4-Wire Kelvin probes

[todorp]

Hello, are 4-wire Kelvin probes for measuring resistance different from the 4-wire Kelvin probes used for LCR meters (apart from the connector of course)? I wanted to buy cheap 4-wire Kelvin probes to measure very low values of resistance but all I find are probes which explicitly mention LCR...

Thanks a lot,
    Todor

[Gyro]

No, they're the same - resistance is just the DC special case of Impedance.

[t1d]

Kelvin probes are often homemade. That does not mean necessarily cheaper, but usually somewhat cheaper than a big brand name set. The reason folks build them is to achieve some function, or combination of functions, that they desire... Like the choice of end clip... Or, wanting shielded cables having a separate banana plug to ground... They can be made from all discrete parts, or by making adaptations to sets that are close to what you desire.

I bought several different types of parts to build different sets with different features to try. I purposefully paid for the well made clips. However, it turns out that those clips are just plain too big. ~$15-20USD


I did also buy smaller clips. They are not as heavily constructed. But, I may finish building them up, because they are smaller and should be easier to attach to the DUT.

Hmm... I wonder if I could file down the wide nose of the better clips to make them thinner? Well, they don't much work, now, so there really can't be any loss in trying.

Usually, it best to stay away from Ebay and Amazon. However, for these parts, if you go with parts others have already proofed, it will probably be okay.

There are lots of these projects on this Forum, YouTube and Google. And, there are lots of things you need to know, before setting out to build them. Its not a tough build, but it would be easy to miss some point that other folks have already sorted out.

[todorp]

Thanks a lot for the detailed answer. I am planning to build my own but also to buy a ready made set (I think I will use AliExpress). I have seen the other posts here in the forums: very instructive. I am a little bewildered by the number of possible clips to use. I might get a few different types and experiment :-).

[Gary350z]

I purposefully paid for the well made clips...

Just a warning.

I bought these same clips (ETA3105 Kelvin Clips) about 10 months ago, from 99centHobbies (ebay seller f-t-2000), and they are crap. The metal didn't look right, so I measured the continuity of the tips of the kelvin clips with an ohm meter (the ohm meter had good gold plated probes) and many times there was no connection at all, even when pressing firmly with the ohm meter probes. Other times the connection was marginal at best, but no where close to zero ohms. They are advertised as gold plated, but I don't think so.

These kelvin clips are highly recommended by many people, so I don't know what is going on here. Are these counterfit? I don't know. I have bought other items from this seller and they were good, and this seller is highly recommended on this forum.

I eventually bought some BK Precision TLDK1 Kelvin Clip Probes and the make very good connection.

[todorp]

Thanks for the warning. Unfortunately BK Precision TLDK1 are out of budget for me :-(. Also I am not trying to make something metrology-grade - I will buy a few clip probes from AliExpress and see how they behave. My target price range for the clip probes will be from around 10 Euro to 20 Euro max...

[Gyro]

I've had reasonable success with this style. The leads are soldered directly to the jaw pieces - one less screw joint/potential thermocouple effect. They're also cheap on ebay. No issues with ruggedness...

[Electro Fan]

Thanks for the warning. Unfortunately BK Precision TLDK1 are out of budget for me :-(. Also I am not trying to make something metrology-grade - I will buy a few clip probes from AliExpress and see how they behave. My target price range for the clip probes will be from around 10 Euro to 20 Euro max...

+1 for the BK Precision TLDK1

You might think you are not doing anything metrology-grade but 4-wire measurements are like a gateway drug on the way to metrology .

I spent a fair amount of time looking at various popular entry level clips and leads and what I found was that after abandoning a couple attempts I could have saved a few $ by just starting with the TLDK1.  Having said that, it depends somewhat on the meter you are using and your budget, of course.  If you haven't already considered a DER EE DE-5000 that might be worth a look-see as a good way to not only get a low resistance meter with 4 wires but L and C measurements too.

