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.
Solved! Go to 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/
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!
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?
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?
Wiring diagram attached. Tried measuring the voltage drop, tired the NI Max resistance configuration and a bunch of others. Nothing seems to work.
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?)
saying "no luck" will not help you and us to fix the problem…
Please provide your VI and exact error messages you encounter!
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.