Measuring Instruments and Testing Techniques
What this domain tests
Correct connection of a multimeter, interpretation of readings, basic oscilloscope use, and recognition of measurement error. Practical checks often include continuity, voltage rails and current draw.
Rules
- A voltmeter goes in parallel with the component; an ammeter goes in series, breaking the circuit; an ohmmeter is used only on a dead, isolated circuit.
- With a manual meter, select a range above the expected value; start high. Digital meters show OL for over-range.
- Continuity mode beeps below a low resistance; it is a quick check, not an accurate resistance measurement.
- On an oscilloscope, vertical scale is volts/division and horizontal is time/division. Frequency = 1/period. A 2 V peak-to-peak sine on 1 V/div occupies two vertical divisions.
- Measurement error includes meter tolerance, loading, lead resistance and parallax on analog meters. A meter with ±1% accuracy on a 12 V reading can be out by 0.12 V.
- Test the meter on a known source before trusting a fault diagnosis.
Worked example
A 9 V battery feeds a 1 kΩ resistor. Predict I = 9/1000 = 9 mA. To measure it, switch the meter to mA, place it in series, and expect about 9 mA. If you instead connect the meter in parallel across the resistor while in current mode, you create a near short, blow the fuse and may damage the battery. For voltage, select DC V and place the probes across the resistor.
Traps
- Measuring resistance in a live circuit.
- Leaving the meter on a current range and then probing a supply.
- Reading peak-to-peak as RMS.
- Forgetting the meter's own burden voltage in low-voltage circuits.