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

fmilburn

This post documents the first working 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.

image

 

It's Alive!

Those who were following the project may have given up it has been so long since the last post.  The reason for the delay has been the Chinese New Year and delivery took longer than expected.  I salvaged the more expensive parts from the Version 1.0 PCB with my hot air reflow station and will use them to experiment with the PCB.  I have one more set to build a clean version if I mess this one up.

 

Here is the experimental PCB measuring a 1 ohm resistor and displaying it on an inexpensive panel meter:

image

 

Refining the Design

Version 1.3 contains a number of refinements from Version 1.0.  They include:

  • 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
  • Replaced op amp with comparator for detecting out of range condition
  • Changed PCB footprint to fit enclosure used by Shabaz
  • Added through hole 5V alternate input
  • Relocated on / off switch

 

What Next?

Additional testing needs to be done but the new PCB seems to perform as well as the previous one - i.e. better than 1 milliohm results all the way down to zero.  Some tweaking needs to be done on the high range since I don't have the required multi-turn potentiometer.  A typo was found on the schematic that needs to be fixed.

 

There is a known problem where occasional instability occurs when measuring resistances below 20 milliohms.  It has been hypothesized this could be corrected with additional capacitance / diode protection and footprints have been provided on the PCB to experiment further.  Once testing is complete the schematic will be updated as needed.  I plan to put everything on github and provide a link.  I have a limited number of extra PCBs that I will share with active element14 members who are interested.

 

Longer range it would be nice to have automatic range selection and consistent display units.  This could be done with a microcontroller and the board provides sufficient access to allow prototyping.

 

I need to put it in a nice enclosure :-)

 

Related Links

Kelvin (4-Wire) Milliohm Meter:  Version 1.3

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

  • in reply to Andrew J

    Thanks Andrew,

     

    I agree and have learned a lot from making this instrument. There is a lot of good material in the links above btw, much of it from the suggestions and advice of other element14 members.

  • Nice project, I’m bookmarking to come back to later.  I like making these tools rather than buying them.

     

    Andrew

  • This has been a fantastic project to keep following and appears to be getting better and better.

     

    Many thanks foresting this.

     

    Kind regards.

  • in reply to fmilburn

    Hi Frank,

     

    I missed seeing this blog post earlier, this looks great!

    Also, I've received the PCB that you sent, so I'll try to assemble it soon too. 

    Many thanks,

     

    Shabaz.

  • Edit:  Replace updated Version 1.3 schematic with higher resolution copy 8 Mar 2019

     

    image

    The only change is that values for R18 and R20 have been put on the sheet.  In truth, you could probably just use a 180 ohm resistor for R19 and a 160 ohm resistor for R21, then leave R18 and R20 unpopulated.  This is because the current sources have a fair tolerance around the calculated values and the trimming is done on the amplification of the instrument op amp.

     

    I am using different values for the trimming pots R15 and R16 since I only have 100K pots on hand.  This means that I had to use a larger value for R14 but I recommend the values on the spread sheet.

     

    Today I quickly checked both ranges and values from around 2 milliohms up to 4 ohms.  Everything seems to be working and I am not seeing the instability often and only down at very low values.  It is intermittent as before although possibly less frequent.  I have not had time to check whether capacitance / diodes can remove it completely.  The non-linearity that occurred at very low values now appears gone completely, fixed by setting Vref a bit above ground.  I am using standard resistors as opposed to low tempco and have not seen issues but also am not using an enclosure and have not quantified...

     

    I have too many projects open at the moment and will put this one aside for now since it is at an easy stopping place.  Too be continued...