AVR-Chat

Steven Hodge wrote:
> I have used VP3203 enhancement mode P-channel mosfets (actually DMOS
> FET's, I just loosely call them mosfets) on PCB's in the past and I
> see no reason why one of these, with its gate driven directly by a uC
> output pin, could not be used to turn on/off the voltage to the
> divider.  Their threshold voltage is only 1 V so a 3 V signal from
> the uC will do the trick. Leakage current when off should be less
> than 10 uA.  Connect the gate with a     
> pull-up to Vb, source to Vb and drain to the top of the divider.   I
> always 
> but TVS's and big input capacitors on the power input to any PCB I
> build and (so far, knock on wood) haven't had any problem with
> transients.  
> 
> Am I missing something here?
> 

Yes, the input protection diodes which Graham just pointed out to me
will pull down the gate of the fet to the micro's vcc which will cause
it to be turned on all the time.

You'll need to put in a n-channel fet or an npn pulldown, as Graham
originally suggested, to switch the gate of the p channel fet.

-- 
Tim Mitchell
tim@sabretechnology.co.uk http://www.sabretechnology.co.uk
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