i-snail ct

Hi guys, just a generic question on this ct:



I had a discussion on the italian arduino forum about this ct, and its supposed capabilities but I really am out of my depth when discussing electronics so I expressed my doubts and then quietly backed off when contested.
In short, from the site and the supposed datasheet this ct is able to 'self power from line current' (and I'm already lost ) and provides a voltage from 0 to 5 v dc directly proportional to the rms value of the current passing through the wire
My understanding was that it was an expensive (about 50 euro in italy) version of the ct used by the oem project (with probably an internal circuit where the burden resistor is placed), that is, the values provided by it should still be sampled by an algorithm like the one in emonlib in order to obtain a meaningful current measurement, while the theory of the experts on the forum was that since the value provided is a sine wave rms calibrated then one single reading would be enough to sample a whole interval.
I really was out of my depth so I just backed off and thought that since I really didn't know enough about electronics in general and cts in particular to keep arguing, but it's been nagging me since so I thought that maybe someone with more knowledge about this could point me to some literature or datasheets that could explain how what in oem requires a ct and an arduino and lots of calculations could be achieved with a circuit that doesn't even require power....
Please feel free to point out that I'm so ignorant on the matter that I couldn't even understand what a sine wave rms calibrated signal is :)

dBC's picture

Re: i-snail ct

Because that CT is actually outputting a DC voltage, you've lost all information about the AC current waveform.  It's that level of detail you need if you want to measure real power (see: http://openenergymonitor.org/emon/buildingblocks/ac-power-introduction)

If you just want to know RMS current, that CT will do the trick.  You could then multiply that by RMS voltage (either measured or nominal) and you'll have apparent power.... which depending on the load you're measuring, can be quite different from real power.

Robert Wall's picture

Re: i-snail ct

"The i-Snail-VC is a self powered AC current transducer that provides a 0-5V dc analog signal proportional to the AC current flowing through the device wire window (sine wave RMS calibrated)."

This sentence contains all the clues. From that, it's pretty clear to me that it contains a burden, a (probably full-wave) rectifier, a load and some filtering. The output is scaled so that for a sine wave only, the output is proportional to the rms current. For any other shape of wave, the output will not be correct. And because of the rectifier, there won't be any directional information either, so you won't know whether you are exporting or importing.

If you take a mechanical moving coil meter movement and put a full-wave bridge rectifier in front of it, the movement will respond to the average value of the rectified wave. That's not the same as the rms value, which is the value that's most useful because it represents the heating effect of the current. The ratio rms / average - it's called the form factor and it's a property of the shape of the wave - works out to be 1.11. You can do the maths if you want (or easier, read this - skip to the table at the end if your eyes glaze over). Most budget multimeters read the rectified average value of the wave, and the reading is scaled so that the display is correct for a sine wave. They will write "Average scaled rms" and the better ones are "True rms".

So, dBC, sorry but you're right only if it's a sine wave (or more precisely a wave with a 1.11 form factor). Give it a phase-controlled (lamp dimmer) load and it's wrong.

And madmatt, you can no longer claim that you're so ignorant on the matter that you can't even understand what a sine wave rms calibrated signal is.

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