The automatic stabilizer is used to keep the secondary load voltage as constant say 220±9
While the supply voltage is high the zener diode operates and off the relay. Now A.C. can be obtained from the upper relay point which is actually 200V point. It can be operated to function with the help of preset If the supply voltage is always on high voltage side the middle and top connections of the transformer may be interchanged to obtain the constant voltage on secondary side.
Fig. 18.24. Automatic Voltage Stabilizer. Input—IB0 V—250 V, Output—220 9