What Is the BT139? Triac (TRIAC): Pinout and Uses
The BT139 is a 16 A-class triac (TRIAC); a power switch that can conduct both half-cycles of AC. It is used to switch AC mains loads (lamps/heaters/motors) and in dimmer circuits. Pinout TO-220: MT1 – MT2 – Gate (tab = MT2). When driven from an MCU it is isolated with a triac-driver optocoupler like the MOC3021; inductive loads need a snubber (RC).
The BT139 is a 16 A-class triac (TRIAC). A triac is a power switch that can conduct both half-cycles of AC; a small current into its gate turns it on and it conducts until the current reaches zero. For the concept, see what is a triac.
What is the BT139 used for?
Switching AC mains loads (lamps, heaters, motors) and dimmer circuits. When driven from a microcontroller it is usually isolated with a triac-driver optocoupler such as the MOC3021 for safety.
Pinout (TO-220)
Front face forward, left to right: MT1 – MT2 – Gate. The metal tab is connected to MT2.
| Pin | Function |
|---|---|
| 1 | MT1 (main terminal 1) |
| 2 | MT2 (main terminal 2) |
| 3 | Gate |
Watch out: gate triggering and snubber
The triac gate is triggered with a suitable resistor (referenced to MT2 where needed). With inductive loads (motors/transformers) a snubber (RC) circuit is usually added to prevent premature turn-off. Because it works at mains voltage, isolation and safety are critical.
Equivalents
- BT136/BT137/BT138/BT139: same family, different current (4/8/12/16 A).
- BTA16: a triac with an insulated tab.
- MOC3021: a triac-driver optocoupler (isolated trigger).
Find it in our catalog
Use the BT139 search.