using a mosfet for current sensing?

[justDIY]

Is it possible to use the on resistance of a mosfet as a current sensing resistor?

I'm interested in monitoring the current demands of a load, most likely under 5 amps, and rather than buy an expensive milliohm resistor, or wire a bunch of regular resistors in parallel, I thought about using the on resistance of a mosfet?

the irf3708 has an RdsON of 8 milliohms at 10v, but they don't show a graph of what it would be at 12v. The difference between 4.5 and 10 is only 1.5 mOhm, so the difference between 10 and 12 is probably too small to worry about?

anyway, here's my idea in diagram form:

 

[audioguru]

You don't like accuracy?
The on-resistance changes with temperature, voltage and is different for each Mosfet.
The typical resistance is 8 milli-ohms, max is 12 milli-ohms (a 50% difference) under certain conditions but they don't say what is the minimum resistance.

 

[Roff]

Typical is 8, max is 12, minimum is not specified, and the tempco is fairly high. You would have to calibrate (cow liberate?) it. A piece of wire might be more stable, and just as easy to calibrate.
EDIT:
Is there an echo in here? :lol:

 

[audioguru]

EDIT:
Is there an echo in here? :lol:

I beat you to it, Ron. :lol:

 

[Roff]

EDIT:
Is there an echo in here? :lol:

I beat you to it, Ron. :lol:

You filthy snake! :lol: