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  • Author Author: fmilburn
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LCR 4-Wire Kelvin Adapter

fmilburn

This is a follow-up to my review of the Multicomp Pro Handheld LCR MeterMulticomp Pro Handheld LCR Meter.  In that review it was noted that the meter does not come with Kelvin clamps and that the manufacturer did not appear to offer them.  In this post I describe a set of DIY Kelvin clamps.

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Thanks to Shabaz and Jan Cumps for the design suggestions in the comments of the original post.

 

Design Features

 

The design consists of a PCB for adapting the Kelvin clamps to the LCR meter and a 3D printed enclosure.

 

PCB Design

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  • ENIG finish
  • 4-Wire connection of clamps
  • Guard connections for shielded cable
  • Connections on bottom of PCB, Faraday shield on top with stitching to bottom
  • Openings for zip-ties for strain relief
  • 4-wire connection footprint allows 0603 and larger parts to be soldered directly to the PCB for testing
  • Designed so that adapter cannot be inserted incorrectly

 

The Multicomp Pro Handheld LCR is a rebranded Hantek 1833C meter.  Because the "fingers" that attach the adapter to the LCR vary from instrument to instrument this adapter may not fit other LCR models.  The PCB was fabricated by OshPark and the Gerber files can be found here.  The cost for 3 is approximately $16 including shipping if ordered from OshPark.

 

Kelvin Clamps

 

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There is a very good post by Shabaz on how to build your own Kelvin (4-Wire) Test Leads that can be found here.  I bought a shielded pair with BNC connections off of Amazon for $13 (with a coupon) and was surprised to find they were well shielded and the core wire of a decent diameter.  The clamps themselves are OK for the use I will give them but not of professional quality.  The BNC connections were removed and the leads were cut to 40 cm and soldered onto the PCB.

 

I recommend soldering the shielding to the guard footprints first.  Be careful as the connections are close together and test for undesired shorts between connections after soldering.

 

Enclosure

 

{gallery} Enclosure

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Bottom

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PCB

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Top

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  • Print consists of two parts, top and bottom.
  • Requires two 3mm bolts and threaded inserts.
  • Bolts are inserted into a recess in the bottom, must be cut to approximately 18 mm in length

 

The enclosure was designed with Fusion 360 and the STL files can be found here for those who would like to print it.  The closest bolt length I had at hand was too long so they were cut to length.  Threaded inserts were used to allow multiple opening and closing of the enclosure but threaded screws could be used if that is not required.

 

Completed Project

 

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The project looks and works well.  If this was something that was to be done on a larger scale the enclosure could be modified for a 20 mm long bolt (which I did not have on hand) but otherwise there isn't much I would change for now.  Comments and suggestions are always welcome.

 

Associated Post

 

Quick Review: Multicomp Pro Handheld LCR Meter

  • I used 1.6 mm PCBs above and they work OK, but if available I might try slightly thinner, say 1.2 mm if fabricating new ones. That might make it easier to guide the fingers into the slots yet still allow them to be firmly held in place.

     

    Forgot to put the STL files on github.  That has been done now.

  • Nice build to extend the capabilities of an instrument. Thanks for sharing.

     

    Kind regards.

  • in reply to Jan Cumps

    That is really rather intricate.  Thanks for showing and explaining the detail. I would not of thought to use PCB to route to the tip but that is clever.

  • in reply to fmilburn

    ...  I think I will try and make some tweezers next.  They may not be 4-wire though.

     

    Mine relies on the flex of the plastic for the flex action. There are no hinges or springs:

    image

    The little plastic lips on the inside keep the action vertical only. The lips of one arm are closer to each other than those on the other lip.

    They avoid that you can get sideways force and let the tip match up perfectly.

    They also avoid that you can push beyond a point. To avoid that you can put real force on the tip once contact is made.

     

    On both sides, there's a full length PCB inside to bring the wire contact to the tip.

    I think (it's hard to measure because the resistance is low and the 2 wires are shorted at the tip) that each side of the PCB is a full copper plane.

    It's fixed with screws.

     

    That tip is made of metal that's at least a mm thick.

    The best I can compare it to, is the tips of this type of tweezers:

    imageimage

    At the tip end of the PCB, the copper is revealed. Where tip and PCB touch, a separate little piece of copper overlaps both tip and PCB, and is soldered down on PCB and tip.

    I can't see, but it looks like it is rivetted from side to side too. There's a solder blob there...

    Mechanically, the tip fits snug   - the arms are formed in a way that the tip lays flat with the outside and they have ridges that fit like a glove. The tips are held in place in the grip with melt points.

  • Great job Frank.  I think tweezers would be good for small SMD parts.