On Wednesday, April 20, 2011, Olin Lathrop <olin_piclist@embedinc.com> wrot=
e:
> 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. =A0Until you understand the circuit, all you can do is poke at i=
t in
> the hope of randomly stumbling accross something that works.
>
> In this case, the feedback is messed up. =A0Let the opamp drive the base =
of
> the darlington directly. =A0The current sense voltage from R2 then goes i=
nto
> the negative input, with the desired voltage on R2 fed into the positive
> input. =A0If you can trust your V3 voltage source, then this can be a div=
ider
> from V3. =A0You may need a small cap (a few 10s of pF) from the opamp out=
put
> to the negative input to keep it stable, and a resistor between R2 and th=
e
> negative opamp input to give this cap something to work against. =A010K O=
hms
> should be fine. =A0You don't need fast response since the battery
> characteristics will only change slowly.
>
> Also, you forgot the bypass cap accross the opamp power leads.


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.

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