Cable for DIY Kelvin probes

[calin]

A while ago I got a nice meter of the fleabay -  Fluke 8842A - (RMS option and all for 75$ shipped) .. very good deal because the meter is bang on (compared with a new 61/2 Agilent matches down to the last digit .. well one count here & there but I can say is spot on).


Now ... of course I am not going to spend 100 plus $ on kelvin probes that I will probably use only once in a blue moon .. so as a good DIY-er I want to make my own. I've been digging around and I noticed that many meters use a twin wire shielded cable like the one used for microphones.  If you do a search for an "XLR cable" you will find plenty of examples around the web; decent quality cables are pretty nice and flexible with good/decent shielding. For clips will use some ebay kelvin/china made clips .. "gold plated" . For my light use these will be more than OK and last long enough.


I am thinking @ something like this cable: http://www.mogamicable.com/category/bulk/microphone/lavalier/


For the more experienced .. Is this type of cable OK for use in Kelvin probes? Shielding is there (will use cable with mesh shielding not foil shielded) ... flexible enough ... Are there any downsides which I should be aware of ?

[Alexei.Polkhanov]

  You did not notice how thin these cables are: 0.08mm² (#28 AWG)   0.054mm² (#30 AWG).
AWG 8 probably is what you want to start with. Also your multimeter uses banana connectors not BNC so I am not sure shielded cable is useful here. You will need to make a special box-like fixture with 4 BNC connectors on one side and banana connectors on other side.

DIY style probes on Ebay are $17: http://www.ebay.com/itm/1pair-1M-Gold-Kelvin-Clip-to-Insulated-Stackable-Banana-Plug-Silicon-Test-Cable-/400486938037?pt=LH_DefaultDomain_0&hash=item5d3ee1b5b5.

[Richard Crowley]

It is not at all clear why you need any "special cable" for 4-terminal Kelvin measurements?
And certainly not 2-conductor shielded cable.  A shield is a nice thing to have if you have somewhere to ground it.
But if you have no ground terminal, the shield is a receiving antenna and essentially worse than no shield at all.

My first choice for 4-terminal Kelvin clips would be ordinary lamp cord (or "zip-cord").
Or if you want to get fancy, a single-conductor shielded cable where the outer braided/spiral shield is the current (low-impedance) side, and the inner conductor is the voltage/sense (high-impedance) side.  That gives you essentially "guarded measurement".

I would even consider making my own Kelvin clips. Here is an elegant design with a few simple bits of metal and plastic..

[calin]

Richard you actually do have a good point ... i have some really nice speaker wire I can use. As for clips ... I may go for test hooks that are glued together like ROBRENZ did (not the uber fancy "ultimate kelvin probes" he did). That seems a decent enough option.



Yes the meter does not have a GND/shield hook .. DOH !!  ...  I got carried looking at kelvin probes which are shielded and all

[iloveelectronics]

Just a little heads-up that I'll be carrying this particular style of Kelvin clips in my store soon They will be packaged as a DIY kit with the usual silicone leads common found in my shop, some banana plugs and heat shrink tubings.

[con-f-use]

Will you remind us here, once they are available?

[richard.cs]

Or if you want to get fancy, a single-conductor shielded cable where the outer braided/spiral shield is the current (low-impedance) side, and the inner conductor is the voltage/sense (high-impedance) side.  That gives you essentially "guarded measurement".

I like this idea, you get a fairly high current carrying ability in the braid, the sense wire is shielded by the low impedance output and it's quite small and flexible. + I like round sheaths on my test leads, they don't get knotted up as easily.

[jlmoon]

Be sure to keep in mind the mechanical noise properties cables can introduce.  Try to find low noise non-coaxial shielded cable, coaxial cable tends to generate quite a bit of noise which translates to millivolts of error in your measurements.  This can create problems when using the test setup for really low level voltages.  Sometimes I use some braid shielded cables on my hp3458A and I can bounce them on the bench and see all kinds of spikes / impulses generated.  Just a FYI!

JLM

[coldframe]

Hello,

This is My Pomona 5940 Kelvin Clip
Photo #1,2:  Model 5940 Kelvin Clip * Cable Assembly (RG-174 Coxial Cable)
Photo #3:  Another RG-174 Cable Applications. (GPS Antenna, Pomona 303 4-Wire Kelvin Probe & SMD Test Tweezer)
FYI.

[MosherIV]

Not sure why you think you need to sheild the cable for resistance measurement, you use normal instrument cable.

https://www.eevblog.com/forum/projects/diy-4wire-probes/msg1245671/#msg1245671