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

Excellent suggestion.  I completely missed the fact that I have a 3.3V ref
that can be halved.

I originally chose to dismiss the use of a voltage divider as I expected it
would affect and/or be affected by the biasing around Vin+ and the 12.8V as
suggested would not be stable.

But a relatively low impedance (1k ohms) divider across 3.3V is simulating
nicely.

Thanks for the input.

Justin



On Sat, Sep 13, 2025 at 7:48 AM Brent Brown <[email protected]> wrote:

> Hi Justin,
>
> LM324 is ok with directly sensing near GND, but directly sensing near +ve
> supply
> not so good - input common mode voltage spec 0 - (Vcc-1.5V). Perhaps
> LTSpice
> doesn't know this. Appreciate the limited parts available, but a
> specialist current
> sensing chip makes things easy and good. Or a different Op-Amp with rail
> to rail
> input stage. CGPT suggests TLV2372, MCP6002, OPA2333, TLV9002, LM324-RRI
> (is that last one even a thing?).
>
> Your 2 x diode 1.2V reference workable but not expected to have excellent
> regulation (12.8V supply not precise, temperature effects on diodes). What
> you're
> aiming for is an offset of around half the 3.3V supply for your uC ADC
> input... an
> improvement might be a resistor divider across the uC 3.3V regulated rail.
>
> Brent
>
> On 12 Sep 2025 at 10:44, Justin Richards wrote:
>
> > I would like to condition the output of a 1.1mV/A current shunt to drive
> > the analog input of a WEMOS D1 Mini (0 - 3.3V).
> >
> > The target application is easy monitoring (wifi) of anything 12V with an
> > Anderson plug. i.e. Vehicle to Caravan charge line, Solar Panel to
> Battery,
> > Battery to Electric Blanket/Fridge etc.
> >
> > The device is to be powered from the circuit it is monitoring which will
> be
> > around 12.8V.
> >
> > I am a long way from a bench so have resorted to LTspice.  The attached
> > circuit (5KB so I hope it gets thru) appears to behave exactly as
> desired.
> >
> > However, 2 diodes in series to raise the reference to allow for the
> > negative swing seems to work in LTspice but I am curious what others
> think
> > about this approach.
> >
> >
> > Justin
> >
>
>
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