The Ultimate Kelvin grabbers - Page 1
Forum section: electronics / testgear

author: jchw4
author_rank: Regular Contributor
timestamp: November 23, 2020, 05:27:32 pm
content: I was fascinated by this video:and surely I wanted one for myself!But I am not a machinist and beginner in electronics, so it should be something cheap and made with hand tools only. And easy of courseThis way I started to look for something that could be easily adapted.And the answer is "the cheapest grabbers that are usually referred to as 'total crap'".The final result is in the photo. Not bad for 34401a last calibrated in 1996!The DIY instruction below.

author: jchw4
author_rank: Regular Contributor
timestamp: November 23, 2020, 05:30:05 pm
content: What you need:1. Cheapest grabbers. See photo. Like a dozen for $5 delivered from aliexpress.2. 1mm PTFE tube. This can be ordered for cheap everywhere. Heat shrink version (you can order both) is better.I used aliexpress, so it's been a long project...3. Kapton tape.4. Hand tools: tweezers, pliers, hand file, sandpaper. Dremel is very useful, but the steel is so mild that you caneasily do everything with the file.4.1. Sharp paper knife is important!4.2. Small drill is useful but not required. You can cut a hole in the plastic cap for the second wire with the knife if needed.5. These grabbers are designed to be crimped and are very hard to solder. If you decide to solder to them,there is a whole thread on it.https://www.eevblog.com/forum/beginners/soldering-to-strange-metal-contacts/.My favourite solution is to electroplate. Just google "copper plating".You need copper sulfate which you can buy in any gardening store. Unfortunately you will have to buy it atgardening scale, like a few pounds, and you will only need a little. Maybe you can start gardening6. If you decide to electroplate, you will need a constant current power supply capable of 50-200ma at ~1V, or AA battery + resistor.(Maybe AAA can be used without a resistor?)

author: jchw4
author_rank: Regular Contributor
timestamp: November 23, 2020, 05:32:24 pm
content: Disassemble the grabbers. You will need 4 of themYou can see that the end is folded, and you need to undo the fold.

author: jchw4
author_rank: Regular Contributor
timestamp: November 23, 2020, 05:34:29 pm
content: I used my tweezers and pushed them with pliers. Not the best use for tweezers, but the steel is very mild, tweezers were not damaged.Now you need to flatten it with your pliers. See that it cracks at the end, which is actually very useful!So you have 4 of them. Note that one contact is a little bit shorter than the other.Now you need to align them and select pairs that match better. You will need to snap one side of each, so select which side looks more straight.You can easily snap the contact that you don't need with your fingers. Just hold the center with your pliers and wiggle the part that you don't need. It will fall off.

author: jchw4
author_rank: Regular Contributor
timestamp: November 23, 2020, 05:35:48 pm
content: Now you need to carefully straighten both sides (remember that you now have a matched pair of contacts).Try to put spring on both contacts together. File away (carefully!) everything that prevents it from moving. Assemble the grabber.Now you basically have it, but you need to prevent contacts from shorting.You will do this by putting 1mm heat shrink on it. But you also need to prevent spring from shorting contacts.To do this you need to make sure that tube extends past the spring. (See photo.)Basically you need to file one of the contacts so that you can push the tube further than the spring.Note: don't try to push it too far, about 3mm is enough. If you make a narrow part of the contact too long, it will freely move along the other one and you don't want this.You should be able to put a tube on it and assemble the grabbers. It should move freely. Or nearly-freely, because once you shrink the tube, it will move easier.Don't shrink the tube yet!

author: jchw4
author_rank: Regular Contributor
timestamp: November 23, 2020, 05:38:23 pm
content: Here you need to make sure you know how to connect wires to it.If you decide to electroplate it, do it now.If you decide to crimp, do it last.For electroplating you will need a single battery, copper sulfate and water.The whole setup took me an hour (with cleanup).I waved the tip of the contact for about two minutes, so it does not need much time.

