>  I am interested in using the linear region of the MOSFET as a
potentiometer.
>
>  My target is a tiny 0.2 ohms max.
>
>  Looking at the local electronics distributor, decided to choose an
IRF640,
> N-channel trenchmos. It has a 180 miliohm Rds (on), Vdss=200V , Id=16 A.
>
>  The initial idea is connecting a 10K potentiometer at the gate, and
allowing
> the drain and source as the terminals of the variable resistor.
> This is going to be battery-supplied (4.5 V), so I expect to use minimum
> current on it.
>
>  Is this correct? What other scheme could I use?

The FET should transition thru various resistances, however these will be
unpredictable and very nonlinear as a function of the gate voltage.  It
will probably be too "touchy" at an operating point you care about.  Take
a look at "digital pots" by Microchip and others if you want more
predictable and repeatable operation.


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