BNC DC voltage limits?

Things at the limits.

Re: BNC DC voltage limits?

Postby chrismb » Fri Jun 03, 2011 1:55 pm

Doug Coulter wrote: If anyone finds a non creeping formulation, I'll be all ears.
My suggestion would be to look around for some very old stock - or keep new stock for a few years. Some formulations begin to polymerise and go pretty solid after a while. I have a tube of silicone so stiff that I have to all but put it in a vice to get any out, but it does come out under considerable pressure, albeit very slowly (viscocity now just a little higher than solid plastic!)

Actually, once it begins to come out, you can wipe it with a finger and push it around on your finger pad like chewing gum and it begins to flow a bit more that the impression given during the 'pushing-out'.

(It might also be because the tubes I bough a few years ago were the cheapest I could find, whereas the more expensive stuff is more stable and doesn't polymerise like this?)
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Re: BNC DC voltage limits?

Postby Doug Coulter » Fri Jun 03, 2011 2:35 pm

Hmmm. I have an old tube of Dow stuff (no obvious number or application on the green tube) and in that case, yes, there's some thick stuff that has separated from some runny stuff -- they both come out with the vise technique. I'd used it as heat sink grease some, and even the harder stuff wound up creeping all over the place in time, not that I could achieve a perfect separation of the two components it seemed to have. It almost seems like mineral oil would be easier to keep in place (still hard, just not as difficult) as something water tight will do that, but not the silicone -- that stuff is pretty sneaky, or at least, what I've tested. Can't use "real" heat sink grease as the fillers are semi conductive - and it creeps some too. Makes me wonder if some old Li based auto grease wouldn't be OK -- something stringy.

The need to be able to take it apart is the killer. Else I've found some hardening silicones that are just fine. A real good one is the blue Permatex gasket former (auto store), which is very unlike the usual acetic acid curing "goop" from the hardware store -- I'm not sure how it cures, but it's odorless and doesn't corrode everything around like the hardware store stuff does, and it's good for HV - but as its name suggests, it's also pretty good glue. Any of the one part silicones won't cure in big stuff like that HV connector anyway -- they stay gooey where not exposed to air for quite awhile -- can be years.
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 » Fri Jun 03, 2011 2:58 pm

I don't offer any experience of polymerised silicone with heat. I guess there may never be enough polymerisation to avoid that scenario. My uses for this stuff have only been cool environments. Maybe what you'd prefer is some hi temp vacuum wax? You'd still be able to get it to melt and release, with enough heat. I think there are certain grades to 180C or so?
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