Olin Lathrop wrote:
>> I have a circuit using a 16f874 , with a total power
>> consumption of 500 mA, the circuit have some 7805 voltage
>> regulators in parallel that reduce the voltage from 12 V to 5 V.
>> I'd like to add a backup rechargable battery circuit, that
>> gives me 6 hours of backup when the main power goes off.
>> If somebody knows about some reliable and cheap solution
>> i'd appreciate it.
>
> I would replace the linear regulator with a buck regulator
> so that you are no longer wasting 60% of the power.  Even
> if not, you are only asking for 3AH from the battery.
> Sounds like a good application for a lead-acid gell cell.

   Developing the idea I'd suggest not "backup" but rather
"on-line" idea with two DC/DC converters.
   First DC/DC converter - Step-up DC/DC converter from
batteries' 3V to PIC 5V.
   Second DC/DC converter - Step-down DC/DC or Buck converter
from external (12v or any, even mains(Viper, TOPSwitch)) to
batteries' 3V.

Good Luck,
Mike.

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