Apply a systematic fault-finding method to a circuit, identify common faults, and test a circuit against its specification

What this dot point is asking

SEAB wants you to apply a systematic method to find a fault in a circuit, to recognise common faults, and to test a finished circuit against its specification. The central insight is that fault finding should be methodical, not random: by checking the circuit stage by stage and comparing each measurement with what it should be, you can pinpoint the one stage that is wrong instead of guessing.

The answer

Why a systematic method

A circuit that does not work can have any of several faults, and changing parts at random wastes time and can introduce new faults. A systematic method works through the circuit in order, narrowing down where the problem is. The key idea is to compare what you measure at each point with what the design says should be there; the first place they disagree is near the fault.

A stage-by-stage method

A reliable routine, working from the power inwards:

1. Check the power. Use a voltmeter to confirm the correct supply voltage reaches the board, and that $0\ \text{V}$ is connected. Many faults are simply no power.
2. Check the input stage. Measure the sensor or divider output and change the input (the light or temperature) to confirm it varies as expected.
3. Check the process stage. Measure the key voltages, such as a transistor base reaching about $0.7\ \text{V}$ to switch on, or a comparator output switching.
4. Check the output stage. Confirm the output device gets the right voltage and current, and that components like LEDs and diodes are the right way round.

The first stage whose measurement is wrong is where to look closely.

Common faults

Most faults in a newly built circuit fall into a few types:

• A short circuit, often a solder bridge joining two strips or a stray wire. Found with the continuity tester between points that should be separate.
• A break or dry joint, such as a cracked solder joint, a missing link wire, or a cut track. Found by a continuity test along the path, or by a missing voltage downstream.
• A reversed component, such as a diode, LED or electrolytic capacitor fitted the wrong way round. Found by checking orientation against the diagram.
• A wrong value, such as the wrong resistor, found by checking the colour code or measuring it.

Testing against the specification

Once the circuit works, test it against its design specification. Predict the voltages and currents from the circuit theory, then measure them and compare. Small differences are expected from component tolerances; large differences point to a remaining problem. Honest comparison of measured against calculated values is what good testing and good coursework demand.

Examples in context

Example 1. Coursework testing. In the practical coursework, a student predicts the divider output and the LED current from the circuit theory, then measures them on the built circuit. Where the measured and predicted values agree within tolerance, the stage is confirmed; where they differ greatly, a fault is hunted down. This honest compare-and-explain is exactly what markers reward.

Example 2. Repairing a dead torch. Faced with a torch that will not light, a systematic approach checks the battery voltage, then the switch with the continuity tester, then the bulb. The fault is found in seconds, perhaps a corroded contact breaking the path, rather than by replacing parts hopefully. The same method scales from a torch to a complex board.

Try this

• Cue. State the first thing to check when a circuit does not work, and why. The power supply: confirm the correct voltage reaches the board and $0\ \text{V}$ is connected, because many faults are simply no power.

• Cue. Describe a test for a suspected short circuit between two strips. Use the multimeter continuity setting between the two points with the circuit unpowered; a beep reveals the unwanted connection.

• Cue. Explain why measured values may differ slightly from calculated ones in a working circuit. Components have tolerances, so their actual values vary a little from the nominal, giving small expected differences; only large discrepancies indicate a fault.