A triac (triode for alternating current) is like a diac with a gate terminal. A triac can be turned on by a pulse of gate current and does not require the breakover voltage to initiate conduction, as does the diac.
Basically, a triac can be thought of simply as two SCRs connected in parallel and opposite directions with a common gate terminal. Unlike the SCR, the triac can conduct current in either direction when it is triggered on, depending on the polarity of the voltage across its \(A_1\) and \(A_2\) terminals.
As with other thyristors, the triac ceases to conduct when the anode current drops below the specified value of the holding current \(I_H\). The only way to turn off the triac is to reduce the current to a sufficiently low level.