Voltage Stabilizer The figure below shows the working model of a Voltage Stabilizer that contains a step-down transformer (usually provided with taps on secondary), rectifier, operational amplifier/microcontroller unit and set of relays. Operation of Voltage Stabilizer In this, op-amps are tuned in such way that they could sense various set voltages such as lower cut off voltage, boost condition voltage, normal operating voltage, higher cut off voltage and buck operating voltage. A set of relays are connected in a manner that they trips the load circuit during higher and lower cut off voltages and also they switch buck and boost voltages to the load circuit. A step-down tap changing transformer has different secondary voltage tapping which are helpful for operating operational amplifier for different voltages and also to add-up and deduct voltages for boost and buck operations respectively. A rectifier circuit converts AC supply into DC to power-up entire electronic control circuit as well as relay coils. Let us assume that this is 1 KVA single phase stabilizer that provides stabilization for voltage range of 200 to 245 with a boost-buck voltage of 20-35 V for input voltage of 180 to 270 V. If the input supply is, say 195 V, then operational amplifier energizes boost relay coil such that 195 + 25 = 220V is supplied to the load. If the input supply is 260 V, corresponding op-amp energizes buck relay coil so that 260-30 = 225 V is supplied to the load. If the input voltage is below 180 V, corresponding op-amp switches lower cut off relay coil such that load is disconnected from the supply. And if the supply is beyond 270 V, corresponding op-amp energizes higher cut off relay coil and hence load is terminated from the supply. All these values are approximate values; it may vary depending on the application. By this way, a stabilizer operates under different voltage conditions. Servo Controlled Voltage Stabilizers In case of automatic voltage stabilizers, the speed of voltage correction is very less. The high speed voltage correction at a greater precision is achieved with servo controlled stabilizers. In servo controlled stabilizers, voltage correction is done very precisely, i.e., closer to the base voltage value. The main components of a servo stabilizer include servo motor driven continuously variable auto transformer, buck-boost transformer and solid state control circuit as shown in the figure below. Servo Controlled Stabilizer In this stabilizer, solid state control circuit sense voltage fall and rise from a predetermined value and correspondingly operates the servo motor. The primary winding of buck-boost converter is connected to the motorized auto transformer whereas its secondary is connected in series with incoming supply. Whenever motor operates the autotransformer, appropriate voltage is supplied to the primary of the buck-boost transformer and hence corresponding secondary voltage corrects the supply voltage to the load. Here comparators (nothing but op-amps) in solid state control circuit senses the voltage changes and activates servo motor to a desired location so that variable transformer increase or decrease the output voltage to the load. When the control circuit finds output voltage higher than a reference voltage, it feeds the positive signal to the servo motor controller and hence the arm rotates till the two voltages are equal. If the output voltage falls below the reference value, a negative signal goes to the servo motor such that arm rotates the contact in other way so as to reduce the voltage. Servo stabilizers can produce output regulation of ±0.5% with high efficiency about 98%.