4-Wire Kelvin probes - Page 1
Forum section: electronics / beginners

author: todorp
author_rank: Regular Contributor
timestamp: September 17, 2021, 08:21:36 am
content: 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

author: Gyro
author_rank: Super Contributor
timestamp: September 17, 2021, 08:46:56 am
content: No, they're the same - resistance is just the DC special case of Impedance.

author: t1d
author_rank: Super Contributor
timestamp: September 17, 2021, 09:45:54 am
content: 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. ~$15-20USDI 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.

author: todorp
author_rank: Regular Contributor
timestamp: September 17, 2021, 09:59:16 am
content: 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 :-).

author: Gary350z
author_rank: Regular Contributor
timestamp: September 18, 2021, 04:45:02 am
content: Quote from: t1d on September 17, 2021, 09:45:54 amJust 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.

author: todorp
author_rank: Regular Contributor
timestamp: September 18, 2021, 04:51:32 pm
content: 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...

author: Gyro
author_rank: Super Contributor
timestamp: September 18, 2021, 05:39:16 pm
content: 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...

author: Electro Fan
author_rank: Super Contributor
timestamp: September 18, 2021, 06:58:53 pm
content: Quote from: todorp on September 18, 2021, 04:51:32 pm+1 for the BK Precision TLDK1You 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/#msg3240888And 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 relevanthttps://www.eevblog.com/forum/testgear/franky_s-kelvin-clip-kit/

author: Gary350z
author_rank: Regular Contributor
timestamp: September 18, 2021, 09:27:18 pm
content: 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, $71https://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 eachhttps://www.adafruit.com/product/33132x Pure Copper Kelvin Alligator Test Clip R &B Gold Plated for LCR Meter, $2 for 2https://www.ebay.com/itm/283109177832------------------------------------ProbesAST Labs: Test Lead Kelvin Probes (3 feet), $23https://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 shippinghttps://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

author: WattsThat
author_rank: Frequent Contributor
timestamp: September 18, 2021, 09:57:12 pm
content: QuoteWow, 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

author: figurativelythedevil
author_rank: Contributor
timestamp: September 18, 2021, 10:04:03 pm
content: 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

author: Electro Fan
author_rank: Super Contributor
timestamp: September 19, 2021, 02:40:54 am
content: Quote from: figurativelythedevil on September 18, 2021, 10:04:03 pmVery nice project!Your adapter expands the possibilities since some kelvin clip leads come ready made with BNC connectors. Nice work and thanks for posting!

author: magic
author_rank: Super Contributor
timestamp: September 19, 2021, 06:10:07 am
content: 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 troubleBTW, 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.

author: todorp
author_rank: Regular Contributor
timestamp: September 19, 2021, 02:45:03 pm
content: 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.

author: Electro Fan
author_rank: Super Contributor
timestamp: September 19, 2021, 07:25:57 pm
content: Quote from: magic on September 19, 2021, 06:10:07 amYes/agreed, I think shielding is another part of the overall discussion regarding 4 wire that is worth having, either here or in another thread.

author: Electro Fan
author_rank: Super Contributor
timestamp: September 19, 2021, 07:39:30 pm
content: Quote from: todorp on September 19, 2021, 02:45:03 pmYour 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/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/#msg814791Additionally, 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/#msg843021In 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.

author: Neomys Sapiens
author_rank: Super Contributor
timestamp: September 19, 2021, 08:05:46 pm
content: 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.

author: bdunham7
author_rank: Super Contributor
timestamp: September 19, 2021, 08:15:03 pm
content: Quote from: magic on September 19, 2021, 06:10:07 amIf 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.

author: mawyatt
author_rank: Super Contributor
timestamp: September 19, 2021, 09:43:48 pm
content: 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,

author: magic
author_rank: Super Contributor
timestamp: September 20, 2021, 08:19:13 am
content: Quote from: bdunham7 on September 19, 2021, 08:15:03 pmFair point, but I think the shields would still benefit from being grounded or guarded in such case.

author: tooki
author_rank: Super Contributor
timestamp: September 20, 2021, 04:43:09 pm
content: Quote from: Gyro on September 18, 2021, 05:39:16 pmI 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.

