your skills to other applications and
increase your employability.
At this level it is not necessary to have an in-depth knowledge of how the system
works. Familiarity with the block diagram is enough. Also, research has shown that as
technicians become better at fault finding they tend to forget most of the theory they
learned.
Finally, the practical exercises do not require you to fix the fault, only to identify it. Here
the emphasis is on choosing a good strategy for finding the fault. In real life, you would
fix the problem, and then check that the repair has been effective. There might be more
faults still to be dealt with.
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Safety
Before you start any practical fault finding, we need to consider risks and safe working
practices. We can classify safety as:
•
primary safety, which protects the user from injury, and
•
secondary safety, which protects the equipment from damage.
Cynics suggest that there is also tertiary safety, which protects the manufacturer from
litigation. This explains the labels which read ‘No user-serviceable parts inside’ and
warn the owner not to open equipment. If you do dismantle the apparatus and then
injure yourself, you cannot sue the manufacturer for damages.
In our circumstances, we should be qualified personnel, competent to investigate faults
and trusted to use safe working practices. Be aware, however, that if you do not follow
the correct procedures and are then injured, your employer may not support you. This
may mean loss of pay while off work, but you may also lose your job or be prosecuted
for endangering others.
Primary safety
The obvious risk when working on electrical equipment is one of electrocution.
However, electronic devices usually run off low voltages, such as ±5 or ±12 V. In some
cases this is supplied by an external transformer. If there is an internal power supply, it
may be a sealed unit. Provided the power supply is working correctly, there is no need
to have access to mains electricity. In any case, the power supply may be designed to
be impossible to dismantle, so that it has to be replaced if faulty.
However, bear in mind that low voltages can still be harmful, particularly if large
currents are drawn. For example, batteries can be dangerous if shorted. NiCads have a
low internal resistance, so shorting them can cause wires to melt. Lead acid batteries
generate hydrogen when sourcing a large current. This can explode, showering the
surrounding area with acid. Lithium polymer batteries are damaged by being driven into
deep discharge, and can also explode or catch fire if mistreated.
Cathode ray tubes as found in televisions, monitors and oscilloscopes require a highvoltage supply, thousands of volts, to operate. This voltage can be stored in capacitors
and still be lethal after the supply has been switched off. Be aware that some designs
of television have a chassis which floats at high voltage.
There are also mechanical risks to be considered, such as moving parts and sharp
edges. Take care to avoid motors, gears and levers. Video recorders in particular have
complex mechanisms for feeding tape to the heads. Be aware that watches, jewellery,
clothing and long hair can be trapped. Also take care when removing PCBs and
connectors. These are often stiff and require force to take out. If a component comes
undone suddenly it would be easy to injure oneself on sharp edges.
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There can also be chemical risks, although these are not so common with electrical
equipment. Avoid chemicals leaking from damaged components, as they are often
toxic. Your employer should have procedures for safely disposing of hazardous
materials.
Secondary safety
Testing circuits can endanger them. Clearly, it will make your job harder if you
accidentally introduce new faults while attempting to repair old ones. Also, we are
trying to complete repairs quickly and efficiently, not create unnecessary work.
Much equipment, especially consumer goods, is designed to be reliable and cheap to
build, with little thought given to making it easy to dismantle. One method is to solder
ICs direct to the PCB instead of using sockets. They cannot work loose with time, but
can then be very difficult to remove. Parts and connecting cables can be jammed in
tightly – removing one unit can risk damage to the cables. Plugs are often clamped
onto the wire directly without solder. Pulling the connector out by the wires risks
damage.
It is easy to accidentally short two tracks together on a circuit board or multiway
connector when attaching a meter probe. Shorting out the supply, even briefly, can
destroy sensitive semiconductor devices.
Static is also a hazard. Humans can easily be charged to thousands of volts. If you
touch an IC, this voltage is discharged in a tiny fraction of a second. It causes you no
more than discomfort, but can destroy the device. Use a static discharge wristband if
handling static-sensitive devices (they are usually labelled as such in their packaging).
In some cases it is enough to touch the chassis of the equipment while it is earthed.
If you need to inject a test signal, it will normally be necessary to disconnect the
existing signal to avoid a conflict. However, some circuits can be damaged if they do
not have a load. Audio amplifiers are an example – they should not be operated with
the loudspeakers disconnected. Basically, a current generator with an open circuit
output has the same risk of damage as a shorted voltage source. With no load, all the
power is dissipated internally, by current flowing through the equivalent output
resistance. Unused current sources should be shorted, but check this very carefully.
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Also, some equipment does not operate without a load. Devices with feedback were
mentioned at the end of the last section. Switching mode power supplies are another
example. One method sometimes used when testing them is to substitute a light bulb
for the load.
