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  • Author Author: mcollinge
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4-Wire Resistance Measurements -- Workbench Wednesdays 19

 

 

Bench DMMs have an extra set of banana jacks called "sense." Known as a Kelvin or 4-wire measurement, these inputs accurately measure small resistances. Like, milliohms small. In this video, learn how to use a multimeter with 4-wire capability, prove 4-wire is more are accurate (in some cases), and a trick to get more accurate measurements with traditional 2-wire techniques.

 

Supplemental Material:

 

 

Bill of Material:

 

Product NameManufacturerQuantityBuy KitBuy Kit
MP730028 US Bench DMM, 5.5 DigitsMulticomp Pro1Buy NowBuy Now
Through Hole Resistor, 0.1 ohm, 3 W, ± 5%, Axial LeadedMulticomp Pro1Buy NowBuy Now
Through Hole Resistor, 1 ohm, 250 mW, ± 5%, Axial Leaded, 250 VMulticomp Pro1Buy NowBuy Now
Handheld DMM (72-7780)Tenma1Buy NowBuy Now
Test Lead Set, Banana Plug, Test Clip, Black, Red, 914 mmTenma1Buy NowBuy Now

 

  • in reply to axelschmidt

    Hi Axel,

     

    Glad to see you on active on the community!

     

    Since a couple of people were upset about my use of "small caps," we decided to change the font used in future diagrams. The one we used up until now did not support lowercase letters. So I used "small caps" to indicate lowercase. You'll notice with the video Workbench Wednesday 21: DMM Half-Digits Explained, the font in the diagrams is different. (And it has native lowercase letters.)

     

    As for the units mix up, that was a mistake on my part. Good catch. Even though the previous shot clearly showed milliamps, I put microamps in the graphic.

  • Using a capital letter "M" for milli  ( 1/1000) is quite confusing. Also at 4:35 you show R = 800mV/797uA = 1.0025Ω ????

    mV divided by uA should be in a kilo Ohm range  (10^-3/10^-6 = 10^3 )

     

    Axel

  • Nice video, . Thank you for the explanation, I haven't used a 4-wire measurement yet.

    By the way, DMMs are not the only devices that feature "sense" plugs. I noticed those on some power supplies and even electronic load. After seeing the video, it makes perfect sense.

     

    David

  • Nice to see this,  Not a lot of people know about it and make errors as a result.  I used to use the PSU method a lot on cable assemblies. I used to design retail display products for mobile phones and so the charge leads for the phones were much longer than standard leads and incorporated recoiling mechanisms to keep the cables tidy on the display.

    Even after demonstrating that specifying too long a cable meant that the voltage drop in the cable would cause the phone to charge more slowly or fail to charge at all, the management and sales team couldn't quite get their heads around a cable having a resistance and kept selling the dream!

    In the end I designed a PSU that would compensate for the voltage drop in the cable.  They then moaned about increased build cost instead.

    I guess sometimes you just can't win!

    Dan.

  • in reply to DAB

    So, true story. When I started teaching "Measurement Basics" classes at my hackerspace, we had a bench DMM with 4-wire capability. I had never made a 4-wire measurement before, but I vaguely remember hearing about them in school (at that time, 10 years before.)

     

    Someone asked, "Hey, what is that for?" I said, "you know, I don't know. Maybe something to do with an h-bridge?"

     

    That day, I learned two things:

    1. The proper reason for 4-wire measurements, because, I looked it up later that night; and

    2. Be very careful what I say, when I'm not certain. The person wrote "h-bridge" down in their notes.

     

    That said, I was genuinely excited to make use of the capability for my upcoming DMM project.

    (stay tuned.)

  • Very good post.

     

    I had not really looked into very low resistance values, though I knew they were important from a hot wire wind speed instrument I used about 45 years ago.

    I had never been shown the four wire method before, but it makes perfect sense.

     

    Thanks for bringing my knowledge up a little.

    DAB