Using a labelled circuit diagram and waveforms, show how a centre-tapped full-wave rectifier uses both half cycles of AC, and explain how a capacitor filter smooths its output.

1. 1 · State the principle. A diode conducts only when forward biased, so it can turn AC into one-way current.
2. 2 · Draw the circuit. [Diagram: centre-tapped transformer with ends X and Z and centre Y; D1 from X and D2 from Z, cathodes joined at B; load RL from B to Y; current direction marked]
3. 3 · First half cycle. X is positive and Z negative: D1 conducts and D2 is off, so current flows through RL from B to Y.
4. 4 · Second half cycle. Z is positive: D2 conducts and D1 is off, and current again flows through RL from B to Y.
5. 5 · Draw the waveforms. [Waveforms: input sine wave; output humps in every half cycle with no gaps; filtered output nearly flat with small ripple]
6. 6 · Explain the filter. A capacitor across RL charges to Vm and discharges through RL in the dips, so the output is nearly steady. Without it, Vdc = 2Vm/π.
7. 7 · Real life. This is how a phone charger gets steady DC from 50 Hz mains.
8. 8 · Close in one line. So, both half cycles are used, and the output is larger and smoother than a half-wave rectifier's.

Scoring tip: Line up the input and output waveforms one below the other on the same time axis. Examiners check that each hump sits under a half cycle.