Safe working practices
First of all, you should be familiar with the correct use of test equipment as covered in a
separate unit, Electronic Testing Skills. Check the manufacturer’s documentation for
safety advice and other information which may help identify hazards.
Switch the power off while removing and replacing the cover – screws could easily fall
inside and cause a short circuit. Also, it may be necessary to have the power on while
performing tests but there is no point endangering yourself when unscrewing the lid.
It may be possible to use an isolation transformer to reduce the risk of electrocution. If
you have one hand inside a unit with exposed mains terminals, make sure the other
hand is nowhere near earthed metal. Do not use a static discharge wristband in these
circumstances.
Watch out for moving parts and keep fingers, hair and test probes out of the way.
Computers can still be partially ‘live’ while not running. If you want to use a static
discharge wristband clipped to the chassis, leave the computer plugged into the mains
but turned off with the switch on the PSU so it is isolated from the live and neutral but
still earthed. Just remember to turn it back on again, as most users never use this
switch.
In a workshop shared with other people, take care turning on and off equipment in case
you accidentally turn on the mains supply to a unit someone else is working on.
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Fault location strategy
The strategy we will be using is based on the objectives for this section. The plan is to:
•
identify risks and use safe working practices
•
identify fault symptoms in terms of system operation
•
interpret fault symptoms using test equipment and/or diagnostic aids
•
locate faulty circuits using system documentation and test equipment.
When you attempt the practical assessment, this all needs to be documented. It is a
good idea to do this in a real-life application also. The advantages of documentation
include:
•
it will save time if you encounter a similar problem later
•
if the repair turns out to be unsuccessful, you will need to refer to this record
•
colleagues can benefit from your expertise
•
you may forget what you have done, especially if the system is complex or
numerous tests are required
•
your employer may need this information for billing purposes
•
lastly, in the event of an accident or a complaint from the customer, you will need
a record of what procedures were followed.
What you need to do is:
•
Identify potential risks. These could include high voltages, hazardous chemicals,
damage to static-sensitive devices and moving parts. If the equipment needs to
be tested while live, identify which parts are at mains voltages.
•
List safe working practices used. Equipment should be disconnected from the
mains where possible. Otherwise, consider some way of ensuring you cannot
accidentally touch live contacts. There should be a procedure for safe disposal of
hazardous chemicals.
•
Describe the fault location strategy implemented. This may be one of the
strategies covered in Section 1, such as input to output, or may have to take into
account diverging or converging paths.
•
Detail the tests carried out and the test equipment and/or diagnostic aids used.
This may be vital for later reference – when you measure voltages and signals,
write down the results so you can refer to them later. You will forget otherwise.
•
Sketch appropriate block diagrams. The unit’s documentation may already
contain block diagrams, but make up your own based on what is important to you
while testing.
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•
Refer to the documentation used. Much time can be wasted by searching through
manuals to find information whose location you have forgotten.
•
Log fault-finding activities. Do this as you go along, rather than waiting to the end,
by which time you will have forgotten important details.
This information is all required as evidence that the assessment has been performed
properly, but it is also for your own benefit, and that of any of your colleagues and
employer in the future.
Worked example
Suppose we have a motor controller used in an industrial plant. It uses a
microcontroller to operate a conveyor belt motor at a fixed speed, set by a dial. If
necessary, a modified program can be downloaded from a computer, through a USB
link. A tachogenerator connected mechanically to the motor measures the actual
speed, and a software algorithm uses that information to adjust the speed. There is
also a separate relay for switching the motor on and off, and a safety cut-out in case
the motor overheats. This is independent of the microcontroller, as it is good
engineering practice to avoid relying on software for safety-critical tasks.
One of our first tasks will be to acquire a block diagram for the system. It is possible
that units from different suppliers were used, or various parts were hand built
specifically for this application. For these and other reasons, a suitable block diagram
might not exist. In any case, it is often best to sketch your own diagram to help make it
clear in your mind how the system works. That way you can include information you
need for fault finding, and miss out extraneous detail.
(You will be required to submit diagrams as part of the assessment.)
Figure 2.1 shows a block diagram for this motor controller. It may seem rather
complicated, but it helps to see a system of realistic complexity – too simple a system
makes the task trivial.
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MAINS
MAINS
plug-top
transformer
power
supply
transformer/
rectifier
+12V
-12V
5V
speed
control
PC
USB
relay
microcontroller
board
power
transistor
amplifier
electric
motor
tachogenerator
temp
sensor
warning
light
Figure 2.1 Motor speed controller.
