Fully automatic contactless AC voltage regulator circuit

This circuit can adjust the output voltage accurately and quickly when the mains voltage changes within the range of 150V to 300V. The circuit is as shown in the figure. The mains power enters VD1 to VD4 through L1 and is rectified by the full bridge and then applied to the thyristor VT. The conduction angle is controlled by BT33. BT33, C, R1, R2, R3 and KP form a relaxation oscillator. Changing KP can change the oscillation period. When the mains voltage rises, the voltage applied to BT33 rises accordingly, the oscillation accelerates, the VT conduction angle decreases, and the voltage on VT rises, leaving the voltage on L1 unchanged; conversely when the voltage on VT drops The voltage on L1 remains constant; so the voltage coupled through L2 also remains constant. Due to the use of full-bridge rectification, the voltages on L1 and L2 are both sine waves. The current on VD1, VD4 and VT should be selected according to the capacity of the transformer and should have a large margin. The withstand voltage should be above 600V. Use 3W resistors for R1 and R2, and it is best to use an adjustable resistor above 2W for KP.