What is the difference between an SCR and a Triac?
An SCR is a unidirectional thyristor and a triac is bidirectional thyristor. A positive SCR gate signal turns that device on. The triac is similar to two reverse parallel SCRs, and can conduct in each direction. See the figure below. The main SCR circuit is anode to cathode and is controlled by a positive gate voltage. The triac conducts in both directions. When MT2 is positive relative to MT1, a positive gate current will cause conduction; and when MT1 is positive relative to MT2, a negative gate signal will cause conduction. See figure below.
