The thyristor (also called a silicon-controlled rectifier or SCR) is a solid-state semiconductor device similar to a diode, with an extra terminal which is used to turn it on. Once turned on, the thyristor will remain on (conducting) as long as there is a significant current flowing through it. If the current falls to zero, the device switches off.
The thyristor is a four-layer semiconducting device, with each layer consisting of an alternately N or P-type material, for example NPNP. The main terminals, labelled anode and cathode, are across the full four layers, and the control terminal, called the gate, is attached to one of the middle layers. The operation of a thyristor can be understood in terms of a pair of tightly coupled transistors, arranged to cause the self-latching action.
Thyristors are used where rapid switching and high currents are desired, and are often used to control alternating currents, where the change of sign of the current causes the device to automatically switch off. For example, a dimmer switch for lights is easily implemented using a thyristor, where the turn on point is controlled to occur at a particular point on the sine curve of the AC supply. The thyristor stays on for the remainder of that cycle. The drawback of a thyristor is that, like a diode, it only conducts in one direction. A similar self-latching 5-layer device, called a triac, is able to work in both directions.
