Milliohm Meter

MilliOhm Meter Display 0.025ohmThe Milliohm Meter Project

I built this mil­liohm meter sev­er­al months ago which is based on a project by Scullcom Hobby Electronics and pre­sent­ed on YouTube.

Louis at Scullcom Hobby Electronics has a new video updat­ing the mil­liohm meter, and fea­tures the board I pro­duced in this post.
NEW Link to updat­ed video on YouTube
NEW Version 1.5 board avail­able at OSH Park requires addi­tion­al 0.01 uF ceram­ic capac­i­tor, 2 pin head­er, and 250 mA PTC fuse.
NEW Updated project post with Milliohm Meter ver­sion 1.5 board.

It uses a 4 wire kelvin mea­sure­ment sys­tem that sup­plies a pre­cise 100 mA cur­rent to the device under test using one pair of leads, and then mea­sures the volt­age drop across the device under test using a sec­ond pair of leads.
As designed it can be used to mea­sure low resis­tances of under 2 ohms (Ω).   Some of the design changes I made to fit my needs were:

  • Change the num­ber of resis­tors need­ed in the cur­rent source cir­cuit
  • Use high­er pre­ci­sion resis­tors with a low tem­per­a­ture coef­fi­cient
  • Add sep­a­ra­tion and shield­ing between input, out­put, and sup­ply cir­cuits
  • Use low­er ESR and high­er val­ue capac­i­tors for the MAX680 pow­er sup­ply
  • Add addi­tion­al decou­pling capac­i­tors on the INA106, and MAX680 IC’s
  • Change the volt­age reg­u­la­tor from a TO220 pack­age to a SOT-223 pack­age

MilliOhmMeterV1.4boardOSH Park gen­er­at­ed ren­der of top cir­cuit lay­out of the V 1.4 board

OSH Park link to shared project page for V1.4 Milliohm meter

MilliOhm-Meter-V1.1Components installed in ver­sion 1.1 board

MilliOhm Meter V1.1-03Angled view of Milliohm meter cir­cuit board

MilliOhm Meter Display OffFront pan­el with switch, 4.5 dig­it dis­play, and dual banana jacks for 4 wire con­nec­tion to device under test.

MilliOhm Meter Display OnMeter turned on with blue LED dis­play.

The LED dis­play was pur­chased on Ebay from sell­er cold­fu­sionx and is a 4 1/2 dig­it blue LED dig­i­tal 2V Volt meter. The pre­sent­ed spec­i­fi­ca­tions are:
Accuracy: +/- 0.1% «
Power: 5V
Range: 0–1.9999V
Resolution: 0.0001V

I was very hap­py with the per­for­mance of the dis­play meter, and it looks much bet­ter than is shown in the pho­tos. The bat­tery assem­bly is com­prised of 6 AA 1.5 volt Energizer Ultimate Lithium cells mak­ing a nom­i­nal 9 volt sup­ply to the board. The lithi­um cells do very well with the + 130 mA load and had a very low volt­age drop in use.MilliohmInsideCase02Component place­ment inside case.

To pow­er the dis­play meter I added a sep­a­rate buck con­vert­er to sup­ply a ded­i­cat­ed 5 volts from the 9 volt bat­tery.
On the V1.1 board I end­ed up with a bad foot­print for the LM2940 5 volt reg­u­la­tor and bodged a TO220 ver­sion to the board. This has been cor­rect­ed in ver­sions after V1.1.

MilliOhm Meter Display 0.025ohm25 mil­liohm 1% resis­tor con­nect­ed with 4 wire leads.

The com­plet­ed meter works well for my pur­pos­es and I have found that it is used much more than expect­ed for things that I would nor­mal­ly use my Fluke 189 mul­ti­me­ter for, and I don’t have to put up with the low res­o­lu­tion.

Link to EagleCAD Milliohm V1.41 schemat­ic and board files ZIP

A big Thanks! to Louis from Scullcom Hobby Electronics for design­ing and pre­sent­ing this project in an infor­ma­tive and pro­fes­sion­al video.

There is an updat­ed project post with Milliohm Meter ver­sion 1.5 board.

17 Replies to “Milliohm Meter”

  1. Wow, that looks impres­sive­ly nice. Is this pure­ly a one-off per­son­al project, or are you con­sid­er­ing mak­ing them to sell? I for one could be inter­est­ed in buy­ing one if they were avail­able…

    1. Hi LeoNerd,
      It was intend­ed as a per­son­al project to fill a need, but I want­ed to doc­u­ment it to make it eas­i­er for the next per­son to build.
      I do have two un-pop­u­lat­ed ver­sion 1.1 boards avail­able, and the ver­sion 1.41 boards are avail­able from OSHpark https://oshpark.com/shared_projects/UWvehNO2
      Thanks

      1. I’d be inter­est­ed in buy­ing one of those unpop­u­lat­ed boards then if it’s going spare. I don’t see an email address pub­lished we could dis­cuss that on — maybe you could email me?
        Thanks.

  2. Cool!!!
    Im also build­ing one myself but your cir­cuit is look­ing far way more sim­pler than my so far.
    Great job, I gonna use your as a ref­er­ence.
    Thanks for share.

