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Thread: Battery backed operation
www.piclist.com/techref/power/batterys.htm?key=battery
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face BY : Dan Michaels email (remove spam text)



Nicholas Irias wrote:
>I want to implement battery backed operation, wherein a PIC based device
>ordinarily runs from a 9v to 12v wall wart, and switches to a 6 volt battery
>pack (AA alkalines) when the external power supply is unavailable.
>
>My initial plan is to use diodes between each of the two power sources and
>my 5v regulator.  I would then use a resistor, a 5v zener, and perhaps
>decoupling capacitor so that one of the PICs input pins could monitor the
>external power supply.  In the event that external power is lost, program
>logic would invoke a low power operation mode (dont try to run relays, LEDs,
>etc until external power returns).
>
>Can I expect this scheme to work flawlessly, or did I miss something?


Not quite flawlessly. When operating off the 6v battery, you'll
get a v.drop across the diode, leaving only a couple of tenths/volt
overhead on the 5v regulator. You can use schottky diodes and
LDO v.reg, but you will still have pretty marginal overhead.
Batteries discharge a little below 6v, and wham. This may be ok,
if the batteries only supply maintenance current, rather than
operating current.

2 possible improvements - use a 5th AA, and use a power jack
with internal switch, as Mike Mansheim suggested, to eliminate
the diode.

- danM

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