author: mawyatt
author_rank: Super Contributor
timestamp: September 20, 2021, 05:52:21 pm
content: Quote from: tooki on September 20, 2021, 04:43:09 pmThese 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,

author: tooki
author_rank: Super Contributor
timestamp: September 20, 2021, 08:15:03 pm
content: Those look good, where are they from??

author: mawyatt
author_rank: Super Contributor
timestamp: September 20, 2021, 11:10:40 pm
content: 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,

author: todorp
author_rank: Regular Contributor
timestamp: September 23, 2021, 08:58:16 am
content: 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?

author: todorp
author_rank: Regular Contributor
timestamp: September 24, 2021, 07:42:16 am
content: Quote from: todorp on September 23, 2021, 08:58:16 amBump - no one?

author: mawyatt
author_rank: Super Contributor
timestamp: September 24, 2021, 02:03:40 pm
content: Don't normally require highly accurate reading down in the sub-ohm region, and if we do then use the Kelvin Probes (TH26011CS) that came with the Tonghui TH2830 LCR meter.With the cheap Kelvin clips mentioned, first is to get a good Zero Ohm reference. A short section of solid 12AWG Cu wire or thick piece of Cu plate/sheet acts as a good short reference, be sure to clean the Cu for a good contact, then "Null" this with the 34401A. We found this gives good results that compare well with the TH2830 in lower value resistors. One difficulty is getting the same location for the clips on leaded resistors when measuring low values, so try and position the clips in the same location for comparisons, with higher values resistors, this isn't a problem.For reference resistors, one should have a quality standard. We don't, and just use our KS34465A as the "Reference" reading instrument to compare other instruments, components and probes. Not exactly "Metrology Standards", but works OK for our needs.Awhile back we did design a custom PCB to host some precision low TC resistors that we can use for relative measurement comparisons, but this fixture plugs directly into the DMM without cables, has PCB mounted banana plugs. This uses header clips to set the resistor under measurement and arranged so the clips resistance doesn't influence the readings, and has a set of pins for external measurement. However, these pins will affect the external reading and partially negate the advantage of the external Kelvin Probes since the measurement current and sense now will both "see" a small series resistance due to the resistance setting header clips.Think these cheap Kelvin clips are OK for GP use, and cheap enough to give a try, but don't recommend them for any precision low ohms work.Hope this helps.BTW for Precision Kelvin Tweezers for use with SMD chip components, the Tonghui TH26009B Test Fixture is excellent. These are very smooth and easy to use, hard to describe just how good they perform. They aren't expensive (~$120) but do have the 4 fixed BNC interface you find on precision LCR meters, so not compatible with a typical DMMs.Best,

author: Ground_Loop
author_rank: Frequent Contributor
timestamp: September 24, 2021, 08:28:53 pm
content: I'm giving these away for shipping cost, US only. PM me if interested. The Pot and Cur leads connect to opposite sides of the clip jaws.

author: Electro Fan
author_rank: Super Contributor
timestamp: September 24, 2021, 10:04:53 pm
content: Quote from: todorp on September 24, 2021, 07:42:16 amI adopted this approach to making a reference "DUT". What I like about it is that you can research what the theoretical measurements should be for various gauges, lengths, and compositions of "copper" wire. Then you can see how close you can get your clips, leads, and meter (working together as a system) to measuring the theoretical values. If you look at enough web sites you might find that some web sites claim different specs for the same wire values - generally small differences, but something to stay on the lookout for. Some web sites provide values and others have calculators - either way, watch for different claimed or assumed values. By the time you dial in the supposed theoretical values you will then find that various other factors will influence the test results - such as how and where exactly you place the clips on the wire ends, the ambient temperature (including any fluctuations) in your test area, how long you let the test setup settle before beginning the tests, how long you run the test, your meter's ability to collect data over some test period (and your meter or your ability to average the samples), and so on and so forth.I think once you get down to a measured value of about a milliohm or maybe hundreds of microhms you will find that this becomes a somewhat tedious undertaking, at which point you will have to decide whether to press onward in search of more measurement accuracy or declare good enough. One of the things I like about the copper wire test fixture approach is that it's very easy to adjust the length of a wire (assuming you can measure and cut it accurately and assuming you were able to find the correct values for the theoretical wire), and then you can make the math easy when you do your hypothesis testing by shortening or lengthening the wire for more tests. I think this offers some advantages over using a reference resistor as it's somewhat hard to know just how accurately that resistor was manufactured. With just the copper wire it's easier to figure out what the value should be, plus unlike a reference resistor you can tweak the reference wire's resistance by cutting different lengths.Once you get your baseline "DUT"-wire (or several wires of different lengths) established with your initial set of clips/leads, then you can move onto your next set of clips/leads and see what variations you get in the measurements, and if you have the desire you can compare lots of clips/leads. Of course, you could also use a preferred set of clips/leads to then compare meters. By this time you will probably need a way to control or at least account for changes in the ambient temperature.This approach can be enlightening and fun as long as it maps reasonably well to your level of OCD.

