Test Procedure: Fault Finding Circuits

Transcription

1 Test Procedure: Fault Finding Circuits 1

2 Test Procedure: Fault Finding Circuits Known as TROUBLE SHOOTING in USA and Far East 2

3 BEFORE STARTING SAFETY TEST EQUIPMENT DOCUMENTATION 3

4 BEFORE STARTING SAFETY Possibly testing live circuits 4

5 BEFORE STARTING SAFETY Possibly testing live circuits Death is permanent and causes lots of messy paperwork 5

6 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) 6

7 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. 7

8 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 8

9 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 9

10 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 10

11 2. Identify the faulty module 1 st : BASICS Is it switched on Are all leads required correctly connected Is it set up correctly 11

12 2. Identify the faulty module 2 nd : POWER Is the power supply functional Is there power on each module 12

13 2. Identify the faulty module 3 rd : 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 13

14 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. 14

15 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 15

16 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 16

17 3. Identify faulty components 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 17

18 3. Identify faulty components 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 18

19 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 19

20 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 20

21 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 21

22 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. 22

23 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 23

24 7. Test to specification Test module functionality Test overall system is up to specification Soak test if necessary to identify infant mortality / overheating 24

25 8. Document Fault: what was wrong Cause: why the failure occurred Fix: how and with what it was fixed Test results: after repair 25

26 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!! 26