Thyristor, also known as thyristor rectifier, was once referred to as thyristor; in 1957, General Electric Company of the United States developed the world's first thyristor product and commercialized it in 1958. The thyristor is a PNPN four-layer semiconductor structure, which has three poles: anode, cathode and gate; the working conditions of the thyristor are: forward voltage is applied and the gate has a trigger current; its derived devices are: fast thyristor, bidirectional thyristor, reverse conduction thyristor , Light-controlled thyristor, etc. It is a high-power switching semiconductor device, which is represented by the text symbols "V" and "VT" in the circuit (represented by the letters "SCR" in the old standard). Thyristor has the characteristics of silicon rectifier device, can work under high voltage and high current conditions, and its working process can be controlled, and is widely used in controllable rectification, AC voltage regulation, non-contact electronic switches, inverters and frequency conversion. in the circuit.