Encapsulating OPTs

[aardvarkash10]

I have some particularly ugly and dangerous OPTs that I would like to use externally mounted and I have been wrestling with how to make them visually appealing and safe.

Last night I was idly gazing around our lounge (probably drooling slightly from one side of the mouth too - Homer Simpson-esque) when my eyes lit on a clear glass vase. Square too.

My thought goes like this - find a suitable sized glass jar or vase that looks appealing - maybe the faux-antique glass stuff - and sit the OPT in it, then fill with clear epoxy, leads hanging out of course. When all is no longer gooey, drill and tap a couple of mounting threads tip upside down on the chassis and bolt on...

Concerns - coefficient of expansion. And thats about it...

Your experience and thoughts?

[BudP]

Obtain a clear two part epoxy with a reynolds # below 45 or so. Make sure it's cooking temperature is less than 130 C. Fill it 1/4 at a time, just to keep the thermal mass down. Look around your area for someone who can pull a 1 Tor vacuum (30 inches of mercury / 0 psi) and hold it on the two liquids for five minutes, before you begin this process, just to get the always present bubbles out. Have fun. Wear a good fumes mask and vinyl gloves at all times.

Bud

[ChrisA]

Read that post above about the need to pull vacuum.

If you simply flood the transformer you will trap air inside. Then when the transformer gets hot the air will expand, or at least it will try to do so. But worse you will be trapping heat as the air inside is a good thermal insulator.

One source for vacuum pumps is an old refrigerator.

Also think about the coefficient of thermal expansion of all materials involved. You do not want to crack the parts after some number of hot/cold cycles.

Better to just make a cover with sheet metal.

[BudP]

One serious problem with home brew vacuum systems is the possibility of catastrophic pressure inrush, like the makeshift tank or a hose breaking seal. This will likely kill everyone in the room and a few outside as well. It is an incredibly powerful explosion, right after the implosion.

If you have a vacuum systems contractor in your area you can probably get enough info to look into this, but really, best to have someone already set up do it for you. They might even have a system for applying the two part material to the container under vacuum. This will help with the overall stability of the package through heat cycling.

Might also look for a clear silicone resin,with a reynolds # in the 20's. This would eliminate all concern about expansion mismatch. You can also look for a glass blower in the area and ask them to look at the container and see if they think they can stabilize it with an annealing process.

Just how much voltage are you contemplating here?

Bud

[wrenchone]

I use a clear plastic dessicator for pulling down on plastic resins. There's no danger of implosion/explosion with the plastic as compared to glass. I also found that 70 torr of vacuum was sufficient for pulling a lot of the entrained air out of silicone or polyester resin. This is well within the means of a small rotary vane compressor run backwards.

[dsavitsk]

http://www.head-fi.org/forums/5490903-post19.html

[wrenchone]

Using the stainless film cans is a nice touch, as long as you don't have to pay full price for them. Ebay and garage sales would be the way to go. My sensibilities go quiver a bit though, as I was a photographer at one point, and probably will be again... Having said that, given the scarcity of drawn metal cans, this works. Another expedient would be to use tureens and steam table pans from a restaurant supply house in case larger covers are needed.

[aardvarkash10]

thanks guys - some great ideas! Especially like the film canister one, with sympathies to wrench... Can I take it that no-one has encapsulated a transformer in a glass container?

[BudP]

I have been in the transformer business for 37 years and have yet to run across a single transformer potted in glass. So you do get an A for originality.

Most finish cast transformers, where the final shape is not determined by the transformer shape to any great degree, are cast in silicone molds, with slits and reliefs for various metal bits for mounting etc.

Bud

[ChrisA]

Originally Posted by aardvarkash10

thanks guys - some great ideas! Especially like the film canister one, with sympathies to wrench... Can I take it that no-one has encapsulated a transformer in a glass container?

OK how about this: You get the same "see through" look but don't need vacuum, or moulds. Use a clear lexan container or even a small goldfish bowl, mason jar or whatever then fill it with "fluorinert" and just dunk the trany in the liquid.

Years ago Cray when they build the Cray-2 put all six equipment racks in a fluorinert tank and flooded the entire computer system, cards, buses, cables and all. I think they even included an air pump to make bubbles so the fluid would circulate.

So your glass idea just might work if you used liquid rather then epoxy. Actually it is common (maybe even universal?) for vary large transformers used for power distribution to be "potted" in oil. Almost all of them are flooded with what looks like #10 motor oil. Most all of those transformers you see up on utility poles are inside a drum of "transformer oil".