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PCB for a Kelvin (4-Wire) Milliohm Meter

fmilburn

EDIT 24 Oct 2018  The schematic in this post contains an error.  It will be reposted after receipt of the PCB and testing.

 

I am developing an inexpensive but reasonably accurate meter for measuring resistance in the milliohm range.  The previous posts are listed in the related links at the bottom.  In this post the design for the working prototype is expanded to include a second current source for measuring resistance up to 400 ohm and provision made for future auto-ranging.  A new schematic and a PCB design are presented along with an update on meter options.

 

Revised Schematic

The revised schematic is shown below:

image

The added current source is identical to the first but designed for 1 mA current.  A dual N-Channel MOSFET with pull-down resistors is connected just above ground to allow selection of a source.  It will add maybe an ohm to ground.  There are jumpers and bypass resistors so that the second current source and MOSFETs can be omitted if desired.  This has not been incorporated into the working prototype so a second eye and comments on the design are welcome.  An additional 5V set of pins was added so a source other than the USB connection could be used as well.  Thanks for ideas and suggestions   image.

PCB Design

The PCB was done in KiCad (5.0.0) which is a new version for me and I am still finding my way around the changes.  The layout was done with ease of hand soldering in mind which led to what I consider poor routing so I may redo it and hope send it to the board house tomorrow.  There were a couple of unfinished wires when the renders below were made but those are finished now and it passes electrical check.

image

image

 

Panel Meters and Alternate Current Sources

The panel meter I ordered from China came in earlier in the week but they were a disappointment.  Although they advertised 5 digit resolution they are not much better than 3 and show zero when down in the low millivolt range so they aren't much use for this project.  There is a zeroing procedure which I tried without much luck and a small pot which doesn't seem to do much.  I also ordered a ammeter but haven't had a chance to really test it.  It looks like I will be using patch cables and a multimeter for the display.

image

 

I also finally got the other current sources I was going to test for the project but time is running out and the LM334 really works good so I will put those aside for a later time.

 

Next Steps

This week I also plan to finish preparation of the BOM and order the parts that aren't in hand but there probably won't be any more posts until after the PCBs are back in about 2 weeks time.  As always, comments and corrections are appreciated.

 

Related Links

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

More on Current Sources and a Kelvin (4-Wire) Milliohm Meter

Even More on Current Sources and a Kelvin (4-Wire) Milliohm Meter

Working Prototype of a Kelvin (4-Wire) Milliohm Meter

  • Meter mod:

    The meter has two white connectors. One connector is for the meter supply voltage to power it up. The other connector is for the measurement input to the meter.

    This modification is required on that measurement related connector.

    Using a pair of pliers or cutters, carefully pull off the white socket as shown in the photo below, which is the meter measurement input connector.

    It will slide off and leave the two pins remaining.

    You can desolder the pins now quite easily, without needing to remove the PCB from the meter enclosure.

    Next, with a knife/scalpel, cut the track indicated with a purple circle in the photo below.

    image

     

    Solder two wires to act as the meter input in lieu of the previous connector. The positive connection will be the corner pin, same as it was with the connector.

    However the negative connection is to be soldered to the resistor shown in the photo below. By doing this, the input to the meter module becomes a differential input.

    image

  • It was quicker to just photograph the mods:

    image

  • Hi Frank,

     

    Good news, tonight I've built the V1.3 board, with a few minor component value changes, and it's working pretty much spot-on, with no adjustment. I still need to do more tests, but I measured a resistance wire which I calculated to be 608 mohm based on its dimensions, and it measured 606 mohm with the meter.

     

    This time around I deliberately did not use any high accuracy component, all resistors were 1% thick film. I had to adjust a few values due to the components I had, but nothing was measured and calibrated out, so that I've got a very typical board if someone else were to solder with standard components too.

     

    Also, it measures all the way down to 0.0000 ohms now (I tested with a gold-plated zero-ohm link, it measured 0.0001 ohm) with a slight circuit mod to the display module. I'll write up the mods in the next day or two. Currently I am only using the 0-4 ohm range, because I didn't have a trimmer pot to install on the PCB for the 0-40 ohm range. However since it's working well without any trimming, I might just stick in a fixed resistor for now so I can use that range too.

     

    And the instability is fixed.. I think it was being caused by the long wires, being inductances, causing the constant current source to oscillate. Putting a resistor (10k is fine) between the two high side input connections, and another between the two low side inputs, made the issue completely disappear. I've not seen it once after doing the mod, yet if I remove the resistors, I can make the problem appear every few readings with the length of wiring I happened to have. So, in summary, I think the design is reliably providing measurements, and at least if the room temperature doesn't change much then it looks like no accurate parts are required, beyond 1% normal thick film, and for those who want to, they can use the trimmer to calibrate with a known resistance. More tests would have to be done to confirm the effects of temperature change, but since a significant part of that is automatically compensated, I'm hoping it's a non-issue for reasonable lab temperature differences experienced over time.

  • in reply to shabaz

    This project is a great example of one that grew without much planning and was left without the best of documentation :-).  I can’t access things right now but will try and find the latest gerbers within the next week or so when I am able.  The performance of the new version is similar to the first version but has accommodation for further tweaking and testing.  It has been on my backlog to revisit the project but not something I can get back to right away. 

  • in reply to andbro

    Hi Andre,

     

    The project works well, with typical error of around 1 mohm.

    I like using it because it has low current (10 mA max, so it doesn't heat up much the resistance-under-test) unlike some other projects which use 100 mA).

    Making it is straightforward, they are all low-cost parts (no high-accuracy parts are needed, although low ppm resistors are preferred), and a single trimmer is adjusted after completion, by using the project to measure a known resistance (two trimmers are used for two ranges).

    The values I used for the early revision PCB are in the blog post Frank referred to, but there is a newer revision ( I'd misplaced it, but I've finally found it and can assemble it).