The unit has failed in operation. The symptoms are that the motor does not run at all,
and adjusting the speed control has no effect.
A primary concern is that of safety. There are two obvious hazards here. Firstly, this
apparatus is mains powered, so there is the possibility of electrocution. Secondly, there
are moving parts, so we could be injured by the motor or anything connected to it, such
as the conveyor belt, gearbox and transmission.
The block diagram shows separate supplies for the controller and motor. It should be
possible to test the system with the supply to the motor disconnected. It pays to be
thorough here in case someone else accidentally turns the power back on while you
are working. Take out the plug, tape over the switch, add a big sign with ‘DO NOT
TURN ON’ in big writing – whatever it takes to ensure your own safety.
If the motor is disabled, then we can perform our tests without running the conveyor
belt and affecting the rest of the plant. The motor will only need to be operational if we
find that everything else works.
The controller runs off a mains-powered transformer, but this is a sealed unit with +5 V
and ±12 V outputs, so this should not be a hazard.
Microcontrollers are static-sensitive devices, so a static discharge wrist band might be
necessary if we have to remove any component from the microcontroller board,
otherwise it will not be required.
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We already have a description of the fault, and in this case there is not much else we
can record. Check that the overheat warning light is not on – you could also test the
bulb in case that has failed, but then we would have two faults, including the source of
the overheating.
With a more complex system there will be other indicators to check, and we could
check which functions work correctly and which ones do not. Otherwise, the next stage
is to start making some measurements.
We could apply the input to output method described in Section 1. This clearly is not a
linear system, but it can be treated as:
•
the low-voltage power supply
•
the speed control, microcontroller and power transistor
•
the tachogenerator and its amplifier
•
the over-temperature warning system
•
the high-voltage side with the transformer and motor.
Each of these can be dealt with separately.
Note that this system also has feedback; we need to bear this in mind but hopefully it
will not cause us too much difficulty.
It is often a good idea to start off by testing the power supply. It is something which can
be done quickly and easily, plus power supply faults can cause consequential damage.
You want to eliminate this possibility immediately.
If the power supply is providing +5 V and ±12 V outputs correctly, the next stage would
be to test the microcontroller side. Record what you do as you proceed, including any
measurements. A log might read:
•
Tested power supply: +5.01 V, +11.96 V, −12.01 V.
•
Tested speed control: 0.01 V at minimum, 4.97 V at maximum, ≈2.5 V in the
middle.
•
Tested microcontroller output with speed set to half (Figure 2.2).
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5.7ms
5V
0V
1.05ms
Figure 2.2 Microcontroller output at half speed.
•
Tested microcontroller output to relay – steady 5 V.
Now the microcontroller is clearly trying to run the motor. This is an example of pulse
width modulation, where motor speed is controlled by turning the power on and off very
quickly (100 Hz–10 kHz is a typical speed.) If the power is on half the time on average,
the motor runs at half speed.
The next task is to check the power transistor. This presents a problem as it cannot be
easily tested without the high-voltage supply, and we are trying to avoid turning that on.
Therefore, test other parts of the system first, and only return to this if everything else
checks out.
Next we check the tachogenerator. This should produce an output voltage proportional
to the speed of rotation. With the motor stationary this will be 0 V. Check that the
outputs of the tachogenerator and its amplifier are both zero – a fault here could cause
the system to conclude that the motor is already running too quickly, and switch off the
power.
The over-temperature system would be tested next. It has two outputs: one which turns
on the warning lamp and one which triggers the relay to cut off the power. Alternatively,
the relay may be wired in such a way that a broken wire from the temperature sensor
causes it to drop out – check the documentation for details.
Lastly, if none of these tests shows suspect results, we would have to reconnect the
high-voltage side and try running the motor. Proceed carefully, avoiding any exposed
terminals and moving parts. Take care when connecting the voltmeter probes.
The fault-finding log might continue:
•
Tested transformer/rectifier output – 75 V dc.
•
Tested voltage at relay input and output – 75 V dc.
•
Tested connection between power transistor and motor with CRO – steady 0 V.
We seem to have identified the problem – the power transistor is faulty.
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2.1
If the fault symptoms are that the motor ran at full speed while the speed control setting
has little effect, how would you set about testing the system? If the source of the
problem is that the tachogenerator output has gone open circuit, what signals would
you expect to see?
2.1
The unit specification recommends that you are given repeated opportunities to fault
find, locating faults to block level in a wide range of electrical circuits and equipment.
The centre will supply you with details of fault finding activities at this point. No detailed
information can be here as it depends on the equipment which is available at your
centre.
