The purpose of this short course is to teach you basic electronic fault finding. The unit which covers this is Unit 60 at level 3. Initially we will be looking at an analog circuit and running through some basic fault finding routines. The teaching and learning for this will be covered over two sessions.
After the initial sessions you will be given time to practice building and fault finding on your circuit and also to formulate a fault location strategy. Once completed you will be required to carry out a fault location examination which could take up to an hour.
For this session we will be covering the following learning aims :
Read a circuit diagram for an electronic system and identify two different types of analogue circuit, the components that make up each circuit and the circuit input and output signals.
Prepare a written fault location strategy for a given analogue electronic system and identify the fault-finding and signal tracing techniques to be applied.
The Analog Circuit
The circuit you will be fault finding on is a relatively simple power supply, temperature sensor. Its not too complicated and is therefore ideal to start out on. The circuit is shown below.
Basic Fault Finding
Before you can carry out effective fault finding on any circuit, it is really helpful to have an understanding of how the circuit works and what readings one should expect at different test points on the circuit. One helpful technique is too break the circuit down into blocks, each block will carry out a function and will have a known input and output signal/level.
Looking at the circuit above I show you how you can break the circuit down into blocks with the next diagram.
I have broken out circuit into blocks in the next diagram. As an exercise can you describe the function of the 4th block (the one I haven’t shown) in terms of function and input/output signal levels?
Once you have broken the circuit down into functional blocks you are ready to begin fault finding on the circuit. In the exam there will be several faults you are required to find they are:
- Component failures
- Components out of specification
- Short circuits
- Open Circuits
- Continuity faults
- Component substitution
- Intermittent faults
This is usually linked to a circuit fault such as a short circuit which causes the component to overheat and fail. Other main reasons for failure are components which are out of specification, through prolonged exposure to excessive heat or through aging (especially electrolytic capacitors
Short circuits are the exact opposite of open circuits. They offer a very low resistance path for the electrons to flow through. This causes too much current to flow in a circuit which leads to overheating and failure.
In the circuit above the lamp has been unintentionally short-circuited. A very large current flows out of the battery which blows the circuit fuse. If a fuse was not fitted to this circuit a large current would continue to flow which would eventually melt the insulation around the conductors and lead to a fire.
Testing for a short circuit
There are three main reasons to suspect a short circuit
- Hot components
- Fuses blowing
- Circuit breakers tripping
We carry out an insulation test to detect the presence of a short circuit. The next picture shows how to measure the insulation between the live and neutral conductors of a domestic electrical mains cable. This is exactly the same as the resistance test carried out to find open circuits. The difference here is that the resistance reading taken should ideally be higher than the maximum reading you can take with the meter. When this happens we call the measurement infinity the reading taken is infinitely high.
For insulation tests between electrical cables such as the one shown here, the minimum acceptable resistance reading should be in excess of 1 million ohms i.e. 1 MΩ.
When you test electronic circuits for shorts you must expect the readings taken to vary depending on the components in the circuit. However as a rule of thumb in almost all electronic circuits the resistance between power and ground would usually be a minimum of 1000 ohms and is often a great deal more. When an electronic circuit does not work, a short circuit is usually one of the first things you will test for.
Isolating short circuits
In small circuits, shorts are relatively easy to find and isolate. In larger circuits which have many building blocks they can be more difficult to find. We can use a method called half split as a way of tracing a short circuit.
Consider the block diagram below:
We can isolate larger circuits by organizing each circuit function into a block. We make a break in the middle of the chain (half split) and test for a short circuit across the first half of the chain. If we find no short circuit, it means that the short must exist in the other half of the chain. That means we have isolated the short circuit. We can continue this process as many times as needed in order to find the short circuit. Half split works very well in larger circuits.
