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resistance measurement

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Hi,
i want to measure a resistance with my PXI System.
I have the PXIe-6356 module.
Is it possible to measure this with VeriStand?

 

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Solution
Accepted by topic author ache

Hi Ache,

 

sadly the PXIe-6356 is not designed to to resistance measurements. For these tasks our PXI-DMMs are usually used. But you can of course use an external power-supply (constant current) and source the resistor with a fixed current, you can then read back the Voltage drop over the Resistor using the PXIe-6356 and calculate the resistance from this.

 

I hope this helps, let me know if you have further questions!

 

Cheers,
Jan Göbel

Staff Technical Support Engineer

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Hello,

is there an opportunity to source the resistor with fixed current by an AO of the PXIe-6356,PXIe-6738, or PXIe-6378?

Because until now i was not able to give out fixed values of current.

I also checked it out with a fixed voltage and a current measurement, but it was not possible to make some correct current measurement.

Can you imagine why ? How i can understand the shunt resistance, is it a parallel shunt resistance and how can i set the value then to Inf?

 

Thank you!

Best wishes ,

Michael

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Hi Michael,

 

no sadly the devices you wrote all don't support current output, so they can't be used for this. But the PXIe-6356 supports a current measurement (thats what the shunt is used for). It should also work, if you output a 10V voltage to the resistor and then measure the current, this should work with your setup.

 

Cheers,
Jan Göbel
Staff TSE

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Hi,

yes, that is what i think as well.

But i have a Problem doing this, because i dont want to have a shunt resistance.

My resistor is about 10kOhms and it should not be a problem to measure this current, sourcing with 10 V Output.

So witch Settings do i have to make? There is an error, when i set as shunt resistance Inf.

 

Best regards,

Michael 

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Hi MIchael,

 

why do you set the shunt resistor to inf? Can't you just source 10V with an analog output task, and then do a Current Input Task to read the current?

 

Cheers,
Jan Göbel

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Hi,

the Problem is the value for the shunt resistor is preset to 0 Ohms.

When I run the project, I get an error that says that the requested value 0.0 cannot be reached.
As a result, I have to set a great value to run the entire current over the AI, right?

 

Best wishes,

Michael

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Hi Michael,

 

the value "inf" is not supported for the shunt, but theoretically the shunt can have a very high resistance. This is coupled to the input-range you are trying to measure. Try to set the shunt resistor to something similar as your measured resistor (10kOhm) and then adjust the range of the current measurement.

If you source the Resistor from an analog output, be aware that there is a maximum sourcing current of +-5mA. With 10V and 10kOhm you should get 1mA which is fine, adding the Shunt (100 Ohm, 1kOhm, 10kOhm ..) to it this should result in an even lower current which should be fine for your setup.

But be aware, that if you are measuring smaller resistors you may reach this limit. In this case you need to set the shunt resistor high enough so that both resistors together don't pull more than 5mA.

Let me know if this helps.
Cheers,
Jan Göbel

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Hi,

So do I get it right that you can't do a current measurement without shunt resistance?
I have now connected a 150kOhms resistance and measure after calculation a resistance of 160kOhms. Is the measurement correct?

(so do I subtract the 150 kOhms?)

 

Best wishes

Michael

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Message 9 of 16
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Solution
Accepted by topic author ache

Hi Michael,

 

sorry, I oversee, that the PXIe-6356 does not has an internal Shunt resistor, now I get your struggle.

 

First of all, here is the basic getting started for Current measurements:

http://www.ni.com/getting-started/set-up-hardware/data-acquisition/current

In Figure 7 you see how the Shunt (R) needs to be connected. The only modification to the diagram is in your case, that you don't have a current-source, but a Voltage source with a resistor. In this case the current flowing through the Shunt causing a Voltage-drop is caused by both resistors in a row, so you're right. You need to subtract the resistance of the Shunt to get your result.

 

After drawing the circuit on a piece of paper I realized, you can also just do a Voltage Input on the Shunt (thats what DAQmx does anyway in the background), and then use the voltage-divider-formula to calculate the resistors value based on the known shunt value. Both ways should work the same way, use what ever suits you better.

 

I hope now it works, let me know how it goes!

 

Cheers,
Jan Göbel

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