V G wrote:
> Thanks. This makes sense to me. But I forgot that I want some way to
> control the current with the computer as well. In either case, the
> circuit will be monitored by a microcontroller which will report to
> the computer for logging, so I might as well just have the
> microcontroller control the base of the Darlington.

I should still let the analog electronics regulate the current.  You can
have the micro produce the control signal for the current regulation
circuit.  If you connect the opamp as I said, then this would be the voltag=
e
fed into its positive input.  The circuit will make sure that whatever
voltage you put there will show up accross the current sense resistor
(assuming the V2 source is capable of that much current).

You can easily generate this control voltage by filtering a PWM output.  Tw=
o
R-C filters in a row with a rolloff frequency much less than the PWM
frequency would do it nicely.  At the same time, you can add another resito=
r
as a divider so that the maximum output from the micro (it's Vdd level)
results in the maximum current you want to be able to sink.  For example, i=
f
the max current sink you ever want is 2A, then the control voltage should b=
e
limited to 780mV with your 390mOhm sense resistor.  The voltage divider tha=
t
is part of your PWM filter would then attenuate such that 100% duty cycle
(straight Vdd) results in 780mV, or maybe a little more to account for part
tolerance and to allow a little overrange.


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