Measuring Instruments and Transducers
This domain tests electrical measurement principles, meter extension, oscilloscope use, bridge methods and common transducers. Questions often require a shunt or multiplier calculation or an instrument-loading judgement.
Core rules
- A PMMC meter responds to average current and has a uniform scale; it cannot measure AC directly without a rectifier.
- To extend current range, connect a low shunt resistance in parallel. To extend voltage range, connect a high multiplier resistance in series.
- A moving-iron meter can measure AC and DC and reads rms, but its scale is non-uniform.
- An ideal voltmeter has infinite resistance; an ideal ammeter has zero resistance. Real meters load the circuit.
- A CRO displays voltage against time. Time base sets horizontal scale; vertical sensitivity sets volts/division.
- A Wheatstone bridge measures resistance; with an AC source, bridge arrangements can measure inductance or capacitance.
- Transducers convert physical quantities to electrical signals: thermistor for temperature, LDR for light, strain gauge for strain, thermocouple for temperature, piezoelectric element for pressure or vibration.
Worked example
A PMMC meter has full-scale current 1 mA and coil resistance 100 Ω. To make a 10 V voltmeter, a series multiplier is required. Total resistance = V/Ifs = 10/0.001 = 10,000 Ω. Multiplier = 10,000 − 100 = 9,900 Ω. The meter then draws 1 mA at full scale and loads the circuit under test.
Marks-losing traps
Do not connect a shunt to extend voltage range; a shunt extends current range. Do not assume a PMMC reads true rms on AC. A real voltmeter can change the voltage it measures because of finite resistance. In a CRO, check the probe attenuation and time/div before calculating frequency. A thermistor’s resistance may rise or fall with temperature depending on type.