Silicon Controlled Rectifier

The SCR conducts from anode to cathode once a gate pulse triggers it, and stays on (latches) as long as the current exceeds the holding current. On AC it turns off naturally at each zero crossing.

By delaying the gate pulse by a firing angle $\alpha$ in each half cycle, the average output of a controlled rectifier becomes:

$V_{avg} = \frac{V_m}{2\pi}(1 + \cos\alpha) \quad \text{(half-wave, resistive load)}$

Uses: controlled rectifiers, motor speed control, crowbar over-voltage protection.