Voltage Divider and Supply Noise?

Searching like crazy, finding opposing answers on using 10k to 470k voltage divider resistors.

My first CT sampler is pretty accurate dividing the 5v supply with 2x 100k 1% resistors.  Below .15a accuracy isn't great but above that it's near perfect to my Kill-A-Watt.

I built a second CT sampling circuit with different resistors in for the voltage divider and ended up with crazy readings.

Second circuit is using 52.3k 1% resistors, same burden resistor, yet it's all over the place.  It's showing +/-5a at no usage.

By accident I noticed that when using a switched 5v supply vs the arduino USB supply the reading shows +/-2a at no usage.

Further investigation with a battery supply shows +/-1a at no usage.

I no longer have a scope, so I am forced to try and find+filter this noise myself. 

What's the best way to condition this noise out?  Should I go to 470k resistors?

As always, thanks for the input!!!





Robert Wall's picture

Re: Voltage Divider and Supply Noise?

First, take a look at Measurement implications of ADC resolution at low current values, this and this. Trystan did some wonderful animated pages a few months ago demonstrating how the errors vary here http://vis.openenergymonitor.org/dev/adc/noiseless, here http://vis.openenergymonitor.org/dev/adc/noisy and here http://vis.openenergymonitor.org/dev/adc/noise.mp4.

From all that, I think you'll see that there are many possible sources of inaccuracy, and you'll appreciate how the results are affected.

And I've not mentioned so far the obvious: pick-up from the mains wiring that the c.t. is clipped onto, and power supply noise which you've already identified.

I think you've got to steadily and systematically eliminate all the contributions that you have control over. Good luck.

bellboy.tech's picture

Re: Voltage Divider and Supply Noise?

Thanks!  I've been working through this site for the past year+ and haven't seen those last three links!! 

I expected the noise on the CT and Arduino side of the circuit, not on the biasing side.  I expected the internal readVCC coupled with the capacitor to account for the noise on the Biasing voltage divider.

More over, I expected the higher the Ohm on the divider the more noise we would see, and it's actually the opposite.

Wait, while I say that the light went on in my head.  It would make sense if the source of the noise was on the CT side of the divider... Ohhhh Thank you !!!!

Again thank you so much!!!




richmc's picture

Re: Voltage Divider and Supply Noise?

An example of CT noise is here-


This is Calypso_raes design with the divider buffered with an op amp, it shows the inherent noise pick up through the CT, the last image is with the CT i/p open circuit, in this case it looks like the CT is picking up noise from the Arduino filtering in Robins design is done in the sketch and is very effective but it seems to be the case that you will have noise at low levels of I/P, fortunately in this application its largely irelevant.



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