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TRIAC, from triode for alternating current, is a generic trademark for a three terminal electronic component that conducts current in either direction when triggered. Its formal name is, bidirectional triode thyristor or bilateral triode thyristor. A thyristor is analogous to a relay in that a small voltage and current can control a much larger voltage and current. The illustration on the right shows the circuit symbol for a TRIAC where A1 is Anode 1, A2 is Anode 2, and G is Gate. Anode 1 and Anode 2 are normally termed Main Terminal 1 (MT1) and Main Terminal 2 (MT2) respectively. TRIACs are a subset of thyristors and are related to silicon controlled rectifiers (SCRs). However, unlike SCRs, which are unidirectional devices and only conduct current in one direction, TRIACs are bidirectional and conduct current in both directions. Another difference is that SCRs can only be triggered by a positive current at their gate, but, in general, TRIACs can be triggered by either a positive or negative current at their gate, although some special types cannot be triggered by one of the combinations. To create a triggering current for an SCR a positive voltage has to be applied to the gate but for a TRIAC either a positive or negative voltage can be applied to the gate. In all three cases the voltage and current are with respect to MT1. Once triggered, SCRs and thyristors continue to conduct, even if the gate current ceases, until the main current drops below a certain level called the holding current. Gate turn-off thyristors (GTOs) are similar to TRIACs but provide more control by turning off when the gate signal ceases. TRIACs bidirectionality makes them convenient switches for alternating-current (AC). In addition, applying a trigger at a controlled phase angle of the AC in the main circuit allows control of the average current flowing into a load (phase control). This is commonly used for controlling the speed of induction motors, dimming lamps, and controlling electric heaters. ==Operation== To understand how TRIACs work, consider the triggering in each of the four quadrants. The four quadrants are illustrated in Figure 1, and depend on the gate and MT2 voltages with respect to MT1. Main Terminal 1 (MT1) and Main Terminal (MT2) are also referred to as Anode 1 (A1) and Anode 2 (A2) respectively.〔 The relative sensitivity depends on the physical structure of a particular triac, but as a rule, quadrant I is the most sensitive (least gate current required), and quadrant 4 is the least sensitive (most gate current required). In quadrants 1 and 2, MT2 is positive, and current flows from MT2 to MT1 through P, N, P and N layers. The N region attached to MT2 does not participate significantly. In quadrants 3 and 4, MT2 is negative, and current flows from MT1 to MT2, also through P, N, P and N layers. The N region attached to MT2 is active, but the N region attached to MT1 only participates in the initial triggering, not the bulk current flow. In most applications, the gate current comes from MT2, so quadrants 1 and 3 are the only operating modes (both gate and MT2 positive or negative against MT1). In other applications, where gate is directly controlled by a digital circuit, usually MT1 is connected to positive voltage (e.g. +5V) and gate is pulled down to 0V, so operatiing in quadrants 2 and 3. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「TRIAC」の詳細全文を読む スポンサード リンク
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