Electronics > Beginners

Help with AC current level detection over sense resistor

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Back2Volts:

--- Quote from: Ian.M on August 20, 2018, 03:29:06 am ---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.

--- End quote ---

The resistor is used for continuity/short detection.   I was just hoping that the parallel effect of the transformer with something like a 50-100 ohm series resistor would not be significant, but you have a point.
 

--- Quote ---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?
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No, two bit status would be fine.


--- Quote ---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.

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Funny you mention that.  Yesterday afternoon, I took your PS LTspice and added a precision rectifier, thinking to make a rectifier-peak-detector-dual-comparator with a quad opamp.   The rectifier seemed to work on a single voltage power supply using a couple of LT1413.  I will finish it tonight or tomorrow. 

Thank you for posting the LTspice because it pushed me to use the simulator, which I had not touched in a long while.



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