Thyristor or Silicon Controlled Rectifier SCR - how it works

A thyristor or SCR (Silicon Controlled Rectifier) can be compared, as the circuit symbol already indicates, with a controllable diode. The three terminals of a thyristor are called cathode, anode and gate - the gate terminal forms the control port of the thyristor.

Thyristor or SCR symbol - p-type and n-type Thyristor

In the basic state, a thyristor blocks in both directions. However, if it is to conduct in the forward direction, it must be set to the conducting state by a small current pulse via the control terminal ("gate"). This is also referred to as "firing" the thyristor. In the reverse bias, a thyristor behaves like an ordinary diode.

Another difference to a transistor is that a thyristor remains conductive even when the current pulse at the gate has passed again - at least as long as the thyristor is still in forward bias and the holding current is not undershot.

Thyristor or Silicon Controlled Rectifier SCR - how it works

When does the thyristor switch off again?

• As already indicated, when the necessary holding current is undershot. If the thyristor is connected to AC voltage, this happens automatically with every zero-cross over of the AC voltage.
• If the thyristor is connected to DC voltage, the current path must be interrupted.
• Special thyristors, so-called GTO thyristors (Gate Turn-Off) can also be switched off via the control terminal without the need to fall below the holding current.