ComponentsActive / Semiconductor

TRIAC

Thyristor

A three-terminal semiconductor device that acts as an electronic switch for alternating current (AC), capable of controlling power in both directions.

Schematic symbol

Thyristor (SCR)

How triac appears in a circuit diagram.

Types & variants

BT139BT151BT137T410MAC97A8BTA16BTA12TRIAC-SSnubberless TRIAC

Key specs

Voltage rating

Volt (V)

Maximum voltage the TRIAC can handle (e.g., 600V, 800V). Must exceed circuit peak voltage.

Current rating

Ampere (A)

Maximum current the TRIAC can handle continuously (e.g., 4A, 16A). Must exceed circuit current.

Gate trigger current

mA

Minimum current needed at gate to turn on the TRIAC.

Gate trigger voltage

V

Minimum voltage needed at gate to turn on the TRIAC.

Hold current

mA

Minimum current to keep the TRIAC conducting after triggering.

Markings

Usually printed with part number (e.g., BT139) and sometimes voltage/current ratings. SMD versions may have abbreviated codes.

Standard values

Common voltage ratings: 400V, 600V, 800V. Common current ratings: 4A, 6A, 8A, 12A, 16A.

How to choose

1) Select voltage rating > circuit peak voltage. 2) Select current rating > load current. 3) Consider gate trigger requirements for your control circuit. 4) Choose snubber type based on inductive/capacitive loads.

Pinout & package

MT1MT2G123

TO-220 — front view, tab at back

Check first: Shown for the BT136/BT137 family; the tab is MT2. Gate current is referenced to MT1, so MT1 is the terminal your trigger circuit shares.

3 pins: Main Terminal 1 (MT1), Main Terminal 2 (MT2), Gate (G). Packages: TO-220 (most common), TO-92, TO-126, SMD (e.g., DPAK). Gate is usually between the two main terminals.

Example circuits

  • AC dimmer switch for lighting
  • Speed control for AC motors
  • Temperature control in heaters
  • Solid-state relay (SSR)
  • Phase-angle control circuit

Common failures

Short circuit (fails ON) due to overcurrent, open circuit (fails OFF) due to overvoltage, or failure to trigger. Visible signs: burn marks, cracks, or discoloration on the package.

How to test

Set multimeter to diode test. Between MT1 and MT2: should show open in both directions. Between Gate and MT1/MT2: should show diode-like behavior in one direction only. Use a gate current to trigger and check conduction.

Substitutes

Use TRIAC with same or higher voltage/current rating. Ensure gate trigger characteristics match your circuit. For higher power, use multiple TRIACs in parallel with current-sharing resistors.

Where to buy

DigiKeyMouserLCSCRobuAmazon