author: tooki
author_rank: Super Contributor
timestamp: September 25, 2021, 01:09:57 pm
content: Quote from: todorp on September 23, 2021, 08:58:16 amI think nobody answered because there’s no simple answer, and the question itself suggests you may not really understand the situation.The entire purpose of Kelvin resistance measurement is to essentially eliminate from the measurement the effects of the test leads and clips. I don’t think any simple test with a resistor will be able to show any differences, because it shouldn’tby definition.So in the end, the “build quality (materials), ruggedness, ergonomics, etc.” are what differentiates one from another.Now, for capacitance and inductance measurements, it is probably different since they are frequency-dependent, but I’m honestly not sure to what extent. The test frequencies in a typical LCR meter (a few hundred KHz at the most) are simply too low for the cabling to be critical. Heck, the original Kelvin clips that came with my old Hioki LCD meter use proper 50 ohm BNC plugs, but with 75 ohm coax cable. It just doesn’t matter in this application.

author: todorp
author_rank: Regular Contributor
timestamp: September 27, 2021, 08:30:00 am
content: Quote from: tooki on September 25, 2021, 01:09:57 pmI understand this, but this is theoretically. I immagine no leads and cables are perfect so there should be some way to say this pair is better than this one because it introduces less noise and gives a reading which is closest to the true value of a resistance with less variations... Of course "the true value" is also something abstract: that is why I am asking how people perform this kind of measurements and what they use as "the true value".

author: GlennSprigg
author_rank: Super Contributor
timestamp: September 27, 2021, 11:40:10 am
content: Quote from: todorp on September 27, 2021, 08:30:00 amFor my old 2c worth, and just for interest!!.. haha...Back when I was a Tech with Honeywell, field "temp Sensors" were wired using a '3-wire' system, that the electronic/digital instruments REQUIRED,to counteract the cable resistance to the Field. Some sensors had values of about say 100-ohms, at room-temp. A 5 or 10 Ohm cable resistance, for saya hundred Metre cable to the field sensor obviously introduced huge errors, for the sensors temp range. So, 2 of the wires were shorted together at thesensors. The 3rd wire went to the other side of the sensor. The 'Instrument' would measure the resistance of the shorted Loop up & back, and thensubtract that resistance from the resistance of one of the shorted loop wires, returning on the 3rd wire, as that should be the same! That always cancelledout any cable resistance, no matter how long the cable is!!, so it then only saw the actual sensor resistance for calculations...But THAT was all done within the digital instrument/device, with nothing special out in the field. Hope I didn't bore anyone!!

author: tooki
author_rank: Super Contributor
timestamp: September 27, 2021, 11:42:09 am
content: Quote from: todorp on September 27, 2021, 08:30:00 amAgain, talking only about resistance:So… you want to measure something that, while measurable, doesn’t matter because it doesn’t influence the measurements you’ll make?

author: todorp
author_rank: Regular Contributor
timestamp: September 27, 2021, 12:14:37 pm
content: Quote from: tooki on September 27, 2021, 11:42:09 amNot so - if your sense cables are made from nickel I suppose your measurement wouldn't be very good, would it?Also why would people go to great lengths to do the following if it didn't matter at all?https://www.eevblog.com/forum/testgear/4-wire-2-wire-resistance-tests-various-cheap-kelvin-clips-various-meters/msg3240888/#msg3240888https://www.eevblog.com/forum/testgear/the-ultimate-kelvin-grabbers/https://www.eevblog.com/forum/testgear/small-kelvin-clips-that-actually-grab/https://www.eevblog.com/forum/testgear/franky_s-kelvin-clip-kit/Remark, am no expert or anything. Just trying to understand...

