Modern A.V.R. (Zener Bridge)

                               Modern A.V.R. (Zener Bridge)

                   

           Voltage across the Zener diodes remains almost constant independent of current variations. Smoothed D.C. output is applied to the voltage reference bridge. This bridge is balanced at the correct generator voltage output with no potential difference between 'A' and 'B'.

If the generator voltage fails, current through the bridge arms falls and current flows from 'A' to 'B' through the amplifier.

If the generator voltage falls, current through the bridge arms falls and current flows from 'B' to 'A' through the amplifier.

If the generator voltage rises, Current through the bridge arms rises with current flow from 'A' to 'B' through the amplifier.

The signal from the amplifier will automatically vary the field excitation current, usually through a silicon controlled rectifier ( Thyristor) control element. 

The Silicon Controlled Rectifier (Thyristor) is a four layer, three terminal, solid state device with the ability to block the flow of current, even when forward biased, until the gate signal is applied. This gate signal could come from a Zener diode Voltage reference bridge. The gate signal will switch on the forward biased S.C.R. and current flows through the exciter field. When reverse biased the S.C.R. will again block current flow. Due to inductance of the field winding the S.C.R. would continue to pass current for a part of the negative cycle. By fitting a 'free wheeling' diode the current though the Thyristor falls quickly at the end of the positive cycle. In some circuits the excitation current is designed to be excess of requirements, so that the gate signal reduces flow.