Question

Discuss the working method of a half-wave rectifier using a p-n junction diode.

Hint:

A diode conducts only when forward biased: it passes current during one half of the AC cycle and blocks the other, giving a pulsating DC output across the load.

Answer 1

Step 1: Purpose of a rectifier.
A rectifier converts alternating current (AC), which reverses direction every half cycle, into direct current (DC), which flows in only one direction. A p-n junction diode is used because it conducts appreciably only when forward biased and blocks current when reverse biased.

Step 2: Circuit arrangement.
The AC supply is connected to the primary of a transformer. The secondary of the transformer is connected in series with a p-n junction diode and a load resistance \( R_L \). The output DC voltage is taken across \( R_L \).

Step 3: Positive half cycle.
During the half cycle in which the p-side of the diode becomes positive with respect to the n-side, the diode is forward biased. It offers low resistance and conducts, so current flows through \( R_L \) and a voltage appears across it.

Step 4: Negative half cycle.
During the next half cycle the polarity reverses, the diode becomes reverse biased, offers very high resistance and does not conduct. Practically no current flows and the output voltage is nearly zero.

Step 5: Output.
Thus the diode allows current only during alternate (positive) half cycles. The output across \( R_L \) is a pulsating unidirectional voltage present for one half of each cycle. Since only half of the AC wave is used, the device is called a half-wave rectifier. Its efficiency is low and the output has a large ripple.
\[\boxed{\text{Diode conducts on one half cycle only} \Rightarrow \text{pulsating DC output}}\]