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Re: [TCML] need help

On 2/16/13 6:19 AM, terry zollner wrote:
Hello to all,

This is my 1st post at this club!

I'm an avid admirer of tesla coils and the people that make them!

I need some help with a tesla project for a school musical.
I have seen several tesla coils in action and know what they can do and
the dangers they produce.

Here is what I'm trying to do
I need to build a tesla coil that can be attached to 42 inch vertical
ring that has a 6 inch plate in the center.The discharge would go from
the small center plate to the outer ring.
So here are my questions.

question 1 What size tesla would I need to make to get discharges
(roughly) 20 inches away?

A rule of thumb for a spark gap tesla coil (as opposed to the solid state DRSSTC) is that the spark length is 1.7*sqrt(power in watts).. That's the BEST you can do.. So a fairly small coil (say one run by a 15kV 30 mA neon sigh transformer) can potentially do it.. (I have a smallish coil 4" secondary diameter, about 3' high that runs off such a NST that does about that length..)

However, your particular setup (disk inside a ring) might make it easier or harder. 6" diameter is pretty small in TC terms, but maybe it would work. Could you go with say, 12" diameter sphere or toroid in the middle of your 42" hoop?

question 2 Roughly what would be the cost for the construction?

Are you looking at parts costs or including cost of labor. If you know what you are doing, you could build such a coil in a day or a weekend, not counting "scrounging up parts" time.

You'll definitely want to go the MMC route (not beer bottle capacitors) because you're probably weight limited (are you flying it above the cast?)

Does it matter what it looks like from the side? that is, can you basically build your 42" diameter ring and have a cylinder or truncated cone of chicken wire that extends back about 3-4 feet from the ring? The coil would sit inside the cylinder.

I'm envisioning something that you look at head on (the plane of the ring and center electrode is perpendicular to the line of sight of the audience)

3rd question. If I put plexiglas on both sides of the ring will that
contain the discharge?
in other words can plexiglass shield you from the discharges.

no..  but see below...

4th question. If I do go with the plexiglass and make it air tight what
would happen if I put argon gas inside?
Will that make the bolts act differently?

You bet.. that's getting closer to the commercial crackle neon kinds of things. Argon ionizes a lot easier, and sparks travel over a surface much easier than in free space. you may not need a tesla coil...

If you had a 12" disk in the center of a 42" round piece of polycarbonate (don't use acrylic. Lexan, not plexi.. it's a lot stronger and easier to work with: doesn't shatter, etc.)

with a ring round the outside, and another piece of plastic. Pump it down a bit and fill it with argon, and you need a LOT less voltage to get it to spark across.. In numbers:

air breaks down at 70kV/inch.. pump it down to 1/100th atmosphere (not too tough.. that's easy pumping) and you're down to 700V/inch. A 15kV NST is actually 21kV peak, your 15kV neon sign transformer can go the 20 inches without too much trouble.

Gotta be careful about how far you can pump down, though.. at some point it just glows and doesn't "spark" But at 7-10 torr, you're still in the spark territory, I think.

Argon breaks down at roughly 1/5th the voltage of air.. so if you can pump down the gap to maybe 1/10th atmosphere (that's about 26" of vacuum) and backfill with argon, you can probably spark through 20" of free space with argon (I haven't tried it myself)..

Even better, sparks along a surface have about 1/3 the voltage requirement as through free space. So you can get your 20" sparks along a surface pretty easily, especially in argon without needing to pump down very much...

5th question Can those bolts go thru wood?

Tesla coil.. you bet, or more likely, around the surface of the wood.

I've read thru your safety sheet, and believe I can create a safe
environment, however that all hinges on what the
static discharges will and will not go thru.
My number one concern is safety of the actors and crew, which is the
reason I'm contacting your club.
I've been told this site has the best people with the most knowledge of
tesla coils. So I'm relying on your expertise.
So what do you think? Should I find a safer way( video or  lights), or
do you think this is possible?

Well, there's lots of safer ways.. a video rear projection on a screen springs to mind.. Not as bright or noisy though...

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