Question
Explain how a p-n junction diode works as a rectifier. Draw circuit diagrams and output waveforms for (a) half-wave rectification and (b) full-wave rectification. Compare their efficiencies.
(NCERT Class 12, Chapter 14)
Solution — Step by Step
A p-n junction diode conducts in forward bias (current flows) and blocks in reverse bias (negligible current). This one-way behaviour converts AC (alternating current) to pulsating DC — this process is called rectification.
In a half-wave rectifier, a single diode is connected in series with the load.
- During the positive half-cycle: diode is forward biased → current flows through the load.
- During the negative half-cycle: diode is reverse biased → no current flows.
Output: only the positive halves of the AC waveform appear across the load. The negative halves are clipped entirely. The output is pulsating DC with frequency equal to the input AC frequency.
Efficiency: (maximum theoretical).
Uses two diodes and a centre-tapped transformer.
- During the positive half-cycle: Diode conducts, blocks.
- During the negative half-cycle: Diode conducts, blocks.
Both halves contribute to the output — the negative half is “flipped” to become positive. Output frequency is twice the input frequency.
Efficiency: — double that of half-wave.
Uses four diodes in a bridge configuration (no centre-tap needed). In each half-cycle, two diodes conduct and two block. The output is the same as the centre-tap full-wave rectifier, but the full secondary voltage is utilised.
Adding a capacitor filter across the load smooths the pulsating DC into a nearly constant DC output.
Why This Works
The rectifier exploits the diode’s asymmetric V-I characteristic. In forward bias, the barrier potential drops and current flows easily. In reverse bias, the depletion region widens and blocks current. This is fundamentally a property of the p-n junction — majority carriers can cross the junction in one direction but not the other.
The full-wave rectifier is preferred because it uses both halves of the input cycle, giving smoother output and higher efficiency.
Alternative Method — Quick Comparison Table
| Parameter | Half-Wave | Full-Wave |
|---|---|---|
| Diodes used | 1 | 2 (centre-tap) or 4 (bridge) |
| Output frequency | ||
| Efficiency | 40.6% | 81.2% |
| Ripple | High | Low |
CBSE and NEET love asking for the output waveform. For half-wave: draw positive humps with gaps. For full-wave: draw continuous positive humps (like a series of arches). Always label the time axis and mark the period clearly.
Common Mistake
Students often draw the full-wave rectifier output with the same frequency as the input. The output frequency of a full-wave rectifier is twice the input frequency because each input cycle produces two output pulses. If input is 50 Hz, output pulsating DC is at 100 Hz.