One could also use regular diodes to limit voltage if that's the issue now
... for instance you have 5 volts and want around 3.3 volts here's what to
do when you are playing it simple and cheap.

3x1N4148 in series will reduce the voltage from 5v to 3,2V

and as Vasile said Germanium diodes (prefix A) consumes 0,2V where Silicium
(prefix B) consumes 0,6v

An example of a germanium diode would be AA127 and a Silicium would be BA127

Best regards.
Lasse Madsen

----- Original Message -----
From: "Vasile Surducan" <vasile@L30.ITIM-CJ.RO>
To: <PICLIST@MITVMA.MIT.EDU>
Sent: Monday, October 15, 2001 8:27 AM
Subject: Re: [EE]: Battery Bakup Circuit for PIC12c508


> Rao, your design looks ok. 4.4 v at output with mains supply and 3.2V with
> backup. I've guess you have a rechargeable battery.
> Schottky have almost the same dropout ( about 0.6V ) so if you need more
> than 4.4V use a germanium diode and you may have 4.6V ( which is almost
> the same )  12C508 is working at 4.4V
> The better thing ( 78l05 or 2v7 zenner and a 0.6 Vbe transistor = 3.3V
> output ) is what you have in your hands !
>
> Cheers, Vasile
>
>
>
> On Sun, 14 Oct 2001, Jeethu Rao wrote:
>
> > Hi,
> >         I've got no experience in battery bakup circuits for PICs.
> >         Now, I've got an application with a 12c508 which needs battery
> >         bakup. The application is a dusk timer so, its supposed to run
> >         for years without intervention.
> >
> >         I've designed a trivial circuit for the purpose using a 3.2
volts
> >         lithium battery. I guess there are quite a few shortcomings in
it.
> >         could you guys please look up the circuit and suggest any
improvements.
> >         One drawback I sense is that using 4148s is gonna cause quite a
> >         bit of voltage drop. Do I use Schottky diodes ? The other one is
the
> >         78L05. I could'nt find a small equavelent for 7803. Since the
size is
> >         constrained, I could use a transistor - zener regulator. Whats
better a
> >         3.2 v simple transistor based reg or a 78L05 ?
> >
> > Thanks,
> >
> > Jeethu Rao
>
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