daqmx resistance measurement 6251

[MickF]

I'd like to take a resistance measurement with a NI USB-6251.  It seems possible but I'm stuck on the excitation source.  I'm using the simple VI example Resistance - Continuous Input.vi.  

The Current Excitation source cannot be set to anything other than External without runtime errors.  Selecting an External source allows the vi to run but I expect to have to supply an external source.  What specs does it need to be and where do I connect it?  Also, What pins are resistance measured across?

My project is eventually going to include an NI USB-6343 instead of the 6251 I'm using for proof of concept.  Will the solution for the 6251 apply to the 6343?  One is M series and the other is X series.

Thanks,

Mick

[Sam_Sharp · Accepted solution]

I think that the 6251, as it is an M-series (i.e. medium range) DAQ device, I don't think it has an internal current source which is used for resistance measurement. For this device, you need to pass a known current through the device and it will measure the voltage drop over the resistor, giving you the R from R = V/I.

There is also a difference between 2-wire and 4-wire resistance measurement - for low resistances, the 4-wire method helps to account for the resistance of the measurement leads.

I am not sure if the X-series device has an excitation source either...I couldn't see anything obvious from the datasheet.

Edit: There is a guide here which describes a method to measure resistance - http://digital.ni.com/public.nsf/allkb/408C3D76B0B5690C86256E4500056DDF and a general article here - http://www.ni.com/white-paper/3981/en/

[GerdW]

Hi Mick,

some basic understanding of (electrical) measurements would help here!

Your NI6251 provides some analog inputs able to read a voltage signal.

Do you know Ohm's law? You know resistance is voltage devided by current?

So if you would supply a known current to your resistor you could measure the voltage across it. Using both values it is very easy to calculate the resistance…

To get started: use the simple ReadVoltage example VIs!

[Jim12345678]

I'm trying to make a resistance measurment using the usb-6281 and using the +5V supply from the DAQ.  I ran a 27.2K resistor from the +5V supply expecting that to generate 0.183 mA, put that current across the unknown resistor and tied the negative differential channel to the analog ground.  No luck.  Any ideas?

[GerdW]

Hi Jim,

can you supply a schematic of your wiring?

Additionally: using a resistor to have a certain current is nice. Using a second resistor in series will change that current (thanks to Ohm's law) so why do you expect your measurement still valid?

[Jim12345678]

Wiring diagram attached.  Tried measuring the voltage drop, tired the NI Max resistance configuration and a bunch of others. Nothing seems to work.

[GerdW]

Hi Jim,

you need to measure the voltage drop as you created a voltage divider! You cannot use a "resistor" measurement here!

Attach your VI! (How do you set up the DAQmx channel?)

[Jim12345678]

Tried measureing the voltage drop. No luck.

[GerdW]

Hi Jim,

saying "no luck" will not help you and us to fix the problem…

Please provide your VI and exact error messages you encounter!

[Jim12345678]

It appears that I cannot use the +5V supply from the DAQ to provide the voltage for the circuit wired into the DAQ (somehow +5V supply is not isolated or other such thing I'm not going to pretend to understand).   If I use a battery, they systems behave as expected.