Milliohm resistance adapter

[grenert]

Bob Nuckolls wrote up a very simple but effective adapter to allow low ohms measurements on pretty much any multimeter.  It uses the LM317 voltage regulator as a 100 mA constant current source for 4-wire resistance measurement.  His very nice documentation is available on his website at:
www.aeroelectric.com/articles/LowOhmsAdapter_3.pdf

I built one of these, making some small changes for my preferences and improved accuracy.  The basic design uses a single 12R resistor to determine the 100 mA current.  I measured my 12R resistor and found it was a little low, so I added another resistor in series and paralleled it with a trim pot to dial in 100 mA.  If the 12R resistor was too high, you would add the resistor and trim pot (in series with each other) in parallel to the 12R, to drive the value down.  I could've just used the trim pot, but I didn't want the lousy tempco and drift of the pot to affect things too much.  As it is now, the effect of the pot on the current is minimal.  I used a very oversized wirewound resistor for 12R so that it wouldn't heat much.  I also put a little heatsink on the LM317 so it doesn't heat up too much (though it would probably be minimal with 1.25 V at 100 mA anyway).

Rather than hardwire the Kelvin leads, I installed banana jacks.  I modified a couple of Pomona cables with double banana plugs by replacing the lugs at one end with the Mueller BU-75K Kelvin clips that I got on eBay.  The device runs on three AAA batteries, and there is some variation in current depending on the battery voltage.  The little HP meter on top is reading the battery voltage, and you can see the current dips a little bit as you go from 4.5 V (new alkalines) to 3.6 V (rechargeables).  The entire variance is only .03% however, so not a big deal for me.  You can see in the trend plot that the current is pretty stable over time, only varying by .01% over an hour or so.

[grenert]

My most accurate low ohms resistor is a 10R Vishay bulk foil with .02% tolerance.  The adapter runs 100 mA current through the DUT, so 10R will ideally appear as 1 V on the multimeter.  I got pretty close    My lowest value resistor is a 10 mOhm current sensor with 1% tolerance.  This should give 1 mV.  I got 0.982 mV on my uncalibrated 8842A, a cool 0.9979 mV on my in-cal 8846A, and just for fun, I plugged into the little HP 3476B (0.5% + 1 digit accuracy, whopping 1100 counts!) and got a respectable 1.1 mV.

[robrenz]

Nice job.

If you have room, adding a switch that reverses the current direction will allow you to remove thermal EMF errors of the connections that are significant at milli ohms and below. You just take the average of the absolute values of the readings of the two current directions.

[grenert]

Thanks for the suggestion.  I probably can fit a switch in there somewhere...

[quarks]

Very nice done. Thanks for sharing.

That is a good and really easy to build approach to go for.
I have tested (but only on breadboard) a similar construction from edn designideas
http://www.edn.com/contents/images/6566536.pdf
(see 70 EDN | june 12, 2008)
This design is "programmable" via BCD thumb wheels, from 1mA to 999mA and works quite well.

bye
quarks