thyristorer

Thyristors are solid-state semiconductor devices used to control high power. A thyristor is a four-layer PNPN structure that acts as a latching switch. It has three junctions and is normally in a blocking state in either the forward or reverse direction. Conduction begins when a gate pulse triggers the device; a brief gate current injects carriers to enable regenerative feedback that latches the anode-to-cathode path on. Once conducting, the device remains on until the current falls below a holding value or is forced to turn off by circuitry.

Common forms include the silicon-controlled rectifier (SCR), which conducts in one direction; the triac, which conducts in both directions; and gated devices such as gate-turn-off thyristors (GTOs) and light-activated SCRs (LASCRs). Each type offers different bidirectional or unidirectional conduction and turn-off characteristics suitable for various applications.

Triggering is typically done with a short gate current pulse; some devices can be triggered by voltage or light. The turn-off mechanism depends on the circuit: natural commutation in alternating-current systems when the current crosses zero, or forced commutation in direct-current or high-power circuits using capacitive or inductive networks. In DC applications, turning off requires additional circuitry to reduce or interrupt the current.

Applications include AC power control such as light dimming and motor speed control, controlled rectifiers for DC power supplies, traction power, welding equipment, and high-power switching. They offer high blocking voltages and currents but have slower switching speeds and require proper heat sinking. Protection against false triggering due to dv/dt, noise, and overcurrent is important.

In design practice, thyristors are chosen for high-power switching where simple, robust control is desired, and for applications needing latching behavior without continuous control signals. Manufacturers specify ratings such as blocking voltage, current, di/dt, and gate sensitivity.