What is Rectifier ?

The process of converting an alternating current into direct current is called rectification. The device used for this purpose is called rectifier.

Junction diode can be used as a rectifier in which single diode is used in a half wave rectifier and two diodes are used in full wave rectifier.

Working of half wave rectifier

A rectifier which conducts only during the positive half cycle of input a.c. supply is known as half wave rectifier.

A half wave rectifier consists of a diode connected in series to the secondary of the transformer. The output is taken across the load resistance \text{R}_\text{{L}}.

During the positive half cycle, the diode is forward biased and current flows through the diode. During the negative half cycle, the diode is reverse biased and current does not flow through it. Thus current flows through the diode during the positive half cycle only efficiency.

\text{Rectifier efficiency} \, \eta = \frac{\text{dc power output}}{\text{ac power input}}

For half wave rectifier, η = \frac{0.406 \, \text{R}_{\text{L}}}{r_{f} + \text{R}_{\text{L}}} where r_{f} is the diode forward resistance and \text{R}_{\text{L}} is the load resistance.

In half wave rectification, a maximum of 40.6% of ac power is converted into dc power.

Working of full wave rectifier

The rectifier which conducts current, during positive and negative half cycle of input ac supply is known as full wave rectifier. In a full wave rectifier a center tapped transformer is connected to diodes \text{D}_{1} and \text{D}_2 together at one side of load resistance. The other end of load resistance is connected to center tapped wire.

During the positive half cycle of ac input, the diode \text{D}_1 is forward biased and diode and \text{D}_2 is reverse biased. So, \text{D}_2 will not conduct and the current that flows through the load resistances is due to \text{D}_1. Similarly during negative half cycle, the diode \text{D}_2 is forward biased and diode \text{D}_{1} is reverse biased. So, \text{D}_1 will not conduct and the current flows through the load resistance is due to \text{D}_2.

The ratio of dc power output to the applied input power is known as rectifier efficiency.

Rectifier efficiency η = \frac{\text{dc power output}}{\text{ac power input}}

For full wave rectifier, η = \frac{0.812 \, \text{R}_\text{L}}{r_{f}+\text{R}_\text{L}} where \text{r}_{f} is the diode forward resistance and \text{R}_\text{L} is the load resistance.

In a full wave rectification, a maximum of 81.2% of ac power is converted into dc power.