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jpsmith123

Sun Jul 17 2011, 01:46AM

Registered Member #1321 Joined: Sat Feb 16 2008, 03:22AM
Location:
Posts: 843

Well I just potted the other coil (the layer-wound coil with vertical leads). I found out that I couldn't go below a few torr pressure because something boils - I think it's the hardener.

It's still curing but it's looking good so far.

Here's a picture of the first coil (the 600 turn, 32 gauge segmented coil with radial HV output leads) in the vacuum chamber.

The epoxy was a learning experience. In a way it was "fun", but I hope I never have to learn it again.

I suppose the next thing to do will be to see how much the epoxy changed the frequency response of the coils.

Patrick

Sun Jul 17 2011, 04:56AM

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

Yep, i think ill stick with bio-oil, more difficulty in liquid containing, but fewer unexpected's.

EDIT: if your vacuum was lowest at a few torr, does that mean the vapor pressure of your hardener is a few torr?
5 Torr = 0.1 PSI, yikes!

jpsmith123

Sun Jul 17 2011, 06:36PM

Registered Member #1321 Joined: Sat Feb 16 2008, 03:22AM
Location:
Posts: 843

Patrick, I don't blame you for deciding to go with oil. Epoxy sux, especially the epoxy I got. Yes the "specs" were good but working with it was nightmarish. Most annoyingly, the hardener apparently has a high vapor pressure and it's rather toxic (I think). I was supposed to get an MSDS with it but I didn't.

I hate the mess it makes. And when it started boiling on me, it splattered all over my chamber and I had to clean it all up later...what a PITA.

Anyway, I'm planning on trying my bud box idea, using teflon wire. I really think that might be the way to go. In fact, I'm thinking about putting the whole assembly inside the bud box i.e., HV secondary and multiplier, and filling it with RTV.

Patrick

Sun Jul 17 2011, 06:49PM

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

jpsmith123 wrote ...

...Most annoyingly, the hardener apparently has a high vapor pressure...

Shit.

Steve Ward

Mon Jul 18 2011, 07:21PM

Registered Member #146 Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
Posts: 1055

For HV potting at work (fermilab) we use this silicone based stuff called Sylgard:

Its fairly thin, and is used for vacuum potting. You must allow for some overflow when you vacuum it out. I believe that they actually pull the air out of the stuff, after mixing (or maybe even before mixing) before they put it in the part to be potted. Then of course they cycle the vacuum on and off to drive the air out of the potting.

Though, i just checked the price of this stuff now, damn its expensive. Doesnt surprise me, we seem to always pick the most expensive materials possible

.

As mentioned before, oil is pretty nice stuff once you work out the containment details. I avoided it for a long time with my HV work, but really, it allows for the best performance possible compared to other insulation options. Ive got a multi-layered transformer that i'm working on at work for a cap charger that easily does 25kVAC at 60khz. It sits in some diala AX transformer oil, and has been evacuated to removed as much air as possible from the winding and the oil.

jpsmith123

Mon Jul 18 2011, 10:21PM

Registered Member #1321 Joined: Sat Feb 16 2008, 03:22AM
Location:
Posts: 843

I think the best or one of the best RTV encapsulants I've seen (in comparing the specs) has to be RTV12 (formerly a GE product, now Momentive Performance Materials).

That's the stuff I originally wanted to use. Unfortunately though, I was not able to find it for sale anywhere, in any quantity under a gallon. And it's expensive. Nor could I get a sample. I pleaded with them on the phone, but they simply wouldn't budge.

Anyway, from what I've learned about potting epoxies, it seems those with the lowest losses (DF) and relatively low viscosities all use some rather nasty chemicals (hardener). For my segmented coil, neither the DF nor the viscosity would have have been too much of an issue, but because of my experimental layer wound coil, I felt I needed both low loss and low viscosity, so I ended up with an expensive product (but luckily the manufacturer was willing to sell me just a pint).

It would've been nice though if the manufacturer had told me beforehand (by way of the data sheet) what the vapor pressure of the hardener was, that way I may have avoided making such a mess when I pumped it down.

Ash Small

Tue Jul 19 2011, 12:27AM

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

Are you sure it was the vapour pressure of the hardener, JP. I once 'sorted out' the vacuum impreg. in a coil winding factory, and was surprised at the 'apparent boiling' of the 'one pack varnish' when it was 'pumped down'. (it was just the air coming out under vacuum).

jpsmith123

Tue Jul 19 2011, 01:42AM

Registered Member #1321 Joined: Sat Feb 16 2008, 03:22AM
Location:
Posts: 843

I'm not 100% sure, but I think so. It seemed to have a sudden onset, and it did not seem to decrease with time. Also, it was large, vigorous bubbles, compared to the "foaming" I saw at higher pressures.

Unfortunately I don't know exactly what the pressure was at the onset, since TC gauges like mine have very low resolution in that pressure range.

Patrick

Tue Jul 19 2011, 03:53AM

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

Its worth figuring out though, even if you have to waste a shot-glass size container of the epoxy and hardener, un mixed in two seperate containers, then pull a vacuum and see!, with no coil or other stuff in the way you can be sure whether its trapped air in the previous attempt or boiling due to low pressure.

Ash Small

Tue Jul 19 2011, 08:43AM

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

I've been thinking about this vacuum potting thing and I've come to the conclusion that maybe the way to go is a two-stage process.

first, immerse the coil in the epoxy/whatever and vacuum impregnate it, then, possibly before it has fully cured, pot it in the final package.

I think this must be how flybacks are made, looking at some of my flybacks. I don't think vacuum is used for the second stage. (maybe the potting compound is put under vacuum before being poured to remove any air bubbles)

Using this technique should avoid the 'boiling over' mess.

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