author: jchw4
author_rank: Regular Contributor
timestamp: November 23, 2020, 05:40:36 pm
content: Ok. Now You need to isolate touching sides of the contacts. Just stick some kapton to it and cut around with a knife.Look at the pictures. You will see that I wrapped kapton around the top of the contacts leaving only a small area for soldering.This simplifies further assembly, because the solder area is small and you prevent shorts to the other side via the attached wires.

author: jchw4
author_rank: Regular Contributor
timestamp: November 23, 2020, 05:43:34 pm
content: Put on heat shrink, do final assembly, check that everything is smooth. Contacts should be well isolated.Attach wires. Enjoy!

author: Electro Fan
author_rank: Super Contributor
timestamp: November 24, 2020, 02:36:06 am
content: Quote from: jchw4 on November 23, 2020, 05:27:32 pmIs the photo showing 4.6 milliohms?

author: Vgkid
author_rank: Super Contributor
timestamp: November 24, 2020, 02:46:31 am
content: Quote from: Electro Fan on November 24, 2020, 02:36:06 amYes, though the resistors accuracy is 5mOhm +/- .05mOhms assuming the resistor is the same as the one on the bag.

author: jchw4
author_rank: Regular Contributor
timestamp: November 24, 2020, 03:09:01 am
content: Quote from: Vgkid on November 24, 2020, 02:46:31 amYes, it's showing 4.6 milliohms. And resistor is from the bag.I guess this is is good for 34401a. Even 24 hour spec is 0.003% of range, so I guess it's best is 100*0.003%= +- 3mOhm. There is no specific spec for 24 years

author: Electro Fan
author_rank: Super Contributor
timestamp: November 24, 2020, 04:06:00 am
content: What resistance measurement do you get if you just measure the two Kelvin clip leads clipped together (without the resistor)?

author: tautech
author_rank: Super Contributor
timestamp: November 24, 2020, 04:13:39 am
content: Quote from: Electro Fan on November 24, 2020, 04:06:00 amDoes it matter as with any half decent meter you can null it out.

author: Electro Fan
author_rank: Super Contributor
timestamp: November 24, 2020, 05:33:35 am
content: Quote from: tautech on November 24, 2020, 04:13:39 amIf nulling was the answer to everything maybe 4-wire measurements & Kelvin clip leads wouldn’t exist. Apparently Siglent thinks there is a reason to include 4-wire measuring capabilities with their DMMs, yes?In this case we’re talking about trying to understand the performance of a particular design of Kelvin clip leads so the idea is to measure their contribution to the measuring system’s performance.As to whether it “matters”, YMMV, but it would seem you can try doing anything you want with a “half decent meter”, includingmeasuring stuff. Just sayin’

author: tautech
author_rank: Super Contributor
timestamp: November 24, 2020, 05:49:32 am
content: Yes well of course, any measurement accuracy is determined by 2 or 4 wire and for either you still need to null (Rel) the meter to zero before doing exacting measurements.Who cares what the leads measure unless you're measuring significant currents.

author: Warhawk
author_rank: Frequent Contributor
timestamp: November 24, 2020, 06:08:22 am
content: I like it, thanks for sharing. I may give it a try too. Everything is at home.

author: Electro Fan
author_rank: Super Contributor
timestamp: November 24, 2020, 06:24:01 am
content: Quote from: tautech on November 24, 2020, 05:49:32 amActually in 4-wire mode on some DMMs with some Kelvin clip leads you can get pretty close (u ohms) without null (or Rel). If you didn’t care about the contribution of the leads there would be no 4-wire systems - there would just be null. Might depend on how much accuracy you are trying to achieve. In any event, if you wanted to know what your meter was doing it might be nice to understand what your leads are doing.

author: Mechatrommer
author_rank: Super Contributor
timestamp: November 24, 2020, 06:31:31 am
content: Robrenz is a member here you can check his machinist topics..https://www.eevblog.com/forum/profile/?area=showposts;sa=topics;u=6155

author: Berni
author_rank: Super Contributor
timestamp: November 24, 2020, 06:41:35 am
content: That is a pretty neat trick. Lovely compact kelvin clip.I just use some cheep chinese "clothes pin" style kelvin clips that works fine enough. A bit bulky to use, but so far they did the job just fine for the few times i needed them.

