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Making liquid tight or silicone resistor containers....

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Patrick

Tue Nov 30 2010, 03:29AM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

i just bought a boat load of really nice vishay/caddock/TRW and no name brand HV resistors, but i want them under oil, the problem is how do i make the plastic body remain liqiud tight where the two leads exit?

i really dont know what else to do, i wonder if PVC can be glued with PMMA epoxy and made suffciciently liquid proof, i just dont want a pool of oil under my equipment each week.

i was leaning towards what i did with my oil ball bearing cap thread in which i threaded with a tap, a tight thread screw fitting, then reassembled them with excess PMMA. but i need other opinions.

Ash Small

Tue Nov 30 2010, 04:11AM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Maybe use bolts/studs with washers, maybe with 'O' rings.

Make the connection to the studs on the inside, then connect suitable wires to the 'terminals' on the outside?

just a suggestion.

Patrick

Tue Nov 30 2010, 04:21AM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

im thinking of making a tray of hard epoxy, filling it with Silicone tub and tile caulk, then pressing the resistors in to the wet silicone. if neccasary the silicone can be removed, and the resistors reused, this removes the hazard of springing a oil leak. since the silicone and epoxy are both solids.

i was advised by senior app engineer Richard Drawz, from Caddock not to hard encapsulate the resistors, as a difference in TCo would cause gaps between the resistor body and insulator, where corona will surely pop up. if expansion doesnt cause gaps, then shrinkage may cause cracking of the resistor Al2O body/core. so he advised only oil be used. He speculated that silicone would reduce heat dissipation over what is normally air, oil would increase effective wattage.

i wonder if silicone would conform to any dimensional changes while not being a liquid.

for background see these previous threads:

this is a great pic, from LutzH, his FuG.

Pinky's Brain

Tue Nov 30 2010, 11:02AM

Registered Member #2901 Joined: Thu Jun 03 2010, 01:25PM
Location:
Posts: 837

Patrick wrote ...
i wonder if PVC can be glued with PMMA epoxy and made suffciciently liquid proof

JB Weld (and other two component epoxy glues) is PVC and metal compatible (not sure about the insulation of HV wire though). Completely oil resistant once cured and vacuum tight. Why wouldn't that work?

PS. what is PMMA epoxy? Epoxy with PMMA filler?

radhoo

Tue Nov 30 2010, 03:16PM

Registered Member #1938 Joined: Sun Jan 25 2009, 12:44PM
Location: Romania
Posts: 701

Small PVC pipes and Hot glue ? Much cheaper than epoxy!

Like this:

It holds the seal very good!

Careful when you put the oil in, not to put oil on the open end of the pipe - it might affect the adherence of glue to pipe. Also the glue must be extremely hot.

When done, carefully bend the wires outside the tube to 90 degrees, and cover that with glue - to make sure when/if you pull them you won't get oil out. (like in the last picture, and put some glue on that)

EDIT: a hot glue gun with white glue bar:

Ash Small

Tue Nov 30 2010, 03:43PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Pinky, PMMA is Poly MethylMethAcrilate, perspex to you and me.

Pinky's Brain

Tue Nov 30 2010, 04:17PM

Registered Member #2901 Joined: Thu Jun 03 2010, 01:25PM
Location:
Posts: 837

I know what PMMA is, acrylic ... but I didn't quite understand what he meant with "PMMA epoxy". Unless he meant glueing a PMMA plug with epoxy, which would work.

Radhoo, aren't you worried the oil will slowly dissolve the hot glue? Epoxy isn't that expensive, two 15 ml tubes for 5$. You could always put hot glue on top of the oil, press in a plug and then secure the plug with epoxy. No air bubble, no need to worry about pulling the plug out (the epoxy is a bit more structurally sound). You will only need a very small amount of epoxy.

Patrick

Tue Nov 30 2010, 05:12PM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

original thread:

From about a year ago....

Patrick wrote ...

OK about the methyl methacrylate, Now IM CONFUSED!!!
Im back from physics class but i scaned a picture of the chemistry in question.

1256259931 2431 FT78188 Loctiteandhenkel

please read the back highlitged in yellow.

Anyway, I need to know more about oil insulation, what approximate volts/mil would I expect , what fluid mineral oil, hydraulic fluid?
Your sure lutz, that pressure from catastrophic arcing, wont burst with dangerous pressure?

TY,
Patrick

radhoo

Tue Nov 30 2010, 05:48PM

Registered Member #1938 Joined: Sun Jan 25 2009, 12:44PM
Location: Romania
Posts: 701

Good idea with using epoxy as an external layer.

Pinky's Brain wrote ...

aren't you worried the oil will slowly dissolve the hot glue?

Are you sure on that? Can you provide some links explaining this process?
(some hot glue materials use Polyamides, which are resistant to oils)

Proud Mary

Tue Nov 30 2010, 06:10PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

I've been pleased with results using laboratory rubber stoppers, which come in a wide range of sizes, and are available plain, or ready bored with one, two, or three holes, through which wires can be channeled in glass, ceramic, or plastic tubes.

With care, you can thread a wire through a melting point capillary tube, wrap that round with self-adhesive copper foil, and insert that inside a second wider tube also shielded with foil, to make a guarded triaxial connection.

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