Voltage controller

A voltage controller, also called an AC voltage controller or AC regulator is an electronic module based on either thyristors, triodes for alternating current, silicon-controlled rectifiers or insulated-gate bipolar transistors, which converts a fixed voltage, fixed frequency alternating current (AC) electrical input supply to obtain variable voltage in output delivered to a resistive load. This varied voltage output is used for dimming street lights, varying heating temperatures in homes or industry, speed control of fans and winding machines and many other applications, in a similar fashion to an autotransformer. Voltage controller modules come under the purview of power electronics. Because they are low-maintenance and very efficient, voltage controllers have largely replaced such modules as magnetic amplifiers and saturable reactors in industrial use.

Modes of operation
Electronic voltage controllers work in two different ways; either through "on-and-off control" or through "phase control".

On-and-off control
In an on-and-off controller, thyristors are used to switch on the circuits for a few cycles of voltage and off for certain cycles, thus altering the total RMS voltage value of the output and acting as a high speed AC switch. The rapid switching results in high frequency distortion artifacts which can cause a rise in temperature, and may lead to interference in nearby electronics. Such designs are not practical except in low power applications.

Phase angle control
In phase angle control, thyristors are used to selectively pass only a part of each AC cycle through to the load. By controlling the phase angle or trigger angle, the output RMS voltage of the load can be varied. The thyristor is turned on for every half-cycle and switched off for each remaining half-cycle. The phase angle is the position at which the thyristor is switched on.

Applications

 * Light dimming circuits
 * Temperature control of electrical heating systems
 * Speed control of motors
 * AC magnet controls