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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 shabaz

    Hi Shabaz,

     

    That is a nice way to do it.  It isn't any problem to do it and I enjoy laying out PCBs.  These are the kind of extra touches that make a project special.

  • in reply to fmilburn

    Hi,

     

    You have restarted the project of milliohm meter?

     

    Thank

  • in reply to andbro

    Shabaz found the fix for the issue we had at very low resistances.   I will update with a new schematic and PCB gerbers as soon as I have time. 

  • in reply to shabaz

    Hi Shabaz,

     

    I made a TO-DO list based on your improvements above plus what I gathered up from some of the other posts scattered about.

     

    1. Add pads for 0805 resistors (10k) between LSENSE and LCURRENT and also HSENSE and HCURRENT.  This fixes the instability issue.
    2. Add a snap off PCB for the DPDT switch (e.g. MHPS2283 or ESB30B102) and a LED.  This aids in moving the switch and LED to a panel.
    3. Modify the footprint of the trimmers to allow use of Bourn 3299W trimmers.  Label the trimmers HI and LO to indicate which range they trim.
    4. Add pads for 0805 resistor and 0805 capacitor between pins 4 and 5 of the voltage regulator.  This will allow use of the REG101NA-A voltage regulator which can hand higher voltage supply such as 9V battery.
    5. Rename JP1 TEST instead of CAL since it is no longer possible to trim the current source.
    6. Place bypass caps closer to comparator.
    7. See sketch from Shabaz and others in comments of Jan 24th post for ideas on improved layout, especially around amplifier.

     

    Did you find the PCB quality OK?  The last version was from Elecrow.

     

    Frank

  • in reply to fmilburn

    Hi Frank,

     

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

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