TRIAC is an abbreviation for a TRIode AC switch. The triac is equivalent to two back to back SCRs connected with one gate terminal as shown in figure. It is a bidirectional device that can pass the current in both forward and reverse biased conditions and hence it is an AC control device. The triac is an important member of the thyristor family of devices. These are often used in motor speed controllers, AC circuits, pressure control systems, light dimmers and other AC control equipments. The TRIAC is a bidirectional switching device that can control the AC power efficiently and accurately. However, this structure can be replaced by special semiconductor device known as a TRIAC to accomplish the bidirectional control. To achieve this with SCRs, two SCRs must be connected in anti-parallel to control over both positive and negative half cycles of the input. But for many applications, bidirectional control of current is required, particularly in AC circuits. Mode 4: MT2 is Negative, Negative Gate CurrentĪs we know that the SCR as a unidirectional device and has a reverse blocking characteristics that prevents the current flow in reverse biased condition.Mode 3: MT2 is Negative, Positive Gate Current.Mode 2: MT2 is Positive, Negative Gate Current.Mode 1: MT2 is Positive, Positive Gate Current.
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