on for a greater or smaller portion of the half cycle.
The SCRs accomplish power control because they
Figure 1-46 shows how power can be increased as the
are rectifiers and in an ac circuit conduct only during
firing point is moved along the phase time axis.
half of each cycle and then only after being turned on
by a positive gate pulse (from the trigger amplifier).
The firing point is determined by the position (or
timing) of a spiked gate pulse. When applied to the SCR,
Power control is accomplished by switching the power
Figure 1-46.-Phase shift of gate pulse.