Hi Forrest,

As others have said, it's definitely something that is done. No accurate
measurement can be made but if you need to know if current is flowing or
if the pass element is in pain, it works fine. You need to deal with the
open circuit voltage either in software, as I have done here:
http://bobblick.com/techref/projects/sv2hb/sv2hb.html

or in hardware, here:
http://bobblick.com/techref/projects/mp3book/mp3book2/mp3book2.html

Cheerful regards,

Bob

On Thu, Mar 8, 2012, at 02:14 AM, Forrest Christian wrote:
> I have an application where I'm considering just using the voltage drop=20
> produced by Rds(on) across a FQB22P10 fet in the place of a current=20
> sense resistor.   This is one of those applications where the exact=20
> current draw is not really needed, but instead it is more being used as=20
> a 'load present' detection.   The load is actually around 0.5A - which=20
> based on the just under 0.1 Ohm Rds(on) resistance, I'm right where I=20
> need to be for my 20V/V current sense amplifier....
>=20
> Whenever I consider something like this, I worry that I've missed some=20
> critical piece of 'semiconductor physics' which would make this not work.
>=20
> Is there a reason I shouldn't do this, or why I wouldn't get at least=20
> somewhat accurate results?

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