Electronic Measuring Instruments and Transducers
This domain tests analogue and digital instruments, bridge methods, oscilloscope use, and the sensors that convert physical quantities into electrical signals.
Core concepts
- The PMMC movement works on DC only, has a uniform scale and needs a rectifier for AC. A shunt extends current range; a series multiplier extends voltage range.
- Moving-iron instruments read AC and DC with a non-uniform scale. The dynamometer wattmeter uses a fixed current coil and a movable potential coil.
- The CRO displays amplitude against time via vertical amplifier, time base and trigger; frequency = 1/(time per division × divisions per cycle).
- Bridges: Wheatstone for resistance, Maxwell for inductance, Schering for capacitance.
- Transducers: LVDT for displacement (needs AC excitation), strain gauge for force (gauge factor ≈ 2), thermistor (NTC, non-linear), thermocouple (Seebeck effect), piezoelectric for vibration, Hall sensor for magnetic field.
- DMM accuracy and resolution are separate specifications; loading effect occurs when a meter draws significant current from the circuit.
Worked example
A 1 mA, 100 Ω PMMC movement must read 10 V full scale. Multiplier R = V/I_m − R_m = 10/0.001 − 100 = 9900 Ω. With this in series, the meter draws only 1 mA at 10 V, so the loading error on a high-resistance circuit stays small.
Traps
Using a PMMC meter directly on AC; expecting DC excitation to drive an LVDT; confusing accuracy with resolution; ignoring voltmeter sensitivity when measuring high-impedance nodes.