Causes of a short circuit
- Poor soldering
- Incorrect component choice leading to overheating
- Old components such as electrolytic capacitors ‘break down’ over time
- Ageing insulation, overheating greatly increases the ageing process
- Mechanical damage such as accidentally cutting through an electrical cable
An open circuit is simply a break in a circuit which prevents electrons flowing (stops the circuit working). Some open circuits are intentional such as switches which allow us to electrically isolate a circuit or fuses which blow and ‘open circuit’ if too much current has flowed through a circuit.
In small circuits we can use input to output fault tracing techniques to isolate the fault.
As with all continuity tests the power is disconnected
We simply disconnect the circuit at the first convenient connection (here I have disconnected after the bulb) and measure the continuity between those two points. If the circuit is good between these two points i.e. we get a reasonably low reading, then we move between the next two points and measure as shown next.
We carry on using this technique until the fault is isolated and fixed.
Consider the simple circuit below
The battery cells create a potential difference (voltage) between the positive and negative terminals. Electrons flow from the negative terminal of the battery, through the bulb, through the variable resistor and arrive at the positive terminal of the cell.
Electrons flowing through the bulb filament produce heat and incandescent light. The amount of electrons flowing through the bulb can be controlled by the variable resistor and therefore the amount of light produced by the bulb can be controlled.
The electrons need a continuous path or circuit in order for the circuit to be able to work. Any break in the path/circuit will cause the circuit to stop working.
So a continuity test is a simple check to make sure that there are no breaks in the circuit.
The easiest way to carry out a continuity test is to use a multimeter. This is an electrical measurement instrument that can be used to measure:
- AC voltage
- DC voltage
- AC current
- DC current
- Resistance (Ω)
Carrying out a continuity test
The most important thing you need to bear in mind when testing electrical circuits is the need to work safely.
- Check the physical condition of the multimeter. Is there any mechanical damage? Is the insulation surrounding the probes in good condition?
- Test the multimeter before connecting it to a real circuit
- Connect the probes to the correct ports as shown
- Set the meter to read resistance using the lowest setting on the resistance range
- Short the probe terminals together
- Read the resistance of the probes from the meter display
This checks the multimeter to make sure the probes are working correctly and that the meter is reading resistance properly. If you get a high reading e.g > 1Ω then you need to double check:
- The condition of the probes
- The probes are properly connected together
- The probes are connected to the correct ports
- The probes are fully plugged in
If you get a reading that is higher than the range set by the multimeter you will get a reading that looks like this:
Here the multimeter is set up to read voltage but the range is set to a maximum of 2 volts. The power supply has a voltage of 7.06 volts which is higher than the maximum reading in this range. When this happens, regardless of whether you are measuring voltage, current or resistance you will always get a reading of 1.
1 indicates that the reading you are trying to take is out of range.
- Check that the multimeter reads voltage correctly by testing it on a known power supply. In the photograph above the multimeter is connected to a power supply of 7.06 volts. However it is reading out of range. If you adjust the range one notch up to 20 volts then the meter should read correctly. If it does not read correctly carry out all the usual checks.
If the multimeter fails either of these tests you should NOT use it. Report the multimeter as defective and select another one to carry out the test.
- Disconnect the circuit from the power source
- With the multimeter set to read voltage, check the circuit under test to ensure it is electrically dead i.e. no voltage present. In some circuits capacitors can store large electrical charges even when the power is switched off. In other circuits there may be more than one power supply. It is very important that you double check.
- Now with the test set up in exactly the same way, set the range of the multimeter to measure resistance. Adjust the range of the meter until you get a reading. Remember this circuit may have considerable resistance depending on the value of the variable resistor. You can adjust the resistor as the test is under way to see if it affects the reading.
- If the meter reads 1 regardless of the range set on the meter then you have an open circuit. The whole point of continuity testing is to track down open circuits
As you can see there is an awful lot of information to take in and much of it is for reference later as you may find yourself revisiting this session a few times.
Next session we will look at some explanation of circuit function and operation and common fault location techniques which can be employed to find and fix faults.