author: tooki
author_rank: Super Contributor
timestamp: September 27, 2021, 04:31:22 pm
content: Quote from: todorp on September 27, 2021, 12:14:37 pmWhat makes you think your measurementswouldn’tbe good if it used nickel wire? The resistance of the leads basically does not matter (within reason; the 34465A manual lists a maximum resistance per lead of 10% of the range on the lower ranges (so 10 or 100 ohms), 1k ohm for the higher ranges.) Whether the leads have 100milliohms or 1 ohm resistance literally does not matter.You have to understand that the drive lines drive a controlled, known current through the DUT, meaning that the current is a known value regardless of what resistance it’s going across (DUT and leads). The sense lines, in turn, measure the voltage drop across the DUT only. The sense inputs (the V input of the meter) have a very high input impedance: 1Mohm on cheap meters, 10Mohm on good handheld meters, and much, much higher on bench meters: “>10Gohm” on the 34465A. The error introduced by a few ohms in the leads literally does not matter: essentially no current flows, so there is essentially no voltage drop in the leads.Quote from: todorp on September 27, 2021, 12:14:37 pmPeople do all sorts of things that aren’t necessary.Most of those reviews are about the build quality anyway.Quote from: todorp on September 27, 2021, 12:14:37 pmNor am I, and should an actual expert chime in and tell me I’m wrong, I’ll listen.

author: bdunham7
author_rank: Super Contributor
timestamp: September 27, 2021, 05:44:32 pm
content: Quote from: tooki on September 27, 2021, 04:31:22 pmI won't assume the 'expert' mantle, but due to the practical construction of 4W ohms circuits, specifically that the SOURCE and SENSE are not completely isolated, the resistance of the SOURCE leads does matter--so you're just a bit 'wrong'. As you say, any error due to resistance of the SENSE leads should be infinitesimal. The good news is that unless that resistance is unreasonably high, your error is a couple of digits off the right hand side of the scale, so nobody will know. The maximum resistances allowed by the specifications of the bench meters I've seen are much higher than they need to be--there's no reason to exceed 1 ohm, IMO. I suppose they are just basing that on the point at which the error will become apparent. The big issues seem to me to be contact resistance/noise/thermal voltage and picking up magnetic fields in the lead loops. So I'll rate your statement 99% correct.

author: todorp
author_rank: Regular Contributor
timestamp: September 27, 2021, 05:48:09 pm
content: @tooki - right, I understand all of this. In my previous post I was talking about the SENSE leads. My next question would have been: why not use some of the crappiest and cheapest conducting materials for the SOURCE and SENSE leads if they do not matter?Thanks to @bdunham7 this question is answerd.

author: Shock
author_rank: Super Contributor
timestamp: September 27, 2021, 06:05:17 pm
content: The answer to your question is *everything* counts but it entirely depends on what you are measuring, how low/high the measurement is, the performance of your instrument and technique. It's such an involved subject it's easier to just go research by reading than explain it all.If you look at various commercial offerings you will note some are guarded which gives them a higher bandwidth and reduces some of the problems associated with longer leads. Genuine cable (not cheap chinese) and quality clips/plating can get quite expensive. Considering it's outside your budget, avoid worrying about that until you need it. I would start by making or buying a set of $20 leads.Aside from good clips, keep an eye out for chinese leads that only solder to one side. Cable flexibility is important to some as well.

author: Electro Fan
author_rank: Super Contributor
timestamp: September 27, 2021, 08:06:00 pm
content: Quote from: Shock on September 27, 2021, 06:05:17 pmThese last several posts and this post are why I think it makes sense to get yourself a piece of copper wire that you can reasonably characterize and quantify and use it as a benchmark test bed as you try various off the shelf and/or DIY 4 wire clips/leads. You can see how the clips/leads perform relative to the theoretical spec, how they compare to other clips/leads, and even how they compare to themselves as you strive for repeatability - which will reveal some of the challenges (clip grip on the wire, precise clip locations on the wire, settling time, temperature fluctuations, test durations, and possibly other variables).Once you get your benchmark test wire and your benchmark clips/leads established with the first meter you might find it interesting to then try another meter. This can go on and on until you are pretty far into a metrology rabbit hole. On the upside, there is a lot to be learned by just doing measurements, developing a hypothesis, and testing some more.Net, net: The copper wire test bed costs close to nothing and it can be a fun opportunity to learn from both research and from doing.