author: Electro Fan
author_rank: Super Contributor
timestamp: November 24, 2020, 04:49:19 pm
content: Here is a measurement of a ~5 cm piece of 18 gauge copper wire that should be pretty close to 1 milliohm. The Kelvin clip leads are BK Precision TLDK1. The meter was set to use offset compensation but the null function was not used. It's hard to say if the the 5 cm wire is exactly 5 cm from clip to clip so the length could be a contributing factor but I think the larger factor is that the air conditioning cycles on and off and the temperature varies.

author: Electro Fan
author_rank: Super Contributor
timestamp: November 24, 2020, 10:50:05 pm
content: Here is a measurement from just the TLDK1 Kelvin leads clipped together - so it's just measuring the Kelvin clip leads. Same settings as previously with offset compensation turned on (and still with no null function). Seems to show an average of about -40 u ohms with a minimum of -0.6 milliohms and a max of 0.4 milliohms over about 19k samples.Edit: added another, also with just the TLDK1 Kelvin leads clipped together but with offset compensation turned off (this one is also with no null). Seems to show an average of abut 50 u ohms with a minimum of -0.6 milliohms and a max of 0.7 milliohms over about 62k samples. As nifty as offset compensation might be it's slightly easier to get my head around 50 u ohms as a plausible resistance contribution from the two Kelvin clip leads.

author: jchw4
author_rank: Regular Contributor
timestamp: November 25, 2020, 01:51:33 pm
content: Quote from: Electro Fan on November 24, 2020, 04:06:00 amIt's about 0.9mOhm - 1.9mOhm. But for nulling it seems that connectors matter (I used cheapest "breadboard style" as you can see on the last photo), and likely twisted pair too (is ~1m too long / hard to balance for zero measurements?).I'll try to solder wires directly to the clips and check what I get.

author: Electro Fan
author_rank: Super Contributor
timestamp: November 25, 2020, 06:51:49 pm
content: Quote from: jchw4 on November 25, 2020, 01:51:33 pmYep, the lead length and soldering might be good measuring experiments; probably want to try them one at a time. Not sure if the number of twists per unit of length might make any difference but might also be worth a look-see. You could probably try each experiment with and without null to see if the meter reports anything interesting. My guess is that at some point the ambient temperature becomes a key variable. Plus you might have to account for some settling time before taking the measurements, and then you might have to sample somewhat over time. This stuff starts out simple but then variables from all over the place start to come into view. For just picking a resistor for most circuits it's over the top but to get a better glimpse into the millis and micros it becomes somewhat more tedious, or fun- depending on how you look at it.

author: mawyatt
author_rank: Super Contributor
timestamp: November 25, 2020, 06:53:58 pm
content: Quote from: Electro Fan on November 24, 2020, 10:50:05 pmJust did a quick test with the modified Kelvin leads shown.Best and Happy Thanksgiving,

author: Electro Fan
author_rank: Super Contributor
timestamp: November 25, 2020, 08:47:38 pm
content: Quote from: mawyatt on November 25, 2020, 06:53:58 pm10 u ohms avgVery Nice!

author: mawyatt
author_rank: Super Contributor
timestamp: November 26, 2020, 12:00:59 am
content: Quote from: Electro Fan on November 25, 2020, 08:47:38 pmThanks, that wasn't staged as we see so oftenIf I do this again it has shown a higher but still respectable value, think I just got lucky with that connection. Gotta love these KS 34465A DMMs, they have DNA from the 34401A days (why I have it)Happy Thanksgiving.Best,

author: Electro Fan
author_rank: Super Contributor
timestamp: November 28, 2020, 05:06:49 am
content: Tried another clip to clip test. Shows -10 micro ohms as the average for just the Kelvin leads but it bugs me that the average between the min and max values results in a negative value. I get that it's a sampling process with temperature variations and that overall the whole thing is somewhat random but still it kinda bugs me. Long story short I think the Kelvin leads are a pretty small factor in the measurement of most typical resistances.