Measuring Current with USB 6009

[tdb867]

Helly everyone,

I've been attempting to measure the current across a circuit in LabVIEW with the usb 6009, but the DAQmx express vi seems to be giving me some trouble. When I configure the channel to read out a voltage input it is spot on, however when I change the channel so it converts to current it appears to be saturated at a level below what I am looking for. Essentially I am inputing around 3 V, but after converting to current it will only read a little over 1 V. The Ohm's Law calculations here are very simple, so I'm confused as to why this could be happening. I've played around with all the values in the DAQmx express vi's channel configurations, but nothing seems to change excepting the external shunt resistance, which is supposed to be 10m.

I can very easily just do the calculations programmatically, but if there's a way around that I'd certainly be interested. I'll come back after my lunch break to check if you guys have come up with anything.

Thanks,

Travis

[P@Anand]

How are you converting Voltage to current and how you have connected it to the DAQ? Did you change the VIs to current measurent and did the configuration?

[crossrulz]

Can you post your VI or at least show your settings?  Are you using a DAQmx scale?  What is the maximum current it could see?  3V/10mOhms = 300A!!!!!  Are you sure about the units you stated?  How do you have the current shunt connected?  Is it a high-side shunt or a low-side?

[tdb867]

Yes, the units are correct. I'm a little shaky with the exact mechanisms behind the circuit, but I believe it's a high side shunt. Basically it's seeing around 27 V and then feeding 2.7 V to the input (differential configuration, ai +/- pins). I've verified that this is actually happening using a few different methods. 300 Amps is essentially the target current draw for this circuit.

When I mentioned DAQmx express vi I was referring to the DAQ Assistant VI, in case I wasn't clear. I've made a png to detail the configuration I was using. I'm not sure my VI will help you out any, but I've includeda picture of the block diagram as well. If you want to see my code I can post that as well, but it seems to be working just fine.

Let me know if you have any other questions.

-Travis

[tdb867]

So I've realized that the current sensing setting for the daq channel is something entirely different. I changed my program to simply read a voltage and calculate the current as I had said.

Thanks anyways!

[johnsold]

The maximum voltage you can apply to the USB-6009 is ~10 V. SO DO NOT ATTEMPT TO MAKE A HIGH SIDE CURRENT MEASUREMENT ON A 27 V SYSTEM! 

Even though the differential measurement may be within the range the voltage to ground wil be 27 V on one input and 24.3 V on the other.

If you have not destroyed your DAQ device, consider these options:

1. Use a low side current measurement.

2. Use a High Side current sense device which produces a ground referenced voltage from a high side current. I do not recll the part numbers but Linear Technology sells such devices.

3. Use a differential amplifier designed to handle 27 V inputs as a signal conditioner.

4. Use a current transducer.

You might also consider getting advice from a competent engineer, technician, or electricain with experience at making such measurements. The cost of that advice could be less than the cost of replacing damaged equipment.

Lynn

[tdb867]

Like I said, I'm a little shaky on the specifications of the circuit. I'm working under supervision though, so it's all good.