author: guymo
author_rank: Regular Contributor
timestamp: September 28, 2021, 09:44:43 am
content: Quote from: bdunham7 on September 27, 2021, 05:44:32 pmI am feeling stupid right now because I can't figure out what error this could cause or how it would arise. Would you (or someone) mind explaining a bit please?

author: tooki
author_rank: Super Contributor
timestamp: September 28, 2021, 11:00:32 am
content: Ditto. That doesn’t make sense to me. Eliminating the effect of resistance in the source leads is critical to the concept of kelvin resistance measurement.

author: mawyatt
author_rank: Super Contributor
timestamp: September 28, 2021, 01:58:22 pm
content: Using a very simple model for the Kelvin resistance measurement where the DMM meter voltage reading impedance is assumed to be infinite, and the DMM current source output resistance is Rs. Using equal lead resistances from the current source of Rl, one arrives at a simple equation for the Resistance under test Rdut as:Rdut ~ (Vm/Is)*(1 + 2Rl/Rs), where Vm is the measured voltage across Rdut and Is is the source current.If Rs >>> 2Rl then Rdut = Vm/Is as expected.The change is Rdut as dRdut is:dRdut = dRl/(Rs/2 + Rl), where dRl is the change in source lead resistance.Again if Rs >> Rl then dRdut is small approaching ~>0 as Rs ~> infinity.Don't know the value of Rs but suspect it's quite high in quality DMMs, then the error introduced by the source lead resistance should be small.However, all this "assumes" the DMM current source is ideal and constant except for the output resistance Rs. It's likely the current source value does vary some small amount with the variation in Rl which causes a variation in current source output voltage.So it seems the common sense approach as mentioned by many would be to keep the source lead resistance small but not go to extremes.An interesting experiment would involve a precision, stable, low value test resistance and the Kelvin clip source leads arranged to add a series resistance in each lead to observe the effects.Best,Edit: Just did a quick test with the old HP34401A and small cheap Kelvin clips. Added a series R in each leg of the Source Lead of 1 and 10 ohms. DMM was Nulled for zero reading with clips shorted to device lead in first reading and not again.Results:R value Kelvin Reading 1 ohm Rlead 10 ohm Rlead1 ohm 0.9966 0.9966 0.9964second 0.9962 0.9965 0.99621K 0.996596K 0.996596K 0.996596K1 ohm 0.9454 0.9450 0.9453120 ohm 119.9989 119.9988 119.99892 ohm 2.0002 2.0002 2.0002Added some reading from KS34465A1 ohm 0.9972 0.9972 0.99722 ohm 2.0020 2.0020 2.002010 ohm 9.9469 9.9469 9.9469120 ohm 119.9941 119.9941 119.99371K 0.996636K 0.996636K 0.996636K1 ohm* 997.13302m 997.13513m 997.12309m1 ohm* is using scaling y =mx +b and stat mean of 10 readings

author: bdunham7
author_rank: Super Contributor
timestamp: September 28, 2021, 04:36:41 pm
content: Quote from: guymo on September 28, 2021, 09:44:43 amQuote from: tooki on September 28, 2021, 11:00:32 amDidn't make sense to me at first for the same reason, and although whatmawyattsays is true, it isn't what I ran into. The base expectation is that the current source will provide a precise current up to a compliance voltage and that the SOURCE and SENSE are isolated, with the meter determining the reading by measuring the SENSE voltage and then doing some math and calibration corrections. You might do a similar measurement with an independent power supply and millivolt meter.I'm trying to make (as in produce) a type of 4W probe fixture and what I ran into on at least some meters I worked with is that the SOURCE and SENSE are not completely isolated nor independent. If you short the SENSE leads with the SOURCE open, you do not get a zero or very low reading, as you might expect. I have not fully characterized or disassembled this particular meter nor do I have a schematic, but I can tell you that it does not seem broken and works very, very well when used as intended. The specifications call for the 4W SOURCE lead resistance to be "less than 10% of range" for the 10R, 100R and 1K ranges, so 1 ohm in practice. The test current in the 10R range is 5mA and the current source OCV is over 5 volts, so I have to assume that this is not in any way a source impedance or compliance voltage issue.I'm sorry I don't know more, but if I ever get my probe samples made I'll have another look in detail and report back.