Various electronic systems may be used for fault finding. A commercial/industrial
electronic system would be particularly appropriate but it is recognised that this will not
always be possible. Other suitable systems could include:
•
a programmable logic controller operating external equipment
•
a desk-top computer driving a multiapplications board
•
a microprocessor driving a multiapplications board
•
an alarm system
•
a multistage AF or RF amplifier
•
a static inverter.
While working on the practical exercises, it will be helpful to refer to Section 1 again.
Theory is always easier to understand once you have some experience to relate it to.
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Summary of this section
So far we have only narrowed down the location of the fault to one unit. This may be
one piece of apparatus in a large system, or a single PCB in a piece of equipment, or
one section of the PCB.
The next stage is to fault find to component level. This is covered in Section 3.
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2.1
Fault find one system and submit a report. This need not be as thorough as the report
for the assessment but will need to include:
•
a description of the fault location strategy implemented
•
details of tests carried out and the test equipment and/or diagnostic aids used
•
sketches of appropriate block diagrams
•
a log of the fault-finding activities.
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Answers to SAQs
SAQ 2.1
As before, we would attempt to test the system with the high-voltage side
disconnected. Check the outputs from the speed control, microcontroller board,
tachogenerator and amplifier.
There is little point in testing the over-temperature warning system.
If the tachogenerator output has gone open circuit that would explain the symptoms –
the feedback indicates that the motor is stationary, so the controller is applying
maximum power. We would expect to see a nearly 100% duty cycle at the output from
the microcontroller on a CRO. The speed control would be working normally, but the
tachogenerator output would be 0 V instead of a voltage corresponding to the motor
speed.
The amplifier would have 0 V at the output, which is incorrect, but this is because the
input is faulty.
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Section 3: Locating faults to component level
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Introduction to this section
What this section is about
This section covers the process of locating faults to component level in digital and
analogue circuits.
Outcomes, aims and objectives
At the end of this section you will be able to:
•
identify risks and use safe working practices
•
identify fault symptoms in terms of system operation
•
select a suitable fault location method
•
locate a fault to component level on an analogue system
•
locate a fault to component level on a digital system
•
use appropriate test equipment
•
correctly use a circuit diagram.
Approximate study time
25 hours.
Other resources required
The systems used in Section 2 could be tested here. It is now necessary to have
access to an individual PCB or unit so that fault finding to component level can be
performed.
In order for the candidate to practice fault finding, several systems need to be available
to test. A wide variety of systems would be helpful.
This section contains a couple of examples of simple circuits. It may be practical to
build multiple versions of each design, with different faults on them. A simulation
package could also be used to simulate a working circuit, then implement various
shorts or open circuits to confirm the effect of these faults. Simulations are not
recommended for the practical assessments.
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Assessment information for this section
How you will be assessed
Candidates will submit evidence to satisfy this section in the form of two reports, one
for fault finding on an analogue circuit and one on a digital circuit.
When and where you will be assessed
The assessment will normally take place at the end of this section.
Centres are recommended to develop and use appropriate checklists to monitor the
candidate’s fault-finding activities and provide a check on the authenticity of the report.
What you have to achieve
The reports should include:
•
the identification of risks
•
appropriate safe working practices used
•
a description of the fault location strategy implemented
•
details of tests carried out and the test equipment used
•
circuit diagrams used
•
reference to the documentation used
•
a contemporaneous log of the fault-finding activities.
Opportunities for reassessment
If one of the reports is incomplete or erroneous, it is likely that you will be asked to
correct and resubmit it. It may be necessary to perform further tests in the laboratory,
adding the results to the report as required.
In extreme cases you may be asked to fault find on a different system and submit a
new report.
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Introduction
In the previous section we used fault-finding techniques to identify the faulty unit. This
is known as fault finding to board level if it stops at identifying the damaged PCB.
Sometimes the faulty board will be discarded and replaced with a working one. It may
be passed on to someone else to repair if this is considered financially worthwhile. If it
is covered by the warranty, it will be returned to the manufacturer. However, you may
need to perform the repair yourself if no replacement unit is available. Also, you may be
asked to fix the fault in a unit someone else has identified as malfunctioning.
In this section, we will continue on downwards, fault finding to component level.
A key factor in fault finding at this level is the ability to understand how the circuit is
supposed to work. Exact details are not necessary, for example the designer will have
calculated resistor and capacitor values, and picked suitable components. (They may
not be critical anyway, or the designer may have simply adapted someone else’s
design.)
With experience, you can tell from a schematic diagram which components are likely to
have a bearing on the fault, and identify roughly what they are for. Hopefully, the
documentation will include an outline explanation of the circuit.
As before, we will tend to concentrate on general principles. Your centre will need to
have some circuits for you to fault find on, but details of these cannot be given here.
