Measuring resistance in the milli-ohm range ?

[compengr]

When you're doing current measurements, you often use a very small resistor value in the milli ohm range so that you don't effect the system behavior.

Right now I'm using a 0.005 ohm current sensing resistor (value given by manufacturer).

Would I be able to build a VI that measures down to the milli-ohm range with my current hardware ?

I have the M-series (High speed) PCI-6259, 16-bit resolution, 1.25MS/s(single channel),32 analog in, 4 analog outputs,48 digital IO .

Do you think it's theoretically and practically feasible ?


Ed

[tbob]

Your setup is not clear. Are you sending a current through the 0.005 ohm resistor and measuring voltage with the 6259 to calculate resistance? The best way to measure very low resistances with accuracy is to use a DMM in the 4 wire resistance measurement mode. In case you are not familiar with 4 wire measurements, two pairs of leads are connected to the point where the measurement is to be made. The leads from the DMM are called Hi, Lo, Hi Sense, and Lo Sense. The Sense wires will measure the drop across the Hi and Lo wires, and this value will be subtracted from the reading. The result is that you will get the true resistance of the component without the resistance of the DMM Hi and Lo leads added in. Using a DAQ board to make low resistance measurements is not practical in my opinion. you will be measuring the resistance of the wires from the DAQ plus the resistance of the component. 4 wire measurements is the only way to go for milli-ohm measurements. There are several DMMs that are accurate at that range. Keithley makes one, and I think that Agilent also makes one. Check their specs.

[AnalogKid2DigitalMan]

What is the current that is passing through the 0.005 ohm resistor? That is what is the key to success. If it is 10A, then it will generate 50mV, fairly feasible but a wee bit low in my opinion. If it is 0.001A, then it will generate 5uV, not so feasible. Is it DC or AC, any harmonics, what is the maximum frequency content, what is acceptable level of range and accuracy? All these need to be considered to determine if it can be done reasonably.

Consider using 4 wire Kelvin connection, it is a must have for this range of resistance.

Your vi will need to control a precision current source and read the voltage developed across the resistor to calculate resistance. Not too hard.

Alternative would be to use an off-the-shelf milli-ohm meter or DMM that has milli-ohms range (again 4 wire kelvin connection) which also has a serial or GPIB port. The vi would then setup the instrument to do the test and poll it for the results. Again, fairly straightforward.

If you just need to measure this one resistor once, just borrow a milli-ohm meter and skip the LabView.

Please post more details as to your needs and intent.

[compengr]

guys,

Thanks a lot for the help.

I think you guys provided more help than I was really expecting.

I will look into the DMM with the Kelvin probes. That seams what everybody is recommending for milli-ohm measurements.


Ed

[AnalogKid2DigitalMan]

You are most welcome. It's Fridayyyyyy, time to go 'OHM' now 🙂