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

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.

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:

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