TRIAC

A TRIAC is effectively two SCRs back-to-back with a single gate, conducting in both directions. Learn its quadrants, phase-control dimmers, zero-cross and burst-fire control, snubbers and solid-state relays.

A TRIAC (triode for AC) switches AC loads in both half-cycles with one device and one gate. It is the heart of lamp dimmers, fan-speed controllers, heater controllers and AC solid-state relays.

Like an SCR it latches and turns off at current zero, but its gate can trigger it with either polarity, in any of four quadrants.

An analogy

If an SCR is a one-way turnstile that locks open once pushed, a TRIAC is a revolving door that can be pushed from either side.

Bidirectional switch

A TRIAC conducts current in either direction between MT2 and MT1 once triggered, and turns off at each current zero. One gate pulse per half-cycle, at angle α, controls the power delivered to the load.

Trigger quadrants

Quadrants I–IV combine MT2 polarity with gate polarity. Sensitivity differs: quadrant IV (MT2−, gate+) is least sensitive or unsupported. Driving the gate with negative pulses (quadrants II and III) works for both half-cycles and is common with MCUs through an opto-TRIAC.

Phase control vs burst control

Phase control (lamp dimmers, universal motors) varies power within each cycle, which produces harmonics and EMI and needs a filter choke. Burst-fire / integral-cycle control switches whole cycles at zero crossing, producing no harmonics — ideal for heaters with long thermal time constants but causing flicker for lamps.

Commutation with inductive loads

With an inductive load, current lags voltage, so at current zero the voltage across the TRIAC jumps immediately — a large commutating dv/dt that can retrigger it and prevent turn-off. RC snubbers, 'snubberless' TRIACs (high (dv/dt)c rating) or back-to-back SCRs for large motors solve it.

Solid-state relays

An AC SSR contains an opto-isolator, zero-cross detection and a TRIAC or back-to-back SCRs. It switches silently, with no contact wear or arcing, but it leaks a few mA when off, drops ~1–1.5 V when on (so needs a heat sink) and fails short more often than open.

Formulas

• RMS output (R load): V_o,rms = V_rms · √[(π − α + ½ sin 2α)/π]
• Power fraction: P/P_max = (π − α + ½ sin 2α)/π
• Burst (integral-cycle) control: P/P_max = N_on / (N_on + N_off)

Worked example: Dimmer at 90°

Given: 230 V, 100 W resistive lamp (treat as linear); α = 90°

Find: Power and RMS load voltage.

• P/P_max = (π − π/2 + ½ sin 180°)/π = 0.5
• P = 50 W
• V_rms = 230 × √0.5 = 163 V

Answer: 50 W at 163 V RMS

Worked example: Burst-fire heater control

Given: 2 kW heater; Pattern: 3 cycles on, 5 off

Find: Average power.

• P = 2000 × 3/(3 + 5) = 750 W

Answer: 750 W, with no harmonic distortion

Common mistakes

• Wrong: Using phase control for a large heater on a shared supply. — Right: Use burst-fire / zero-cross control to avoid harmonics and EMI (subject to flicker limits).
• Wrong: Driving an inductive motor load with a standard TRIAC and no snubber. — Right: Check commutating dv/dt; use a snubberless TRIAC or an RC snubber.

Applications

• Lamp & fan dimmers: Leading-edge TRIAC dimmers control incandescent lamps and universal-motor fans (LEDs need compatible drivers).
• Appliances: Washing machines and vacuum cleaners control universal motors with TRIACs; MCUs fire them via opto-couplers.
• Industrial heaters: Zero-cross SSRs with PID controllers regulate oven and extruder temperatures.

Key takeaways

• TRIAC = bidirectional thyristor with one gate.
• Phase control power fraction: (π − α + ½ sin 2α)/π.
• Burst-fire gives harmonic-free heater control.
• Inductive loads need attention to commutating dv/dt.

Going further

• Trailing-edge dimmers: MOSFET/IGBT dimmers turn off mid-cycle instead of turning on late. This suits capacitive LED drivers, reduces EMI and allows soft switching.
• Firing from a microcontroller: Detect the zero crossing (an opto or resistor to a pin), wait (α/360)·T with a timer, then pulse the opto-TRIAC. Re-trigger or use a long pulse so inductive loads latch.
• Thermal design: A TRIAC dissipates ≈ V_T0·I_avg + r_T·I_rms² (~1 W/A). Junction temperature = T_ambient + P·(R_θjc + R_θcs + R_θsa).