Triac-Construction and Operation

  
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The triac is another three-terminal ac switch that is triggered into conduction when a low-energy signal is applied to its gate terminal. Unlike the SCR, the triac conducts in either direction when turned on. The triac also differs from the SCR in that either a positive or negative gate signal triggers it into conduction. Thus the triac is a three
Triac-Construction and Operation - schematic

terminal, four layer bidirectional semiconductor device that controls ac power whereas an SCR controls dc power or forward biased half cycles of ac in a load. Because of its bidirectional conduc ­tion property, the triac is widely used in the field of power electronics for control purposes. Triacs of 16 kW rating are readily available in the market. Triac  is an abbreviation for three terminal ac switch. Tri`-indicates that the device has three terminals and ac` indicates that the device controls alternating current or can conduct in either direction. As mentioned above, triac is a three terminal, four layer bilateral semiconductor device. It incorporates two SCRs connected in inverse parallel with a com ­mon gate terminal in a single chip device. The arrangement of the triac is shown in figure. As seen, it has six doped regions. The gate terminal G makes ohmic contacts with both the N and P materials. This permits trigger pulse of either polarity to start conduction. Electrical equivalent circuit and schematic symbol are shown in figure. b and figure. c respectively. Since the triac is a bilateral device, the term anode  and cathode  has no meaning, and therefore, terminals are designated as main terminal 1. (MT1), main terminal 2 (MT2) and gate G. To avoid confusion, it has become common practice to specify all voltages and currents using MT1 as the reference. Though the triac can be turned on without any gate...



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