Thyristors or SCRs (silicon controlled rectifiers). The Phase Control SCR provides phase-control of ac signals with a line frequency of 50-60 Hz. The Inverter SCR is used in pulsed power applications connecting higher frequencies. The main difference between the two families is the turn-off time (tq). Inverter SCRs typically have a tq of less than 30 microseconds (µs). Similar phase control SCRs has tq ratings of several hundred µs.