Electrical and Electronic Measurements
Measurements rewards knowing which instrument responds to which quantity. GATE EE tests analog instruments, bridge circuits, instrument transformers, potentiometers, CRO and digital meters, and error analysis.
Analog instruments
- PMMC responds to the average value and works on DC; moving iron responds to RMS and works on AC and DC; rectifier instruments are calibrated in RMS for sine waves only.
- Ammeter shunt: R_sh = R_m I_m/(I − I_m). Voltmeter multiplier: R_s = V/I_m − R_m.
Bridges
- Wheatstone for medium resistance; Kelvin double bridge for low resistance; Maxwell for inductance at medium Q; Hay for high Q; Schering for capacitance and dissipation factor.
Worked example: a PMMC movement of 100 µA full scale with R_m = 1 kΩ must read 1 A. Full-scale voltage across the movement is 100 µA × 1 kΩ = 0.1 V. The shunt carries 1 A − 0.0001 A ≈ 0.9999 A, so R_sh = 0.1/0.9999 = 0.10001 Ω. The multiplying factor is 1 A/100 µA = 10,000, and that factor is what you use when converting the scale reading.
Traps that cost marks
- Loading effect: a voltmeter's resistance sits in parallel with the element under test.
- Crest-factor error when a rectifier-type meter reads a non-sinusoidal waveform.
- Energy meter constant (revolutions per kWh) is inverted when computing energy.
- Instrument transformers introduce ratio and phase-angle errors, not just scaling.