Why a DIAC Fires the TRIAC

Lesson notes

Key points

• A DIAC is a two-terminal bidirectional device that stays off until its voltage reaches the breakover value, then suddenly conducts.
• In a dimmer, a resistor charges a capacitor each half-cycle, and the variable resistor sets how fast it charges.
• When the capacitor reaches breakover, the DIAC dumps a sharp current pulse into the TRIAC gate, firing it cleanly.
• Its symmetrical switching gives equal firing in both half-cycles, reducing DC components and uneven triggering.

Definitions

DIAC

Gateless bidirectional trigger diode that switches on sharply at its breakover voltage in either polarity

Takeaway: The RC network sets timing; the DIAC delivers a clean gate pulse.

DIACRC delayTrigger pulseSymmetry