Diode Circuits and Applications

A diode is a semiconductor device that conducts current readily in one direction and blocks it in the other. This one-way behaviour comes from the junction between p-type and n-type material, which allows current to flow only when the diode is forward-biased above a small threshold voltage. That simple property makes the diode the basic ingredient of rectification, the conversion of alternating current into direct current that nearly every mains-powered device relies on.

The simplest rectifier is the half-wave circuit, in which a single diode passes only one half of each input cycle, leaving the output as a series of one-sided pulses. A full-wave rectifier uses the negative half-cycles as well, either with a centre-tapped transformer and two diodes or, more commonly, with a bridge rectifier of four diodes that needs no centre tap. Full-wave rectification doubles the pulse rate, which makes the output easier to smooth into steady direct current with a filter capacitor.

Diodes appear in many other roles. Voltage multiplier circuits, including voltage doublers and triplers, combine diodes and capacitors to produce a direct-current output several times the peak input voltage, which is useful where a high voltage at low current is needed. The flywheel (or freewheeling) diode is placed across an inductive load such as a relay coil or motor winding to provide a safe path for the collapsing current when the supply is switched off, protecting other components from the resulting voltage spike.

Frequently asked questions

What is the advantage of a bridge rectifier over a half-wave rectifier?

A bridge rectifier uses both halves of each input cycle, giving a higher and smoother output that is easier to filter, and it does not require a centre-tapped transformer.

What does a flywheel diode protect against?

When current through an inductive load is suddenly interrupted, the collapsing magnetic field produces a large voltage spike. The flywheel diode gives that current a safe path and clamps the spike.

How does a voltage doubler work?

It uses diodes and capacitors to charge on alternating half-cycles and add the stored voltages together, producing a direct-current output near twice the peak input voltage.