DAQ not outputting current signal.

[rslewis96]

Hello and thank you for reading my post.

I have a circuit that I created to show the linear relationship of a slide potentiometer with respect to voltage and current. I have a current source set to 0.5mA running parallel to the potentiometer. I'm using the analog inputs on the ELVIS II to input voltage and current into LabVIEW. What I am trying to do is set it up so that both the voltage and the current get put into the formula V/I =R where R should be the resistance of the potentiometer. I know that there is some error in the accurate measurement of the resistance, but that's not a problem.

What I'm struggling with is whenever I go to check the vi for the readings, I can click on both the DAQs I have for the voltage and the current to see them working. I'll slide the potentiometer to see voltage changing in the DAQ as well as do the same for the current. Now, when I go to run the whole vi, voltage will output, the current will output 0A whereas when I was running it in the DAQ, I got a change in current. I'm not sure what I have done wrong. I've saved the setting, so I'm not sure what I am missing.

[Bob_Schor]

It is unclear whether your problem is related to LabVIEW (a problem, say, with your Block Diagram), with your use of DAQmx (I'm guessing you are using the Dreaded DAQ Assistant, which can really obscure things), or your lack of understanding how to make voltage, current, and resistance measurements (and a circuit diagram of what you are doing).

We could definitely provide more help if you would attach the following:

• The VI (the actual .vi file, not a picture of it) that you are using.
• A picture of the circuit diagram, showing the various components, the values that are fixed, and the measurement points (where you have attached the inputs for recording voltage, current, and/or resistance).  It is not clear how many things are being measured and how many are constants.

Bob Schor

[rslewis96]

Here is both my vi and the block diagram of my circuit. The value that should be constant is the current and the value that is changing is the resistance which also would change the voltage reading. I'm not sure what I am missing.

[GerdW]

Hi Lewis,

in your (to cite Bob: "dreaded" :D) DAQAssistent you set an external shunt of 1kOhm.

I don't see this shunt in your circuit diagram…

Why do you want to measure the current when you use a current source? Is this source so unreliable?

[Bob_Schor]

I'm going to assume that the circuit that you posted is, in fact, the circuit that you are using.  You are using an A/D (Analog-to-digital) converter, which converts analog volts into digital numbers.  With a little circuitry (E = IR), you can measure a voltage and interpret it as the current flowing through a resistor.  However, your circuit does not have such a resistor.  Hence it cannot measure current.

Bob Schor