Products in the DIAC and SIDAC family are two-terminal devices most commonly used as triggering mechanisms in AC phase control applications. They act to oppose current flow until the voltage applied across them reaches some critical value, at which point current flow through them increases significantly. The two differ in the shape of their characteristic curves; DIACs (Diodes for Alternating Current) generally exhibit higher forward voltages in their conductive mode, and thus tend to be less suitable than SIDACs (Silicon Diode for Alternating Current) when substantial current flow is an application requirement.