Your tutor will have more information.
We will begin with two simple circuits, one analogue and one digital, to show basic
principles. Later you will have some more complex circuits to fault find on, and finally
the assessment. This will also feature an analogue and a digital circuit.
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Analogue example
+9V
R1
Rc
100kΩ
3.9k Ω
10µF
input
R2
22k Ω
C2
c
Q1
BC184L
b
C1
+
+
10µF
output
e
Re
1k Ω
+
Ce
47µF
gnd
Figure 3.1 Common emitter amplifier.
Figure 3.1 shows a single transistor common emitter amplifier. It should be familiar.
Basically, the transistor needs to have a dc bias to work. The base needs to be about
0.7 V above the emitter, and the dc level of the collector is half-way between the supply
and the emitter voltage. This maximises the signal amplitude that can be output without
clipping. Capacitors C1 and C2 are the coupling capacitors. They permit an ac signal to
enter and leave the circuit, while removing the dc bias.
Figure 3.2 shows the signals which should be present at various points in the circuit
when there is a sinusoidal input.
V
V
V
1.7V
V
b
0.9V
c
5V
Ve
output
voltage
time
(a) Signals at base and emitter
time
(b) Signals at collector and output
Figure 3.2 Typical signals at key nodes in circuit.
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Resistor Re is present to permit ‘thermal runaway’. The value is usually chosen to make
Ve 10% of the supply voltage. As the transistor heats up, it conducts better and the
emitter voltage rises. This causes the emitter current to reduce, acting as a negative
feedback effect and stabilising the circuit. Without Re the transistor could become too
hot and be damaged. Ce effectively disables the feedback for ac signals, as otherwise
the gain would be much reduced.
Now, if we are testing this circuit, there are only three points or ‘nodes’ in the circuit
which can be usefully measured, namely the base, emitter and collector of the
transistor. The input, output and power rails are external connections. We assume that
these are intact, although it is a good idea to check. There is a chance that a working
unit has been handed in for repair, when actually one of the connecting cables was at
fault. Also, if the power rails are shorted somewhere on the board, you would want to
find out quickly before further damage is done. (This can be easily checked by
measuring the resistance between the power rails with no power or signal supply
connected.)
Table 3.1 lists the voltages at the transistor terminals, with no input signal connected.
These are the dc levels. The values shown as ‘expected’ are calculated roughly based
on our understanding of the circuit. The measured values are taken from an actual
circuit built in the laboratory.
Table 3.1 DC voltages for common emitter amplifier.
Terminal
Measured (V)
Expected (V)
Emitter
0.91
0.9
Base
1.56
1.6
Collector
5.43
5
If we test the circuit and find substantially different values then we have evidence of a
problem. The actual values give an indication of the source of the fault. We should be
able to work backwards and work out what effect a possible fault might have. If the
measurements agree with this we have probably found the fault.
Be aware that some faults can give similar symptoms.
All dc faults can be classified as:
•
faults on the PCB – shorts and open circuits
•
resistors faults – usually open circuit (shorts are rare)
•
transistor faults – short or open circuit on collector–base or emitter–base junction.
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Capacitor faults are considered separately, as a capacitor appears as an open circuit to
dc anyway.
A dc fault will cause the transistor to be either fully off or ‘saturated’ (fully on).
If the transistor is fully off, the emitter will be at 0 V and the collector at the supply
voltage.
If the transistor is saturated, the emitter and collector will be at approximately the same
voltage. This can be estimated by treating Re and Rc as a voltage divider, giving about
1.8 V. The base cannot normally be more than about 0.7 V above the emitter; the
transistor will be damaged otherwise.
Resistor faults
We will consider open circuits only.
•
If R1 is open circuit, R2 will pull the base down to zero and turn the transistor off.
Hence the emitter will be at zero, and the collector at the supply voltage.
•
If R2 is open circuit, R1 will pull the base up and saturate the circuit. Hence the
emitter and collector will be at 1.8 V, and the base at 0.7 V above this.
•
If Rc is open circuit, no collector current can flow, and the base and emitter
currents will be the same. The emitter current will then be much less than normal,
approximately zero.
•
If Re is open circuit, the transistor cannot conduct so the collector will be up at
the supply voltage. The base will sit at whatever voltage the R1/R2 bias resistors
give.
Table 3.2 shows actual measured voltages from a real circuit in the lab. Note that
connecting a voltmeter to components which are open circuit can have an effect. This
explains why the collector is not at 0 V when Rc is open circuit. Similarly, connecting
the meter across an open circuit emitter resistor effectively replaces it with a large
resistance.
