We can control the power delivered to an AC load using a phase control circuit. The AC waveform is controlled by the phase control circuit by cutting the cycle to give zero cycle, half cycle, full cycle, or somewhere in between. This circuit is looks like a dimmer circuit, but it has a litle difference. In this circuit, the switching is synchronized with the zero crossing of the waveform. Here is the circuit :
In this circuit, the triacs doesn’t suffer power dissipation because of switching the power in zero crossing condition. so that the the overall efficiency rises. This phase control circuit is suitable for heater filament, incandescent lamps or brushed AC motor.
U208B is designed as a phase control circuit in bipolar technology. It has internal supply-voltage monitoring. This internal monitoring is used to avoid uncontrolled output pulse. Furthermore, it has voltage and internal-current synchronisation. We can use it as a low cost open-loop control. [Circuit's schematic diagram source: TEMIC TELEFUNKEN Microelectronic Application Notes]
