Cheap triax connector ?

[michaeliv]

Hello,

I'm looking for a triax connector for the output of a Keithley 220.
The female end ( the one I want to plug into ) looks like this: http://i.ebayimg.com/images/g/OBMAAOSwstxVNT-6/s-l1600.jpg
Picture is attached of what I want to purchase.
The cheapest I can find on eBay is $40 which seems excessive considering a similar coax connector is under $1.
Am I searching for the wrong name or is this expected pricing ?

Best!

[pmcouto]

You’re doing nothing wrong – Triax connectors are *really* expensive!

Sometimes there are good deals on eBay.
Keep searching and good luck.

Be careful:
There are 2 lug and 3 lug triax connectors. Be sure to pick the right one!
(linked picture shows a 3 lug connector)

[michaeliv]

Out of curiosity do they use rare/expensive materials, or are they difficult to manufacture, or there is little demand for them ? What's the reason behind the pricing ?

[Hendry Born]

Please check : http://www.pomonaelectronics.com/pdf/d_ai_5218_2_03.pdf

[pmcouto]

Michaeliv,

As you can see in the Pomona 5218 assembly instructions, these connectors have a lot of small precision machined parts that are not cheap to make.

Also, demand for triax connectors is low and there aren’t many manufacturers producing them (competition is very low).

Last but not least, triax connectors are mainly used in professional high-end laboratory instruments and video (broadcast grade) equipment. Compared to equipment price, triax connectors price is insignificant…
Additionally, companies using this kind of equipment can easily afford high-priced accessories such as cables and connectors, so there’s no real “pressure” on the manufacturers to lower the prices.

Now, you probably have a better idea of the reasons behind the pricing. 

[TiN]

If you think triax is expensive, look at SHV version of it (e.g. ones Keithley using for high-voltage SMU and SCS). 
Also all those bits in triax connector are made from teflon and require extra cleaning, to ensure high insulation resistance. I don't think regular BNC are Teflon-insulated. And metrology-grade BNC's are expensive too.

[michaeliv]

Thanks for all the replies. A somewhat unrelated question, why is teflon used either in these connectors or in PCB standoffs. Its resistivity is ridiculously high when compared to air, according to https://en.wikipedia.org/wiki/Electrical_resistivity_and_conductivity, so most of the leakage would be through air ( for PCB standoffs ). Why not use PET which costs nothing, has a bit lesser resistivity than teflon but still much much higher than air.
It seems to me like plugging little cracks in a barrel that is missing it's bottom.

[smithnerd]

Thanks for all the replies. A somewhat unrelated question, why is teflon used either in these connectors or in PCB standoffs. Its resistivity is ridiculously high when compared to air, according to https://en.wikipedia.org/wiki/Electrical_resistivity_and_conductivity, so most of the leakage would be through air ( for PCB standoffs ). Why not use PET which costs nothing, has a bit lesser resistivity than teflon but still much much higher than air.
It seems to me like plugging little cracks in a barrel that is missing it's bottom.

It has other attractive properties, such as a relatively high melting point and low chemical reactivity. UV stability maybe?

[vk6zgo]

Thanks for all the replies. A somewhat unrelated question, why is teflon used either in these connectors or in PCB standoffs. Its resistivity is ridiculously high when compared to air, according to https://en.wikipedia.org/wiki/Electrical_resistivity_and_conductivity, so most of the leakage would be through air ( for PCB standoffs ). Why not use PET which costs nothing, has a bit lesser resistivity than teflon but still much much higher than air.
It seems to me like plugging little cracks in a barrel that is missing it's bottom.

It has other attractive properties, such as a relatively high melting point and low chemical reactivity. UV stability maybe?

Another advantage of PTFE is that it can be machined.
Probably not a concern with small connectors,but it becomes important in large ones.
Yet another is that it does not deform when heated.(within reason-smoke & flames mean all bets are off )

[pelud]

One easy hack for the 2 lug triax connectors if you don't mind modifying it is to file off one of the lugs.  That way you can use 3 lug cables which are much easier to find.

Also when searching ebay, they are sometimes listed as "trompeter" connectors, but be careful that they are not twinax which is different.

Here is a 3 lug triax cable: eBay auction: #311547280972

[macboy]

I have a few extra Trompeter (Cinch) brand PL75-55 connectors. I'll part with one for USD $15 or two for $25 shipped. PM if interested.
These are technically made for "Twinax" cable which is a shielded twisted pair, but the all important mating surfaces are identical to Triax. They are 3-lug (un-keyed, 120 degrees per lug), as in your pictures. I don't think it would be too difficult to mate these to Triax cable... the most difficult part would be getting a good connection with the inner braid, which would likely require soldering since it is not designed to crimp to it (it will crimp to the outer braid when assembled). You also need to solder the pin. Here is a photo of the exploded connector shamelessly linked from digikey:



p.s. Please check if you need 2-lug or 3-lug connectors, as the Keithley 220 came with 2-lug connectors originally. Yours may have been modified by a previous owner, as 2-lug triax is especially rare.