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Table 3.2 Voltage measurements with resistor faults.
Fault
Emitter
voltage (V)
Base
voltage (V)
Collector
voltage (V)
R1 open circuit
0.0
0.0
9.0
R2 open circuit
1.9
2.5
1.9
Rc open circuit
0.0
0.7
0.1
Re open circuit
1.2
1.6
9.0
3.1
Confirm that Re = Rc = 1.8 V if the transistor is saturated. Calculate also the base
voltage if the base current is zero. This should agree with the symptoms with Re open
circuit.
3.1
Calculate the emitter, base, and collector voltages if a fault on the PCB shorts out each
of the resistors.
3.2
If time permits and you have access to a simulation program such as Electronics
Workbench, try simulating the circuit of Figure 3.1. Confirm the symptoms of each of
the resistor faults listed in Table 3.1 and check your calculations for SAQ 3.1.
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Transistor faults
The transistor can fail by internal damage. The base–emitter or base–collector
junctions can appear as open circuits or short circuits. Similar symptoms can appear if
there is a short or open circuit where the transistor is soldered to the PCB. Note that
there is no emitter–collector junction in a transistor, although the terminals could still be
shorted externally.
The effects these various faults have on the amplifier circuit can be worked out the
same way as with resistor faults, so these faults can be traced by checking the dc
voltages with no signal connected.
•
If the base–collector junction is open circuit, then the transistor cannot conduct.
There will be virtually no emitter current, so the emitter will be at 0 V with the
base sitting at 0.7 V.
•
If the base–collector voltage is shorted, then the base will be pulled up, saturating
the transistor.
•
If the base–emitter junction is open circuited, the transistor cannot conduct, but
this time the emitter will sit at 0 V when measured with the meter.
•
If the base–emitter junction is shorted, they will be at the same voltage, forcing
the transistor off. The collector will be pulled up to the supply voltage.
•
If there is a short between the emitter and collector, they will both be at 1.8 V,
with Re and Rc acting as a potential divider. The base will then sit at 1.6 V, not
loading the bias resistors.
Table 3.3 lists some actual measurements taken by disabling a working circuit. Note
that a short between the base and emitter gives similar symptoms to R1 being open
circuit. If these voltages are measured and there are no obvious signs of a short or
component damage, then remove the transistor. Check that the R1/R2 potential divider
is giving the correct base voltage of 1.6 V; if it does, the transistor was at fault.
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Table 3.3 Symptoms of transistor faults.
Fault
Emitter
voltage
(V)
Base
voltage
(V)
Collector
voltage (V)
Base–collector junction
open circuit
0.0
0.7
9.0
Base–collector junction
short circuit
1.7
2.3
2.3
Base–emitter junction
open circuit
0.0
1.6
9.0
Base–emitter junction
short circuit
0.1
0.1
9
Short circuit between
emitter and collector
1.8
1.6
1.8
Capacitor faults
Open circuits in the capacitors do not affect the dc operation of the amplifier, so the
base, collector and emitter voltages will all appear correct with no signal connected. A
test signal is required to fault find in this case.
C1 and C2 are coupling capacitors, effectively allowing ac signals to pass while
eliminating dc offset. If either capacitor goes open circuit, then the ac component of the
signal will disappear at some point. Use an oscilloscope to test for this problem.
If C1 is open circuit, there will be no ac signal at the base of the transistor or anywhere
else in the circuit. If C2 is faulty, there will be an amplified signal at the collector, but
none at the output.
Figure 3.3 shows these symptoms.
V
V
V
b
1.6V
c
5V
output = 0V
time
time
(a) C1 open circuit
(b) C2 open circuit
Figure 3.3 Symptoms of open circuit coupling capacitors.
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As described previously, the emitter capacitor means that Re can eliminate thermal
runaway without affecting the ac gain. If the gain is much reduced, it indicates that Ce is
open circuit. The gain of this circuit is controlled by the gain of the transistor, quoted as
β or hfe in the manufacturer’s data sheets. This is of the order of 100 for general
purpose transistors such as the BC184L.
If Ce goes open circuit, the gain will drop to around 5.
Capacitors are more likely to be shorted than to go open circuit. Their operation
requires the presence of an extremely thin insulating layer, the dielectric, between the
two conducting plates. In the case of an electrolytic capacitor, this takes the form of a
coating of aluminium oxide. Internal damage can lead to a gap in the dielectric,
allowing current to pass.
For a capacitor to go open circuit, one of the leads would need to break.
3.2
What effect will a short in one of the capacitors have? How would you test for such a
fault?
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Digital circuits
Some things need to be taken into consideration when testing digital circuits. A few are
described here.
