Thanks guys, it appears the diode method is the simplest and easiest way
to go.  It's sort of like the old diode OR gate....why wasn't I thinking
of that?  Once again, this list has been very helpful.

Rob


On Wed, 1 Jul 1998, Quentin Lewis wrote:

> -- [ From: Quentin Lewis * EMC.Ver #3.0 ] --
>
>
> > seems like a good method.  However, I wanted to add a battery backup
> to the
> > PIC, as this chip will normally be powered by a 9V DC power supply
> that is
> > connected to AC.  The 9V will go through a 5V fixed regulator before
> the PIC of
> > course.  My question is, what kind of circuit can I build that would
> switch in
> > or out a 9v battery as a backup?  I
>
> How about just diode isolating each supply....that is, a diode from each
> supply to the regulator....then when the battery voltage is higher then
> the 9v AC supply, it supplies the regulator.
>
> Ok....so you might want to ensure that the 9v AC supply is always going
> to be higher than the battery, so that the battery only supplies when
> the AC supply is out.
>
> So two diodes on the battery....or a schotkly on the AC supply and a
> silicon diode on the battery.
>
> Just a thought,
> -Quentin
>