[michaeliv]

I have a few extra Trompeter (Cinch) brand PL75-55 connectors. I'll part with one for USD $15 or two for $25 shipped. PM if interested.
These are technically made for "Twinax" cable which is a shielded twisted pair, but the all important mating surfaces are identical to Triax. They are 3-lug (un-keyed, 120 degrees per lug), as in your pictures. I don't think it would be too difficult to mate these to Triax cable... the most difficult part would be getting a good connection with the inner braid, which would likely require soldering since it is not designed to crimp to it (it will crimp to the outer braid when assembled). You also need to solder the pin. Here is a photo of the exploded connector shamelessly linked from digikey:



p.s. Please check if you need 2-lug or 3-lug connectors, as the Keithley 220 came with 2-lug connectors originally. Yours may have been modified by a previous owner, as 2-lug triax is especially rare.

Thanks, but I do indeed need a 2-lug triax connector. I didn't know that there were different types so I just slapped in a photo that I could find   .

[rastro]

Keithley and possibly Trompeter make  triax to BNC adapters.  There are obvious trade-offs going to BNC (2-conductor vs 3-conductor) but it may be acceptable for some applications.  The Triax cables are also very expensive. 

zucca also wrote about converting his 220 to 3-spade triax.  Looks like some of the original pictures are missing???
https://www.eevblog.com/forum/testgear/keithley-220-output-triax-connector-mod/

-rastro

[woodchips]

Where are you? Not really worth posting overseas.

[michaeliv]

Where are you? Not really worth posting overseas.

I'm in the US, and already checked and there is an ocean in between US and UK , thanks anyways.
I don't _*REALLY*_ need it, I'll manage with a hacked in connection, I wouldn't benefit that much from the extra isolation.

[PedroDaGr8]

This is a thread I am very interested in. Would be nice to find some affordable triaxial setups to test with. In particular, I wish there were some affordable triax to bnc adapaters even those seem damn expensive. I have a Keithley 236 that uses these things and it would be nice to utilize the sense lines and what not. I don't anticipate doing measurements taht are sensitive enough to need the guard of the triaxial cable. A BNC should work fine.

[(*steve*)]

I also have a piece of equipment with a 2 lug triax connector and cables that are 3 lug.  This thread had more useful information than the rest of the internet combined, so maybe I can add a little to it. :-)

I finally discovered that a Pomona 5299 is a 2 lug to 3 lug adapter and A couple of sellers on ebay have these for reasonable prices. (I got one for less than $US20)

There's also a seller with over 200 "Trompeter PL75-24A Triax Triaxial Cable Connector" for $US10 each.

It's also worth looking for "Trompeter BJ72" as there are a few bargains to be had at the moment.

These are reliable brands (I am not connected to any of the sellers), so might be a cheap option to fix some triax connector woes.

[alm]

I finally discovered that a Pomona 5299 is a 2 lug to 3 lug adapter and A couple of sellers on ebay have these for reasonable prices. (I got one for less than $US20)

You sure you got that model right? Pomona 5299 is a 3 lug triax (male) to BNC (female) adapter with shield shorted to guard (not good for anything with an active guard like a SMU).

There are also adapters from triax to BNC with either triax shield connected to BNC shield with guard open (e.g. Trompeter ADBJ20-E2-PL75) or guard connected to BNC shield with triax guard open (e.g. Trompeter ADBJ20-E1-PL75). The former is safer (shield tied to ground), especially on the high side, but the latter performs better for low currents (keeps guard) and can be fine for the low side where guard is very close to ground, low voltage work or setups where the connector and cable shields are protected from touching.

All of this is three lug. Two lug triax is legacy and my suggestion is to either change the connector to two lug (there is an old thread about changing a Keithley 220 output connector from two lug to three lug on this forum), or get a 3-lug to 2-lug adapter (e.g. Keithley 6172), or make one from a 2-lug male, short length of triaxial cable, and 3-lug female.

There's also a seller with over 200 "Trompeter PL75-24A Triax Triaxial Cable Connector" for $US10 each.

It's also worth looking for "Trompeter BJ72" as there are a few bargains to be had at the moment.

