Measuring Instruments and Testing
This domain tests correct use of multimeters and basic test instruments, including range selection, connection mode and interpretation of readings. Most marks are lost through wrong connection, not wrong arithmetic.
Instruments and connections
- Digital multimeter (DMM): voltage measured in parallel, current in series by breaking the circuit, resistance only with power removed. Continuity and diode-test modes are useful for checking tracks and semiconductor junctions.
- Analog multimeter: observe polarity and zero the meter before resistance measurement.
- CRO/DSO: volts/div sets vertical scale, time/div sets horizontal scale; use triggering for a stable waveform.
- Clamp meter: measures current without breaking the circuit by sensing the magnetic field.
- LCR meter: measures inductance, capacitance and resistance.
Worked example
A 12 V supply feeds a 1 kΩ resistor. Expected current is I = V/R = 12/1000 = 0.012 A = 12 mA. Select the mA range, break the circuit, connect the meter in series with correct polarity, and read 12 mA. If the display shows 0.012 A, that is the same current in amperes, not a fault.
Traps that cost marks
Connecting an ammeter across a supply (it acts as a near short circuit); measuring resistance in a powered circuit; using the wrong range and reporting an overload as zero; forgetting that a DMM's current terminals may need to be moved; reading peak-to-peak when the question asks for RMS.
Exam focus
Know which quantity is measured in parallel and which in series, and be able to convert between A, mA and µA.