SCR Controlled Rectifier

Drag the firing angle slider and watch the half-wave output waveform reshape in real time. The circuit schematic, gate pulses, conduction region, and computed V_avg / V_rms update instantly — no equations to solve by hand.

Firing angle (α): 45 °Peak voltage (V_m): 170 V

Firing angle: α = 45°

Conduction angle: 135°

Average voltage

V_{avg} = \frac{V_m}{2\pi}(1 + \cos\alpha) = \frac{170}{2\pi}(1 + \cos 45°) = 46.2\;\text{V}

RMS voltage

V_{rms} = V_m\sqrt{\frac{1}{2\pi}\!\left(\frac{\pi - \alpha}{2} + \frac{\sin 2\alpha}{4}\right)} = 170\sqrt{\frac{1}{2\pi}\!\left(\frac{\pi - 45°}{2} + \frac{\sin 90°}{4}\right)} = 81.0\;\text{V}

Quick experiments

Full conduction. Set α = 0° — the SCR conducts the entire positive half-cycle, giving maximum V_avg.
Half power. Find the firing angle where V_avg drops to roughly half of the α = 0° value (~90°).
Zero output. Push α to 180° — the gate pulse arrives at the zero-crossing so no conduction occurs.
Peak voltage effect. Raise V_m to 340 V (≈ 240 V RMS mains) and note how V_avg scales linearly with peak voltage.

SCR (Silicon Controlled Rectifier)

An SCR is a four-layer PNPN thyristor that acts as a latching switch. A gate pulse turns it ON; it stays ON until the anode current falls below the holding current I_H. Unlike a MOSFET or BJT, it cannot be turned off by the gate — anode current must be interrupted externally.

Phase-angle control

By delaying the gate trigger pulse relative to the AC zero crossing (firing angle α), average power to the load is controlled. Firing at α = 0° passes the full half-cycle; α = 90° passes half the energy; α = 180° passes none. Used in lamp dimmers, motor speed controls, and heating elements.

Crowbar protection

SCRs are commonly used in overvoltage crowbar circuits: when the output voltage exceeds a threshold, the SCR fires and short-circuits the supply, blowing the fuse and protecting downstream circuitry. Simple, fast, and failsafe.

Learn more → Controlled Rectifiers — Learn

Formula reference

Average power with phase control: $P_{avg} = \dfrac{V^{2}}{R} \cdot \dfrac{1}{\pi}\left(\pi - \alpha + \dfrac{\sin 2\alpha}{2}\right)$
α is the firing angle in radians.
Latching condition: $I_A > I_L$
Anode current must exceed the latching current for the SCR to stay on after the gate pulse.
Turn-off condition: $I_A < I_H$
It only turns off when anode current falls below the holding current.
Gate trigger requirement: $I_G > I_{GT}, \; V_G > V_{GT}$
Both the gate current and voltage thresholds must be met.

Symbol	Meaning	Unit
$\alpha$	Firing (delay) angle	° or rad
$I_L$	Latching current	A
$I_H$	Holding current	A
$I_{GT}$	Gate trigger current	A

Common mistakes

Expecting the gate to turn an SCR off.
The gate only turns it on. Anode current must drop below the holding current, which on AC happens at the zero crossing and on DC needs forced commutation.
Using an SCR where a TRIAC is needed.
An SCR conducts one way only, so on AC it passes just one half-cycle. Full-wave control needs a TRIAC or two anti-parallel SCRs.
Applying a gate pulse that is too short.
The pulse must last until anode current exceeds the latching current. With an inductive load, current rises slowly and a short pulse fails to latch.
Ignoring the dv/dt rating.
A fast rising anode voltage can turn an SCR on with no gate signal at all. A snubber across the device limits the rate of rise.
Forgetting that phase control distorts the line.
Chopping the waveform injects harmonics and worsens power factor, which is why phase-controlled loads often need filtering.

Frequently asked questions

How does an SCR turn on?

A brief gate current pulse latches the SCR into conduction, provided the anode is positive with respect to the cathode. Once latched it stays on even after the gate signal is removed.

How do I turn an SCR off?

The gate cannot turn it off. Anode current must fall below the holding current, which happens naturally at each AC zero crossing. In DC circuits a forced-commutation circuit is required.

What is the difference between latching and holding current?

Latching current is the anode current needed to stay on immediately after triggering. Holding current is the lower value needed to remain on afterwards. Latching current is always the larger of the two.

What is phase-angle control?

Delaying the gate trigger within each AC half-cycle controls how much of that half-cycle conducts, and therefore the average power delivered. It is how lamp dimmers and heater controllers work.

What is the difference between an SCR and a TRIAC?

An SCR conducts in one direction only, so it handles a single half-cycle. A TRIAC conducts in both directions and can control the full AC waveform, which is why it is used in mains dimmers.