Wednesday, July 2, 2014

Auto Power Off Circuit To Protect Home Appliance


In this post I will be showing you how to make a simple circuit for protecting your appliances from sudden power loss and restoration. What often times happen is when the power returns the voltage level can be a bit too high. What this circuit will do is once the power is gone it will keep the load off until you manually switch it on. You can modify this circuit in many different ways.


What we will need?


Transformer: 

1 12-15V or anything suitable for chosen relay.

Diode:

5 1N4007

Capacitor:

2 100μF 25V
1 0.1μF 100V

Regulator:

1 7812

Relay:

1 12V SPDT relay.

Switch:

1 Momentary switch.



Circuit Diagram



Mechanism:

When the 220V AC IN is hooked up to power source, the load won't turn on because one of the wires is connected to the Normally open terminal of the relay. If we hit the "Press Button" once it will provide power to the load and high side of the transformer momentarily. But in the mean time the relay will turn on as the power was available for the transformer to drive it. If the press button is released afterwards the load will continue to run.

But if the main 220V AC IN is lost the relay will automatically go to normally closed and thus if the power is back again the load won't turn on unless the press button is pressed again.

Note and Warning: 

1. As this circuit has 220VAC connection with it make sure to be careful.
2. Before connecting this circuit to AC main power check the circuit thoroughly for shorted connection or wrong connection.
3. Before changing anything remove AC main power.
4. Make sure to use a relay that can handle the load you want to turn on.
5. All capacitors should be rated higher than source voltage. If you use 5V Relay and power supply 16V Capacitors will be fine.
6. Regulator 7812 shouldn't be that hot. In case its hot, attach a heat-sink to the regulator with some thermal paste.
7. If you don't want to make a full circuit you can simply use a 5/9/12V ready made power supply with the relay. Make sure to put the diode that is with the relay. Make sure to use current input voltage for the relay as well.
8. Press button should be such that it will withstand the load current.
9. Use Neon or LED indicators if you want.


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2 comments:

  1. what type of switch should i use at the relay side? is it the type used in automatic volume control? thanks

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    Replies
    1. You mean with the one in the right side of the relay? You can simply use a calling bell switch or some sort of press button that can withstand your load's current.

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