Introduction to The Triac
Introduction to the Triac:
- The Triac is a member of the thyristor family.
- But unlike a thyristor which conducts only in one direction (from anode to cathode) a triac can conduct in both directions.
- A triac is similar to two back to back (anti parallel) connected thyristors but with only three terminals. As in the case of a thyristor, the conduction of a triac is initiated by injecting a current pulse into the gate terminal.
- The gate looses control over conduction once the triac is turned on.
- The triac turns off only when the current through the main terminals become zero.
- A triac can be categorized as a minority carrier, a bidirectional semi-controlled device. They are extensively used in residential lamp dimmers, heater control and for speed control of small single phase series and induction motors.
- A TRIAC acts much like two SCRs connected back-to-back for bidirectional (AC) operation.
- TRIAC controls are more often seen in simple, low-power circuits than complex, high-power circuits. In large power control circuits, multiple SCRs tend to be favored.
- When used to control AC power to a load, TRIACs are often accompanied by DIACs connected in series with their gate terminals. The DIAC helps the TRIAC fire more symmetrically (more consistently from one polarity to another).
- Main terminals 1 and 2 on a TRIAC are not interchangeable.
- To successfully trigger a TRIAC, gate current must come from the main terminal 2 (MT2) side of the circuit.