V G wrote:
> I made a constant current source (for battery drain testing,
> charging, pretty much any application). But there's so many
> variables that I don't know how to calculate for it. So many of the
> components influence the current through Q1 (power BJT) that I don't
> know how to select the values and which one to vary to get the
> widest current selection range.

Yes, this is the problem of "solving" a circuit with a simulator instead of
a brain.  Until you understand the circuit, all you can do is poke at it in
the hope of randomly stumbling accross something that works.

In this case, the feedback is messed up.  Let the opamp drive the base of
the darlington directly.  The current sense voltage from R2 then goes into
the negative input, with the desired voltage on R2 fed into the positive
input.  If you can trust your V3 voltage source, then this can be a divider
from V3.  You may need a small cap (a few 10s of pF) from the opamp output
to the negative input to keep it stable, and a resistor between R2 and the
negative opamp input to give this cap something to work against.  10K Ohms
should be fine.  You don't need fast response since the battery
characteristics will only change slowly.

Also, you forgot the bypass cap accross the opamp power leads.


********************************************************************
Embed Inc, Littleton Massachusetts, http://www.embedinc.com/products
(978) 742-9014.  Gold level PIC consultants since 2000.
--=20
http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist
.