Thyristors (also known as Silicon Controlled Rectifier, SCR), are 3-terminal pnpn-junction devices, as shown below.
Thyristors are half-controlled devices. They can be turned on by a control signal, but are turned off by the power circuit.
In the off state, thyristors can block positive voltages below their forward breakdown voltage and reverse voltages below reverse breakdown voltage.
Thyristors can be turned on by applying a positive current pulse to its gate when the device is forward biased. When it turns on, the gate current pulse can be removed.
To turn the thyristor off, its current must be brought down to zero and a negative voltage has to be maintained across its terminals for longer than a specified minimum period of time.
