Nay Lasse,
        Using diodes to drop the voltage down is ridiculous for
        an app like this one.
        Vasile's zener / transistor idea is good. But ripple
        rejection ain't gonna be as good as the 3 terminal Regulator ICs.
        I'm going in for a LM317L based regulator @ 3.6 volts.
        The battery is a 3.6 volts 60mAh Nickel Cadmium battery.

Jeethu Rao


> -----Original Message-----
> From: pic microcontroller discussion list
> [mailto:PICLIST@MITVMA.MIT.EDU]On Behalf Of Lasse Madsen
> Sent: Monday, October 15, 2001 1:48 PM
> To: PICLIST@MITVMA.MIT.EDU
> Subject: Re: [EE]: Battery Bakup Circuit for PIC12c508
>
>
> 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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