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1  Using Arduino / Programming Questions / Re: ACS714 30A on: February 11, 2013, 11:28:53 am
Thanks for the help, its still not perfectly clear how you got the 73829 from the range but at least it works. It just has to be converted to A and remove the negative.

As I'm currently noodling using the ACS714 for current measurement, I went back to figure out why 73.829 mA was the number he gave. It ends up it's just how an ADC works.

10 bits = 1024 steps (from 0 for 0v to 1023 for full AVCC) - theoretically, mind you; there can be plenty of external reasons (read: noise) that can prevent a consistent and precise reading on the ADC pins
0.52V would approximately read as 106 in the ATmega (assuming AVCC is 5V)
4.88V would approximately read as 917, meaning that the actual range of numbers you'd read would be between 106 and 917 (or 811 steps)
Divide 60A (the range of +/-30A) by 811, you get 73.829 mA per step.

Finally, as a best practice, always oversample the ADC and average across several measurements. I've never used a single sample reliably.
2  Using Arduino / Sensors / Re: Split-phase current sensing - best practice? on: March 29, 2011, 09:41:00 pm
Hello nearby bay area arduino fan!

Greetings! I hear we are legion here in the bay area, but all I hear up here in Santa Rosa is the crickets!

I think the two CTs with secondary wired in series is going to give some problems. Household power as you stated around here is split phase 220. There is a L1, L2, and a neutral wire. All the 110 circuit breakers  are usually wired alternately to the L1 and L2 buses and neutral and any 220 loads are wired just to L1 and L2 branches. So if you have a CT on each of L1 and L2 and series their outputs that would give you a total of what all the 110 branches are consuming, however won't the 220volt loads be 'double counted' in that scheme, as the 220 load current will be passing through both CT's?

I would figure that the 180º phase difference between the legs would work in favor of tying them together. Perhaps parallel instead of serial...?

I wish I had paid more attention in physics...

3  Using Arduino / Sensors / Re: Anybody using Rogowski coils for current sensing? on: March 29, 2011, 07:50:42 pm
I am trying to use one fed directly to the adc; the coil produces a derivative of the
current; for a sine wave that's a cosine wave.

Does being 90º out-of-phase bother you in terms of reading the value directly into the ADC?

Just having trouble getting the spec - it produces ".404 mV/A" which is a useless fact because
that can be peak volts, peak-to-peak or rms.

My guess would be that any AC-related voltage spec (which a CT or Rogowski would be) would be given as RMS. But a voltmeter should be able to ascertain the reality for you.
4  Using Arduino / Sensors / Re: Split-phase current sensing - best practice? on: March 11, 2011, 12:24:13 pm
If you assume that the phase voltages are equal and opposite you can wire current transformers in series - multiply the current you measure by 110 rather than 220V.  And get e polarity right (!)

Thanks, and I gather that if you didn't get the polarity right, you'd see seriously out-of-whack results (i.e. it wouldn't damage anything in particular or cause a fire). I'm pretty sure a voltmeter/ammeter will be able to sort this out before you wire it into an actual circuit.

I intend to do most of the breadboarding and proof-of-concept using "light bulb" loads for 120V and perhaps rigging both a symmetric and an asymmetric 240V load (though I will have to put in a 240V circuit in order to do that, I have a sub panel in my workshop with which I can make that happen).
5  Using Arduino / Sensors / Split-phase current sensing - best practice? on: March 08, 2011, 10:22:31 am
Okay, this is something of a follow-up to my previous post, since it looks like Rogowski coils are a dead-end for measuring current accurately in household electrical service.

Considering most current transformers (at least the affordable ones) are sized such that you can only wrap them around a single conductor on the incoming mains, how do you deal with both legs of a U.S. split-phase service? I only have one place to wire a current sensor to an Analog Devices ADE7753.

I read in a blog[1] that you should have a CT around each leg of the split phase, then wire the CT's in series in order to sum the current measurements. Does this seem correct? I'd like opinions on this in order to reach a sensible course of action.


6  Using Arduino / Sensors / Re: Anybody using Rogowski coils for current sensing? on: March 08, 2011, 09:42:46 am
Yeah, my reading was sending me to that conclusion as well. I would need to wind this thing umpteen thousand times with 60 AWG magnet wire in order to be sensitive enough at low amplitude. Wrapping it around a ferrite core was what I was trying to avoid, since I wanted to wrap it around both legs of my home's split-phase 240V mains conductors at the same time.

I was also drawn in by the attractiveness of the air core, I admit it.

I may still give it a try, but I'm considerably less excited about Rogowski coils now. bummer.  smiley-neutral
7  Using Arduino / Sensors / Anybody using Rogowski coils for current sensing? on: March 07, 2011, 09:38:38 pm
I'm researching a home power meter project, and despite all that I've read on Open Energy Monitor [] and a nice if difficult-to-read Instructable [], I haven't seen anybody put Rogowski coils forward as a viable alternative to other current sensors. For my version of the power meter, I'm planning on using the Analog Devices ADE7753, which has the integrator on the current input that would be necessary to get the correct value from the Rogowski coil, perhaps it's that folks are looking to use a simpler (if ultimately more expensive) way to measure current directly with an Arduino.

So, the question is: is anybody else using Rogowski coils for current sensing? Are they DIY or purchased? Opinions about accuracy, efficacy, etc.?


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