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INDUSTRIAL CONTROL HANDBOOK - 3.3 TRIACS

The bidirectional triode thyristor (triac) is a solid-state device that acts like two SCRs that have been connected in parallel with each other (inversely) so that one SCR will conduct the positive half-cycle and the other will conduct the negative half-cycle. This means that the triac can be used for control in ac circuits. Before the triac was designed as a single component, two SCRs were actually used for this purpose.

Fig. 3-13 shows the symbol for the triac, and its pn structure. The terminals of the triac are identified as main terminal 1 (MT1), main terminal 2 (MT2), and gate. The multiple pn structure is actually a combination of two four-layer (pnpn) junctions.

FIGURE 3-14 Two SCRs connected in inverse parallel configuration to provide the equivalent circuit for a triac.FIGURE 3-14 Two SCRs connected in inverse parallel configuration to provide the equivalent circuit for a triac.

FIGURE 3-15 (a) A graph of the positive and negative voltage and current characteristics of a triac. (b) The four quadrants identified for the graph to describe the voltage polarity of the triac when it is in conduction.FIGURE 3-15 (a) A graph of the positive and negative voltage and current characteristics of a triac. (b) The four quadrants identified for the graph to describe the voltage polarity of the triac when it is in conduction. (Courtesy of Philips Semiconductors.)

The triac is required in circuits where ac voltage and current need to be controlled like the SCR controls dc current. Another difference between the triac and SCR is that the triac can be turned on by either a positive or negative gate pulse. The gate pulse need only be momentary and the triac will remain in conduction until the conditions for commutation are satisfied.

 

3.3.1 Operation of the Triac

The operation of the triac may best be explained by the two-SCR model in Fig. 3-14. From this figure you can see that the SCRs are connected in an inverse parallel configuration. One of the SCRs will conduct positive voltage and the other will conduct negative voltage. Unlike the two SCRs, the triac is triggered by a single gate. This prevents problems of one SCR not firing at the correct time and overloading the other. In the 1960s and 1970s when triacs were not available or were too small, two SCRs were actually connected together and used as a device to control current in an ac circuit.

The firing of the triac can be described by the diagram in Fig. 3-15. In this figure you can see that the triac can conduct both positive and negative current. The graph uses typical identification for its four quadrants. Voltage is shown along the horizontal x-axis, and current is shown along the vertical y-axis. This diagram also shows a second graph with the four quadrants identified. These quadrants will be used to explain the operation of the triac as polarity to its MT1, MT2, and gate changes.

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