two vs three lug triaxial connectors? - Page 1
Forum section: electronics / projects

author: coppercone2
author_rank: Super Contributor
timestamp: July 23, 2022, 04:38:39 am
content: Is there any electrical difference between the two lug and three lug triaxial connectors?I got something with connector problems and I wanna replace em, and for a small cost I can move my other instruments to 3 lug triaxial (their old connectors are shoddy anyway). I only have one triaxial cable (with one triax only) so keeping it in service is hardly prohibitive.Are the 3 lug ones just better mechanically with no other differences?

author: ivaylo
author_rank: Frequent Contributor
timestamp: July 23, 2022, 05:04:29 am
content: I am curious to see the answer to this one as well. What is the equipment you have with two lug connectors? I have some experience with Keythley and HP instruments and they were all three lug. Whenever I’ve came across two lug cables always took them as a curiosity and lack of standardization.

author: coppercone2
author_rank: Super Contributor
timestamp: July 23, 2022, 05:15:34 am
content: keithley

author: Swainster
author_rank: Frequent Contributor
timestamp: July 24, 2022, 03:31:33 am
content: I have a Keithley 616 with 2 lug connector, however i filed 1 lug off to allow me to use my 3 lug cables. They fit fine... sort of - unfortunately it looks like someone had already jammed a BNC cable into it (its possible to do with 2 lug triax) and damaged the centre conductor, so it only works if you hold it in the correct position. If it wasn't for this it would work fine, however I will be replacing it with a keithley 3 lug connector... which incidentally cost more than what I paid for the whole 616.One potential gotcha is that standard triax connectors are only suitable for something like 500V. Keighley 1000V SMU cables seem to have a slightly lengthened insulator in the male connector, which I presume is due to the higher voltage specification. That said, I'm not aware of any 1000V 2-lug triax applications.

author: donlisms
author_rank: Frequent Contributor
timestamp: July 24, 2022, 06:06:06 am
content: As I understand it, the switch from 2 to 3 lugs was exactly due to the " jammed a BBC" issue. It's not hard to imagine lots and lots (and lots) of such events!My impression is that you can get "good" (i.e., Teflon, etc) and "bad" (not so special) triad connectors in 3-lug form; maybe it was so for 2-lug also. In other words, I don't guess that 2- vs 3-lug is a quality thing.Surely, a 1-lug connector is universal! Seems a little shaky, though... maybe not so good for precision measurements.

author: coppercone2
author_rank: Super Contributor
timestamp: July 24, 2022, 06:54:47 am
content: yeah the connectors I have on something have wobbly plastic dielectric and the impedance reads 4meg unless I press on the cable, center pins look bad too

author: Hydron
author_rank: Super Contributor
timestamp: July 24, 2022, 10:43:29 am
content: Keithley 237 SMU goes up to 1100V yet uses normal (3 lug) triax, though they do also seem a bit more relaxed than I would expect about dealing with the HV internally (e.g. some very skinny insulation, albeit teflon, on some of the wires that can have HV on them, and huge sections of the instrument casually float at the output voltage).I believe that the special HV triax might be for later post-brown-era instruments.I did have issues with the output voltage once - one of my home made triax cables had a stray strand from the guard braid too near the outermost earth connection in one of my connector terminations - it would arc at >~500V and the impulse would reset the SMU! Have had no issues with connectors at 1100V since fixing that though.

author: Swainster
author_rank: Frequent Contributor
timestamp: July 24, 2022, 12:49:09 pm
content: Actually the K237 was what I had in mind when talking about high voltage triax. I noticed that genuine Keithley triax cables have 1 or 2mm more PTFE on the insulator between 1st and 2nd shield rings, and require more force when "locking" them up, compared to say a pomona triax cable.Edit: just to add that before I noticed the difference I had my K237 at full whack with the yellow pomona cable and nothing bad happened.

author: TimFox
author_rank: Super Contributor
timestamp: July 24, 2022, 02:11:07 pm
content: The three-lug version was invented to prevent students from forcing BNC connectors into an unnatural act with the triax connector, damaging the guard contact.The insides seem to be identical.

author: Hydron
author_rank: Super Contributor
timestamp: July 24, 2022, 06:37:51 pm
content: Quote from: Swainster on July 24, 2022, 12:49:09 pmHuh, I didn't realise that the 237 mates with the extended insulator connectors. I've only used trompeter cables/connectors on mine, the Keithley tax is way too high, triax in general is bad enough!

author: Swainster
author_rank: Frequent Contributor
timestamp: July 26, 2022, 01:37:25 am
content: Back in the office, I just had a chance to more closely compare Keithley vs "standard" triax connectors. Actually its the contacts which are recessed rather than the insulator being extended. This would make them compatible with standard triax receptacles, so Keithley instruments would be able to use either kind of cable.Oops, seems to have wandered a bit off topic, though I guess still related to triax connector types.