Testing Current Sources for a Kelvin (4-Wire) Milliohm Meter

RE: Testing Current Sources for a Kelvin (4-Wire) Milliohm Meter

fmilburn — Tue, 18 Sep 2018 01:42:37 GMT

Update on accuracy and precision....

I decided to do another test before moving on to the improved design. I took the current source out of the loop and connected the bench power supply through my good meter to measure current through the wire. The current was set to 0.974 amps and the voltage drop measured with a separate meter. This moves the measurements into a better range for my meters.

[View:/resized-image/__size/620x315/__key/commentfiles/f7d226abd59f475c9d224a79e3f0ec07-6513dd6a-c138-464b-8abf-df66a6b2550e/5732.contentimage_5F00_188437.jpg:620:315]

The meter on the left gives the voltage drop in millivolts for the full length of wire in the left hand picture and for the half length in the right hand picture. Thus with a fixed current of 0.974 amps the full length of wire is determined to be 0.0941 ohms and the half length 0.0476 ohms (this compares to 0.090 ohms and 0.044 ohms measured with the 10 mA current source).

At least part of the difference is due to the fact that I was down to the last two digits of resolution before and I am encouraged to continue.

RE: Testing Current Sources for a Kelvin (4-Wire) Milliohm Meter

jw0752 — Sun, 16 Sep 2018 20:51:56 GMT

Hi Frank,

Great start on a device to measure low resistances. It is so nice how the ideas and projects of others inspire us to learn and try new things. A combination of recent blogs has inspired me to try to build a simple DC load that will be good down to 1 mA. I built a larger load a couple years ago that will handle 6 amps up to 35 volts but whenever I try to use it for producing a load below 10 mA it is not stable. So far I have incorporated ideas that I got from [mention:b0bc65b9ecdc4307bd967592f00e340a:e9ed411860ed4f2ba0265705b8793d05] , [mention:b60dcf9c31a8492ea6badbcc611934d0:e9ed411860ed4f2ba0265705b8793d05] , and [mention:6e474abc33b64a82aa78c9813503c962:e9ed411860ed4f2ba0265705b8793d05] with small dashes of ideas from many others. I want to follow your project as I would also like to be able to improve my ability to measure milliohm resistances.

Thanks for your post.

John

RE: Testing Current Sources for a Kelvin (4-Wire) Milliohm Meter

shabaz — Sun, 16 Sep 2018 10:45:14 GMT

Hi Frank,

That's impressive results you're getting! It's a good idea to use this level of low current, since it will also reduce heating effects causing drift over a long measurement.

There's a nice circuit I saw recently, which is for 10mA that you may be interested in investigating, because it relies on only one part (resistor) to be accurate - this is in figure 1 here:

https://www.electronicdesign.com/analog/what-s-all-lm334-stuff-anyhow

The LM334 is very low cost, so it's a handy chip for constant current. That figure 1 circuit reduces the current through the LM334, so there is tiny self-heating.

However, the datasheet shows there is a variation with temperature (since the LM334 can also be used as a temperature-to-current converter), but that can be near-eliminated, using a single low-cost diode and another resistor.

So, that figure 1 circuit can be improved in the same way basically.

I've not tried that circuit, but maybe its worth a try if you're interested.

RE: Testing Current Sources for a Kelvin (4-Wire) Milliohm Meter

michaelkellett — Sun, 16 Sep 2018 10:09:09 GMT

You'll find that the op amps built into micro-controllers have pretty dismal characteristics.

If you are happy with the floating load (ie neither end of the resistance you are measuring is connected to ground) then the things that affect the accuracy of the measurements are the reference voltage, the resistors (R1 to R3) and the offset voltage of the op amp.

If you use a chopper (zero drift) type of op amp the offset will be below 20uV and the drift of offset less than 60nV/C (Microchip MCP6N16 - not the best but pretty cheap). If you don't mind calibrating out the scale error caused by the current being to high or low then you just need low temperature drift in the resistors. 10ppm/C parts are affordable, 1ppm (and better ) can be obtained but get kind of pricey.

In your design the reference voltage 300mV will result in error of 1.2e-6/0.3*100 = 0.0004% due to a 20C temperature drift of offset of the Microchip amp (a typical in processor op amp STM32F301xx is 83x worse, giving 0.03% drift error). If you used 10ppm resistors they would add 20ppm * 20 ppm drift (for 20C temperature change) = 400ppm = 0.04%. (bog standard 100ppm resistors would take this up to 0.4%).

From the above, the resistors are much more important than the op amp, so you could get the drift against temperature very low by using good (10ppm resistors ), a zero drift op amp would be probably be overkill but using an external op amp will give you a much bigger range of processors to choose from. If you use the same reference voltage for the current source and the ADC you measure the voltage drop with then the two will track and the absolute accuracy of the reference doesn't matter. Otherwise you will need a good (10ppm) voltage reference which will be quite pricey.

You could use perhaps 1V across R2 and scale R3 accordingly which would reduce the effect of op amp offset and offset drift by a factor of 3.3 - allowing you an even better choice of cheapo op amps.

If I were building this as a a feature in a bit of custom test equipment I would go for precision reference and resistors if it got round having to calibrate. It would be possible to get perhaps to within 0.2% by dead reckoning but would need a 0.05% resistors, the zero drift amp, and a 0.1% reference.

If you don't mind adjusting to compensate for errors in the resistors then you can put a pot at the junction of R1, R2 and the op amp. The pot will have a stinky temperature coefficient but you only need it to cover 2.5% of the range if you use 1% resistors - so 400ppm/C for the pot would be OK.

You could calibrate by using an accurate DMM to measure the current, or using a precision resistor and measuring the voltage drop (or if using the combined reference for the ADC and current source, it has to be the precision resistor.)