3 ms·

> calibration uncertainty What is the uncertainty? I thought you just need to zero each ADC after it stabilizes on boot before powering up the load?

michaelt 4y ago
For a start, if you're monitoring your whole house, getting to zero load while keeping the sensor powered on is not trivial :) For another, in my experience a lot of current sensing options are just a bit.... suspicious on the accuracy front. I've had resistive sensors tell me 1W is flowing through a plug with nothing plugged into it. Turning off a resistive load but the measurement slowing falling back to zero over several seconds, instead of dropping immediately. Solar monitoring current clamps that claimed zero current when the power company's meter said we were exporting 20W. Fancy £800 fluke clamp meters tell me 2A is flowing when there isn't even a cable in the clamp. Clamp ferrites getting broken in transit and producing bad results. Installers putting the CT clamp the wrong way. Installers installing the power meter the wrong way around, because the installation instructions assume you want to meter import not export..... Current sensing is surprisingly hard to get right :)
- FatActor 4y ago
  > For a start, if you're monitoring your whole house, getting to zero load while keeping the sensor powered on is not trivial :) Hahahaha! Duh. I just assumed you could power cycle the target (which you could but not without potentially drastic consequences). thanks for the follow-up.
kragen 4y ago
i should preface this by saying that my knowledge here is entirely theoretical, and i'd be interested in knowing if i'm mistaken. as i understand the situation: your ±1% 1Ω shunt resistor might actually be 0.99Ω, maybe depending on temperature (which changes after you power up the load), but you can get ±0.01% resistors pretty cheap these days (maybe not if they have to handle 150 amps tho) typically the bigger issue is the reference voltage the adc is comparing it against, because a voltage reference with 1% precision that's stable over time is pretty challenging, and also needs to be calibrated but i'd think that would also be the case for calibrating the driver for the cancellation current in the hall-effect sensor?

michaelt 4y ago

For a start, if you're monitoring your whole house, getting to zero load while keeping the sensor powered on is not trivial :) For another, in my experience a lot of current sensing options are just a bit.... suspicious on the accuracy front. I've had resistive sensors tell me 1W is flowing through a plug with nothing plugged into it. Turning off a resistive load but the measurement slowing falling back to zero over several seconds, instead of dropping immediately. Solar monitoring current clamps that claimed zero current when the power company's meter said we were exporting 20W. Fancy £800 fluke clamp meters tell me 2A is flowing when there isn't even a cable in the clamp. Clamp ferrites getting broken in transit and producing bad results. Installers putting the CT clamp the wrong way. Installers installing the power meter the wrong way around, because the installation instructions assume you want to meter import not export..... Current sensing is surprisingly hard to get right :)

FatActor 4y ago
> For a start, if you're monitoring your whole house, getting to zero load while keeping the sensor powered on is not trivial :) Hahahaha! Duh. I just assumed you could power cycle the target (which you could but not without potentially drastic consequences). thanks for the follow-up.

kragen 4y ago

i should preface this by saying that my knowledge here is entirely theoretical, and i'd be interested in knowing if i'm mistaken. as i understand the situation: your ±1% 1Ω shunt resistor might actually be 0.99Ω, maybe depending on temperature (which changes after you power up the load), but you can get ±0.01% resistors pretty cheap these days (maybe not if they have to handle 150 amps tho) typically the bigger issue is the reference voltage the adc is comparing it against, because a voltage reference with 1% precision that's stable over time is pretty challenging, and also needs to be calibrated but i'd think that would also be the case for calibrating the driver for the cancellation current in the hall-effect sensor?