BNC DC voltage limits?

Things at the limits.

BNC DC voltage limits?

Postby Joe Jarski » Wed Jun 01, 2011 6:27 pm

I didn't know where to put this one, so it ended up here...

What's a reasonable DC voltage limit for BNC connectors for things like GM tubes and scintillators? I know they're rated for 500V with a minimum breakdown of 1500Vrms, which is ~2100Vp-p. So, where do you draw the line and move up to a higher voltage connector?
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Re: BNC DC voltage limits?

Postby Doug Coulter » Wed Jun 01, 2011 7:24 pm

Maybe someone can give a better answer than mine which is "when I can't get it done with a BNC". Which varies. For an HV supply, a little noisy leakage is not a big deal, and I can usually get a couple kv or so fine -- not 5, but usually 2-3kv. But if there's signal riding on that, less, maybe half that. The leakage and almost-arc corona are noisy and mess up small signals.

I think most of us roundly hate the UHV and SHV series since they are expensive and hard to find at all. I use N connectors at up to 3kv so far (3He kinds of volts) since they work well and are priced right and fit fat cable I like to use for the bigger stuff -- RG-8 size. I'm not sure of their ratings, this is just experience from trying stuff. I know from experience that you can really push the fat coax much higher than the ratings if its solid dielectric - the foam stuff not so much. 20 or 50kv no problem at all (rated at 11 kv).

Above that, I make my own, often something like a banana plug at the end of a rod that fits down a tube receptacle that has a banana jack at the end (main supply on my fusor now is one). Tight fit rod<>tube and grease it works up to 50-70kv pretty well, maybe higher. You ought to see those connectors Spellman uses for 160kv. Cliff warned me off trying to make my own, and having seen them (he donated a couple of these $1200 things to the cause) I agree - those are just the right squishy to make an airtight fit along a long slow taper...

How clean the BNC is makes a big difference, as can painting the backside of it with glyptal. But it's flakey and some dust and humidity can kill that scheme.
Posting as just me, not as the forum owner. Everything I say is "in my opinion" and YMMV -- which should go for everyone without saying.
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Re: BNC DC voltage limits?

Postby chrismb » Thu Jun 02, 2011 12:12 am

The issue is how the cable is manufactured into the plug. I think the plug is good for a big voltage, but there will be some form of discontinuity in the dielectric, and it can spark across that if it is an air gap and is not filled somehow, or continuous into the plug.

FWIW: I use F-type satellite connectors and high quality German 4-screened (2 solid, 2 braided) satellite coax. I put an account up on fusor.net. I modify a female-female coupling so that the cable dielectric fits snugly into it. These days, I am regularly running these to 15kV and have never had any indication of a breakdown.

http://www.fusor.net/board/view.php?bn= ... 1258401294

hv_f_connectors[1].pdf
(702.68 KiB) Downloaded 499 times


(I've not shown refitting a shell, but once it is screwed up it just looks like any other F-connection.)

It is my standard coupling now. Because I have a need for so many RF couplings in my system, it was the only thing to do to avoid big cost. (Besides, you can't get 15kV BNC!) I've struggled to work out a means for a suitable, robust 'conversion' back to BNC at those voltages, thought, so I just don't use BNC's for any high voltage work at all.

When I did testing on these, I ran totally unmodified connectors - just to see what the 'minimum' was, and they still took 3.3kV before breaking down (in the air gap between the central wire and the coupling - which this mod fills).
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Re: BNC DC voltage limits?

Postby johnf » Thu Jun 02, 2011 2:01 am

Voltage limits on BNC's
All depends on which brand and how clean they are
Recently we had a few probs with one of the implanters --one of the quadrupoles was intermittant I finally traced this to a coax join where when the beam line was extended one of the techs put in a section of coax using BNC's, joiners ETC little did he know that each end was on SHV's from 10kV supplies using Shuhner RG58 coax.
Long and short of it all on humid days the Quadrupole played up tracking in one or other BNC connector.

