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
Sabre Technology (Hull) Ltd, 3a Newlands Science Park, Hull HU6 7TQ
Registered in England and Wales no.3131504
t:01482 801003 f:01482 801078

Ah, yes, I did miss that.  My scenario would have Vb >> Vcc applied to the
uC output pin when off.  I like the N-channel  fet approach to switch the
gate of the VP3203.   However, I do need to clarify one point.  In Graham's
post he says "To turn the whole thing on, we apply a high level to the drain
of the N-channel."   I assume he meant to the gate, not the drain?   Steve

From: AVR-Chat@yahoogroups.com [mailto:AVR-Chat@yahoogroups.com] On Behalf
Of Tim Mitchell
Sent: Friday, December 14, 2007 8:41 AM
To: AVR-Chat@yahoogroups.com
Subject: RE: [AVR-Chat] Re: ADC Vref


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





[Non-text portions of this message have been removed]

--- In AVR-Chat@yahoogroups.com, "Steven Hodge" <stevehodge@...> wrote:

> In Graham's post he says "To turn
> the whole thing on, we apply a high
> level to the drain of the N-channel."
> I assume he meant to the gate, not
> the drain?

Argh!  And I read that post through THREE TIMES before I sent it!  Yes, 
yes, gate not drain.

Now that we know more about your application, it's clear that other 
solutions would work equally well (including those that sacrifice some 
accuracy).  For example, you could have a voltage divider with high 
value resistors to get a very low (but continuous) current and use an 
op-amp in voltage follower configuration between that and the 
microcontroller.  Power the op amp from a port of the microcontroller 
and turn it on only when you need it.  A multi-megohm resistor between 
the supply and your circuit would go a long way towards protecting it 
from damage and the tiny current when the op amp is off would surely 
just bleed harmlessly away.

As this is a one-off, what you end up doing might have a lot to do with 
what you have in your parts drawer so the fun can continue and we can 
collect more ideas.

Graham.

Thanks for confirming.  As for other techniques, I thing I'll just stick
with the simple voltage divider, turning it on only to sample.  I've already
bread-boarded up the divider/dual-mosfet thing and it works just fine.
Steve

From: AVR-Chat@yahoogroups.com [mailto:AVR-Chat@yahoogroups.com] On Behalf
Of Graham Davies
Sent: Saturday, December 15, 2007 6:08 AM
To: AVR-Chat@yahoogroups.com
Subject: [AVR-Chat] Re: ADC Vref

 

--- In AVR-Chat@yahoogroups.com <mailto:AVR-Chat%40yahoogroups.com> ,
"Steven Hodge" <stevehodge@...> wrote:

> In Graham's post he says "To turn
> the whole thing on, we apply a high
> level to the drain of the N-channel."
> I assume he meant to the gate, not
> the drain?

Argh! And I read that post through THREE TIMES before I sent it! Yes, 
yes, gate not drain.

Now that we know more about your application, it's clear that other 
solutions would work equally well (including those that sacrifice some 
accuracy). For example, you could have a voltage divider with high 
value resistors to get a very low (but continuous) current and use an 
op-amp in voltage follower configuration between that and the 
microcontroller. Power the op amp from a port of the microcontroller 
and turn it on only when you need it. A multi-megohm resistor between 
the supply and your circuit would go a long way towards protecting it 
from damage and the tiny current when the op amp is off would surely 
just bleed harmlessly away.

As this is a one-off, what you end up doing might have a lot to do with 
what you have in your parts drawer so the fun can continue and we can 
collect more ideas.

Graham.

 

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[Non-text portions of this message have been removed]