Electronics > Beginners
Help with AC current level detection over sense resistor
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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