A thyristor is a four-layer semiconductor device consisting of alternating P-type and N-type materials (PNPN). A thyristor usually has three electrodes: an anode, a cathode and a gate, also known as a control electrode.

The most common type of thyristor is the silicon-controlled rectifier (SCR). When the cathode is negatively charged relative to the anode, no current flows until a pulse is applied to the gate. Then, the SCR conducts current until the voltage between the cathode and anode is reversed or reduced below a certain threshold or holding value. Using this type of thyristor, large amounts of power can be switched or controlled using a small triggering current or voltage.

Devices using alternating currents can be turned on and off by sending a signal to the control gate. This device is called a gate turn-off, or GTO, thyristor. Previously, thyristors needed the current to be reversed to turn off, making them difficult to use with direct current systems. Thyristors are useful in switching applications because they can be fully on or off. This two-state capability differs from transistors, which operate in between on and off states, waiting for a signal to conduct current.

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