Triac is an abbreviation for triode for alternating current. Terms such as bidirectional thyristor triode or symistor can also be found. It is an electronic component with a semiconductor layer structure, which in principle represents an anti-parallel connection of two thyristors. This makes it possible to switch both half-waves with alternating current, whereas a single thyristor can only switch one half-wave and act like a diode in the reverse direction with the other half-wave. The triac is ignited via the gate and remains conductive until the holding current falls below the limit.
A triac has one control electrode G (gate) and two main electrodes H1 and H2 (in the main terminal, MT1 and MT2), with main electrode H2 (MT2) usually having a direct connection to the housing. To ensure that one control connection is sufficient for the two thyristors, two ignition or auxiliary thyristor sections are installed in Triacs so that it can be tilted into the low-impedance state with positive and negative control pulse.
The opto-triac (also called photo-triac) consists of two anti-parallel photo thyristors that are optically switched on. Instead of the control electrode, a light-emitting diode is used to flood the p-n transitions with charge carriers via the photoelectric effect and thus make them conductive. This is a way of creating a galvanic isolation between the control and load circuits, which means that loads such as the mains-powered light bulbs of a light organ can be controlled without the mains voltage posing a danger to the control circuit.
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Application of Triac
The areas of application are mainly in the area of phase angle controls as a typical variant of power control in the AC voltage range, such as for brightness control of incandescent lamps (dimmers), light or speed control of universal motors. The latter are very often used, for example, in power tools, electromotive household appliances and much more, where low-loss speed setting and high starting torques are required. Opto-triacs are used in semiconductor relays, where they are used for galvanically isolated switching of the actual switching element (power thyristors or triacs). Due to their compact design, low manufacturing costs and the fact that an arbitrarily high insulation voltage can be achieved by using fiber optics, opto-triacs have replaced the ignition transformers previously used for this purpose. In rare cases – when only a small current (up to approx. 100 mA) needs to be switched – an opto-triac can also be used to switch the load directly.
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