Playing it safe in damp conditions I would not use a BNC / RG58 above 5kV with clean connectors

this rules out handling any part of the connector during assembly without gloves on --I also think that the crimp connectors are better as no soldering fumes condense on the coax tail or connector insides.
To ward off humidity problems use Dow Corning silicon stop cock grease when assembling the crimpp connectors ie inside the connector and again inside the mating bits ---all a bit messy but you can get to over 20kV this way

over this "N" or "C" connectors will go to 50kV under the above provisos
Modified teflon UHF connectors ala the Glassman method will get you to over 80kV using good RG8
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Re: BNC DC voltage limits?

Postby chrismb » Thu Jun 02, 2011 4:08 am

Further to your comments on Glassman, John, there is also a post on this in fusor.net:

http://www.fusor.net/board/view.php?sit ... 1190343950
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Re: BNC DC voltage limits?

Postby Doug Coulter » Thu Jun 02, 2011 9:06 am

Yes, my banana plug dealy was more or less inspired by the Glassman trick, but in some cases I've instead used sparkplug wire (Accel 8.8 mm racing stuff) and depending, even put that inside a plastic tube, drilled for tight fit, with silicone grease. I swipe the braid off RG8 class coax for that, it works well past 100kv. The main downside (assuming you can tolerate the series R which is low - nichrome wound over graphite string) is that the Accel wire has a high dielectric coefficient, so you pretty much have to put it in braid, else all the charge on the inside finds its way to the outside and charges up stuff in the lab by induction.

I've never managed 5kv on a bnc, though - the chassis mount females will arc on the backside unless "painted'" with something. For things like small signal from detectors, I limit it to much lower as long before you have arcs, you get corona noise.

I'm surprised you do that well with type F's -- that must take some careful greasing, it's a short path to arc in there.
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Re: BNC DC voltage limits?

Postby chrismb » Thu Jun 02, 2011 9:35 am

It's all to do with the cable. It is foamed PE, so when you push the wire in and tighten it up, it seals into the female part like an ear-plug. No grease, except of the elbow variety!

There is some devil in the detail - you should tighten both male shells against the adapted female part at the same time so the centre doesn't push out one way or the other, or, if installed as a 'bulkhead socket', then screw the 'inside' shell on until the core begins to move, then press the core back into the f-f part, which in turn pushes the cable dielectric into the hole.

It all works out beautifully. I have a 30kV 'megger', maybe I should test the coupling higher still, just to see if I can hit the 'max spec' that could be achieved with it.
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Re: BNC DC voltage limits?

Postby chrismb » Thu Jun 02, 2011 9:40 am

f+bulkhd.gif

adapted female f-f connector, installed as a 'bulkhead' socket.


P1290300s.gif

How the chamber feeds currently look, as viewed in the underside 'junction box'.
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Re: BNC DC voltage limits?

Postby Joe Jarski » Thu Jun 02, 2011 7:36 pm

Thanks for all of the great information!

Doug, At least now I have an idea of when I need to go through more effort to get higher voltages, if need be.

Chris, Nice write up on the F-connectors. I guess I would have found some of the information on fusor.net if I did a search for something other than BNC connectors. :oops: After dissecting some of the cable connectors that I have it became more obvious that the air gap where the cable joins the connector is the weak point - as you pointed out. The connector interface looks pretty well insulated, otherwise.

John, thanks for tip on the grease. I was thinking a little more complicated than that, but it seems like it would be easy enough to fill air gap with grease in the cable terminations for a few more kV. Somewhere I read that the D-C silicon stop cock grease has been replaced with "Silicon High Vacuum Grease", do you know if that would work just as well?
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Re: BNC DC voltage limits?

Postby Doug Coulter » Fri Jun 03, 2011 1:03 pm

Chris, I suppose you're increasing the clearance between the uninsulated "pin" on a type F and the shell by putting the female socket way back into the female shell? Else you have a very short spark path (less than 1/8") in those.

For grins, here's what a real HV connector looks like -- no amateurs need apply. This one is 3 pins, two for filament power and one for the HV. They're not cheap. The cone is some silicone that has a little give and fits airtight into a cone in the HV supply, being clamped in there with considerable force after greasing with some silicone grease.
RealHV.jpg
Spellman 160kv+ connector and cable

Which works -- but I hate that stuff. It has a huge tendency to creep out of anything not perfectly hermetic and make a mess (not to mention getting lost from where it belongs). If anyone finds a non creeping formulation, I'll be all ears.
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