Author Topic: STPM32 current reading lower than multimeter reading?  (Read 1338 times)

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Offline jpmkmTopic starter

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STPM32 current reading lower than multimeter reading?
« on: January 08, 2019, 12:53:20 am »
I'm working on a power meter based on the stpm32(https://www.st.com/en/data-converters/stpm32.html), that I'm planning to use on on 24VAC loads at no more than about an amp.  I've gotten to the point of being able to read RMS voltage and current(register 0x48).  Using the formulas, I'm getting pretty close.  With purely resistive loads I'm <1%(which is just calibration) off from my amprobe am-130.   However, when I hook up an HVAC controller(which is why I designed the meter), I'm reading significantly lower current with the stpm32-based meter than with either the amprobe am-130 or a fluke 8808-a(both of which claim true-rms measurement).  For example, with one controller, I was reading around 115-120mA on the fluke, and about 90mA with the stpm32.  Why are the readings so different and which one is correct?  I don't want to believe that the fluke is wrong, but I also don't want to believe the the stpm32, which is used in actual utility power meters, is wrong.  The controller uses a half-wave rectifier(single diode) to a capacitor to the power supply, which I have a feeling has something to do with this.  My stpm32 meter is using a low-side .01ohm current shunt, and 188Kohm/470ohm voltage divider for the voltage side.  Voltage seems to be within calibration no matter what.

Ultimately I'm not really concerned with exact current measurements - I'm more concerned when one controller I'm testing pulls more than the other hundreds that I've tested.  But when my measurements are so different and I don't know why, I don't feel like I can trust a meter I designed over a fluke.  Even though I think the fluke is being less accurate in this case, I can't explain why.
 


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