V G wrote:
> I'm looking to make a current source which will typically test
> batteries at around a 500mA - 2A drain. What gain values should I be
> looking at when selecting a BJT?

That depends on how much error you can tolerate.  You can probably get
transistors with a gain of 30 in that range, especially since you don't nee=
d
high collector voltage ability.  If one part in 30 is good enough, then
that's fine.  If not, you can use a smaller transistor around the big one t=
o
make a darlington, which then will have a combined gain that is the product
of the two individual gains.  If the second transistor has a gain of at
least 50 (easy to get), then the darlington gain would be 1500.

You could also use a FET between collector and base to drive a big fat TO-3
power transistor, like the ancient but brick-outhouse 2N3055.  That channel=
s
the base current to where it becomes part of the overall current, so there
is not error, and a gain as low as 10 or 15 from the big transistor is fine=
..


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