Download Electronic-Fault-Finding-Support

Electronic Fault Finding Support Booklet
The Rules of Electronic Fault Finding are:
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Study the circuit schematic and break the circuit down into a series of blocks to form a block diagram, draw the block diagram
Label each block, list its inputs and outputs and provide a brief description of the function of the block
Use data sheets to work out the pin-out, parameters, tolerances and values of the components on the schematic so you can compare
them with the physical components
With all of the information gathered you can carry out a risk assessment to ensure you are working safely at all times
Devise your plan, which essentially is a list of things to check using the fault finding methods you will learn in this unit, the test and
measuring equipment you are going to use to carry out the checking, the expected result of each check and the actual result
Carry out your plan!!
Consider the circuit below
After some study the circuit can be broken down into a series of blocks
Now we can draw a block diagram of the circuit. This helps us understand the function of each block and its inputs and outputs.
Block 1
Block 2
Block 3
Bridge Rectifier
Voltage Regulator
LM35 Temperature Sensing Chip
15 volt AC power input
13.6 volt DC power input
5 volt DC power input
13.6 volt DC power output
5 volt DC output
Output depends on temperature
Function
Function
Function (Transducer)
Converts AC to DC
Provides power for the IC’s @ 5 volts DC
Converts temperature to voltage
This process needs to be repeated for each block in the system!!
Always remember, this is an aid to understanding what the circuit does. If you do not understand what a circuit does, you will never be able to
carry out fault finding on it.
Fault Finding Techniques
In electronic systems there are two major methods for tracing faults within a system, they are called:
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Input to Output
Half Split
Input to Output
This method works very well on small circuits. Basically you trace the electronic signal from the input of the circuit to the output. It is one
reason why we break large circuits down into smaller sub-circuits or blocks. If we know what the electrical inputs and outputs to a block should
be then we can work out if the block is functioning properly after taking measurements with suitable electronic measuring equipment. Let’s
demonstrate this with the bridge rectifier block.
Here is the bridge rectifier circuit can you tell where to measure the inputs and outputs?
If you are completely inexperienced and have no idea where to start I would advise that the very first thing you do is to list each component in
a table, along with a physical image and a brief description of what the component does. This is a similar exercise to the block diagram except
we are taking each sub-circuit in turn and breaking it down to component level. At first it takes time but as you gather experience and
knowledge things will begin to go much faster.
Component Symbol/Name
Function
Physical Image
This component transforms the mains supply
down from 240 volts AC to 15 volts AC. The
AC input provides all of the power for the
circuit but it has to be turned into DC first.
V1 15 volts AC input
A general purpose diode,
current flows in the direction of the arrow.
An electrolytic capacitor
Recall that a diode only allows electrical
current to flow in one direction (Direct
current DC) So if we apply a current that flows
back and forth (Alternating current AC). The
diode will stop the current flowing in one
direction, this is called Rectification
Capacitors have many uses in circuits and
come in all shapes and sizes. This capacitor is
used to smooth out the rectified DC
waveform before it is fed to the input of the
voltage regulator
Current always flows to earth, ground or
neutral potential.
Electrical Earth
To summarize
The information in our table tells us that:
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The input to the circuit is V1 the transformer
That the electrical current flows in the direction of the arrow
That electrical current always flows to ground
And that the capacitor is used to smooth the output before it is fed to the voltage regulator
From this we can deduce the input to the circuit and the output of the circuit
The expected waveform measured at the input using an oscilloscope should be 15 volts AC
The expected waveform measured at the output using an oscilloscope should be 13.6 volts DC
Ok so we have worked out what the components look like, we have a basic idea of the system function and we know what measurements to
expect at the input and output of the system. We switch the system on and it does not work. Congratulations, you have just arrived at the
point where fault finding skills are needed. In a simple system like this we can use a variety of skills to trace the fault with the circuit. I am
going to do this in the style of a report card.
Fault
Technique – Input to Output
No voltage measured at the output of the
bridge rectifier
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Tests carried out
Comments
Is the system switched on at the mains?
Yes
Measure the output of the transformer
before it is connected to the bridge rectifier
using an oscilloscope
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Connect the transformer to the bridge
rectifier and switch on circuit
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Use a multi-meter to measure the resistance
between the inputs of the bridge rectifier
No voltage measured at the output of the
transformer. However it was discovered that
the transformer had an over-current
protection trip, this was reset and a voltage
was then measured at the output of the
transformer.
When the circuit was switched on there was
an audible click from the transformer. It was
discovered that the over-current trip had
operated again
Over-current trips are usually caused by the
presence of a short circuit. In this case a
short circuit was found at the AC input to the
circuit. The short circuit was fixed and now
the circuit functions correctly
This is the essence of fault finding. Using a range of techniques you will gain both knowledge and experience of finding and fixing circuit faults.
The list such as the one shown here is but a small snippet of a complete fault finding routine. It is your task to prepare and use a fault finding
routine on the circuit shown as a part of this unit. It won’t be perfect at first but each time you apply the things you have learned your skills will
improve.