author: MaTkEOxjC
author_rank: Contributor
timestamp: April 15, 2025, 11:52:05 am
content: I was looking for qualitative clips myself which did not cost 300€ and wanted to share my findings.TLDR: I went for the GWInstek GTL-108A for 74€It seems to use the ETA3105 clips which also seem to appear herehttps://www.eevblog.com/forum/testgear/franky_s-kelvin-clip-kit/What I found on the market (sorry, didn't manage to insert as a table)Manufacturer Modell Note Cable length Price (with 20% tax) WeblinkSourcetronic ST26004S-1 BNC connectors 84cm 50 €https://eleshop.de/sourcetronic-st26004s-1-kelvin-messleitungen.htmlSourcetronic ST26050 for Milliohmmeter -> can carry at least 1A 143 €https://www.sourcetronic.com/shop/en/kelvin-test-clips-st26050.htmlBK Precision TLDK1 similar to sourcetronic 138 €https://at.farnell.com/b-k-precision/tldk1/4-draht-kelvin-messleit-widerst/dp/3405293GWInstek GBM-01 for battery tester ? 110cm 103 €https://www.reichelt.at/at/de/shop/produkt/messleitung_fuer_batterietester_gbm-serie_90_v_kelvin_clip-287652?PROVID=2835#closemodalGWInstek GTL-108A 120cm 74 €https://eleshop.de/gw-instek-gtl-108a-kelvin-test-leads.htmlGWInstek GTL-308 150cm 100 €https://eleshop.de/gw-instek-gtl-308-test-leads.htmlMatrix LCR001 BNC connectors 60cm 36 €https://eleshop.de/matrix-lcr001-kelvin-messleitungen.htmlPomona 5940 291 €Keysight 11059A 283 €https://eshop.xtest.at/keysight-11059a-kelvin-probe-setElectro PJP 440 155 €https://at.rs-online.com/web/p/kelvinklemmen/2639175Extech 380565 83 €https://www.digikey.at/de/products/detail/flir-extech/380565/7786246Hioki L2107 200 €https://eleshop.de/hioki-clip-messleitungen-l2107.htmlStäubli 669795 big clamps -> probably not suited for electronics use, needs two pcs for one set 25cm 114https://www.distrelec.at/de/messleitung-messing-250mm-staeubli-electrical-connectors-66-9795-250/p/30223778There are also some DIY/clamps only options:Keysight 1400-3652 Available in set PS0010A, hard to get individually, maybe current ones are produced by Hirschmann 100€?Hirschmann 972416100https://www.newark.com/hirschmann-testmeasurement/972416100/clip-test-micro-smd-black-mln/dp/67C8450

author: timeandfrequency
author_rank: Frequent Contributor
timestamp: April 15, 2025, 12:44:35 pm
content: Hello MaTkEOxjC,Some more choice :-BK Precision TL8KC1: two Kelvin alligator clips for 4-terminals test slots (+ guard) 73 EUR wo VAT-SMD Kevin tweezerto 4 x BNC (Ali) 21 EUR-ZIBOO LCR-06B LCR-08 Clip Kelvin(Ali) : two Kelvin alligator clips to 4 x BNC in row (Ali) 31 EUR-SMD Kelvin tweezersfor 4-terminals test slots (+ guard) (Ali) 33 / 86 EUR-Keysight 11062A(=BU-75K MUELLER ELECTRIC) Kelvin alligator clip for DIY probe. 78 / 16 EUR : small, strong spring, only one colour available (black).Crappy coloured copies(*) from Ali.I'm gonna order the Keysight ones just because they are 5 times more expensive(*) not sure if those are genuine Kelvin clips providing two isolated connexions