Re: [EE] Isolated AC line current sensors

Thanks! Interesting board. It reminds me of the Kill A Watt, which does a
lot for very little money.

I'd like to find a chip that can sample up to 20A RMS with the normal
hipot isolation (something like 2.5 kV, I think), and output a DC voltage
proportional to the RMS current. As I think more about thermocouple RF
ammeters, the use of the thermocouple is pretty clever. The power
dissipated in the sampling resistor causes a temperature rise across a
thermal resistance. The thermocouple measures this temperature difference
(thermocouple hot junction minus cold junction) to yield the RMS current
through the sampling resistor. The thermocouple automatically does the
temperature difference measurement, though we then have to deal with
linearizing the output.

Another thermal approach is that used in the LT1088 (
https://www.analog.com/media/en/technical-documentation/application-notes/an22.pdf
and
https://www.analog.com/media/en/technical-documentation/data-sheets/lt1088.pdf).

Thanks for the ideas!

Harold


On Mon, July 21, 2025 5:51 pm, Bob Blick wrote:
> PZEM 004T modules are pretty cool if you don't want to roll your own
> design. The 10A versions use a current sense resistor, the 100A versions
> have a current transformer. They have optoisolated TTL serial in and out.
> You get voltage, current, power factor, RMS power. Under $10 from the
> usual cheap goods online stores.
>
> Cheers, Bob
>
>
>
> ________________________________________
> From: Piclist <[email protected]> on behalf of Harold Hallikainen
> Sent: Monday, July 21, 2025 5:05 PM
> To: Pic List
> Subject: [EE] Isolated AC line current sensors
>
>
>
>
> I've been asked about sensing current on the AC line. Current
> transformers immediately come to mind. Also, I see there are Hall effect
> based sensors (similar to current transformers, but using a Hall effect
> device instead of a secondary winding). For isolation, I also see linear
> optocouplers, that have two photosensors so the LED current can be
> servoed. Another method would be a current sense resistor with an
> isolation amplifier.
>
> I remember years ago an LTC RMS to DC converter that measured the
> temperature of a heater. It SEEMS like there could be a chip equivalent of
>  a thermocouple ammeter with an electrically isolated heater and a
> temperature sensor.
>
> Other ideas?
>
>
> Thanks!
>
>
> Harold
>
>
>
>
>
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