Metal tab (heat sink) · Ø 3.7 mm holeEpoxy bodyPins MT1-MT2-G · 2.54 mm spacing

TO-220 body, metal tab with a hole and MT1, MT2, gate pins
TO-220 body ≈ 10 × 15 mm; pin pitch 2.54 mm

Transistors

Triac (BT136)

In simple terms

What is this?

An electronic gate that passes current in both directions and opens with a small nudge.

What is it for?

Turns on an alternating-current load with a small trigger current; it is the working principle of a light dimmer.

Where do you see it?

Inside wall light dimmers.

How do you spot it?

Black body with three pins and one face with a metal tab; the pins are MT1, MT2 and gate.

Caution:

The metal tab can be live too, so don’t touch it; the part does not switch off immediately when you cut the gate current.

Technical definition

A power semiconductor that can conduct current in both directions and is triggered by gate current. Once triggered, it keeps conducting even after the gate signal is removed, as long as the current stays above the holding current. In AC it turns off when the current nears zero. Unlike a transistor, it cannot cut a continuous DC current by turning off the gate.

Load current4 A RMS

Blocking voltage600 V

Gate trigger≈5–35 mA (depends on version)

Schematic symbol Same in IEC and ANSI

IEC 60617: the drawing style used in Türkiye and Europe.

Pins 3

1MT1, pin 1Left when the printed face is toward you; the gate current is referenced to this pinmt1

2MT2, pin 2Middle pin; the metal tab is also connected to MT2mt2

3Gate, pin 3Right pin; trigger current limited by a resistorg

Features typical

Load current: 4 A RMS (with proper cooling)
Blocking voltage: 600 V
Gate trigger (IGT): ≈5–35 mA; varies by version, temperature and region
BT136-600E IGT: Max. 10 mA (I–III), 25 mA (IV), 25 °C
Holding current (IH): ≈10–20 mA class; 600E max. 15 mA, 25 °C
Closing: When the load current drops below IH
Cooling: Depends on power dissipation and ambient temperature
Lab limit: Low-voltage AC only; mains is out of scope
Example part: BT136-600E, TO-220

Common mistakes

Expecting it to turn off when the gate is switched offConduction latches; it does not turn off until the load current falls below the holding current.

Not limiting the gate currentTrigger through a resistor at the gate, relative to MT1; check the IGT limit for the quadrant and version.

Assuming the tab is insulatedThe metal tab is connected to MT2. Check the heatsink insulation; use only low voltage in the Lab.

Test with a multimeter

Test whether this part is good with the multimeter on the measurement screen.

Take a measurement

Where it is used

Low-voltage AC lamp switchZero-current turn-off experimentAC phase control principle

Try it in the Lab