Thanks Dwayne,

I have since hit another problem, the line I am trying to switch is
floating.
One end is connected to a matrix keypad, the other to a SBC which scans
the keypad. I therefore have no source voltage to reference the gate too
(Vgs).

I have decided to use a Photo-Mos Relay (normally closed) to disconnect
the keypad matrix lines. (Wow they are expensive!!!!!!!!!!!)

Incidentally, the reason I am having to do this is because during
power-up, if
you press any key on the matrix keypad, the SBC goes into the bios. Wow,

that is a good idea when it is inside a hand-held instrument! Not!

Thanks

Graham

> -----Original Message-----
> From: Dwayne Reid [SMTP:dwayner@PLANET.EON.NET]
> Sent: Friday, March 01, 2002 2:22 AM
> To:   PICLIST@MITVMA.MIT.EDU
> Subject:      Re: [ee]: Controlling a Depletion Mode MOSFET with a
> PIC?
>
> At 04:03 PM 2/27/02 +0000, Graham North wrote:
> >Hi all,
> >
> >I need to control (turn off) a depletion mode mosfet with a Pic
> output
> >(the Pic is running from a 3.3V supply). Is this possible? As far as
> I
> >am aware a depletion mode mosfet conducts with a zero volt potential
> on
> >the gate, and is turned off when a negative potential is applied to
> the
> >gate. Is there an easy way to do this with a Pic output?
>
> Yes - and no!
>
> First - use a P-channel FET - something like a J174 thru J177.  A
> positive
> gate voltage (with respect to source) turns the FET off.  I use LOTS
> of
> this family!
>
> Your main problem is the low supply voltage.  The closest part from
> the
> J17x family would be the J177 - the maximum rated pinch-off voltage is
> 2.5V.  The problem is the channel resistance is quite high at 300
> Ohms.
>
>         VGS     VGS    ON RESISTANCE
> PART   MIN     MAX        MAX
> 174     5      10         85
> 175     3       6         125
> 176     1       4         250
> 177    0.8     2.5        300
>
> The above data from the Fairchild web site.
>
> Hope this helps!
>
> dwayne
>
>
> Dwayne Reid   <dwayner@planet.eon.net>
> Trinity Electronics Systems Ltd    Edmonton, AB, CANADA
> (780) 489-3199 voice          (780) 487-6397 fax
>
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