Here is a thread with some test results from the TLDK1 and some other entry level clips/leads:

https://www.eevblog.com/forum/testgear/4-wire-2-wire-resistance-tests-various-cheap-kelvin-clips-various-meters/msg3240888/#msg3240888

And a couple other threads in case you want to test your ability to fend off metrology syndrome:

https://www.eevblog.com/forum/testgear/the-ultimate-kelvin-grabbers/

https://www.eevblog.com/forum/testgear/small-kelvin-clips-that-actually-grab/

Edit:  adding one more link that might be relevant
https://www.eevblog.com/forum/testgear/franky_s-kelvin-clip-kit/?PHPSESSID=h41h3jdg9eqpu3afq59s2upad4

[Gary350z]

I was going to say BK Precision TLDK1 Kelvin Clip Probes are not that expensive. I paid $47 at TEquipment.
But then I looked at the current price at TEquipment and it is $71. That's a 51% increase in one year.
BK Precision TLDK1 Kelvin Clip Probes, $71
https://www.tequipment.net/BK/TLDK1/Kelvin-Test-Leads/

The following might be good, but you never know until you get them (I was considering them until I got the BKs):
------------------------------------
Clips only (these appear to be the same)

Adafruit Kelvin Clip, PRODUCT ID: 3313, $2.50 each
https://www.adafruit.com/product/3313

2x Pure Copper Kelvin Alligator Test Clip R &B Gold Plated for LCR Meter, $2 for 2
https://www.ebay.com/itm/283109177832
------------------------------------
Probes

AST Labs: Test Lead Kelvin Probes (3 feet), $23
https://ast-labs.com/shop/test_cables/kelvin/test-lead-kelvin-clip-4-channel-banana-plug-3-feet/

eBay: LCR Meter Cable w/ Banana Plug Connectors kelvin clip SMD, $20 plus shipping
https://www.ebay.com/itm/381460680487?hash=item58d0d40f27:g:tTsAAOSwhcJWPg~e
------------------------------------

Of course you could loose your mind completely and get the Keithley Kelvin Clip Lead Set for only $384.
https://www.tequipment.net/Keithley5806.html

[WattsThat]

Of course you could loose your mind completely and get the Keithley Kelvin Clip Lead Set for only $384.
https://www.tequipment.net/Keithley5806.html

Wow, that makes even the Keysight 11059A kelvin lead set seem cheap at $209.

https://www.newark.com/keysight-technologies/11059a/probe-set-kelvin/dp/92F026

[figurativelythedevil]

I actually just made an adapter board for some of the cheaper coax kelvin probes on ebay to connect to a standard banana jack meter! Probably not worth it since the TLDK1 exists, but it was a fun project nonetheless.

Forum post:
https://www.eevblog.com/forum/projects/banana-jack-to-coaxial-4-wire-kelvin-clip-adapter-board/

Imgur album w/ higher res photos and commentary:
https://imgur.com/gallery/B6nNp7m

[Electro Fan]

I actually just made an adapter board for some of the cheaper coax kelvin probes on ebay to connect to a standard banana jack meter! Probably not worth it since the TLDK1 exists, but it was a fun project nonetheless.

Forum post:
https://www.eevblog.com/forum/projects/banana-jack-to-coaxial-4-wire-kelvin-clip-adapter-board/

Imgur album w/ higher res photos and commentary:
https://imgur.com/gallery/B6nNp7m

Very nice project!
Your adapter expands the possibilities since some kelvin clip leads come ready made with BNC connectors.  Nice work and thanks for posting!

[magic]

I thought the BNC ones would be for guarded instruments. If you don't have that, might as well buy probes with banana jacks and spare yourself the trouble

BTW, the tied shields may be making things worse for high-ohm measurements because internal insulation of sense cables appears in parallel with the resistor under test.

[todorp]

So, I was convinced to look for the BK Precision TLDK1. I am in Europe so I looked in digikey, mouser, farnell and other sites in europe and unfortunately either the price is over 100 Euros or the lead times (mouser) are crazy - 13 weeks. I found these somewhat similar (price) on Mouser: https://www.mouser.it/ProductDetail/685-380465. I wonder if these are similar to the BK Precision ones and could be used with a 34401A?

I also have a DER EE DE-5000 but for this one I surely will get comething from ebay or aliexpress.

[Electro Fan]

I thought the BNC ones would be for guarded instruments. If you don't have that, might as well buy probes with banana jacks and spare yourself the trouble

BTW, the tied shields may be making things worse for high-ohm measurements because internal insulation of sense cables appears in parallel with the resistor under test.

Yes/agreed, I think shielding is another part of the overall discussion regarding 4 wire that is worth having, either here or in another thread.

[Electro Fan]

So, I was convinced to look for the BK Precision TLDK1. I am in Europe so I looked in digikey, mouser, farnell and other sites in europe and unfortunately either the price is over 100 Euros or the lead times (mouser) are crazy - 13 weeks. I found these somewhat similar (price) on Mouser: https://www.mouser.it/ProductDetail/685-380465. I wonder if these are similar to the BK Precision ones and could be used with a 34401A?

I also have a DER EE DE-5000 but for this one I surely will get comething from ebay or aliexpress.

