A TRIAC, or TRIode for Alternating Current is an electronic component approximately equivalent to two
silicon-controlled rectifiers (SCRs/ thyristors) joined in inverse parallel (paralleled but with the polarity reversed) and with their gates connected together. Formal name for a TRIAC is bidirectional triode thyristor. This results in a bidirectional electronic switch which can conduct current in either direction when it is triggered (turned on). It can be triggered by either a positive or a negative voltagebeing applied to its "gate" electrode (with respect to A1, otherwise known as MT1). Once triggered, the device continues to conduct until the current through it drops below a certain threshold value, such as at the end of a half-cycle of alternating current(AC) mains power. This makes the TRIAC a very convenient switch for AC circuits, allowing the control of very large power flows with milliampere-scale control currents. In addition, applying a trigger pulse at a controllable point in an AC cycle allows one to control the percentage of current that flows through the TRIAC to the load (so-called "phase control").
Low power TRIACs are used in many applications such as light
dimmers, speed controls for electric fans and other electric motors, and in the modern computerized control circuits of many household small and major appliances. However, when used with inductive loads such as electric fans, care must be taken to assure that the TRIAC will turn off correctly at the end of each half-cycle of the ac power.
snubbercircuit is often used to assist this turn off. Snubber circuits are also used to prevent premature triggering. For higher-powered, more-demanding loads, two SCRs in inverse parallel may be used instead of one TRIAC. Because each SCR will have an entire half-cycle of reverse polarity voltage applied to it, turn-off of the SCRs is assured, no matter what the character of the load.
* [http://www.du.edu/~etuttle/electron/elect5.htm A site about thyristors]
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