“Usually, our home appliances run on single-phase AC voltage. And in India usually, a house is supplied with single-phase AC voltage with a voltage range of about 220-240 volts at 50Hz(your supply may be different). AC voltage in its first half cycle starts increasing the voltage from 0 to 220-240 as its peak voltage and when it reaches the peak voltage it starts decreasing from 220-240 to 0. Now in the next half cycle, the voltage starts increasing but in the negative direction and ultimately reaches to its peak (say -220v-240v) and again it will start decreasing and continue to reach zero voltage.
If we draw a graph of this voltage nature with respect to time. We will find it as a sine wave. So, in India, the supplied voltage is a sine wave with a 220-240v peak and a frequency of 50Hz. (check with your country’s supply voltage range)
Now controlling AC voltage means we are trying to control the speed of single phase ac motor or trying to control the brightness of an incandescent bulb or any other task which needs to control the AC voltage. So, to control the voltage we can not chop off the required voltage. First, we need to know the zero point of the voltage which means to detect the AC voltage when it reaches the zero point. And when we get the zero point at that time we can chop the voltage as required. To detect zero voltage crossing we will need a zero voltage crossing detector.
This is the main circuit of the project and needs to design it very carefully.”