Your link doesn't seem to be fully rendering (or maybe it's just a temporary thing or just on this end). 

fwiw, I'm a big advocate of the TLDK1 but if they are not available at a reasonable price I wouldn't abandon your original idea.  As shown in this thread the OP made a very good project with what looked like a similar approach and he seemed to get good results.
https://www.eevblog.com/forum/testgear/franky_s-kelvin-clip-kit/?PHPSESSID=h41h3jdg9eqpu3afq59s2upad4

Plus, in this reply he got an attaboy from robrenz who is for sure one of the most serious kelvin clip project makers around here:
https://www.eevblog.com/forum/testgear/franky_s-kelvin-clip-kit/msg814791/#msg814791

Additionally, the OP attracted the attention of Dr. Frank, another very serious metrology forum member.
https://www.eevblog.com/forum/testgear/franky_s-kelvin-clip-kit/msg843021/#msg843021

In summary, I think you were on a reasonable track to start so don't get derailed onto a new track unless you decide you really want to be on a new track. 

I think the TLDK1 provides a reasonable benchmark, so I'd say if someone can make kelvin clips and leads that equal or outperform the TLDK1 for an equal or lower price go for it. 

[Neomys Sapiens]

There is one difference although: 4-wire measurements for resistance are mainly in use for low resistances, i.e. on milli- and micro-ohm-meters. Some of those instruments use rather high currents when compared with the usual LCR-meters. The cheaper test clips might not be able to cope with this. The leads need to have a certain cross section.

[bdunham7]

BTW, the tied shields may be making things worse for high-ohm measurements because internal insulation of sense cables appears in parallel with the resistor under test.

If you are using kelvin leads, you are probably making a low resistance measurement.  I think it is more important to shield the sense lines from external fields because at very low resistances, even with substantial test currents the sense voltages are quite low.

[mawyatt]

Posted on another thread these BK TLDK1 Kelvin clips look like the ones Tonghui provides with their TH26011CS Test Fixture, which is also supplied with their Precision LCR meters. I can't find these specific clips, and they are not the usual types you find everywhere on eBay or Alix.

Here's some images of the Tonghui clips (bottom image), similar cheap eBay types, and cheap smaller eBay types.

Best,

[magic]

If you are using kelvin leads, you are probably making a low resistance measurement.  I think it is more important to shield the sense lines from external fields because at very low resistances, even with substantial test currents the sense voltages are quite low.

Fair point, but I think the shields would still benefit from being grounded or guarded in such case.

[tooki]

I've had reasonable success with this style. The leads are soldered directly to the jaw pieces - one less screw joint/potential thermocouple effect. They're also cheap on ebay. No issues with ruggedness...

I made some leads using that style of clip from eBay. One of the jaws has practically broken off because the serrations were stamped much too deep.

Would not recommend. I’m planning to order some Mueller Electric ones. Or maybe seeing if the machinists at work could make something for me, since there’s absolutely no need for them to be so gigantic.

[mawyatt]

I've had reasonable success with this style. The leads are soldered directly to the jaw pieces - one less screw joint/potential thermocouple effect. They're also cheap on ebay. No issues with ruggedness...

I made some leads using that style of clip from eBay. One of the jaws has practically broken off because the serrations were stamped much too deep.

Would not recommend. I’m planning to order some Mueller Electric ones. Or maybe seeing if the machinists at work could make something for me, since there’s absolutely no need for them to be so gigantic.

These are not as big as the other type Kelvin clips, and you can solder leads directly to the clip faces. We got these for their size, and they work OK, and are cheap, so if they break just replace.

Best,

[tooki]

Those look good, where are they from??

[mawyatt]

The clips are all over eBay and Alix. We made our own cables, but I'm sure you'll find ready made cable using these clips. The clips only cost ~$4 for a pair.

Best,

[todorp]

So I ordered some probes from Ebay and AliExpress and now am waiting for arrivial (will take some time). I started thinking of a test setup/procedure to be able to tell which probes are bad/ok/good. Of course there is build quality (materials), ruggedness, ergonomics, etc. But what I am interested in for the time being is the electrical performance. I have a HP34401A which hasn't been calibrated in a long while and a DE-5000 LCR. I plan to use the kelvin clips with both of these instruments.
The question is: what is a standard procedure that can be used to assess the quality of the 4 wire kelvin connection on each one of these instruments? For example do people use a standard 1% resistor (which value?) or something better and measure it? What parameters do you setup in the instrument (nulling, NPLC)? How long do you people measure for? What post-processing do you apply to the measured data?