03-14-2016 12:02 PM
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
Solved! Go to Solution.
03-14-2016 12:18 PM - edited 03-14-2016 12:21 PM
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/
03-14-2016 12:20 PM
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!
04-15-2016 09:22 AM
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?
04-15-2016 10:16 AM
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?
04-15-2016 10:24 AM
Wiring diagram attached. Tried measuring the voltage drop, tired the NI Max resistance configuration and a bunch of others. Nothing seems to work.
04-15-2016 10:32 AM
04-15-2016 10:37 AM
Tried measureing the voltage drop. No luck.
04-17-2016 10:49 AM
04-18-2016 12:31 PM
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.