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A silicon-controlled rectifier is a four-layer latching switch. A small pulse of gate current (IGT) turns it on; it then stays on (latched) until the current through it falls below the holding current. In AC circuits this happens automatically each half cycle at the zero crossing, so delaying the gate pulse by a firing angle α controls how much power reaches the load — the basis of dimmers and motor speed controls.
| Quantity | Formula |
|---|---|
| Gate resistor | RG = (Vsource − VGT)/IGT |
| Peak voltage | Vm = √2 × Vrms |
| Average output | Vavg = Vm(1+cosα)/(2π) |
| RMS output | Vm√((π−α+sin2α/2)/(4π)) |
| Load power | P = Vrms(out)² / R |
At α=0 the SCR conducts the full positive half cycle; as α increases toward 180° the output falls to zero.
A silicon-controlled rectifier is a latching semiconductor switch. A brief gate pulse turns it on, and it conducts until the main current drops below its holding current, at which point it turns off.
The delay (in degrees of the AC cycle) between the zero crossing and the gate pulse. A larger firing angle means the SCR conducts for less of each cycle, delivering less power.
The gate trigger current and voltage needed to reliably turn the SCR on, from the datasheet. The gate resistor is sized so the drive source delivers at least IGT.
The minimum anode current that keeps the SCR latched on. If the load current falls below it, the SCR turns off — important for light or intermittent loads.
The minimum current needed to latch the SCR on at the moment of triggering. It is usually a few times the holding current.
It cannot be turned off by the gate. In AC it turns off naturally when the current crosses zero (natural commutation); in DC you must force the current below the holding value.
To limit the gate current to a safe value above IGT but below the gate's maximum, giving reliable triggering without damaging the gate.
An SCR conducts in one direction (like a controlled diode); a TRIAC conducts in both directions, so it controls full AC waveforms — common in light dimmers.
Yes, but because DC has no zero crossing you need a commutation circuit to turn it off. For DC switching a MOSFET or IGBT is usually simpler.
Varying the firing angle each half-cycle to adjust the average power delivered to a load — the working principle of dimmers, heater controls, and motor speed controllers.
Yes. This calculator uses the resistive-load half-wave equations. Inductive loads keep the SCR conducting past the zero crossing, changing the waveform and requiring a different analysis.
Noise or fast dv/dt on the anode can false-trigger it. Add a gate-cathode resistor and an RC snubber across the SCR to improve noise immunity.
An RC network across the SCR that limits the rate of voltage rise (dv/dt) to prevent false turn-on, especially with inductive loads. It is recommended in most AC power circuits.
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