Inaccurate Current Output from DAQ 9265 when 250 Ohm Resistor is Wired

[jazz234]

I am working with a 0-20mA Current Output Card (NI 9265), with a simple LabVIEW program controlling the current that is output. The card outputs correctly (checked with a multimeter) until a 250 Ohm resistor is wired into the port: with the resistor, the amount of current across the terminals is dropped by ~0.5-0.9mA. I have tried switching the port I am using but each give the same result. I have also tried wiring another resistor and observed the same behaviour. The maximum load of the card is said to be 600 Ohms - why is this happening?

[jred101]

Hi jazz234,

Can you please explain more your application. If you require a greater a output current you can try reading and using the example from the following knowledge based article:

http://digital.ni.com/public.nsf/allkb/B990B1BB8C6228F286256DDA005526E0

Regards

[jazz234]

Hello, 

The application is to control a voltage driven releif valve. So from LabVIEW I output a 0-20mA current to the DAQ Card. With a 250 Ohm resistor wired into the card (receiving the current)  I can give the valve 0-5V. I tested the current output with a DMM across the port I was using on the card  WITHOUT the resistor, and it gave an accurate reading, however once the resistor was wired in, the current reading dropped. The issue isn't getting a higher current output, rather, it is getting an accurate current output (a reading that will match the output I am sending in LabVIEW). 

Thanks

[Henrik_Volkers]

can you make a simple schematic  (including the DMM)  of your setup?

[jazz234]

Here is a sketch of what the circuit looks like:

Where Rvalve is very big. 

[Henrik_Volkers]

You have to measure current in line (serial) with the sensing resistor!

Read the manual of your DMM or  whatever tutorial on how to measure current.

[jazz234]

Thank you Henrik! I am now reading the current (in series with the resistor) however there is still a current drop. However there is a constant scale factor - a table of values for current outputted vs. current read is attached, as well as a graph. The relationship is: y = 0.9742x - 0.0046 with an R² = 1. Do you know why this is still happening?

Thanks again. 

[Henrik_Volkers]

Again, how does your setup look like?
Please include all supplies an every connection/ device.You should have such kind of documentation anyway 😉
Next thing are the tolerances of both devices.

[jazz234]

Hello,

We were actually able to resolve the issue: it turns out that the internal resistance in the valve we were controlling was not infinite (like we expected) and this resistance was actually causing the drop in current we were reading. The linear relationship between the current output and the current read was sufficient for our use.

Thanks for your help.