TRIAC Symbol
Definition: The TRIAC symbol represents a bidirectional, gate-triggered thyristor used in AC power-control circuits, standardised under IEC 60617 and ANSI Y32.2 / IEEE 315, with three terminals designated MT1 (Main Terminal 1), MT2 (Main Terminal 2), and Gate, capable of conducting current in both directions when triggered.
Also known as: bilateral thyristor, bidirectional triode thyristor, full-wave AC switch, AC power controller.
What the TRIAC symbol means
The TRIAC symbol represents a three-terminal semiconductor switching device that can conduct AC current in either direction between its two main terminals (MT1 and MT2) once a triggering pulse is applied to its Gate terminal. Unlike an SCR (thyristor), which conducts in only one direction, the TRIAC handles both positive and negative half-cycles of an AC waveform, making it ideal for full-wave AC power control without a centre-tapped transformer.
In schematic diagrams, TRIAC symbols appear in dimmer switches, motor speed controllers, and heating element regulators where the phase angle of AC conduction must be precisely controlled. The device latches on when triggered and can only be switched off when the current through it falls below the holding current — typically at the zero-crossing of the AC cycle — giving the TRIAC its characteristic phase-control behaviour.
How to identify the TRIAC symbol
The TRIAC symbol consists of two anti-parallel SCR (thyristor) triangles sharing a common vertical bar, drawn horizontally between the two main terminals MT1 (left) and MT2 (right). Each triangle points toward the shared vertical cathode bar from opposite directions, visually indicating bidirectional conduction. A short diagonal line — the Gate lead — exits from the MT1 end of the symbol, typically drawn downward at 45 degrees. The overall shape resembles a pair of opposing diode symbols fused at the cathode bar with a single gate connection.
Function in a circuit
A TRIAC acts as a fully bidirectional AC switch: when its Gate receives a trigger pulse of sufficient current (regardless of polarity), the TRIAC latches into conduction and passes current from MT2 to MT1 or MT1 to MT2 depending on the instantaneous polarity of the supply voltage. By delaying the gate trigger relative to the AC zero crossing (phase-angle control), the proportion of each half-cycle delivered to the load can be varied continuously from near zero to full power. The TRIAC turns off naturally at each AC zero crossing when load current falls below the holding current, typically 5–50 mA for small devices.
Standards: IEC vs ANSI
| IEC 60617 | IEC 60617-05 (semiconductors): the TRIAC is drawn as two back-to-back SCR triangles with a shared gate, electrode labels MT1, MT2, and G. |
|---|---|
| ANSI/IEEE 315 | ANSI Y32.2 / IEEE 315: uses the same bidirectional-thyristor symbol — two opposing SCR symbols with a single gate lead; designator Q or sometimes TRIAC on the schematic. |
| Key difference | The graphical symbol is identical in IEC and ANSI/IEEE. Terminal designations differ slightly: IEC labels the electrodes A1/A2 (anode 1 / anode 2) in some older texts, while modern practice (and ANSI) uses MT1/MT2 for main terminals. |
Terminals / pins
| Pin | Name |
|---|---|
| mt1 | MT1 |
| mt2 | MT2 |
| gate | Gate |
Typical values
Typical TRIAC ratings: voltage (VDRM) 200 V–800 V; RMS current (IT(RMS)) 1 A–40 A; gate trigger current (IGT) 5–50 mA; holding current (IH) 5–50 mA; on-state voltage drop ~1.5 V at rated current. Common part numbers: BTA16, BTA26, MAC97.
Where the TRIAC symbol is used
- Phase-angle light dimmer switches for incandescent and halogen lamps
- Single-phase AC motor speed controllers for fans and power tools
- Domestic appliance heating element power controllers (irons, heaters)
- Solid-state relay output stages handling 230 V or 120 V AC loads
- Soft-start circuits that ramp mains voltage gradually to a motor on startup
- Industrial programmable logic controller (PLC) output modules switching AC actuators
Example
In a standard ceiling-fan dimmer, a TRIAC (e.g. BTA16-600B) is connected in series with the fan winding across 230 V AC. A DIAC and RC phase-shift network feed the Gate; as the RC time constant is varied by turning the control knob, the Gate fires later in each half-cycle, delivering less average power to the motor — the same principle illustrated on any residential wiring diagram using this symbol.
