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  • Author Author: fmilburn
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Kelvin (4-Wire) Milliohm Meter:  Version 1.3

fmilburn

This post documents Version 1.3 of an inexpensive but reasonably accurate meter for measuring resistance in the milliohm range. The development to date is documented in the links at the bottom of this post.  Version 1.3 contains numerous upgrades and suggestions from Shabaz, Gene Breniman, John Wiltrout, and Jon Clift to which I am indebted.  I intend to send off for new PCBs tomorrow evening and any comments in the meantime are greatly appreciated!  NOTE: This post was revised on 24 Jan 2019 to update the schematic and PCB screenshots per upgrades and comments from Shabaz.

 

Summary of Changes

  • Added biasing for VREF of the instrument op amp which improves reading of resistances around 1 milliohm
  • Added ability to add capacitance and / or diodes in front of instrument op amp
  • Moved trim pots to trim gain of operational amplifier - current sources are not trimmed
  • Modified arrangement of range switch
  • Changed PCB footprint to fit enclosure used by Shabaz
  • Added through hole 5V alternate input
  • Relocated on / off switch
  • Numerous other small improvements

 

Development is a bit behind as I have been struggling to remove the one known bug - intermittent instability when measuring resistances below 20 milliohms.  I have been making modifications to the Version 1 PCB with flying leads to a breadboard.  This seems to introduce unwanted capacitance and perhaps other problems.  To address this and allow quick changing of components I have determined that I can bend the pins on duPont female sockets and solder them to an 0805 footprint.  Then insert passives as desired.  The black 2 pin socket is a bit hard to see but the concept is demonstrated in the mockup using the Version 1 PCB below.

image

 

The new schematic and PCBs are shown below.

 

Schematic

The original schematic posted here was in error and has been replaced with the corrected version.

image

PCB

The original PCB screenshots posted here were replaced with a version that include an upgraded layout based on comments posted by Shabaz below.

image

image

image

 

As always, comments and suggestions are greatly appreciated!

 

Related Links

Building Frank's Milliohm Meter

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

PCB for a Kelvin (4-Wire) Milliohm Meter

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  • Hi Frank,

     

    Wow. I don't know how you manage keeping everything so neat on such a tiny PCB!

    I'll print it all off and do a review, in case I can spot anything. Just initially the red connection needs removing:

    image

     

    I had one suggestion though, I don't know if it will have any effect, but just in case any instability is caused by signal paths, it could be worth attempting to bring the components closer together around the amplifier (it will still be hand-solderable), especially to reduce the feedback path (although that's hard with the trimmers). It would mean the switch connections would have to be moved off the edge of the board but I think it's worth doing.

    What do you think about some layout like this?

    image

Comment

  • Hi Frank,

     

    Wow. I don't know how you manage keeping everything so neat on such a tiny PCB!

    I'll print it all off and do a review, in case I can spot anything. Just initially the red connection needs removing:

    image

     

    I had one suggestion though, I don't know if it will have any effect, but just in case any instability is caused by signal paths, it could be worth attempting to bring the components closer together around the amplifier (it will still be hand-solderable), especially to reduce the feedback path (although that's hard with the trimmers). It would mean the switch connections would have to be moved off the edge of the board but I think it's worth doing.

    What do you think about some layout like this?

    image

Children
  • in reply to shabaz

    Oops - I was in a hurry and that is an old / incorrect schematic which I should delete.  I will repost  with the correct one.  Thanks and you have a keen eye.

     

    Great layout you have proposed.  I need to design in a more modular fashion like that.  To use the duPont header trick to test different passives It may be necessary to space out C9 C10 D1 D2 a bit more.   Will update and repost before sending off the board.

  • in reply to shabaz

    Hi

     

    The correct schematic has now been posted in the original post.  The differences in the old one are the error you pointed out plus the voltage divider biasing VREF also is the negative output to the panel meter.

     

    To keep the clutter down I also removed the old PCB screen shots and posted the revised version in the original post which includes your layout around the instrument op amp.  Bad form I know to erase evidence of my errors :-).  I did not place components quite as tight as you did in your layout but generally followed it. 

  • in reply to fmilburn

    Hi Frank,

     

    Good idea to remove the old screenshots, to keep clutter down! It's great that you managed to incorporate the changes yet keep it looking tidy.

    I'll go through it today, but I only had these few comments meanwhile.

    1. Very minor, the JP1 jumper is labelled CAL on the PCB, but no current adjust can be done to calibrate since there are no current-trimmers any more, so maybe it can be renamed to (say) Test (and could be completely removed in a Rev 3).

    2. For the comparator, the bypass caps can be closer, something like this:

    image

    3. (Super minor): Maybe label the trimmers HI/LO or something, to indicate which trimmer corresponds to the high range, and which to the low range. Similarly maybe the DPDT switch positions could be labelled, although maybe that's not possible due to space.

  • in reply to shabaz

    Hi Shabaz,

     

    I already submitted :-(   If you find anything major I will make a run at OshPark. Othrwise I will start a new improvements log and make these changes so they are ready for the next PCB.  I think the next PCB will need to be a bit larger to incorporate upgrades anyway.

  • in reply to fmilburn

    Hi Frank,

     

    Oh, no problem, I didn't realise it was submitted. These suggestions were minor anyway! The current layout looks great already.