Electronic Measuring Instruments and Testing
The Electronic Measuring Instruments and Testing domain covers choosing the right meter, connecting it correctly and interpreting what it displays. Practical exams award marks for range selection, probe placement and the accuracy statement, not only for the number read.
Instruments and their rules
- A digital multimeter has high input impedance (≈10 MΩ), so it loads a circuit far less than an analog VOM (≈20 kΩ/V).
- Voltmeter in parallel with the load; ammeter in series, never across a supply - an ammeter has near-zero resistance and will blow its fuse or the circuit.
- Continuity and diode tests use the meter's own source; the circuit must be dead.
- Oscilloscope: amplitude = vertical divisions × volts/div × probe attenuation; period = horizontal divisions × time/div; f = 1/T.
- LCR meters or bridges measure inductance and capacitance; a logic probe or logic analyser is used on digital lines.
- Accuracy is stated as ±(% of reading + digits); precision is repeatability, not correctness.
Worked example
A sine wave on a CRO spans 6 divisions peak-to-peak at 0.5 V/div with a 10:1 probe, and one cycle covers 4 divisions at 5 ms/div. Peak-to-peak = 6 × 0.5 × 10 = 30 V, so peak = 15 V. Period T = 4 × 5 ms = 20 ms, so f = 1/0.02 = 50 Hz. Both answers need the probe factor and the division count written down as reasoning.
Traps that cost marks
- Quoting peak-to-peak as amplitude, or forgetting the ×10 probe factor.
- Leaving the leads in the current sockets while measuring volts.
- Measuring resistance in a live circuit and blaming the meter.