Download Test Procedure: Fault Finding Circuits

Test Procedure:
Fault Finding Circuits
Testing Procedures: Fault Finding CR
March 2014
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Test Procedure:
Fault Finding Circuits
Known as
TROUBLE SHOOTING
in USA and Far East
Testing Procedures: Fault Finding CR
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BEFORE STARTING
• SAFETY
• TEST EQUIPMENT
• DOCUMENTATION
Testing Procedures: Fault Finding CR
March 2014
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BEFORE STARTING
• SAFETY
• Possibly testing “live” circuits
Testing Procedures: Fault Finding CR
March 2014
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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
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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
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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
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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
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What are the symptoms
Identify the faulty module /stage
Identify the faulty component
Analyse the failure
Redesign if necessary
Replace faulty component
Test to specification
Document
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1.What are the symptoms
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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
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1st:
BASICS
Is it switched on
Are all leads required correctly connected
Is it set up correctly
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2. Identify the faulty module
• 2nd:
POWER
• Is the power supply functional
• Is there power on each module
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2. Identify the faulty module
• 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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2. Identify the faulty module
• 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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3. Identify faulty components
• 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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3. Identify faulty components
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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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3. Identify faulty components
• Some faults are intermittent
• They do not occur when you look
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Try vibration, heat, cold
Try maximum supply, minimum supply
Try different inputs, loads etc
Test with lid “on”
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3. Identify faulty components
• 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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3. Identify faulty components
• 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
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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!!
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