  3. Hey Barbouri,

    It seems we have very sim­i­lar inter­ests, and vis­it the same sites for some of our projects.

    I was already plan­ning to build the mil­li-ohm meter, and the 6.5 dig­it DMM, when I found the link to your boards.

    Since you’ve made some inter­est­ing and worth­while changes to both designs, one would need to have the BOM to be able to work with the boards that are avail­able through OSH Park.

    Do you by any chance have a BOM for both projects to share?

    Mant thanks,

    Paul

    1. Barbouri,
      In case you don’t have a BOM that will help order­ing the right parts, I went through the exer­cise myself but could use your help in ver­i­fy­ing that I picked the right parts to match the PCB lay­out.
      If you can send me a PM with your email address, I can send you the Excel sheet, and can start on the Milli Volt/6.5 dig­it DMM.

      Many thanks,
      Paul

      Part Value Information Supplier Partnumber

      C1 22uF 16V Tantalum? Or low ESR Mouser / DigiKey Nichicon UMA1C220MDD1TP
      C2 22uF 16V Tantalum? Or low ESR Mouser / DigiKey Nichicon UMA1C220MDD1TP
      C3 22uF 16V Tantalum? Or low ESR Mouser / DigiKey Nichicon UMA1C220MDD1TP
      C4 22uF 16V Tantalum? Or low ESR Mouser / DigiKey Nichicon UMA1C220MDD1TP
      C5 220uF 10V Electrolyte LOW ESR Mouser / DigiKey Panasonic EEU-FR1A221B or EEU-FR1A151B
      C6 0.1uF MLCC X7R Mouser / DigiKey Vishay K104K20X7RH5TL2
      C7 0.1uF MLCC X7R Mouser / DigiKey Vishay K104K20X7RH5TL2
      C8 0.1uF MLCC X7R Mouser / DigiKey Vishay K104K20X7RH5TL2
      C9 0.1uF MLCC X7R Mouser / DigiKey Vishay K104K20X7RH5TL2
      C10 0.1uF MLCC X7R Mouser / DigiKey Vishay K104K20X7RH5TL2
      G1 AB9V
      IC1 LM2940‑5 LM2940IMP‑5.0/NOPB Mouser / DigiKey TILM2940IMP‑5.0/NOPB
      IC2 LT3092 IST is bet­ter than the EST DigiKey LT3092EST#TRPBF or LT3092IST#TRPBF
      IC3 MAX680CSA Mouser / DigiKey MAX680CSA (end-of-life) or MAX680CSA+
      IC4 INA106U Mouser / DigiKey TIINA106U-2K5
      IC5 LT1634ACS8‑1.25 DigiKey LT1634ACS8‑1.25#PBF
      R1 56R .1% 250mW 15PPM/C Mouser / DigiKey H856RBYA or YR2B56R
      R2 56R .1% Mouser / DigiKey H856RBYA or YR2B56R
      R3 56R .1% Mouser / DigiKey H856RBYA or YR2B56R
      R4 56R .1% Mouser / DigiKey H856RBYA or YR2B56R
      R5 56R .1% Mouser / DigiKey H856RBYA or YR2B56R
      R9 100R .1% Mouser / DigiKey YR1B100RCC or H8100RBYA or RC55Y-100RBI
      R10 499K .1% Mouser / DigiKey YR1B499KCC or 279-H8499KBYA
      R11 10R .1% Mouser / DigiKey YR1B10RCC or H810RBYA
      R12 100K 3266 Bourns Mouser / DigiKey 3266P‑1–104LF
      R13 100R 3266 Bourns Mouser / DigiKey 3266P‑1–101LF

  4. Hi Barbouri,

    Great work on this and your oth­er projects.

    Can you tell me where you got your test clips from? Did they come with the 2 leads per clip or did you make them your­self?

    Thanks,
    Tim

    1. Hi Tim, Thanks!
      The Kelvin 4 wire test leads were pur­chased from Amazon.com. Search for “kelvin test”.
      They were an inex­pen­sive set pur­chased at under $20 USD, but will build my own set next time as their qual­i­ty is not the great­est.

      Greg (Barbouri)

  5. Paul Hi,

    Very impres­sive work, mind shar­ing links for the test leads sock­ets and the enclo­sure ?

    I think­ing of build­ing one based on your lat­est v1.5 PCB — many many thanks for that.

    Eddi

    1. Hi Eddi,
      The V1.5 enclo­sure is a Hammond 1455N1601 extrud­ed box with met­al end plates 6.299″ L x 4.055″ W x 2.087″.
      Sockets are:
      501‑1081-ND, POMONA ELECTRONICS, 1581–3 PJS+, 1, JACK BANA PANEL MT TIN ORG
      501‑1079-ND, POMONA ELECTRONICS, 1581–1, PJS- 1, JACK BANA PANEL MT TIN BRN
      2269–0‑ND, POMONA ELECTRONICS, 2269–0 PJV+-, 1, BANANA JACK DOUBLE BLACK

      Greg (Barbouri)

  6. Thank you for all the info and help on this page .
    One more thing i can­not find it.
    The alli­ga­tor clips where you plug the four wire.
    Any sug­ges­tion ?
    Digikey would be nice.

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