LCD TV Protector. Design Note 25

SIMAGEometimes you need a simple circuit to protect the Electronic gadgets like LCD TV from voltage fluctuations. Normal domestic power supply is 230 volts at 50Hz in most countries. But this voltage may not remain steady and may go as high as 250 volts or as low as 180 volts. Complaints in the distribution transformer, operation of heavy current appliances like flour mill, welding machine etc in the nearby places can cause severe voltage fluctuations in the lines. A good stabilizer with Buck / Boost facility can solve the problem, but still it is interesting to make this circuit and it costs less than Rs.100.It cut off the power supply to the gadget connected when the voltage increases above 230 volts and decreases below 200 volts.

You need the following components
1. 0-12 volt 500 Milli Ampere transformer- 1No
2. IN4007 Diodes- 2 Nos
3. 470 uF 25 volt Electrolytic capacitor-1No.
4. 10 uF 25 volt Electrolytic capacitor-1No.
5. 10 K Pot / Preset – 2 Nos
6. LED Red – 5 mm -1No
7. LED Green-5 mm -1No
8. 1K ¼ Watt resistors – 4 Nos
9. 470 Ohms ¼ Watt resistor – 2 Nos
10. BC 548 Transistor – 2 Nos
11. 12 volt 200 Ohms, 2 Amps Relay – 1 No.
12. 2 Amps Fuse – 1 No.
13. Common PCB- Small Size-1No
14. Connecting Wires – 1mm gauge electric wire, Thin plastic wires.
15. LED Holder – 5 mm- 2 Nos
16. Plastic Switch box with 3 Pin Socket provision- 1No
Circuit working

Power supply to the circuit is derived from a 0-12 volt 500 mA transformer. Note that, only a half wave rectification using Diode D1 is done which is essential to achieve the result. That means, when the input voltage in the transformer Primary increases or decreases, a corresponding change occurs in the Secondary also which reflects in the circuit power supply. Capacitor C1 smoothes the power supply from ripples. Green LED indicates the Power On status. Circuit works on the basis of the switching action of T1 and T2. When the voltage level is normal (200 – 230 Volts), Transistor T1 remains off (as adjusted by VR1). This allows T2 to conducts because, it gets base bias through R4. The relay then triggers and connects the power to the load.


When the voltage rises above 230 volts, more current goes to the base of T1 through R2 and VR1which switches on T1. Since the base of T2 is connected to the Collector of T1, it goes to the ground potential and T2 turns off. Relay then turns off and power to the load cut off.

When the voltage decreases below 200 volts (as adjusted by VR2), both T1 and T2 turns off and relay cut off to break the power to the load.

How to set

You need a Variable AC power supply for precise high and low voltage settings. Don’t worry you can make a crude setting. Before connecting the Phase and Neutral lines adjust VR1 and VR2 to set the voltage. Assuming that the domestic supply is between 200-230 volts, just turn VR1 just the Relay and Red LED turns On. So when the voltage rises above this level , relay will turns off. Similarly, adjust VR2 also till the relay and Red LED turns On.
If a variable power supply is available, provide 240 volts in the transformer primary and adjust VR1 till relay and Red LED turns off. Then provide 180 volts and adjust VR2 till Relay and Red LED turns off.


Phase line from the Primary of transformer should go to the Common point of Relay. From the NO (Normally Open) contact of the relay, solder a wire which should go to the Right pin of Socket. Take the Neutral wire from the second primary wire of transformer which should go directly to the Left pin of socket. So when the relay triggers circuit completes. Ground can be connected to the Screw of Transformer.


You can use a 12 volt PCB relay to make the unit compact. It has 5 pins. On one side ,there are three pins. Middle pin is Common for connecting AC phase. The pins on either side of it are DC connections. You can connect either way round because it is the coil connections. Two pins on the other side are NC (Normally Connected- To the Common pin) and NO (Normally Open- Connects with Common pin when Relay energize. You can easily identify the NC and NO pins through Continuity test using Multimeter. Connect one probe to the Common pin and touch the other ones with second probe. The pin that gives continuity is NC. So use the other NO pin to connect with the Socket.

12 Volt Relay Pins

If you have an old Stabilizer case, it will give enough space to enclose the circuit and transformer. Moreover, you will get AC cord and Socket attached to it.

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