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is allowed to pass. The output waveform if triggered at 90° is shown in Figure 11-32 . Because a triac allows current to flow in both directions, the reverse cycle of AC voltage can also be used and controlled. 

When used in actual circuits, triacs do not always maintain the same phase firing point in reverse as they do when fired with a positive pulse. This problem can be regulated somewhat through the use of a capacitor and a diac in the gate circuit. However, as a result, where precise control is 

required, two SCRs in reverse of each other are often used instead of the triac. Triacs do perform well in lower voltage circuits.  Figure 11-33  illustrates the semiconductor layering in a triac. 

NOTE: The four layers of N- and P-type materials are not uniform as they were in previously described semiconductor devices. None the less, gate pulses affect the depletion areas at the junctions of the materials in the same way allowing current to flow when the areas are narrowed. 

