Olin Lathrop wrote:
> >    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.
>
> The original poster said nothing about a 3V battery.  He
> did say he was using a 7805 regulator, which implies 7V or
> so minimum, and also mentioned a 12V battery.  I believe
> he is linearly regulating the existing 12V to 5V
> for the PIC and other circuitry.


Where have he "mentioned a 12V battery"? Have look at
his text:

> 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. Thanks in advance.

My English is weak, but not to that extent, I hope.
He has 12V power supply and he'd "like to add a backup
rechargeable battery  circuit".

I also "believe he is linearly regulating the existing
12V to 5V for the PIC and other circuitry".

And for "backup" he needs "some reliable and cheap
solution".

I suggested 3V battery pack, because it's cheaper than
12V pack for the same capacity and needs less contacts -
reliability.

Anyway, "when the main power goes off" regulator is needed.
Converters from two cells voltage to 5V are very cheap.

Then, he needs something to charge the battery. I suggested
step-down DC/DC or Buck converter from his 12V to battery's
3V. Or even better from mains to 3V - STM Viper or TOPSwitch
for example.

That's a solution. You think that just telling him to
replace 7805 voltage regulator with a DC/DC converter
helps him very much, don't you?

Mike.

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