At 10:58 AM 14/04/2011, you 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-sourc=
e
>current with the gate. I'm looking to get around a 0.5 - 2 ohm resistance
>between drain and source. Is this possible?

Sure. That's how many electronic loads work. You can actively control
the gate voltage to give constant current, constant wattage or
constant resistance if you like. Having an electronic load in a box
is a great convenience if you do a lot of work with batteries and
supplies. Not generally absolutely necessary, but a good use of $$ IMHO.

Watch the power dissipation and safe operating area of the transistor..
anecdotally.. they don't always do what you might hope from reading the
data sheet- they're not typically used in linear mode. If you have
a minimum resistance that you'd ever need in mind, putting most of
that external to the MOSFET(s) will make things easier. A resistor
can run a lot hotter than a semiconductor, so it's easier to get
rid of the heat. 0.5 ohm on a 12V battery is almost 300W -- so maybe
half a dozen TO-247 or TO-220 units if no external resistor is used.

Of course you'll want a fuse too, as semis generally fail shorted,
and shorting batteries can be expensive and unpleasant.

>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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