Re: [EE]: Bi-Directional 20A current shunt Using Single supply OPAMP

Oh for a Radio Shack (with parts) now that the fall soldering season is here :)

On Sat, Sep 13, 2025 at 4:30 PM Justin Richards
<[email protected]> wrote:
>
> Harold,
>
> Reducing the parts count as you described was an interesting exploration.
> It simulates nicely.
>
> However, with the selection of resistors on hand (listed below) I was
> unable to exactly match the feedback resistor (output to inverting input
> (Vin-)) with Rth of the divider.  If not matched then the output when
> measuring fwd/rev current is no longer symmetrical.
>
> I now have a circuit I am happy to attempt to build.  Except, there are
> three unused modules in the LM324 so obviously feature creep such as
> measuring and conditioning the 12.8V rail by mapping 15V-10V -> 3.3V-0V.
>
> Thanks.
> justin
>
> >
> > I assume you are using a pair of 1k in series to form the voltage divider
> > and then driving the 100k to the non-inverting input from the voltage
> > divider.
>
> Yes
>
> > You could reduce the parts count by just using a pair of 200k
> > resistors, one to +3V3, the other to ground, and the junction going to the
> > non-inverting input. This would have the same Thevenin voltage and
> > resistance. There MIGHT be an issue with noise on the 3.3V line being sent
> > into the amplifier through the voltage divider. This could be avoided
> > using the circuit you are apparently using and adding a capacitor to
> > ground at the voltage divider output. You could increase the resistances
> > used in the voltage divider some (as long as they are small compared to
> > 100k). But, if the 3V3 is quiet enough, I'd just use the two 200k
> > resistors.
> >
>
> Collection of resistors on hand roughly :-
> 10x 1R
> 10x 4R7
> 10x 10R
> 10x 47R
> 10x 100R
> 10x 220R
> 10x 330R
> 10x 470R
> 10x 680R
> 30x 1K
> 10x 4K7
> 10x 6K8
> 30x 10K
> 10x 22K
> 10x 39K
> 20x 47K
> 10x 68K
> 10x 82K
> 20x 100K
> 10x 330K
> 10x 470K
> 10x 680K
> 10x 1M
> 10x 4M7
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