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Topic: Circuit to swap to battery power (Read 2 times) previous topic - next topic

moonunit

Hello.

Have you had any luck with this? I am looking to do the exact same thing. I plan on running a project from mains, but I would like a battery to step in an power everything if there is an outage.

It would be extra great if the mains power kept the battery charged as well. :)

Thanks.

ill_switch

I designed a circuit to do this (though with NiMH batteries), had the PCB made, bought the parts, but never assembled or tested it. Here is that thread:

http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?num=1269960867

Drmn4ea

This sounds like something that can be done with a P-channel MOSFET. Think of it as a transistor* that turns ON when its gate (base) is grounded and OFF when brought near its drain voltage. Tie the gate to the wall-wart input and a pull-down resistor to ground, it should cut off the battery supply whenever the wall wart voltage rises to near the battery voltage or above.

*Unlike a transistor though, the gate of a MOSFET is essentially an insulated capacitor plate and won't transfer any charge to its source/drain (unless something goes seriously wrong!). Also, it won't incur the 0.7V or so diode voltage drop that a transistor does.

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