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PICList Thread
'battery/wall xfmr switching'
1999\02\10@025130 by erik

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Hello,
I'm finishing up my first (?real?) PIC project. A chess clock. I'd like
to power it with either a 9V battery or a little wall transformer.

I'm guessing there is an IC out there to switch between supplies. Or
maybe there is some other trivial way to do this.

Any help would be much appreciated.

Thanks,
Erik

1999\02\10@043157 by Nigel Orr

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At 01:51 10/02/99 -0600, you wrote:
>Hello,
>I'm finishing up my first (?real?) PIC project. A chess clock. I'd like
>to power it with either a 9V battery or a little wall transformer.
>
>I'm guessing there is an IC out there to switch between supplies. Or
>maybe there is some other trivial way to do this.

Assuming there's a regulator on the PIC circuit, use a 9V battery and a 12V
supply, with a diode in series with each.  Then if the 12V drops below 9V,
the battery will take over supply.  Quick and dirty, and usually works just
fine ;-)

Nigel

1999\02\10@115943 by Mark Willis

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erik wrote:
>
> Hello,
> I'm finishing up my first (?real?) PIC project. A chess clock. I'd like
> to power it with either a 9V battery or a little wall transformer.
>
> I'm guessing there is an IC out there to switch between supplies. Or
> maybe there is some other trivial way to do this.
>
> Any help would be much appreciated.
>
> Thanks,
> Erik

 One way would be to use diode "Or"-ing (use a power supply that has
over 9V output - which would include most 9V wall warts!) - Common
cathodes, an anode to +9V and +WallWartIn, and the system picks the
higher voltage automagically.  Cheap, too.  (Do lose a diode drop's
worth of power - an FET or BJT circuit might be better if switching
between 2 battery sources.)  One project used this with a bridge
rectifier they had scads of, as a reverse voltage protection/spike
protector (connect the common anodes to ground, though Battery & Wall
Wart then need to share grounds.)  "Think Devious" <G>

 Mark

1999\02\10@165930 by w. v. ooijen / f. hanneman

picon face
suggestions:
- include a small relay (powered from the wall wart) which switches from
battery to mains
- use a switch: one side to the battery, the other side to the mains:
doubles as on/off when you pull out the mains.
regards,
Wouter.

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{Quote hidden}

1999\02\11@033737 by Michael Rigby-Jones

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Or you could even use a power socket with an NC switch built into one of the
contacts, as most fo them have.  Much easier, it's what the vast majority of
mains/battery appliances use.  (Although their long term reliability is not
terrific)


Mike Rigby-Jones
mrjonesspamKILLspamnortelnetworks.com



{Quote hidden}

1999\02\11@152745 by paulb

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w. v. ooijen / f. hanneman wrote:

> - include a small relay (powered from the wall wart) which switches
> from battery to mains
> - use a switch: one side to the battery, the other side to the mains:
> doubles as on/off when you pull out the mains.

Michael Rigby-Jones wrote:

> Or you could even use a power socket with an NC switch built into one
> of the contacts, as most fo them have.  Much easier, it's what the
> vast majority of mains/battery appliances use.  (Although their long
> term reliability is not terrific)

 Both these (or all 3) have a problem.  I'm just wondering whether he
really wanted a "no-break" switching to retain its RAM contents?

 My feeling is that you use a (Schottky) diode from the battery and
a bridge (or half-bridge) rectifier from the power transformer which
presumably will produce a somewhat higher voltage than the battery.
It doesn't have to be much higher though, as having the reservoir
capacitor *after* the diodes guarantees that any supply from the mains
keeps the battery diode biassed off.

 This presumes either that the PIC can tolerate the higher supply
voltage or you use a low-leakage regulator and that in either case, low
leakage reservoir capacitor after the diodes.  This is where things
actually become complicated.

 You don't generally need a switch in the mains supply; it almost
certainly uses so little power (or shuts itself down) that you can
afford to leave it running.  If it has no shut-down function, you will
however want a battery on/ off switch, but this can be a simple single
pole.  OTOH, you could place the switch between all the diodes and the
reservoir capacitor.
--
 Cheers,
       Paul B.

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