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What is TRIAC?
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TRIAC is two thyristors are connected in parallel and Inverse with only one gate terminal.
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three terminal electronic component that conducts current in either direction when triggered. Its formal name is bidirectional triode thyristor or bilateral triode thyristor
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TRIAC Structure: 


Operating Principle: A triac is a three terminal, five layer semiconductor device. It is equivalent to two separate SCRs connected in anti-parallel with gates common. It acts like a bidirectional switch i.e. it can conduct current in either direction regardless of polarities. It can be turned ON either with a positive or negative current pulse at the gate terminal. Once the triac is fired into conduction, the gate losses all control over the conduction and triac continues to conduct. It can be turned OFF by reducing the current to the value less than holding current. With MT2 terminal positive with respect to MT1 terminal, the junctions J1 and J3 get forward biased but the junction J2 get reverse biased. Due to this reverse biased junction J2 the triac can not conduct. However, when the positive or negative gate current pulse is applied to gate, the carriers are injected at the reverse biased junction J2. Due to this, the junction J2 loses its identity as reverse biased junction, the carriers cross over the junction J2 and the triac is turned on. The current then flows from MT2 to MT1, as shown in the figure. With reversed polarities i.e MT1 positive with respect to MT2, the same phenomenon takes place when positive or negative gate current pulse is applied. 

Applications: i. A high power lamp switch. ii. Electronics changeover of transformer taps. iii. As light dimmers. iv. Speed control for electric fans and other electric motors. v. Heating control vi. Zero voltage switched relay

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