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  • Author Author: fmilburn
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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

Parents

  • 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

    Woohoo!  Shabaz you are a wizard!  I won’t be able to mod mine for several more weeks :-(  On the other hand I should have free time to spin a new board. Then document things in a new blog and put a link at the top of all the tangled old posts pointing people there. Hopefully more people will build it.  It has to be the best diy milliohm meter out there. You made my day :-)

     

    Edit:  with my move on my mind I notice I have not posted gerbers.  Will see if I can do that this weekend.

  • in reply to fmilburn

    Hi Frank,

     

    Great list, I hadn't thought about some of these points. The PCB quality was really good too.

  • in reply to fmilburn

    Here is the modified schematic except the resistors controlling current and gain have not been modified.  That is easy enough to do later and I want to go back through the calculations again.  I plan to redo the PCB design and then do the following:

     

    • Create a new post dedicated to version 2
    • Put a link at the top of all previous posts to the new post since this has become quite hard to follow
    • Document the design by pulling appropriate parts from previous posts
    • Maintain the new post in the main body and not in the comments

     

    I thought about adding auto ranging but will save that for a possible version 3.

    image

  • in reply to fmilburn

    Hi Frank,

     

    I thought you might want to see your V1.3 project design part-assembled : )

    The assembly is still a work-in-progress, I still need to drill the socket locations at the back, and figure out the power supply (I'm going with in-built battery). A low-supply indicator would be nice but I might assemble that on a separate PCB sometime (or probably that exists as an adafruit/sparkfun type module, it sounds like the type of thing that may already exist)..

     

    I permanently glued the PCB into position, since I can easily fix things from the top, there are no components on the underside of the PCB anyway.

     

    Anyway, construction was straightforward as expected!

    image

    image

  • in reply to shabaz

    That looks really nice. One of the nice things about the PCB is that it is small and allows about any case to be used.  Your layout is really neat and tidy.

  • in reply to fmilburn

    Thanks! : )

    Experimented with labeling today:

    image

    I had a low-cost (£50 in the UK) handheld label printer (Brother PT-E300) and I'd been using opaque label tape for ages. However, it turns out that transparent labels exist for it too. I cut them with scissors around the box shape that can be printed. And, since it's transparent, I could put the label on top of the LED hole.

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  • in reply to shabaz

    Hi ,

    This evening I was finally able to dig through some of my boxed things and find the milliohm meter and my 0805 resistors.  Working on a folding table where some day my kitchen table will be I was able to make what were the worst soldering joints I have ever created (and that is saying something) and get the 10k resistors attached between the sense and current pads on the back side of the PCB.  It works!  I haven't been able to test on my "calibrated" resistors yet as they are still in a box somewhere but on short pieces of wire measuring from 10 milliohms down to less than 1 milliohm it is stable and consistent.

     

    I went ahead and ordered a label maker as well.  Time to get organized!  Thanks again for your guidance and help on this project.  I plan to lay out a new PCB when things are a bit more normal for me.

    Frank