Glassmike cap as MMC / Tesla Coils / Forums

Forums

4hv.org :: Forums :: Tesla Coils

« Previous topic | Next topic »

Glassmike cap as MMC

previous 1 2

Move Thread

LAN_403

J. Aaron Holmes

Tue Jun 19 2007, 06:03AM

Registered Member #477 Joined: Tue Jun 20 2006, 11:51PM
Location: Seattle, WA
Posts: 546

I suppose your point is that danger prepared for is not danger at all. Which, whether true or not, is something we must all believe to some extent or we wouldn't be playing with any of this stuff!

And, naturally, we all *think* we're prepared when in reality, to be fully prepared would mean knowing the complete domain of possible outcomes. And if we knew that, we'd have gotten bored with all this silly stuff a long time ago!

And I suppose Terry's point is that, if you know the Right Answer, why shoot for any less? Of course, this generally assumes you have some $$ and not a stockpile of "yesterday's" Right Answers. Although hey! That guy on eBay is asking $15 for just one of those caps Tom540 says he's got several of. Time to cash in, perhaps?

For me, time is the most precious resource, so if building something with materials on hand suggests that I'll probably have to do it again when it blows up three minutes (or three seconds!) in, I'll probably wait and save my $$ and build it when I can buy the Good Stuff.

And generally, I'll side with not using caps the are predisposed to self-destruction. No, I haven't used the glass ones, but I have found myself running for my life from a friend's coil when one of his big forty-pound metal-cased pulse caps turned from a cube into a sphere and sent twenty-foot spouts of boiling oil out from the seal around the ceramic bushing on top. One more second under power and I'm sure we'd have all been in the hospital. A positive experience, on the whole, because now we know what *not* to use! There's no shame in blowing stuff up, really. You just don't want to be caught making the same mistakes twice.

Regards,
Aaron, N7OE

sparky

Tue Jun 19 2007, 07:14AM

Registered Member #530 Joined: Sat Feb 17 2007, 07:56AM
Location: Victoria BC, Canada
Posts: 178

I had some nasty experiences with some Glassmikes running a parallel series for 30 kV 10nF - the caps got hot and blew shards of glass all over the floor. They just couldn't take the discharge current I'm guessing...
This was years ago - I ONLY use plastic MMCs now!

Coronafix

Tue Jun 19 2007, 01:42PM

Registered Member #160 Joined: Mon Feb 13 2006, 02:07AM
Location: Melbourne, Australia
Posts: 938

MMMMmmmmm, I thought I would be eaten, but I know now who's advice to take, that be the man with the experience. And you pale against few Terry, who think their dark might dost shine brighter.

Wolfram

Tue Jul 03 2007, 08:25AM

Registered Member #33 Joined: Sat Feb 04 2006, 01:31PM
Location: Norway
Posts: 971

Sorry, a bit late, I was going to reply to this thread, but forgot all about it.

I'm surprised that no one has commented the impolite behaviour of Hazmatt.

Hazmatt, why the angry tone? Terry only tried to give safety advice. Just because your glassmikes haven't exploded doesn't mean that glassmikes can't explode in tesla duty, as Terry said, he has heard stories of glassmikes exploding in TC duty.

You are free to challenge his statements, but please try to do so in a civil manner.

Hazmatt_(The Underdog)

Tue Jul 03 2007, 08:36AM

Registered Member #135 Joined: Sat Feb 11 2006, 12:06AM
Location: Anywhere is fine
Posts: 1735

Because I think this case is an exaggeration, and what I have seen and done tells me it is.

What surprises me is none of you guys have jumped on the PCB bandwagon yet.

Wolfram

Tue Jul 03 2007, 08:52AM

Registered Member #33 Joined: Sat Feb 04 2006, 01:31PM
Location: Norway
Posts: 971

Still, that's no reason to be rude, you could have just pointed it out.

Hazmatt_(The Underdog)

Tue Jul 03 2007, 08:59AM

Registered Member #135 Joined: Sat Feb 11 2006, 12:06AM
Location: Anywhere is fine
Posts: 1735

You know Anders, this is getting off topic by the way, but go back to the first 4 replys and actually read them. I have tried to 'point' this out, but from your point of view and everyone else's, you just don't take the time to read, so I gave up.

Steve Conner

Tue Jul 03 2007, 09:12AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Will you guys calm down! Terry is *THE* expert on capacitors, period. If you saw the work he has done over the years on characterising different kinds of cap for Tesla coil duty, you would agree.

Also, enough of the arguing over whether things pop, burst, crack, shatter or explode. If things made of glass are doing any of the above, it's unsafe. (well, a potential liability issue at least.) It's also a complete waste of capacitors that would function fine in another project like a C-W multiplier.

The issue with glassmikes is the same as with most other DC filter capacitors, AFAIK. Not only do they have a lossy dielectric, but the lead-outs are simply rolled into the capacitor, or maybe spot welded to the foil in one place if you're lucky. That causes a lot of resistance and very high current density, so they fail where the foil meets the lead-outs, start to arc internally, and the gas generated by the arcing bursts them.

A "real" pulse capacitor has "extended foil" construction where the foil sticks out of the ends of the roll and is connected to the leadout along the entire length of one edge. The path for current is far shorter, thicker, and more even. Low-loss polypropylene dielectric is nice too.

previous 1 2

Moderator(s): Chris Russell, Noelle, Alex, Tesladownunder, Dave Marshall, Dave Billington, Bjørn, Steve Conner, Wolfram, Kizmo, Mads Barnkob