Key facts
- The TRIAC symbol depicts two anti-parallel SCR thyristors with a single shared Gate terminal, showing its ability to switch AC in both polarities.
- Terminal designators: MT1 (Main Terminal 1), MT2 (Main Terminal 2), and G (Gate); schematic reference designator is Q (or sometimes labelled TRIAC).
- IEC 60617-05 and ANSI/IEEE 315 both use the same bidirectional-thyristor glyph; IEC legacy texts may label terminals A1/A2 instead of MT1/MT2.
- A TRIAC conducts in either direction once the Gate current exceeds IGT (typically 5–50 mA) and self-commutates off at every AC zero crossing.
- Phase-angle control: by varying the gate trigger delay within each AC half-cycle, average load power is adjustable from ~2% to 98% of full power.
- TRIACs are sensitive to rapid voltage rise (dV/dt); a snubber RC network (typically 0.1 µF + 39 Ω) is placed in parallel to prevent false triggering.
- Common voltage ratings are 400 V or 600 V (VDRM); always choose a device rated at least 2× the peak supply voltage for safe operation on 230 V mains.
- The TRIAC superseded two back-to-back SCRs in most domestic power-control applications because it requires only one gate drive circuit.
Frequently asked questions
What does the TRIAC symbol look like?
The TRIAC symbol shows two opposing SCR (thyristor) triangles connected back-to-back with a shared vertical bar between the two main terminals (MT1 on the left, MT2 on the right). A short diagonal Gate lead exits from the MT1 side, usually drawn downward. The result visually resembles a double-headed arrowhead with a centre bar and one gate connection.
What does the TRIAC symbol mean in a circuit diagram?
The TRIAC symbol means there is a bidirectional AC switch at that point in the circuit. When the Gate receives a trigger pulse, the device conducts AC current in whichever direction the supply voltage is pushing it, and it continues to conduct until the AC current naturally falls to zero at the end of each half-cycle.
What is the difference between a TRIAC and an SCR (thyristor) symbol?
An SCR symbol shows a single-direction diode-like triangle with one gate lead and conducts in one direction only. The TRIAC symbol shows two mirrored SCR triangles fused together at a common bar, indicating bidirectional conduction. Both share a single Gate terminal in the symbol, but the TRIAC controls both half-cycles of AC without needing a bridge rectifier.
What is the designator letter for a TRIAC on a schematic?
The reference designator for a TRIAC is Q (used for all semiconductor switching devices in IEEE 315 convention) or sometimes the full label TRIAC is written beside the symbol. Some older European schematics following IEC practice also use the designator TR.
What are the three terminals of a TRIAC symbol?
The three terminals are MT1 (Main Terminal 1), MT2 (Main Terminal 2), and G (Gate). MT1 and MT2 carry the main load current bidirectionally. The Gate terminal receives the trigger pulse that initiates conduction; it is referenced to MT1 in most datasheets.
What is the IEC vs ANSI difference for the TRIAC symbol?
The graphical symbol is identical in IEC 60617-05 and ANSI Y32.2 / IEEE 315: both show two back-to-back SCR symbols sharing a gate. The only common difference is in terminal naming: modern schematics worldwide use MT1/MT2, but older IEC documents used A1/A2 (Anode 1/Anode 2).
What standard defines the TRIAC symbol?
The TRIAC symbol is defined in IEC 60617-05 (semiconductor devices) and ANSI Y32.2 / IEEE 315-1975 (graphic symbols for electrical and electronics diagrams). Both standards specify the same bidirectional-thyristor glyph with a Gate lead.
What is a common circuit that uses the TRIAC symbol?
The most common circuit is a phase-angle dimmer or AC motor speed controller. The TRIAC sits in series with the load across the mains; a DIAC-triggered RC phase-shift network drives the Gate, delaying ignition within each half-cycle to reduce average power delivered to the load.
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