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Help with AC current level detection over sense resistor
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Back2Volts:
I want to detect the AC current level on the load driven by an existing device which has a built in sense resistor with externalized pins.   I need to detect load status, which can be either 0 mA, 300 mA or 600 mA on 24VAC.   

In addition, there will be a 1-wire serial line and a handful of input digital signals, all connected via FT232H/USB to a Raspberri Pi.  I am counting with a layer of isolation between FTDI chip and device.

I have attached a simplified schematic.   There is no DC readily available in the device.   One of the two 24VAC legs is tied to ground; the modules in the device have hidden DC supplies drawing from the 24VAC.  The digital signals expose DC levels related to ground.

I am struggling with how to deal with the AC current sense voltage on the 24VAC leg.   The simplest solution I can think of is using a signal transformer, rectifier/filter and two voltage comparators.   Are there micro size transformers for 60Hz?  Will something like this work?  https://www.mouser.com/datasheet/2/281/kmp_da100-185018.pdf .  (It shows up on a Mouser search for 50-60Hz, but obviously is rated higher frequency.) 

   What would be a common solution?   As a last resort, I could put a current sense transformer on the 24VAC line for the whole device, but they are bulkier than I would like...

Are there any specialized ICs for AC current measurement with I2C interface, like INA219 but for AC.   Is there such a thing? 

Comments and suggestions appreciated.
Kleinstein:
One can use a normal toroid core inductor as a crude current transformer - just add a well insulated wire for the primary current through the center.

Those linked small digital signal transformers are not really suitable for 50/60 Hz. The maximum voltage would be really low ( mV range).
MasterT:
Alternatives to current sense transformer are hall effect IC (ACS712 etc.) , so would not be accurate < 1A.
Or Isolation amplifiers, most of them needs power supply at high side, but there are some with integrated dc-dc converters, can't find part number right now.
https://www.mouser.com/Semiconductors/Integrated-Circuits-ICs/Amplifier-ICs/Isolation-Amplifiers/_/N-6j73s/
Back2Volts:

--- Quote from: Kleinstein on August 17, 2018, 02:27:10 pm ---One can use a normal toroid core inductor as a crude current transformer - just add a well insulated wire for the primary current through the center.

Those linked small digital signal transformers are not really suitable for 50/60 Hz. The maximum voltage would be really low ( mV range).

--- End quote ---

I have found/ordered this oddball transformer.   I'll give it a try. 
https://www.ebay.com/itm/5PCS-2mA-2mA-Voltage-transformer-precision-micro-voltage-transformer-ZMPT107/263629041550 

I'll keep in mind the possibility of building my own current sense transformer.

Thanks
Back2Volts:

--- Quote from: MasterT on August 17, 2018, 03:54:15 pm ---Alternatives to current sense transformer are hall effect IC (ACS712 etc.) , so would not be accurate < 1A.
Or Isolation amplifiers, most of them needs power supply at high side, but there are some with integrated dc-dc converters, can't find part number right now.
https://www.mouser.com/Semiconductors/Integrated-Circuits-ICs/Amplifier-ICs/Isolation-Amplifiers/_/N-6j73s/

--- End quote ---

I had looked at the Hall sensors but as you say they are out of range.   The use of isolated amplifiers is a good idea and I did not realize some are at reasonable prices.   I would need a power supply, so for now it is a backup solution.

Thanks   
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