Functionally, a triac may be considered  as  two  parallel SCR (p-n-p-n)  structures oriented  in opposite directions, as shown in Fig. 30. The same approach used to explain gating, latching, and holding currents in the SCR can be extended to include the two-SCR model of a triac.  In triacs, the gate-trigger-pulse  polarity is usually measured with respect to main terminal No.1, which is comparable to the cathode terminal of an SCR. The triac can be triggered by a gate-trigger pulse which is either positive or negative with  respect to main terminal No.1 when main terminal No.2 is either positive or negative with respect to main terminal No. 1. The triac, therefore, can be triggered in any of four operating modes, as summarized in Table I.

The quadrant designations refer to the operating quadrant on the principal voltage-current characteristics (either I or III), and the polarity symbol represents the gate-to-main-terminal-No. 1 voltage. Fig. 31 shows theflow of current in a triac for each of the four triggering modes. The gate-trigger requirements of the triac are different in each of operating mode. The I (+) mode (gate positive with respect to main terminal No.1 and main terminal No.2 positive with respect to main terminal No. 1), which is comparable to equivalent SCR operation, is usually the most sensitive. The smallest gate current is required to trigger the triac in this mode. The other three operating modes require slightly higher gate-trigger currents. For RCA triacs, the maximum trigger-current rating in the published data is the largest value of gate current that is required to trigger the selected device in any operating mode.