Triac or thyristor (SCR)?

The difference between a triac and a thyristor (SCR): conduction direction, triggering, turn-off, typical parts, uses such as dimmers and chargers, safety and common mistakes.

Short answer

Use a triac to control the power of a lamp, heater or brushed motor on mains AC; it conducts both ways. Use a thyristor where you need one-way control (controlled rectification, battery chargers, overvoltage protection).

Comparison

	Triac (BT136, BT139, BTA16)	Thyristor / SCR (BT151, TYN612)
Conduction	Both directions (AC)	One direction (anode to cathode)
Triggering	Gate pulse of either polarity	Positive gate pulse
Turn-off	On its own each time the current crosses zero	When current drops below the holding current; won't turn off by itself on DC
Pins	MT1, MT2, Gate	Anode, Cathode, Gate
On-state drop	≈1–1.5 V (5–7 W of heat at 5 A)	≈1–1.5 V
Typical use	Dimmers, heaters, brushed motor speed, solid-state relays	Controlled rectifiers, battery chargers, crowbar protection, welders

Choose a triac if…

✓You're controlling a lamp or heater on mains AC
✓You're controlling the speed of a brushed (universal) motor
✓You're switching an AC load from a microcontroller (through an opto-triac)

Choose a thyristor if…

✓You're building an adjustable DC output (controlled rectification)
✓You're designing a battery charger
✓You're building a crowbar that blows the fuse on overvoltage

Common mistakes

!Expecting a thyristor or triac to switch off like a button in a DC circuit. It stays on until the current is interrupted.
!Leaving out the snubber on inductive loads like motors. The triac can trigger on its own; add an RC snubber or use a "snubberless" type.
!Wiring a mains circuit directly to a microcontroller. Always isolate it with an opto-triac such as the MOC3021 / MOC3041.
!Forgetting the heatsink. At 5 A a triac produces 5–7 W of heat.
!Forgetting the tab may be at mains potential. BTA-series tabs are insulated, BTB-series are not; touching the heatsink of a non-insulated type is dangerous.

Try it: triac phase control

Change the firing angle and see how the power to the lamp and the waveform change.

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