Open circuit inputs tend to float high on digital gates. That is to say, they act as if they
are at logic 1, not logic 0 as one might expect. This is not always the case, however.
Shorting a digital output to ground or the supply is likely to damage it, as one of the
output transistors will effectively short out the power supply.
AND and OR gates have a peculiar property when it comes to fault finding. Under
certain circumstances, a fault at one input does not affect the output. For example, if
one input to an AND gate is at zero, the output will be zero, even if the other input is
wrong.
With an OR gate, if one input is high the output is high, regardless of any fault in the
other input or inputs.
NAND and NOR gates have similar properties.
You can use this effect to your advantage when testing a combinational logic circuit. It
means you can work out what the output of a gate should be without checking all the
inputs.
Fault finding a digital circuit means identifying a device whose output is inconsistent
with its inputs. A logic probe can be used for testing, although it is quite common to use
an oscilloscope since most engineers will have one near at hand. One method would
be to check each signal in turn as the circuit runs, watching out for signals which never
change. These are likely to be faulty.
It is easier to check logic levels while they are static. A circuit which runs off a clock,
with signals which are continuously changing, is much harder to analyse.
One cannot check an AND or OR gate with a two-channel oscilloscope as the output
and inputs all need to be displayed simultaneously.
A digital storage scope is necessary if we need to examine a signal which does not
repeat continuously, or repeats too slowly to allow a steady trace on a normal CRO.
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A logic analyser would be useful for a digital circuit, as it allows multiple signals to be
sampled and displayed as parallel traces. They tend to be more complex to set up, and
are often used only if simpler methods do not allow the fault to be found.
Digital example
+5V
Green
+5V
Amber
+5V
TP1
+5V
R
IC1
IC3a
74LS93
14
TP2
Clock
Q
A
Q
B
3
1
9
IC2a
R01
R02
4
IC3b 270Ω
IC2b
IC3c
6
R
3
270Ω
C
Q
2
270Ω
5
2
8
Red
R
4
5
3
B
Q
2
6 3
A
1
2
TP3
Gnd
1
12
+5V
1
11
D
Figure 3.4 Sample digital circuit.
The circuit in Figure 3.4 simulates a set of traffic lights. It is quite a simple circuit (it
could be even simpler), but it is a good example to begin with. A 3-bit binary counter
cycles through eight states. The sequence should run as shown in Table 3.4. In order
to represent realistic timing, the lights spend more time at red and green than changing
through amber.
Table 3.4 Output sequence.
State
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Traffic light
displayed
QD
QC
QB
Decimal
0
0
0
0
Red
0
0
1
1
Red
0
1
0
2
Red
0
1
1
3
Red + amber
1
0
0
4
Green
1
0
1
5
Green
1
1
0
6
Green
1
1
1
7
Amber
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The LEDs are wired to the digital logic so that a logic 0 turns the LED on. The
combinational logic used is:
•
red – comes on when QD is low, so the red LED is connected directly to QD.
•
amber – comes on when QB and QC are both high, so the logic is QB ⋅ QC .
•
green – green LED should be on whenever red and amber are both off, so it is
connected to Re d ⋅ Amber .
The integrated circuit used is a 74LS93. This is a 1-bit counter plus a 3-bit counter.
They can be connected to form a 4-bit counter if required. We are just using the 3-bit
part, so there are some spare pins. Inputs R01 and R02 are reset inputs. The counters
are reset whenever R01 and R02 are both taken high. On this circuit they are wired to
ground as this feature is not required.
+5V
TP1
Clock
TP2
Gnd
TP3
IC1
IC2
IC3
Figure 3.5 Traffic lights simulator.
Figure 3.5 shows the circuit board and its connections. The power and clock signal are
supplied from external circuits. An ordinary switch cannot be used for the clock input
as switch bouncing will cause the traffic lights to skip parts of the sequence at random.
The most obvious way to test this circuit is to use the input-to-output method. Note that
it has both converging and diverging paths. The AND gates have two inputs and one
output, so signals are converging. The output from the left hand AND gate goes to two
inverters, so the signals are diverging also.
A logic analyser would be useful for a circuit such as this. We could record the 3-bit
number generated by the counter, along with a selection of other signals, and check for
one which is incorrect. Figure 3.6 shows some sample waveforms.
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Figure 3.6 Sample output from logic analyser.
3.3
Suggest a test strategy for each of the following faults:
1.
amber permanently off
2.
green permanently on
3.
green permanently on, amber permanently off
4.
amber comes on for two extra cycles when green should be on
5.
circuit does not cycle through states.
