TRIAC
A TRIAC conducts in both directions when its gate receives a trigger pulse, latching on until the current naturally falls below holding (typically at the AC zero crossing). Common parts: STMicroelectronics BTA16-600B 16 A 600 V, ON Semiconductor MAC15M 15 A snubberless, NXP BT139-600.
A TRIAC is essentially two SCRs integrated anti-parallel into one silicon die with one shared gate. A gate trigger pulse of either polarity latches the appropriate SCR depending on the main-terminal polarity at that instant, and the device stays on conducting AC current until the next zero-crossing. Trigger configurations are described as quadrants (I, II, III, IV); modern 'snubberless' devices like the BTA series include enhanced dV/dt and dI/dt ratings that allow operation without a snubber across resistive and most inductive loads.
In plain terms
A two-way ratchet that can lock either direction of rotation: a tap on the gate engages it for the next half-cycle whichever way the current is flowing, and it stays engaged until the current crosses zero.
Why designers use it
- Phase-control AC heaters, lamps, and fans with a single device.
- Replace mechanical contactors with a solid-state, silent, vibration-immune AC switch.
- Soft-start AC induction motors by ramping the firing angle from 180° toward 0° each half-cycle.
- Build solid-state relays (SSRs) by optically coupling a TRIAC to a low-voltage drive.
Best for
- AC light dimmers
- AC fan/motor control
- SSRs
Key specifications
- Blocking voltage: Check repetitive peak rating
- Gate trigger: Depends on supported quadrant
- Commutation: Check load and dV/dt limits
- RMS current: Depends on thermal conditions
When not to use it
- PWM-style speed control of DC motors — TRIACs cannot turn off mid-conduction; use a MOSFET or IGBT.
- Highly capacitive loads where dV/dt at zero-crossing exceeds the device rating — the TRIAC can self-trigger; use snubberless variants or a snubber.
Common mistakes
- Underestimating gate trigger current at low temperatures — IGT can double at −20 °C; size the trigger pulse with margin.
- Driving the gate with DC instead of pulses — DC continuously triggers the wrong quadrant after the half-cycle reverses; use 38 kHz pulses or DIAC-fired pulses.
Where you will find it
- A Lutron Maestro 1000 W incandescent dimmer uses a snubberless STMicroelectronics BTA16-600BWRG TRIAC fired by a DIAC each half-cycle: the user's potentiometer sets a phase angle from ~10° to 175°, and the TRIAC chops both half-cycles symmetrically, dimming an incandescent bulb from full to nearly off — a circuit topology that has changed little in 50 years and proves the TRIAC's value.
- A Crydom 25 A SSR (Solid State Relay) uses an optocoupler-driven TRIAC: the input LED isolates the 5 V control logic from a switched 240 V AC heater bank, the random-fire vs. zero-cross variants are selected per application, and the contactor-replacement function has zero mechanical wear over millions of cycles where a relay would fail at hundreds of thousands.
- A residential ceiling-fan rotary speed controller uses a 4 A TRIAC (NXP BT139-600) with a gate snubber sized for the slightly inductive PSC fan motor: the TRIAC chops the AC mains at one of three preset angles, dropping the fan from 1 200 to 800 to 500 RPM — and the same circuit has been used in millions of homes since the 1970s with only minor modernisation.
A short history
The TRIAC extends thyristor-style switching to both directions of an AC circuit. Its single gate can trigger conduction in supported polarity combinations, which is useful for AC switching and phase control. Trigger sensitivity and commutation behaviour depend on the chosen device and operating quadrant.