A Triac is equivalent to two thyristors connected back to back as shown in Fig. Thus, it is a directional switching device, in contrast to the thyristor, which is a unidirectional device, having reverse blocking characteristic, preventing the flow of current from Cathode to Anode. So, when it (triac) is in conduction mode, current flows in both directions (forward and reverse). This switching device is called as TRIAC (TRIode AC switch), with the circuit symbol shown in Fig. The three terminals of the triac are designated as , and gate, shown in the same figure. These are similar to the terminals – A (Anode), K (Cathode) and G (Gate), of the thyristor The terminal, is taken as the reference point for the measurement of the voltages and currents at other two terminals, G (gate) and . The gate (G) is near to the terminal. The thyristor conducts with the current direction from Anode to Cathode (positive), when a positive pulse is fed at the Gate terminal with respect to Cathode, and at that time, with positive voltage applied between Anodeand Cathode terminals, being connected in series with the load. The triac conducts in the MT1 MT2 G MT1 MT2 MT1 positivedirection from to , when a MT2 MT1 positivepulse is applied at the gate (G) terminal with respect to and at the same time, the positive voltage is applied between two terminals, (+) and (-). Similarly, the triac conducts in MT1 MT2 MT1 negativedirection from to , when a MT1 MT2 negativepulse is applied at the gate (G) terminal with respect to and at the same time, the positive voltage is applied between two terminals, (+) and (-). Please note that the voltage between two terminals, and , is MT1 MT1 MT2 MT2 MT1 negative, in this case. So, the triac can conduct in both directions (positive and negative) as given here, whereas the thyristor conducts in one (positive) direction only. Only one triac is needed, whereas it is to be replacedby two thyristors, with consequent change in the control circuit. The V-I characteristics of both thyristor and triac. A thyristor turns off (non-conducting mode), if the current through it, falls below holding current. Similarly, a triac turns off (non-conducting mode), if the magnitude of the current, irrespective of its direction, falls below holding current. As a triac is connected in an ac circuit, and if the load in the circuit is resistive, the triac turns off at the zero crossing points of the voltage in each half (the supply (input) voltagereaches zero at the end of each half cycle). This will be nearly valid, if the load inductance issmall, though the triac in that case turns off, as the current though it goes to zero, after the zero crossing point is reached in each half. The case of higher inductance in the load has been discussed in detail in lesson #26 (module 3). The triac is a low power device, used in voltagecontrol circuits, used as light dimmers, speed control for fan motors (single-phase), etc. Some of the advantages and disadvantages of the triac.
