What is a thyristor (SCR) and how is it turned on?

A thyristor (Silicon Controlled Rectifier) is a four-layer PNPN device with three terminals: anode, cathode, and gate. It conducts when forward-biased AND gate triggered - once triggered, it latches on and continues conducting until current falls below holding current. Turn-on requires gate pulse when forward voltage is applied. Thyristors cannot be turned off via gate (unlike GTOs); current must reduce naturally or by forced commutation. Used in high-power controlled rectifiers and AC controllers.