Description

The thyristor is a four-layer, three terminal semiconducting device, with each layer consisting of alternately N-type or P-type material, for example P-N-P-N. The main terminals, labelled anode and cathode, are across the full four layers, and the control terminal, called the gate, is attached to p-type material near to the cathode. (A variant called an SCS—Silicon Controlled Switch—brings all four layers out to terminals.) The operation of a thyristor can be understood in terms of a pair of tightly coupled bipolar junction transistors, arranged to cause the self-latching action:
RexST
(Thyristor Discrete):
TYN**0C **0A ***0V TO*3
TYN**0C **0A ***0V TO***7
TYN**0C **0A ***0V TO***7
TYN*0C *0A ***0V/***0VTO***7
TYN*0H *0A ***0V TO***7
TYN*5C *5A ***0V/***0V TO***0
(Triac Discrete):
BTA*1 ***0V/***0V/***0V
BTA*1 ***0V
BTA*1 ***0V/***0V/**0V TOP3
BTA*6 ***0V/***0V TO***0

Thyristors have three states:

   1. Reverse blocking mode — Voltage is applied in the direction that would be blocked by a diode
   2. Forward blocking mode — Voltage is applied in the direction that would cause a diode to conduct, but the thyristor has not yet been triggered into conduction
   3. Forward conducting mode — The thyristor has been triggered into conduction and will remain conducting until the forward current drops below a threshold value known as the "holding current"