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).
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/
A power supply is very simple - you've got 24V AC so all you need for a low current DC supply referenced to either rail is a capacitive dropper feeding a charge pump, then regulate.
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).
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
That transformer doesn't look really well suited for the task but maybe you can bodge it to work if you really want.
That transformer doesn't look really well suited for the task but maybe you can bodge it to work if you really want.
Those two current sense transformers are for straight current sense, but with them, I could only measure the total device current and not just the one through the load which would be preferred. The best not intrusive solution is to tap on the existing current sense resistor drop, which will be a fraction of a volt. That is why I am trying a two coil transformer instead of the pass through current sense one.
Thanks for the interest.
I don't really see why you couldn't use "straight" current sense transformers to measure the individual load current(s)
I don't really see why you couldn't use "straight" current sense transformers to measure the individual load current(s)
The load wire is not on the connector I want to tap into. On the other hand, the two pins of the sense resistor are available, so my wishful thinking is that a 1:1 transformer connected in parallel with the sense resistor may allow to read the reflection of the sense voltage on the secondary. May be I can play with it in LTspice. I know the current sense transformer is a backup solution.
Shunting the internal sense resistor with a transformer winding would be unwise if the sense resistor is used for any sort of internal status sensing, fault detection or current limiting.
With a 0.82 ohm sense resistor, you've got less than 0.5V of sense voltage at 600mA load current, and do you really want to have to deal with measuring AC in realtime with an ADC controlled by the Pi?
I think I'd design a split rail +/-2.5V supply riding on the 24V AC line, then *either* use an OPAMP precision rectifier circuit and low pass filter to drive two comparators driving optocouplers, *or* use a Microchip PIC16F1765 which has an integrated OPAMP and ZCD module, and can run at 8 MIPs from its so is well suited to doing whole cycle true RMS measurements of small signals, then either do the thresholding in the PIC and use a dual OPTO to drive two Pi GPIO pins, or use its UART to report the RMS current to the Pi by optoisolated asynch serial.