A triac is a solid-state device that performs the same function as a relay, without the moving parts. Its construction is T-1 (terminal), T-2, and a gate. Normal state of a triac is “open” between T-1 and T-2. Applying a small AC voltage to the gate activates it, which then “shorts” or closes the two terminals and allows current to pass. Because there are no moving parts, a triac has no contacts and will not “pit” and wear out like a relay can.