Conduction angle Firing angle Balance of waveform  applied to load To Applied anode cathode voltage 180° SCR blocks until gate voltage is applied SCR blocks this half cycle 30° firing 90° firing Average voltage Average voltage Shaded area represents voltage applied to the  load. The earlier the SCR is fired, the higher  the output voltage. D$_{1}$ R$_{1}$ R$_{2}$ R$_{3}$ D$_{2}$ D$_{1}$ A C Controlled DC output G SCR Output waveform Power  source MT$_{1}$ MT$_{2}$ Base Main terminal 2 Gate Main terminal 1 Output waveform Figure 11-31.  Phase control is a key application for SCR. By limiting the percentage of a full cycle of AC voltage that is applied to a load,  a reduced voltage results. The firing angle or timing of a positive voltage pulse through the SCR’s gate latches the device open allowing  current flow until it drops below the holding current, which is usually at or near zero voltage as the AC cycle reverses.  Figure 11-32.  A triac is a controlled semiconductor device that allows current flow in both directions.  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. 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  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.  11-15