The automatic stabilizer is used to keep the secondary load voltage as constant say 220±9%, or so, whatsoever the input voltage may be (Generally ranging from 180 V to 250 V). It employs a voltage sensing circui comprising of zenor diode and BC-148. This circuit is used for SK-100 and the relay. The zener diode does not function during normal voltage. This keeps BC- 148B off while second BC- 148, SK- 100 and relay is functioning in ON State.
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% V