Q.What is the function of AC voltage controller?

An AC voltage controller provides a variable alternating voltage with no change in frequency. It is useful for domestic and industrial heating and lighting, where the RMS voltage delivered to the load must be adjusted while the supply frequency stays fixed. A fixed-AC-to-variable-AC converter is therefore called an AC voltage controller.

The variable voltage is obtained with a bidirectional electronic switch — a TRIAC — connected between the AC supply and the load. Because the TRIAC can conduct in both directions, it can control power over both half-cycles of the supply. By varying the firing instant each half-cycle, portions of the supply sinusoid reach the load while the switch blocks the rest, so the RMS load voltage (and hence the power) is controlled. …