test procedure: fault finding...

Testing Procedures: Fault Finding CR

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Test Procedure:

Fault Finding Circuits


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Test Procedure:

Fault Finding Circuits

Known as

TROUBLE SHOOTING

in USA and Far East


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BEFORE STARTING

• SAFETY

• TEST EQUIPMENT

• DOCUMENTATION


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BEFORE STARTING

• SAFETY

• Possibly testing “live” circuits


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BEFORE STARTING

• SAFETY

• Possibly testing “live” circuits

• Death is permanent and causes lots of

messy paperwork


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BEFORE STARTING

• TEST EQUIPMENT

• Typically Multimeter and Oscilloscope

• Know how to use safely

• Working Order (No flat Batteries!)

• Proper probes / clips / IC probes

• Know how to use it! (e.g. AC coupling on

oscilloscopes)


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BEFORE STARTING

• TEST EQUIPMENT

• ALSO:

• Eyes: look carefully for discoloration etc.

• Nose: smoke smells….find source

• Fingers: feel for heat

• Brain: Think about what you are doing.

Random probing achieves very little.


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BEFORE STARTING

• DOCUMENTATION

• System description: What is it meant to do

• Block Diagram: How is the system built

• Circuit diagram: How is each block /

module / stage constructed at component

level


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STEPS IN FAULT FINDING

• 1. What are the symptoms

• 2. Identify the faulty module /stage

• 3. Identify the faulty component

• 4. Analyse the failure

• 5. Redesign if necessary

• 6. Replace faulty component

• 7. Test to specification

• 8. Document


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1.What are the symptoms

• What is not working

• What is working

• When does it not work

• When does it work

• Write it down. Helps to describe fault

accurately


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2. Identify the faulty module

• 1st: BASICS

• Is it switched on

• Are all leads required correctly connected

• Is it set up correctly


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• 2nd: POWER

• Is the power supply functional

• Is there power on each module


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• 3rd: MODULE CHECKS

• Check output of each module at the input of

the next module

• Start half way: Can save time

• Check on input pin of first device in module

• Locate first faulty stage / module


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• 4th: TEST REST OF SYSTEM

• If possible inject signal into next module

after faulty one and check final output. This

may indicate multiple faults.


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3. Identify faulty components

• A LIST OF TYPICAL FAULTS IS

USEFUL:

• Power supply to component

• Dry soldered / loose joints

• Project board sockets

• Under rated components

• Components damaged on insertion


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• A LIST OF TYPICAL FAULTS IS

USEFUL:

• And simply failed components for as yet an

unknown reason

• Remember eyes, nose and fingers


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• Check each component in module / stage

• Resistors must generally have some V / I

• Transistors must generally have some Vbe

• Capacitors must not have Vc = 0

• Transformers must have some Vo

• Diodes must have some Vfoward

• Op amps must have e.g. some gain


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• Some faults are intermittent

• They do not occur when you look

• Try vibration, heat, cold

• Try maximum supply, minimum supply

• Try different inputs, loads etc

• Test with lid “on”


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• SOME HINTS

• Check dc voltages first (supplies etc)

• Check input / output signals with

oscilloscope

• If recent changes have been made,

concentrate on the consequences of the

change


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• SOME MORE HINTS

• If software is involved, try run single test routines

• Measure on components, not on board.

• Do not simply change components. Damage to new and existing usually results

• UNDERSTAND YOUR CIRCUIT DIAGRAM


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4. Failure Analysis

• Why did the identified component fail?

• Over current, over voltage, surge, spike, to little heat sink, under rated, soldering damage etc.

• Try simulate part of circuit separately

• Good analyses might lead to other faults e.g. failed regulator caused over voltage


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5. Redesign if necessary

• Depending on failure analyses, redesign or

reselection of the component may be

necessary

• Other circuitry might be required to be

added e.g. filters, back emf diode, isolation

etc.


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6. Replace faulty component /s

• Remove component

• Test component again if possible, out of

circuit, to confirm diagnosis

• If replacing with component from the same

batch, test new component before installing


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7. Test to specification

• Test module functionality

• Test overall system is up to specification

• Soak test if necessary to identify infant

mortality / overheating


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8. Document

• Fault: what was wrong

• Cause: why the failure occurred

• Fix: how and with what it was fixed

• Test results: after repair


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FINALLY

• Be methodical

• Do not “fixate” on a particular guess you

have made. You will miss the fault

• Remember the more you practice fault

finding the “luckier” you will get!!

test procedure: fault finding...

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