What Is a Thyristor (SCR)?
A thyristor (SCR — Silicon Controlled Rectifier) is a power semiconductor that turns on with a small gate pulse and stays on until the current through it is cut. It's the basic element of high-power switching.
How Does It Work?
It has three terminals: anode, cathode and gate. When a short trigger pulse is applied to the gate, it starts conducting between anode and cathode. Interestingly, even when the pulse is removed it stays conducting (latching).
How Does It Turn Off?
An SCR only turns off when the current through it drops below a certain value (the holding current). In an AC circuit the current naturally goes to zero every half-cycle, so the SCR turns off by itself.
| Terminal | Function |
|---|---|
| Anode | Current input |
| Cathode | Current output |
| Gate | Triggering |
Difference From a Diode
An ordinary diode always conducts in the forward direction; an SCR, even in the forward direction, conducts only when triggered. This makes it a "controllable rectifier".
Where Is It Used?
AC power control, motor speed control, light dimming, controlled rectifiers, overvoltage protection (crowbar) and high-power switching.
Other Members of the Thyristor Family
For bidirectional switching the TRIAC, for turning on and off from the gate the GTO, and for producing triggering/timing the UJT are the other members of this family.
Common Mistakes
- Can't turn off in DC: in a DC circuit an SCR won't turn off by itself without cutting the current; an extra circuit is needed.
- Gate current: an insufficient trigger pulse gives no reliable triggering.
- dV/dt triggering: a sudden voltage change can trigger it unintentionally; a snubber is needed.
Frequently Asked Questions
Is an SCR unidirectional?+
What is the holding current?+
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