Thyristors (commonly known as silicon controlled rectifiers, SCRs) are semi-controlled high-power semiconductor devices. Their core characteristic is that they can only be turned on by a small gate signal to control large anode current, but cannot be turned off by the gate (they turn off naturally when the anode current drops below the holding current). They feature extremely high voltage withstand capability, strong current carrying capacity and very low cost. Common types include unidirectional thyristors (SCRs, for DC and rectification scenarios), bidirectional thyristors (TRIACs, the mainstream for AC control), fast thyristors, reverse conducting thyristors and gate turn-off thyristors (GTOs). They are widely used in power frequency high-power power conversion and control scenarios such as controlled rectification, AC voltage regulation, motor speed regulation, electric welding machines, UPS power supplies, power system reactive power compensation, and dimming and speed regulation of home appliances.