79-A type automatic AC regulated power supply circuit

It uses servo motor and transistor control.
    When the power supply voltage changes in the range of 160-260V, the output voltage can be maintained at 220V±3V. When
    RP is adjusted so that the output voltage is 220V, the potential difference between A and B is zero. At this time, the bridge circuit is in a balanced state,
the transistors VTi and Vtz are cut off, and the motor M does not move. When the voltage rises, VTz turns on and M reverses. It drives
the sliding arm of the voltage regulator to move in the direction of voltage reduction; when the voltage decreases, VTL is turned on. M forward rotation, the voltage regulation process is opposite to the above.