At 01:13 PM 14/04/2011, you wrote:
>V G wrote:
> > Hey all, instead of using a wire wound resistor for testing battery
> > drain, I'm thinking of using a heat sink bound MOSFET and control the
> > drain-source current with the gate. I'm looking to get around a 0.5 -
> > 2 ohm resistance between drain and source. Is this possible?
>
>Sure it's possible, but will require active control and feedback else you'=
ll
>have no idea what is actually going on.  The easiest is probably a low sid=
e
>current sense resistor with a opamp driving the gate to keep the voltage
>accross the resistor at the desired level.

If he wants to simulate a resistor rather than a constant current sink
he'll need to adjust the voltage across the sense resistor in proportion
to the battery voltage. Eg. control Vs =3D Vbat/k where k is a constant

I =3D Vs/Rsense
Req =3D Vbat/I =3D Vbat*Rsense/Vs =3D Vbat*Rsense*k/Vbat
      =3D Rsense * k

Since the battery voltage shouldn't be changing very fast with a
steady load, a PIC could be used to do this rather than an analog
multiplier. Perhaps by deriving the ADC reference from the battery..
So, an inner analog loop that sinks a current proportional to input
voltage and an outer digital loop that commands that current.

>Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the rewar=
d"
speff@interlog.com             Info for manufacturers: http://www.trexon.co=
m
Embedded software/hardware/analog  Info for designers:  http://www.speff.co=
m



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