Session 2 Analog

This session will build upon some of the very basics we covered in session 1. To start with we will walk through how each section of the circuit works. You may also find this video helpful as it gives some circuit explanation and goes through some of the basic fault finding techniques.

Reading the circuit from right to left we have a full wave bridge rectifier whose function is to convert ac to dc in conjunction with the smoothing capacitor. The rectifier will have inputs and outputs which look very much like the following picture.

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A word of caution here, the circuit will work with an ac voltage ranging from 9 – 20 volts ac. The values given correspond to an input voltage of 15 volts, just remember the output voltage when converted to dc will have a peak that is about 1.2 – 1.4 volts less than the ac input.

You can now begin to compile a list of possible faults which could occur with this circuit block. Typically they would be:

  • Incorrect component specification
  • Incorrect component tolerance
  • Component failure
  • Component polarity incorrect
  • Incorrect input signal
  • Output short circuit or open circuited
  • Continuity errors

So a very simple fault location strategy would be to take each block at a time. Draw the schematic and note the input and output signals and their test points (as shown above). From this you can devise a list of things to check for each block.

If you do this block by block, start at the beginning and work to the end, making sure each block functions correctly before moving on then you will be following and input to output strategy.

Working back from the output to the input is of course called output to input. If you split the circuit in the middle and test at the point you will be using a half split strategy. The whole point of half split is to minimize the amount of testing required to find a fault but it does require more experience and confidence to get right.

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The diagram illustrates the idea behind half split. You basically keep dividing the circuit in half until you identify the block where the fault lies. Extremely useful techniques for larger more complex circuits.

The following table is one useful way of identifying the circuit components and their function/operation. It is by no means the only way to do this.

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You could also set out your fault location strategy in a similar manner.

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There are no hard and fast rules about the way you set out your strategy but it should include elements of the following:

  • Relevant diagrams/schematics
  • Component specifications/tolerances/values
  • Details of the expected input and output signals for each block
  • Details of the tests to be carried out and the equipment to be used
  • Pin out diagrams for each of the components
  • Relevant information from the component datasheets

There are several other methods of finding faults which you could also employ as a part of your overall strategy. They are:

Symptom to cause hierarchies

Basically a list of common symptoms displayed by the circuit and the most common causes for those symptoms. Can save time in circuits which regularly display the same type of fault in service.

Unit substitution

Sometimes when there is no other obvious cause of a fault the only way to proceed is by  replacing components.

Visual inspection

A big favourite with my students and useful on any circuit to identify burnt out components, failing components and tracking. However, in a real circuit with subtle, intermittent faults visual inspection is unlikely to be of much use. Here you are far better of reading the schematic, identifying test points and using the equipment to discover the faults. Over reliance on visual inspection and its twin cousin pull testing is unlikely to reward in a real fault finding situation.

Pull test

Despite the mildly amusing name a gentle wiggle of each of components can often yield results. Any component which feels a little looser than you would expect is a candidate for a poor terminal connection (one of the chief causes of intermittent faults) usually because of a cold/dry joint.

Function tables and flow charts

Can be very useful in fault location as they help you build up a logical test pattern for your circuit. I have seen some students complete some very useful strategies this way.

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Experience

There is no substitute for experience that is why once the teaching phase is finished, you are given 4 weeks to investigate the circuit to be tested and come up with a viable strategy for locating the faults. You should use the time wisely and practice schematic reading (not just the circuit under test), practice your fault location strategy and practice using the test equipment. You will only gain experience by doing not by writing about doing. Little wonder that some of the most successful students find all of the faults within 10 minutes yet their written strategies are nothing to write home about.

Its because they invested the time in working with the circuit. You should do the same.