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A.C. rectification

Diodes can be used to convert alternating current (a.c.) into direct current (d.c.). This process is called rectification.

 Half-wave rectifier circuit (left) and half-wave rectified a.c. waveform (right)
Half-wave rectifier circuit (left) and half-wave rectified a.c. waveform (right)

The diagram shows a simple a.c. rectifier circuit.

The diode only allows current to flow in one direction and not the other.

This means that only the positive half of the a.c. is allowed to pass through the circuit, effectively converting the a.c into d.c. This is called half-wave rectification.

Appliances such as mobile phones, computers and radios require direct current to function. A rectifier is needed when they are connected to the a.c. mains supply.

Full-wave bridge rectifier circuit (left) and full-wave rectified a.c. waveform (right)
Full-wave bridge rectifier circuit (left) and full-wave rectified a.c. waveform (right)

A full-wave rectifier converts the entire a.c. waveform to one that is only positive. The process is called full-wave rectification.

Full-wave rectifiers usually use four diodes to change the alternating current. An example of a full-wave rectifier circuit is shown in the diagram.

The rapid changes in voltage can damage sensitive electrical components. To avoid this, the output can be smoothed using a capacitor.

The rms current and voltage of a full wave rectified a.c. are the same as the rms of the original a.c.$$$I_{\text{rms, rec}}= I_{\text{rms}}\quad V_{\text{rms, rec}}=V_{\text{rms}}$$$

When the waveform is symmetrical (which is the case for a sinusoidal or a square wave), the rms current and voltage of a half-wave rectified a.c. are given by:$$$I_{\text{rms}, \text{ half rec}} = \frac{I_{\text{rms}}}{\sqrt{2}}\qquad V_{\text{rms}, \text{ half rec}} =\frac{V_{\text{rms}}}{\sqrt{2}}$$$ The reason is that the power loss of each half-wave is the same and that the sum of the contribution of each of them is the total power loss.

A half-wave rectifier circuit.
A half-wave rectifier circuit.