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DISCLAIMER: This design is experimental, so if you decide to build one yourself then you are on your own, I can't be held responsible for any problems/issues/damage/injury that may occur if you decide to follow this build and make one yourself.
THE DESIGN
I have an HP 4338A Milliohmmeter and have been using the stock Agilent 16143B mating cable with it. However, the 16143B is rather bulky, and where I have the 4338A situated on my bench I really needed something smaller and with a lower profile.
I reversed engineer the 16143B and if you look at the diagram here you'll see it's effectively a current loop and where the voltage is being measured also. It's extremely precise because it measures the resistance at 1kHz and the current through your resistor is controlled very precisely.
I designed a Pcb in KiCad to mimic the 16143B, and as you will see, there are only 3 BNC's on my new board compared to the 4 BNC's on the 16143B, and thats because the 16143B doesn't use the 4th connector, not even for grounding.
16143B connections:
- LCUR (low current)
- LPOT (low potential / sense)
- HPOT (high potential / sense)
- HCUR (high current)
My Pcb connections:
- LPOT (low potential / sense)
- HPOT (high potential / sense)
- HCUR (high current)
The reason is that, in this configuration, the LCUR return path is already internally shorted to LPOT inside the instrument. In low-frequency, low-impedance measurements (milliohm range), the return current path and the potential sense at the low side are essentially the same node. That means carrying a separate LCUR lead through the test fixture provides no metrological advantage — it doesn’t reduce error, and leaving it floating reduces the chance of accidentally creating loops or ground offsets.
The 4338A’s low-resistance fixture, the return current and low-potential sense share the same node, so a separate low current lead adds no benefit.
DOWNLOADS
- Gerbers here.
- BOM here.
- Schematic here.
- 3D printed cover (.stl & .gcode) here.
BOM
- 2off Kelvin Test Clips (copper or gold plated), Aliexpress link here
- 3off BNC male connector PCB mount straight - 1off which needs the external cover removed (I just cut it off), Aliexpress link here
- 1lot Wire 24/0.2mm (silicone preferred)
- 2off Small cable ties
- 2off BNC levers - 3D printed from TheDefPom's Thingiverse project here. Note that a little heat is required to get them to fit over the BNC connectors.
KELVIN CLIPS
The Kelvin clips I used (per the BOM) work great, albeit expensive. I did try much cheaper ones as often seen on Chinese supplied Kelvin leads but they don't work as good because of the spring mechanism and un-equal wire lengths. A 50mohm resistor read as 80mohm. The ones in the BOM are spot on.
PHOTOS
The original 16143B alongside my new Pcb:
Front view:
Rear view:
Cover fitted:
Preferred kelvin clips:
Pcb mounted BNC connectors:
Avoid these kelvin clips, they don't work properly:
