Thyristors are similar to transistors in that they have three leads and a small current at one lead will allow a much current to flow through the two other leads, but the controlled current is either on or off, and so thrysitors do not amplify signals like transistors do. Thyristors are solid state switches. There are two types of thyristors, SCR’s (Silicon Controlled Rectifiers) and Triacs with the former switching direct current and the latter used to switch alternating currents.
Silicon Controlled Rectifiers
These are similar to a bipolar transistor but has a fourth layer, so it has three PN junctions. SCR’s are sometimes refered to as a 4-layer PNPN diode as it only allows current to be passed in one direction.
Triacs
A Triac is equivalent to two SCR’s in parallel, so Triacs can switch both alternating and direct current. Triacs have five layers and an extra N-Type area. All the three leads on a triac make contact with two silicon layers.
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