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'Battery Supply selector'
2003\06\24@021424 by Michael Milner

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Hi,

I'm making a battery powered device which will have the option of
being powered by an external supply.  What I want is to power the
device from the battery, unless a voltage is present on the external
connector.

Normally I would just use some diodes to prevent any reverse current
into the battery.  However, the battery voltage is tight, and I don't
want to have the voltage drop across a diode.

I was thinking of some kind of mosfet switch or mosfet driver IC.

Any recommendations?

Thanks,

Mike

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2003\06\24@044558 by Anand Dhuru

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How about a relay driven by your external supply?

The battery could go in series with the normally clsoed contacts of the
relay; when you apply external power, the relay closes, disconnecting the
battery.

{Original Message removed}

2003\06\24@092830 by Michael Milner

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I'm actually looking for something a bit smaller.

I may have found a solution that works.  I'm thinking of using a
p-channel hexfet from International Rectifier (or equivalent).  They
have a very low on-resistance (< 1 Ohm) an they can be switched with
less than a 3 volt Gate-Source difference.

Anand Dhuru wrote:
>
> How about a relay driven by your external supply?
>
> The battery could go in series with the normally clsoed contacts of the
> relay; when you apply external power, the relay closes, disconnecting the
> battery.
>
> {Original Message removed}

2003\06\25@005035 by cdb

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What about the supply switch chips from Maxim (MAX47xx or 31x ) or
the P-Channel FDC6329 from Fairchild see app note AN-1030 with design
notes for mobile phone/laptop power switching.

Colin
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2003\06\25@104846 by fbabis

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> I'm making a battery powered device which will have the option of
> being powered by an external supply.  What I want is to power the
> device from the battery, unless a voltage is present on the external
> connector.

How about a mechanical solution? Many companies offer DC power
connectors with a switch built in to them. From the following page you
can see what I mean.
http://dkc3.digikey.com/PDF/T032/0236.pdf

Check out fig.35. Connector 3 would go to the batteries while connector 2
would continue on to the rest of the circuit. If the plug is connected  the
contact on connector 2 is depressed away from connector 3. Of course this
also makes it impossible to run the unit from batteries if the plug is
connected,
even without power.

Cheers!

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