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'[EE]: Negative voltage load disconnect.'
2003\02\11@134024 by A.J. Tufgar

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Hey all,
       I'm using a load disconnect circuit for +15V from linear
http://www.linear.com/pdf/3400f.pdf (not using this regulator though).
Attached is a pic of the circuit.

Just wondered if anyone know's how I could add a similair load
disconnect for an additional negative 15V rail that could be controled
by a logic signal preferably.

I'm using the load disconnect for a power supply if too much current is
going through either rail I want to open circuit that rail.


Thanks in advance,
Aaron

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2003\02\11@180839 by Dave Dilatush

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Aaron wrote...

>I'm using a load disconnect circuit for +15V from linear
>http://www.linear.com/pdf/3400f.pdf (not using this regulator though).
> Attached is a pic of the circuit.
>
>Just wondered if anyone know's how I could add a similair load
>disconnect for an additional negative 15V rail that could be controled
>by a logic signal preferably.
>
>I'm using the load disconnect for a power supply if too much current is
>going through either rail I want to open circuit that rail.

Here's a circuit for switching your negative supply arranged so
it can be controlled by the same ON/OFF signal used by the
positive switch you showed.

Dave D.

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2003\02\12@090509 by Jai Dhar

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Just out of curiosity, is this schematic one that basically turns on and off a
positive/negative rail with a logic signal?

Quoting Dave Dilatush <spam_OUTdilatushTakeThisOuTspamCOMCAST.NET>:

{Quote hidden}

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2003\02\12@152031 by A.J. Tufgar

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Dave, thanks for your help hopefully it works.

Question, anyone out there know how possibly to turn off the positive
rail with an N-channel mosfet instead? This would be much better as
typically N-channel have a much lower Ron.

Jai, yes your correct that's what this circuit does.
Thanks in advance,
Aaron

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2003\02\12@164222 by Ned Konz

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On Wednesday 12 February 2003 11:48 am, you wrote:

> Question, anyone out there know how possibly to turn off the
> positive rail with an N-channel mosfet instead? This would be much
> better as typically N-channel have a much lower Ron.

Yes, but you'll need a gate supply that's above the positive rail.
There are "high side drivers" that have a boost supply to do this.
Unless the Rds is too high for you (and there's some very good PMOS
FETs out there now), I wouldn't bother with the hassle of adding one
of these.

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2003\02\12@165927 by Dave Dilatush

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Jai Dhar wrote...

>Just out of curiosity, is this schematic one that basically turns on and off a
>positive/negative rail with a logic signal?

Yes, it is.  The diagram attached to A.J Tufgar's original post
showed a switch for ON/OFF control of a positive 15V rail by a
logic signal; and the diagram I supplied showed how to make a
negative 15V rail switch controlled by the same logic signal.

Dave D.

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2003\02\12@172809 by Olin Lathrop

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> Question, anyone out there know how possibly to turn off the positive
> rail with an N-channel mosfet instead? This would be much better as
> typically N-channel have a much lower Ron.

The current flow direction of an N channel FET makes it unusable to switch
a positive power rail on and off.  However, you could use an N channel FET
to switch the ground side of the load if this is acceptable to the load.


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2003\02\12@185613 by Dwayne Reid

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At 05:26 PM 2/12/03 -0500, Olin Lathrop wrote:
> > Question, anyone out there know how possibly to turn off the positive
> > rail with an N-channel mosfet instead? This would be much better as
> > typically N-channel have a much lower Ron.
>
>The current flow direction of an N channel FET makes it unusable to switch
>a positive power rail on and off.  However, you could use an N channel FET
>to switch the ground side of the load if this is acceptable to the load.

Not if you use the FET in the source follower configuration.  As has
already been pointed out in earlier messages, you need a gate supply that
goes above the positive rail.

This can be generated with a variety of methods, or you can use a high-side
gate driver that has it built in, or it can be supplied from an existing
higher voltage supply if one is already available.

dwayne

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2003\02\13@080805 by Olin Lathrop

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> Not if you use the FET in the source follower configuration.  As has
> already been pointed out in earlier messages, you need a gate supply
that
> goes above the positive rail.
>
> This can be generated with a variety of methods, or you can use a
high-side
> gate driver that has it built in, or it can be supplied from an existing
> higher voltage supply if one is already available.

You're right.  I was trying to keep things simple for the original poster
who clearly didn't have much electronics background.  Sorry, I didn't mean
to leave the impression that it couldn't be done at all.


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2003\02\13@082850 by Jai Dhar

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Quoting Dave Dilatush <dilatushspamKILLspamCOMCAST.NET>:

> Jai Dhar wrote...
>
> >Just out of curiosity, is this schematic one that basically turns on and off
> a
> >positive/negative rail with a logic signal?
>
> Yes, it is.  The diagram attached to A.J Tufgar's original post
> showed a switch for ON/OFF control of a positive 15V rail by a
> logic signal; and the diagram I supplied showed how to make a
> negative 15V rail switch controlled by the same logic signal.
>
> Dave D.
>

Ok, this is what I thought - just trying to tie it in with my thread dealing
with MOSFET's, how come you couldn't just use one logic level MOSFET instead
of that and a Transistor? Are there any advantages to using a transistor?
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