Vol. EE Reference
Chapter 9 Circuit Schematic Symbols

The Thyristor

A thyristor is a device that has a number of unusual characteristics. It has three terminals: Anode, cathode and gate, reflecting thermionic valve / vacuum tube technology.

As might be expected the gate is the control terminal while the main current flows between the anode and cathode.

The DIAC circuit symbol is generated from the two triangles held between two lines as shown.

In some way this demonstrates the structure of the device which can be considered also as two junctions.

The two terminals of the device are normally designated either Anode 1 and Anode 2 or Main Terminals 1 and 2, i.e. MT1 and MT2.

The TRIAC if seen from the outside may be viewed as two back to back thyristors and this is what the circuit symbol indicates.

It is effectively a development of the SCR or thyristor, but unlike the thyristor which is only able to conduct in one direction, the TRIAC is a bidirectional device.

The GTO is sometimes also referred to as the gate turn off switch. This device is unusual in the thyristor family because it can be turned off by simply applying a negative voltage to the gate - there is no requirement to remove the anode cathode voltage.

Silicon controlled switch (SCS), like the SCR, is a unilateral, four layer three junction P-N-P-N silicon device with four electrodes namely cathode C, cathode gate Gx, anode gate G2 and the anode A.

Gate-controlled switch is designed for easy opening with a reverse-biased trigger.

A gate controlled switch (GCS) is closed by a positive trigger and opened by a negative trigger (or by low-current drop out).

Unijunction transistor (abbreviated as UJT), also called the double-base diode is a 2-layer, 3-terminal solid-state (silicon) switching device.

The device has-a unique characteristic that when it is triggered, its emitter current increases regeneratively (due to negative resistance characteristic) until it is restricted by emitter power supply.