TC 9-62
Triac
3-48. The TRIAC is a three-terminal device similar in construction and operation to the
SCR. The triac controls and conducts current flow during both, instead of only one,
alternations of an AC cycle. See Figure 3-23 for a comparison of the schematic symbols for
the SCR and the triac. Both the SCR and the triac have a gate lead. However, in the triac
the lead on the same side as the gate is "main terminal 1," and the lead opposite the gate is
"main terminal 2." This method of lead labeling is necessary because the triac is essentially
two SCRs back-to-back, with a common gate and common terminals. Each terminal is, in
effect, the anode of one SCR and the cathode of another, and either terminal can receive an
input. In fact, by connecting two actual SCRs (see Figure 3-24), the functions of a triac can
be duplicated. The result is a three-terminal device identical to the triac. The common
anode-cathode connections form main terminals 1 and 2 and the common gate forms
terminal 3.
Figure 3-23. Comparison of SCR and Triac Symbols
Figure 3-24. Back-to-back SCR Equivalent Circuit
3-16
TC 9-62
23 June 2005
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