Power Supplies and Electronic Measurements
What this domain tests
This domain covers the chain from mains to regulated DC and the instruments used to check it. Questions combine block-diagram reasoning with meter-connection rules and CRO readings.
Core rules
- Supply chain: transformer (step down and isolate) → rectifier → filter → regulator. Ripple factor = Vripple(rms)/Vdc, and smaller is better.
- Capacitor filter: Vripple ≈ Idc/(f·C), with f = 100 Hz for full-wave rectification on 50 Hz mains.
- Three-terminal regulators: 78xx positive fixed, 79xx negative fixed, LM317 adjustable. Respect dropout voltage and heat-sink at high current.
- Zener regulator: the series resistor drops the difference and the Zener must stay in reverse breakdown.
- Voltmeter in parallel with high resistance; ammeter in series with low resistance. Loading effect spoils accuracy when meter resistance is comparable with circuit resistance.
- CRO: V = divisions × volts/div (times probe attenuation), T = divisions × time/div, f = 1/T.
Worked example
A CRO trace is 4 divisions peak-to-peak at 2 V/div with a ×1 probe: Vpp = 8 V, so Vpeak = 4 V and Vrms = 4/1.414 ≈ 2.83 V for a sine wave. One cycle spans 5 divisions at 0.5 ms/div, so T = 2.5 ms and f = 1/0.0025 = 400 Hz.
Traps that cost marks
Connecting an ammeter across a supply; forgetting the ×10 probe factor; measuring ripple on the DC range; using 50 Hz ripple for a bridge rectifier; ignoring regulator dropout and heat sinking.