How to grab ultra small resistors for 4-Wire measurement - Page 1
Forum section: electronics / projects

author: HighVoltage
author_rank: Super Contributor
timestamp: December 10, 2015, 12:36:37 pm
content: For this project I have to measure about 100 resistors very precisely.But they are so small, I can not even grab the wire with the different tiny grabbers I have at hand.As soon as I grab the resistor, it is falling out again.The resistor itself without leads is about 2.5 mm longThe wire leads are about 6 mm long but the leads are only 0.10 mm in diameter.The resistance is 19.95 Ohms and I need to measure them to 1 mOhm exact.Measuring them exactly with my Keithley Source Meter and 4 Wires is no problem.But:How do I grab them correctly, so they will not fall out?Are there any special tiny grabber available from some manufacturer?Is there a "jig" available for this kind of work?Any other ideas?Thanks

author: Ian.M
author_rank: Super Contributor
timestamp: December 10, 2015, 12:47:44 pm
content: Make DIY kelvin clips from two clothes-pegs and some self-adhesive copper foil. With 100 to measure, its probably worth hotglueing them to a block of wood with the correct spacing so you can load or unload the resistor in one operation.

author: Wytnucls
author_rank: Super Contributor
timestamp: December 10, 2015, 01:08:19 pm
content: These pincers may be more suitable. Sold by Franky in Hong Kong.

author: Ian.M
author_rank: Super Contributor
timestamp: December 10, 2015, 01:27:37 pm
content: They look neat. Are those, or can they be assembled as, kelvin clips?

author: Wytnucls
author_rank: Super Contributor
timestamp: December 10, 2015, 01:48:13 pm
content: They are sold as a Kelvin kit. Easily assembled DIY. The set in the picture is mine (4W to the DUT contact points).

author: HighVoltage
author_rank: Super Contributor
timestamp: December 10, 2015, 01:54:42 pm
content: Quote from: Wytnucls on December 10, 2015, 01:08:19 pmGreat ideaThey look really good and both sided are insulated towards each other and therefore should be a full kelvin arrangement.Thanks, I ordered some of these gold plated clips.

author: tszaboo
author_rank: Super Contributor
timestamp: December 10, 2015, 03:44:04 pm
content: Probably I'm late, but why would you need to have Kelvin clips for them? With regular crocodile clips, grab them with sense where they will be soldered, and put the force slightly away from them. Maybe it is not that convenient, but faster than waiting for a delivery.

author: Ian.M
author_rank: Super Contributor
timestamp: December 10, 2015, 04:14:51 pm
content: Good luck getting two relatively coarse grabbers to hold on a short 0.1mm diameter component lead, then hooking up the other end without mangling the component to the point of unusablety. That's why I suggested the home built Kelvin clips that can both be operated with one hand leaving the other free to insert/remove the component. Another option would be a notch to fit the resistor body cut out of a piece of veroboard, at a rightangle to the tracks. Use the tracks under the leads nearest the body as contacts for the sense wires and the next ones out for the forcing wires. Apply pressure with a plastic pencil eraser, again with a notch cut for the resistor body, to get a good contact when taking the reading.For either home built option it would be best to tin the contacts with lead free solder and wipe off any excess as tin oxide is vastly preferable to copper oxide.

author: HighVoltage
author_rank: Super Contributor
timestamp: December 10, 2015, 05:35:13 pm
content: I got some similar clips, gold plated, in a local store and making my Kelvin test wires now.So, soon I will post some pictures.

author: babysitter
author_rank: Frequent Contributor
timestamp: December 10, 2015, 07:47:38 pm
content: I got pogo sticks of the "multi-prick" / "wafer tip" style, glued them to a plastic pincer, soldered four wires to it. Need to find it to take a photo.you can both use the pinching to grab a little part between them and you can hold it in a constant distance to probe resistor legs or points on a PCB.

author: nidlaX
author_rank: Frequent Contributor
timestamp: December 10, 2015, 11:42:56 pm
content: Quote from: blueskull on December 10, 2015, 05:37:56 pmI do a ghetto version of this where I have two mini-grabbers connected to drive grab onto multimeter probes near the tips. If you have to grab the leads, then maybe 4 mini-grabbers would do?

author: HighVoltage
author_rank: Super Contributor
timestamp: December 11, 2015, 04:06:47 pm
content: OK, I made my own Kelvin clips with gold plated grabbers that I found locally.It works perfectly like this.Thanks for all your advise.

author: Macbeth
author_rank: Super Contributor
timestamp: December 11, 2015, 05:27:21 pm
content: Very nice. I have the same clips which I plan on using with shielded cable for the guard connection and modifying my EE-5000 TL-21 croc adapter.

author: Ian.M
author_rank: Super Contributor
timestamp: December 11, 2015, 05:36:42 pm
content: Nice. Those will do great for anything leaded. Now all you need is some true Kelvin split tip tweezers for SMD stuff!