If MT2 is positive and MT1 is negative the first SCR conducts and if the MT2 terminal is negative and MT1 is positive then second SCR conducts. When triggered one of the two SCR inside, the TRIAC begins to conduct based on the polarity of the MT1 and MT2 terminals. To turn on a TRIAC a positive or negative gate voltage/pulse has to be supplied to the gate pin of the TRIAC. As you can see the TRIAC mostly operates in the 1 st Quadrant and the 3 rd Quadrant. Since the TRIAC is just a combination of two SCR in anti-parallel direction the V-I characteristics graph looks similar to that of an SCR. The turn on and turn off characterises of the TRIAC can be understood by looking at the VI characterises graph of the TRIAC which is also shown in the above picture. The below picture illustrates the status of TRIAC in each quadrant. Negative Voltage at MT2 and negative pulse to gate (Quadrant 4).Negative Voltage at MT2 and positive pulse to gate (Quadrant 3).Positive Voltage at MT2 and negative pulse to gate (Quadrant 2).Positive Voltage at MT2 and positive pulse to gate (Quadrant 1).Thus this puts the TRIAC into four operating modes as listed below For a TRIAC this trigger voltage that is to be applied to the gate terminal can either be positive or negative with respect to terminal MT2. Since the TRIAC is a bi-directional device the current can either flow from MT1 to MT2 or from MT2 to MT1 when the gate terminal is triggered. The symbol and pin out of TRIAC is shown below. It is a three terminal switching device similar to SCR (Thyristor) but it can conduct in both the directional since it construct by combining two SCR in anti-parallel state. The term TRIAC stands for TRIode for Alternating Current. Due to this property TRIAC is exclusively used where sinusoidal AC supply is involved. In this tutorial we will learn about the TRIAC, which is a bidirectional device meaning it can conduct in both the direction. Each device has its own unique property and thus they have their own specific applications. Power electronic switches like BJT, SCR, IGBT, MOSFET, and TRIAC are very important components when it comes to switching circuits like DC-DC converters, Motor Speed Controllers, Motor Drivers, and Frequency Controllers etc.
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