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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

  • 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 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,

     

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

  • 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 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.