Sketch and describe Automatic Voltage Regulator (AVR)

A small transformed, rectified and smoothed power supply is taken from the alternator output in the AVR. The d.c current is applied to a bridge which has two fixed resistance in two arm and two variable resistance (zener diode) in other two arm. The zener diode has a reverse breakdown voltage which is very low. Here, voltage remains constant, but when breakdown occurs, then change in current. At the same time, voltage and current passing through the resistance. As per whitestone bridge principle, imbalance of the resistance will cause a current flow which is error signal.The error signal will be amplified in silicon control rectifier and used to control alternator excitation.

Thyristor is used in this fig. Thyristor has a triggering circuit with gate v/v. If any deviation occurs of the voltage, then that error signal acts on the thyristor gate v/v which increase or decrease the excitation voltage as per required.