Hi,
I have an equipment which works with 12V DC with average current consumption of about 250mA. Made the DC power supply to give power from AC line. But want to make this equipment work with rechargeable battery (should automatically switch over to battery supply) when AC power fails. 4Hrs back up preferred. Can anybody give suggestions on how to do that in a simple way.

 

How fussy is your circuit vis a vis the voltage regulation at 12V? One idea is to select a battery, e.g. 12V SLA which nominally float charges at 13.5V or so, and run it and your circuit in parallel. When the AC goes off, the battery will keep the supply between 13.5V and about 12.5V (assuming you don't completely discharge the battery). When AC is restored, assuming that the regulator is modified for suitable current-limiting, the battery will recharge at the same time the circuit remains powered. This is the way that automobiles and airplanes work...

If your circuit requires a more constant voltage, then "diode or" a battery with the output of the full-wave rectifier upstream of your voltage regulator. The battery voltage would have to be higher than the final output voltage+the dropout voltage of the reg+one diode drop, however. You will have to add a suitable charger for the battery, too.

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Mike ML.

 

You can make a simple 12V battery with ten NI-CAD or NI-MH cells. The AA size will give you anywhere from 700 mA-Hour to 2000 ma-Hr for AA size. The 2000 mA-hr AA size NI-MH batteries are the ones they sell for digital cameras, would give you about 8 hours of run time. You can get a pretty good price on them at places like Frys because of the large quantity that get bought. I think I got four of them for $8 once a while back.