Reading mV from Current Transformer connected to NI 9206 through cRio Chassis

[phdieum]

I am trying to read mV from a current transformer with two leads (+/-) with output between 0.0 - 0.4V.  I have the pos(+) connected to channel AI0 and the neg(-) to channel AI8.  Channel AI0 is set to DIFF, +/- 1V.  The pos(+) and neg(-) are also each connected their own 47Kohm resistor which are both connected to COM.   Using a Fluke 189 Multimeter I get a constant reading off the +/- leads of 90mV.   The 9206, however, returns a floating value between +/-150mV.  Any ideas of what I'm doing wrong so that I can get the 9206 to read similarly to the Fluke measurement?  Any help would be greatly appreciated.

[kfc]

What is the burden resistance?  This would be the resistor connected between + and - terminals of the CT.  From what you have described is seems that it would be 47+47kOhms or 94k.  Usually the burden resistor is of the range 50ohms-1kohm, which will depend on the ratio of the transformer.....here is a datasheet that shows how to calculate the resistance for the working voltage that you desire:

http://media.digikey.com/pdf/Data%20Sheets/CR%20Magnetics%20Inc%20PDFs/CR8300%20Series.pdf?cshift_ck...

I'm not familiar with the 9206, so i can't comment on that yet.

Hope this helps some.

-Kory

[phdieum]

I'm not sure what the burden resistance of the current transformer is.  The current transformer is a 30Amp which sends 0.333V at 100% of 30Amp. The range is linear so the current transformer will send 0.033V at 10% of 30Amp.  The 9206 specifies when configured for differential reading to connect a bias resistor to both the pos(+) and neg(-) which are then connected to the COM (AI Ground) pin.  It states these bias resistors should be equal and between 10kohm<R<100kohm which is why I am using 47kohm resistors.

[kfc]

What do you have driving the CT?  What frequency are you operating the current....or is it a DC/AC CT?  Can you provide a link to its datasheet?

-kory

[phdieum]

Hey Kory, thanks so much for your help.  The CT is measuring amp on one leg of a residential air-conditioner.  I'm assuming the current is at 60HZ?  Here is the link to the CT data sheet:

http://www.magnelab.com/catalog/sct_0750.php

[kfc]

Is this a 110-120V AC unit?  And if so are you monitoring on the proper line....and not the neutral line?  If this is so, you may be reading a near zero current in the neutral line and not the HOT line....which is carrying the current.

It looks as if the burden resistor is "built in" in the 30A CT you are using.

-kf

[phdieum]

Kory, this is a 220 AC unit and I've got the CT around one hot leg.  The burden resistor is indeed built into the CT.  I've measured the current of the hot leg on the AC separately with a Fluke 189 and it reads a constant 8.1 amp when the AC is on.  Also, I used the Fluke to measure the pos(+) and neg(-) lines off the CT when the AC is on and the fluke reads a constant 0.090V.  This voltage reading on the CT corresponds correctly to the amp on the line.  Also, I've compared two individual CTs with the same results with the fluke, so I know the CT is functioning properly.  It's just that when I connect the pos(+) and neg(+) lines from the CT to the 9206, I get floating values between +0.150V and -0.150V.  From what I read this was due to the CT Voltage signal being a floating voltage signal unreferenced to ground. This is why I was attempting to use bias resistors. In this link under Table 1. it shows the connection for a differential unreferenced signal:

http://zone.ni.com/devzone/cda/tut/p/id/3344 

I've tried two 9206s with the same results, so i don't think it's a calibration issue.  Also I don't see how noise could be causing this since the Fluke 189 is able to measure the voltage without a problem.  I must me missing something with the connection, I just don't know what exactly.  If I could get the 9206 to read a constant +0.090 Volt like the Fluke I'd be on my way.  

Thanks again for your help.

-jason

[phdieum]

just a note - the signal is oscillating between +0.150v and -0.150v.  Could it be a sampling rate issue?  Since the CT is capturing current at 60HZ could I be picking up noise? 

[Mr.F]

Hi,

Are you connecting the COM line of the NI 9206 to anything? The card is bank isolated and leaving that connection unconnected can lead to unexpected behavior. Can you connect COM to something and see if that clears up your signal? Depending on the common mode ranges you expect, you can try the COM to your chassis ground or the negative terminal on one of your signals. Have a nice day.

[phdieum]

Hey Matt,  thanks for your help.  The COM line was connected to the pos(+) and neg(-) signal via bias resistors of equal size.  The COM was not connected to anything else.

I tried connecting the COM to chassis ground and to the signal neg(-) with no effect. The signal still oscillates between +0.150v and - 0.150v.  The oscillation is definintely periodic.