Current direction in electrical circuit with NI 9264

[Yabudabi]

Hi,

I would like to measure R1 and R2 in my circuit, using a NI 9264 and NI 9203 module.

For that, I already wrote a VI in LabView which subsequently outputs a voltage and measures the current:

- First, a voltage V0 is applied to AO0 such that a current flows through R1 to the NI 9203 module and

- Then, a voltage V1 is applied to AO1 such that a current flows through R2 to the NI 9203 module.

When running my script just for R1, I realized that I measure a different current when R2 is not connected to AO1 of the NI 6264 module and when it is connected.

The only explanation I have is that when R2 is connected to AO1, the current from AO0 can flow through both the NI 9203 but also to the AO1.

Am I right with my guess? If yes, is there a way to block AO1 such that current cannot flow through this output?

I attached a picture of my electrical circuit.

Thank you very much for your help!

 

[ZYOng]

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@Yabudabi wrote:

is there a way to block AO1 such that current cannot flow through this output?

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I don't think that's possible. Perhaps you should build an external switch circuit or get a Relay Output Module like NI-9482 or NI-9485

[crossrulz · Accepted solution]

Yes, you do have current flowing from AO0 to AO1 through the resistors.  As you have it connected, AO1 will always effect the measurement for R1.  To make it even worse for you, AI channels have a very high impedeance, practically an open.  So you have no current draw to possibly get the resistance measurement.  To measure a resistor you need a Voltage and a Curent measurement and then apply math (R = V/I).  So what you really need is a known resistor to measure the voltage drop to calculate the current (I = V/R) and then measure the drop across the test resistor, divide by the calculated current, and you have the resistor value.  This is how DMMs do it.  This is how you should be connecting things up in order to get your measurement.  I recommend a 1 Ohm current-sense resistor to make the math a lot easier on yourself.

NOTE: Duplicate this circuit with different channels for a second resistor measurement.

[Yabudabi]

Thank you very much for your clear answer!

As I understand from your answer, it is not possible though to add a second resistor R2 to the same circuit and measure first R1 and then R2?

[crossrulz]

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@Yabudabi wrote:

As I understand from your answer, it is not possible though to add a second resistor R2 to the same circuit and measure first R1 and then R2?

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Nope.  You cannot have other possible current paths than your sense circuit to get an accurate measurement.

[santo_13]

To suggest a better test fixture and method, please describe your application and the instruments you have on-hand.