analog input and current shunt resistors

[topaz]

I am  using LabVIEW 7.1, with the DAQ assistant. I have a 0.005 Ohm current sensing resistor connected to a load and am trying to measure the voltage drop across it. I should have current values between 1 and 20 amps (not mili amps).  I am not sure if the device I am using is just not sensitive enough to use this small of a resistor (USB-6259) or if I am inputting some wrong settings.  I go to Analog Input -->  Current --> and i change my resistor value to 5mOhm and my range is from -20 to 20 Amps.  When I try to test just this channel in measurement and automation, it just oscilates from -20 to 20 Amps, not actually displaying my current.  Any suggestions?

[Abhinav T]

topaz,

Thankyou for posting on the National Instruments Forums.

There are a few things I would recommend

- Try measuring the voltage being read instead of the current as that is what the DAQ card actually reads.
- Change the resistor's value to 0.5Ohms as that will utilize the full spectrum of the DAQ card's voltage input limits.
- Confirm the input signal is infact a DC current that is not fluctuating (this can be easily done using an oscilloscope).

What are the other settings you configured your Current Input task for?

Thanks,

Abhinav T.
Applications Engineer
National Instruments

[Conseils]

Is it a DC current?

Are you using single ended or differential measurement?

Do you have an earth (ground) loop?

You should be on the 100mV or 200mv input range to measure the voltage with the DAQ assistant. Debug with this as its the easiest way to confirm operation.