Thyristor
A thyristor (SCR or Triac) is a bistable switch. A brief gate pulse triggers it into conduction; it then latches and continues to conduct regardless of the gate signal. In AC circuits it turns off naturally when the current crosses zero. It's the workhorse of phase-angle AC power control.
For an SCR, four alternating semiconductor layers can be represented by two coupled transistors. Gate triggering starts regenerative conduction. Once latched, removing the gate drive does not normally stop the main current: turn-off requires current below the holding threshold and suitable recovery conditions. Latching current, holding current and turn-off time are device-specific.
In plain terms
A light switch with a latch: push the button (gate pulse) and it snaps on and stays on even if you release the button — the only way to turn it off is to interrupt the main current flow, not the gate.
Why designers use it
- Control AC power to heaters, dimmers, and motor-speed controllers with phase-angle firing.
- Build soft-starters for large induction motors to limit inrush.
- Protect crowbar circuits that short a supply rail immediately on overvoltage.
- Switch large DC currents in pulsed power and electromagnetic forming.
Best for
- AC dimmers
- Crowbar protection
- Soft-starters
Key specifications
- Blocking voltage: Check repetitive peak rating
- Gate trigger: Check required current and voltage
- Latching and holding current: Different thresholds
- Load current: Derate for thermal conditions
When not to use it
- When the circuit cannot provide the current interruption and recovery required by the selected thyristor.
- When its switching, current, voltage or thermal limits do not meet the load requirements.
Common mistakes
- Ignoring gate-drive requirements, latching current, holding current, allowable di/dt or dv/dt for the selected device.
- Expecting an ordinary SCR to turn off merely because its gate signal is removed. Its main current must fall below the holding requirement and the circuit must allow recovery.
- Treating an illustrative phase-control circuit as a safe, complete mains design.
Where you will find it
- In an illustrative phase-control application, a suitable thyristor conducts for part of an AC cycle to control power delivered to a compatible load.
- Littelfuse's AN1001 distinguishes SCR, TRIAC and other thyristor behavior. Their trigger conditions and current directions are not interchangeable.
A short history
The SCR belongs to a family of regenerative semiconductor switches. A gate pulse can start conduction, but removing that pulse does not normally turn the SCR off; the main current must fall below its holding level. This latching behaviour distinguishes it from a transistor used as a directly controlled switch.
Good to know
- An SCR latches on with a brief gate pulse and stays on until the current drops below the holding threshold — a feature, not a bug, for AC line switching where the current reverses every half-cycle.
- Industrial AC drives, electric-train traction (pre-IGBT), and welder front-ends ran on thyristor banks for decades; some still do.
- TRIACs are bidirectional thyristors — a single device that conducts both half-cycles of AC, perfect for light dimmers and motor speed controls.