Current measurement accuracy issues with USB-6009 and SignalExpress

[srao]

Hello,

I am trying to measure and acquire a DC current (5-20mA) in my circuit using the shunt resistor method (tried both 10ohm and 60ohm values). I am using the USB-6009 and Labview signal express 2012 v6.0, and am using the Differential mode, inputting the right shunt resistor value, and I see a current waveform in the display.

But it has the following problems:

1. The sign is wrong (even though I am feeding the higher voltage point to the DF+.

2. The value is wrong. It is showing up as -120mA, where in fact it is 5mA. When the current increases to 20mA, the display changes to -135mA.

 I tried the following:

1. Changing the shunt resistor from 10ohms to 60ohms so as to increase the differential voltage sensed. (didn't help)

2. Changing the Port on the USB-6009 from AI0 to AI1.  (didn't help, in fact, it changed the current now from -120mA to -150mA)

3. Placing the shunt resistor on the high-side of the circuit (Line-voltage side) and also on the Low-side (Ground return side). (didn't help)

I would appreciate any help in solving this issue.

[fblerg]

Hi srao,

I have a couple suggestions for your setup. The USB-6009 does not natively support current measurements so I assume you have an external shunt resistor that you are measuring a differential voltage across and dividing the voltage by the shunt resistor value. For the sign issue, you can simply switch the terminals the wires are going into. This will give you a positive sign. You will also need a high precision resistor for precise current measurement. What type of resistor are you using? Another issue may be the current supply itself. What device are you acquiring current from? 

You may also want to take a look at this forum post.

measuring current in ni usb 6009

http://208.74.204.114/t5/LabVIEW/measuring-current-in-ni-usb-6009/td-p/2945414