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Here we will study the design of a simple automatic mains AC voltage stabilizer which can be applied for safeguarding appliances like TV and refrigerators from fluctuating voltages. A voltage stabilizer is a device which is designed to sense inappropriate voltage fluctuations in AC mains supply inputs, and correct them to produce a stabilized voltage for the connected appliances or gadgets. How the Circuit Functions Referring to the figure we find that the proposed automatic voltage stabilizer circuit is configured with the single opamp IC 741. It becomes the control section of the whole design.The opamp is wired as a comparator, we all know how well this mode suits the IC 741 and other opamps. It's two inputs are suitable rigged for the said operations. Pin #2 of the IC is clamped to a reference level, created by the resistor R1 and the zener diode, while pin #3 is applied with the sample voltage from the transformer or the supply source. This voltage becomes the sensing voltage for the IC and is directly proportional to the varying AC input of our mains supply. The preset is used to set the triggering point or the threshold point at which the voltage may be assumed to be dangerous or inappropriate. We will discuss this in the setting up procedure section. The pin #6 which is the output of the IC, goes high as soon as pin #3 reaches the set point and activates the transistor/relay stage. In case the the mains voltage crosses a predetermined threshold, the ICs non inverting detects it and its output immediately goes high, switching ON the transistor and the relay for the desired actions. The relay, which is a DPDT type of relay, has its contacts wired up to a transformer, which is an ordinary transformer modified to perform the function of a stabilizer transformer. It’s primary and secondary winding are interconnected in such a manner that through appropriate switching of its taps, the transformer is able to add or deduct a certain magnitude of AC mains voltage and produce the resultant to the output connected load. The relay contacts are appropriately integrated to the transformer taps for executing the above actions as per the commands given by the opamp output. So if the input AC voltage tends to increase a set threshold value, the transformer deducts some voltage and tries to stop the voltage from reaching dangerous levels and vice versa during low voltage situations. Complete Circuit Diagram Opamp Calculations If a resistor divider was used instead of a zener at pin#2, the relationship between the reference level at pin#2 of the opamp with the resistor divider and Vcc could be given as: Vref (R2 / R1 + R2) x Vcc Where R2 is the resistor used instead of Z1. Op-Amp Hysteresis Resistor Calculations To calculate the resistor values for an op-amp hysteresis circuit, we'll use the following formulas: Upper Threshold Voltage (VTH): VTH VREF * (R1 + R2) / R1 Lower Threshold Voltage (VTL): VTL VREF * R2 / R1 Hysteresis Voltage (VHYS): VHYS VTH - VTL VREF * R2 / R1 Where: VTH: Upper threshold voltage VTL: Lower threshold voltage VHYS: Hysteresis voltage VREF: Reference voltage (usually the supply voltage or a voltage divider output) R1: Resistor connected between the non-inverting input and ground R2: Resistor connected between the non-inverting input and the output Transformer Relay Wiring Diagram Parts List You will require the following components to make this homemade automatic mains voltage stabilizer circuit: R1, R2 10K, R3 470K or 1M, (lower values will enable slower voltage corrections) C1 1000 uF / 25 V D1, D2, D3 1N4007, T1 BC547, TR1 0 – 12 V, 500 mA, TR2 9 – 0 – 9 V, 5 Amp, IC1 741, Z1, Z2 4.7V/400mW Relay DPDT, 12 V, 200 or more Ohms,Approximate Voltage Outputs for the Given Inputs Stabilized Output Vs UnStabilized Input Voltage Proportions INPUT------OUTPUT 200V -------- 212V210V -------- 222V220V -------- 232V225V -------- 237V230V -------- 218V240V -------- 228V250V -------- 238V How to Set Up the Circuit The discussed simple automatic voltage stabilizer circuit may be set up with the following steps: Initially do not connect the transformers to the circuit, also keep R3 disconnected. Now, using a variable power supply, power the circuit across C1, the positive of the supply goes to the pin#7 line of the opamp while the negative goes to the negative pin#4 line of the opamp. Set the voltage to about 12.5 voltage and adjust the preset so that the output of the IC just becomes high and triggers the relay. Remember, here we have assumed that the DC output 12.5V from TR1 corresponds to around 225V AC input from mains. For your circuit be sure to confirm this before doing this setup procedure. Meaning, if suppose you find that your TR1 DC output corresponds to 13V for an input of 225V, then complete this procedure using 13V....and so on. Now lowering the voltage to about 12 volts should make the opamp trip the relay to its original state or make it de-energized. Repeat and check the relay action by altering the voltage from 12 to 13 volts, which should make the relay flip flop correspondingly. Your setting up procedure is over. Now you may connect both the transformer to its appropriate positions with the circuit, and also restore the R3 and the relay connections across their original points Your simple home made mains voltage stabilizer circuit is ready. When installed, the relay trips whenever the input voltage crosses 230 volts, bringing the output to 218 volts and keeps this distance continuously as the voltage reaches higher levels. When the voltage drops back to 225, the relay gets de-energized pulling the voltage to 238 volts and maintains the difference as the voltage further goes down. The above action keeps the output to the appliance well between 200 to 250 volts with fluctuations ranging from 180 to 265 volts. Warning: A single wrong connection could lead to a fire hazard or explosion, therefore please proceed with caution. Always use a 100 watt protection bulb in series with the one of the mains line which goes to the stabilizer transformer initially. Once the operations are confirmed, you can remove this bulb. 2) The entire circuit is not isolated from mains, therefore users are advised to maintain extreme caution while testing the unit in an uncovered position and while powered ON, to avoid lethal electric shocks.

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