3.3
The unit specification recommends that you are given repeated opportunities to fault
find, locating faults to component level in a wide range of electrical circuits and
equipment. The centre will supply you with details of fault finding activities at this point.
No detailed information can be here as it depends on the equipment which is available
at your centre.
It is acceptable to use the same faulty systems as in Section 2, locating the faults to
component level for Section 3.
The bulk of the time allocation for this unit is allocated to this section, in order to give
candidates the opportunity to locate many different faults prior to proceeding to the
assessment.
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Summary of this section
You should now be in a position where you are able to fault find to component level.
This section covered some examples of fault finding both analogue and digital circuits.
Applying these principles to real equipment can be challenging, but expertise comes
with experience. Familiarity with particular items of equipment makes repair easier, but
you should be able to apply these techniques to a variety of electronic circuits.
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3.1
Fault find one analogue system and one digital system and submit a report. This need
not be as thorough as the reports for the assessment, but will need to include:
•
a description of the fault location strategy implemented
•
details of tests carried out and the test equipment and/or diagnostic aids used
•
circuit diagrams
•
a log of the fault-finding activities.
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Answers to SAQs
SAQ 3.1
Symptoms of shorted resistor.
Fault
Emitter
voltage (V)
Base
voltage (V)
Collector
voltage (V)
R1 short circuit
8.3
9.0
8.3
R2 short circuit
0.0
0.0
9.0
Rc short circuit
0.9
1.6
9.0
Re short circuit
0.0
0.7
0.0
SAQ 3.2
C1 shorted
Signal supply will pull down the base by an amount dependent on the source’s output
resistance. This will eliminate the dc bias, as seen in the diagram below, and prevent
the transistor conducting.
Connect a suitable signal and check the base voltage with a CRO. Note that the
oscilloscope needs to be set to dc coupling, otherwise the trace will look exactly the
same as the correct signal.
V
correct signal
b
1.6V
C1 shorted
time
The effect of a short on C1.
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C2 shorted
The output will have a dc bias of about 5 V. Check the output with a CRO,
remembering to use dc coupling. As seen in the diagram below, the signal will be the
same on both sides of the capacitor, a clear sign of a short.
V
V
5V
5V
c
O
time
time
a) Collector voltage
b) Output voltage
Symptoms of a short in C2.
Ce shorted
This will have the same effect as a short in Re. If there is no obvious sign of the source
of the problem, replace the capacitor first as they are more susceptible to shorts.
Resistors are more likely to go open circuit.
SAQ 3.3
1.
As green LED is operating normally, IC2a must be working correctly therefore
check circuit for amber, namely IC3c and its connection to the LED.
2.
As amber LED is operating normally, IC2a must be working correctly therefore
check circuit for green, namely IC3a, IC2b, IC3b, and the connection to the LED.
3.
IC2a is the obvious candidate for the fault as the green and amber LEDs both
depend on it. (This is an example of divergence.)
4.
IC2a is again the obvious candidate for the fault.
5.
This is very probably a clock problem. Check the clock input to IC1.
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Glossary
CMOS
Complementary Metal Oxide Semiconductor. A type of
integrated circuit which consumes very little power, but is
susceptible to static damage."
Common mode failure
Failure mode in which multiple independent systems fail
simultaneously because of an external factor affecting all
of them.
Converging paths
System structure in which a unit has multiple inputs.
Diverging paths
System structure in which a unit has multiple outputs.
Half-split method
Testing method in which the suspect area of the system
is tested in the middle. The process is repeated,
narrowing down the search until the faulty component is
found.
Input-to-output method
Testing method in which the first unit in a chain is tested,
then the next and so on until a faulty signal is detected.
LiPoly
Rechargeable battery using lithium salt electrolyte held
in a polymer casing. Useful for applications where weight
and charge density are important, such as mobile
telephones.
NiCad
Rechargeable battery using plates of nickel and
cadmium to generate voltage.
Non-sequential testing
Testing strategy in which multiple tests are performed
effectively simultaneously, and the fault is determined by
looking up the test results in a database. Automated test
equipment uses non-sequential methods.
Non-systematic testing
Fault testing by checking units at random.
Output-to-input method
Testing method in which the last unit in a chain is tested,
then its predecessor and so on until a correct signal is
detected.
PSU
Power supply unit. A device which converts mains
voltage to one or more low-voltage supplies, usually
smoothed dc. Often a sealed unit for safety reasons.
Sequential testing
Testing strategy in which tests are done in a sequence,
one after the other. The result of each determines what
test is done next.
Systematic testing
A logical fault-finding strategy.
Tachogenerator
Device used to determine speed of rotation by
generating a proportional voltage. Basically, an electric
motor run in reverse.
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