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Pulling a voltage signal from an AC current transformer...

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chuggins143

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Hello all! New guy here working on a little problem that has me stumped... I'm relatively new to the electronics world and basically know enough to be really dangerous. :D

Ok, the project...
I have an AC current transformer that I purchased from Digikey (Triad Magnetics pn - CST306-3A) This has a primary amp limit of 25A. It has a very nice linear response when I'm measuring mA with a DMM.

For example, I have two Fluke 179's one has an amp clamp and gives me a reading of 1mA/A and my current transformer (CT for short) gives me a nice linear reading that corrisponds back to the clamp. In this case my clamp is reading .84mA and my CT is reading 4.45mA which translates into I'm pulling .84A The CT reading I calculate by multiplying .00445 X 200 = .89A If I increase my motor speed my clamp reading goes to 2.58mA or 2.58A being drawn and my CT reading is now 13.45mA or .01345 X 200 = 2.69 ...a 1/10th of an amp off, ok I can deal with that. ..and, technically, my clamp is only "good" down to 1A so there may be a little fuzz down where I'm reading.

The issue I'm running into is that I need to measure a voltage response, so my first thought is to just put a resistor across the CT and measure the voltage across the resistor, but I can't get any where near a linear response... I've tried a 10 ohm 1% reisistor and a 200 ohm 1% resistor thinking that my hardware is plenty sensitive to measure the difference...

Here are some test points...
200 ohm 1% resistor
2.6A through the clamp = .82mV across the resistor
1.12A = .6mV
.48A = .34mV

Go to a 10 ohm 1% resistor...
2.55A = 130.2mV
1.01A = 52.7mV
.47A = 24.9mV

My thinking was I'm using a 10 ohm 1% resistor... I was figuring V=IR, and if "I" is 1mA (.001) and "R" is 10 then I should get 10mV/A which I should be able to measure quite easily with my hardware.

Is my thinking off here? Once again, I know just enough to be dangerous! I'd appreciate a point in the right direction!
Thanks,
Chad
 
You could be using the wrong value of burden resistor.

Look at the data sheet for the CT. It will tell you the value of burden resistor you need.
 
Thanks for the reply Hero.

I have the spec sheet here in front of me... is there another term for that? The only value that's given in terms of a resistor is "DCR Max Ohms"... not sure what DCR means... I've seen other spec sheets that call out a specified resistor value to read across, but this one does not.
 
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