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Re: [TCML] TC Newbie
Hi Matt,
There's a lot of mythology out there concerning bottle capacitors. Most of it conflicts so badly with my own experience that I can only conclude they are tall tales. For example:
1) Beer bottles quickly overheat in Tesla coil service and often crack.
A) I've built and used over 100 beer and wine bottle caps, and I NEVER had one fail in Tesla coil service, even at powers of up to 7200VA and at NST voltages of up to 15kvac. I've also never had one warm up above room temperature. Never had a leak either--not a drop.
2) Bottle caps are lossy and inefficient, and shorten your sparks.
A) I won't argue with this one. I've not measured it myself, but expert coilers assure me that plastic film pulse caps outperform glass caps by a mile and a half. I have no reason to doubt them. It may depend on what kind of bottles are used, as not all glass is created equal. No doubt some formulations of glass have better RF properties than others. Cap bank configuration probably matters too. A large number of small bottles wired in parallel is probably more efficient than a few large ones. Having said all that, I've been very satisfied with the sparks I generated on a beer-bottle budget! Giving up a couple of inches of spark length to save L50-L100 on capacitors seems like a pretty good deal.
3) Bottle caps sometimes burst while the Tesla coil is running. In fact, one time a bottle cap exploded with so much force, it compressed the water inside it to the fusion temperature of Hydrogen. The resulting nuclear explosion vaporized the poor kid who built the Tesla coil, and he got a big, fat "F" on his science project!
A) OK, I just made that one up. But some of the bottle cap stories I've read are nearly that silly.
Matt, I'm guilty of making a short story long, so let me close with this bit of advice. If you want to make salt water caps, stick with plain old NaCl, and clear glass containers. Table salt may not be the last word in conductive electrolytes, but it is cheap and non-toxic. Likewise, green and brown bottles are pigmented with...what? I don't know! The dye could be metallic salts! Do you really want unknown compounds of Iron or other metals mixed in with the glass? Stick with water-clear glass and avoid the whole issue. For mind-numbing text and grainy visuals on salt water bottle caps, visit my site at:
http://myweb.cableone.net/grcarhunter/
Cheers,
Gregory R. Hunter
--- On Thu, 12/4/08, Matt <tonedeafmessiah@xxxxxxxxx> wrote:
> From: Matt <tonedeafmessiah@xxxxxxxxx>
> Subject: Re: [TCML] TC Newbie
> To: "Tesla Coil Mailing List" <tesla@xxxxxxxxxx>
> Date: Thursday, December 4, 2008, 11:26 AM
> Hello again, I'm back
>
> Just received, my transformer - I don't really feel
> like shelling out the
> money for high voltage caps (or several lower voltage ones)
> though. I was
> thinking of making a 'beer bottle' salt water cap.
> The transformer delivers
> 2x 7.5kv at 40mA. It is a danfoss oil burner transformer. I
> was wondering if
> anyone could provide me with some advice - especially as I
> have heard that
> salt water capacitors can tend to explode when things go
> wrong...
>
> All input and help is valued and appreciated
>
> Thanks a lot,
>
> -Matt
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