Protect your LCD TV from Over voltage. After a power failure or during lightening, very high voltage surge develops in the Mains line which is the major cause that damage TV. Plasma and LCD TV are costly devices and uses SMPS type power supply. Use of Stabilizer is recommended for LCD TV. This low cost circuit can do the job of protection very well. When the mains voltage increases above 250 volts, it will drop most of the voltage around 100 volts passing through the load and the condition remains as such till normal voltage level attains.
A Relay and a single 5K 10 Watt resistor are doing the trick.
1. The Relay driver circuit obtains power from a 9-0-9, 500 mA transformer, diodes IN4007 and smoothing capacitor C1.
2. Actuation of the relay can be set using the pot VR1. Adjust it to actuate the relay at 250 volts. So when the voltage rises above this limit, relay will be activated.
3. The load is connected through the Common and NC( Normally Connected) contacts of the relay, so that when the relay is off at normal 230 volts, load works normally.
4. A 5K 10 Watts resistor is connected between the Common and NC contacts of the relay.
5. Now see the action. When the voltage is below 250 volts, relay will be off and the load gets full power through the common and NC contacts. When the voltage is above 250 volts, relay triggers and the NC contact breaks. Now the current passes through the 5K resistor to the load. The 5K resistor drops around 100 volts from mains. That is ,if the increased voltage is 260 volts, the load gets only 160 volts.
Then how can we set VR1 for 250 volts. Just do this, before connecting AC and load, adjust VR1 till relay clicks and LED lights. This is the normal voltage level. Slightly turn the pot and fix a position ( still the relay is activated). This may be for 250 voltage or slightly high or less. Then connect load.
The circuit can be enclosed in a switch box with 3 pin socket. So that the TV can be plugged into it. Any load such as Computer, Fridge, Printers, Home theater system etc can be protected like this. Select the Relay suitable for the load. Ordinary relay can handle up to 4 amperes. If the load is higher ,use a 16 ampere Industrial relay.