Make sure (as is the case for these two) that whatever you are buying is for TRB connectors. There are also thread, larger and smaller versions that are not compatible. This document shows some of the variants.

[pmcouto]

Keithley 6172 is a 2 slot male to 3 lug female Triax adapter.
But it’s not cheap…

http://pt.farnell.com/keithley/6172/triax-adapter-2slot-male-3lug/dp/2672724

Regards,
Pedro Couto

[alm]

Few things related to triax are cheap. Depending on how you acquire them, a 2-slot male connector, piece of triaxial cable and 3-slot female connector may not be that much cheaper. I believe Pomona and Trompeter may make similar adapters, but I do not have the part numbers handy and I am not sure if they are still current products. Is there any equipment currently in production that uses 2-slot triaxial connectors?

Also you may not have to pay list prices. Things might be available on eBay, or even through regular distributors that have surplus stock. I paid much less for mine brand new from Top Elektronik a number of years ago.

[(*steve*)]

You sure you got that model right? Pomona 5299 is a 3 lug triax (male) to BNC (female) adapter with shield shorted to guard (not good for anything with an active guard like a SMU).

I'm well aware of the triax to BNC converters and I blame <insert favouite blame thing here> for totally failing to spot this, even though I looked up the spec sheet. 

Oh well, I can use it for my Keithley 600B.  I already have a converter to BNC but my regular coax to alligator clip cable has a resistance of 50G\$\Omega\$.  I can probably use this to allow me to connect my nice Keithley triax cables

And I guess I'll be filing off that damn lug after all.

Thanks everyone for the heads-up. 

[ebclr]

https://item.taobao.com/item.htm?spm=a1z10.3-c.w4002-2384871945.56.29ed68dexQhfcl&id=536567327738

[TerraHertz]

I've been looking for affordable triaxial BNC connectors (and cable) for a while. I have a HP 4140B pico-amp meter I need to connect to a test fixture, and it's required to use triaxial 3-lug BNCs and cable.
The triaxial cable is Belden 9222. Low leakage, low microphony.
Suitable connectors are Pomona model number: 5218. See:
   https://www.pomonaelectronics.com/products/connectors/bnc-triaxial-m-3-lug-belden-9222-triaxial-cable
   BNC Triaxial (M), 3 Lug For Belden 9222 Triaxial Cable
But these cost around AU $78 each, and the first price break is at Qty 25. Still not much cheaper. Too rich for me.

Anyway, I finally found an affordable source on Aliexpress.
Store name: Chengtec Store
Triaxial BNC male connector, domestic high quality, JY75-1.5 / JY75-2 / JY75-3 1553B cable
Curently AU$ $23.98 each (15.19 USD), 10% off for 5 or more.
I'd originally ordered just two. They arrived recently and are well made and fit Belden 9222 cable perfectly.
The construction allows for re-using them if you need to replace the cable. A big relief to me, since they are still quite expensive.
Insulating material seems to be PTFE, so hopefully they are suitable for the pA meter.

It appears the store only recently listed these connectors, and I was their first buyer. Perhaps they'll lower the price if they start getting significant sales numbers?

Here are some pics I took of one I received.

Incidentally I also found an ebay seller (wzjworld) with some used Belden 9222 for sale. That cable is OK too, and cheap.

[noreply]

Just be careful with 'ebay' or Amazon - as you usually get what you pay for.

I got some 50 ohm BNC inline terminators from Amazon - these were VERY reasonably priced - but compared to an original Rigol part - which is 5 times more expensive - their quality was reflective of the cost.

I know the Triax connector is not same as what I am referring to - but the concept of an alternative 'cheaper' item stays same.

The Rigol BNC is a machined part - expensive to make - the 'Amazon' BNC is cast metal -poor tolerances - much cheaper to make - not to mention the
Rigol termination resistance is spot-on 50.0 ohms, the 'Amazon' part is 49.8 ohms.

Remember you tent to get what you pay for.

Good luck in your selection 

 

[TerraHertz]

All good points, and thus why out of caution I initially only bought two of these 'cheap' triax connectors. Also why I posted photos of them here, to show that they are not the cheap diecast zinc alloy rubbish that does exist. These are machined brass, nicely plated, seem to be close-tolerance parts.

Whether the 'gold' plating is actually gold, and what's the plating thickness, don't know.
As for impedance matching, for the frequencies involved in the target system (all under 15MHz) that isn't going to be a significant issue. Down in the pico-amp measurements range a far more significant factor will be surface and bulk conductance of the insulating materials. But if those are half-way decent (teflon/PTFE) a fingerprint on the insulator surface would swamp the material characteristics anyway.