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Re: first coil experiments (fwd)





---------- Forwarded message ----------
Date: Fri, 28 Feb 1997 06:31:58 -0500
From: Steve Falco <sfalco-at-worldnet.att-dot-net>
To: Tesla List <mod1-at-poodle.pupman-dot-com>
Subject: Re: first coil experiments (fwd)

Tesla List wrote:
> 
> ---------- Forwarded message ----------
> Date: Thu, 27 Feb 97 07:06:36 UT
> From: William Noble <William_B_Noble-at-msn-dot-com>
> To: tesla-at-pupman-dot-com
> Subject: first coil experiments
> 
> I (for the second time) tried building a small tesla coil - generally
> following the plans I have uploaded from a 1964 popular electronics magazine.

I also built a coil from those plans.  Generally speaking, the article
got everything wrong based on what I have learned from reading this
list.  I cut my secondary in half with a saw, and got much better
performance - much closer to tune, but not quite there yet.  Take a look
at the pictures I posted to tesla-img.

> 1. from what I have read, this transformer (since it has AC ratings) should be
> ok at 15KV, right??? or should I use it with 7.5KV transformers (I have a
> couple of 25 ma 7.5 KV beer sign transformers)??

Your transformer is fine.  No problem.

> 2. from what I have read, it would seem that my secondary coil is about twice
> as long as it should be. so a 4" tube with 20" of 24 gague wire will give me
> between 900 and 1000 turns.  I'm inclined to repeat with such a secondary.
> Comments??

Exactly.  You want no more than 18 to 20 inches of windings on a 4"
tube.
 
> 3. Most of the neon sign xformers have the center tap of the HV windinds
> grounded to the case - certainly this is the case with the 15KV units I have
> looked at - this would seem to be a problem with a standard tesla circuit
> because it prevents you from grounding the Xformer case and also the low end
> of the coil.  I presume that usually this whole mess is left floating???

The schematic in the magazine is wrong and dangerous.  You should have
the center tap of the transformer grounded.  The spark gap will be
connected across the transformer.  The capacitor and primary will be in
series, connected across the transformer too.  Your secondary will not
connect to anything other than a separate ground on the bottom terminal.

The way the magazine shows it, you are modulating the coil base with
6000 volts at 60 Hz.  Not a good thing, and dangerous.

One more thing missing from the article is protection for the neon
transformer.  I used an old secondary to make two air-wound choke coils
(I stripped 3 or 4 inches of turns off the middle of the secondary and
used the two remaining coils for chokes).  Also, I added a pair of
protective spark gaps right at the transformer (one from each high
voltage terminal to ground).  In this arrangement, the main spark gap
and the primary coil/capacitor connect to the air-wound chokes, so the
RF generated by the main spark gap cannot get back into the transformer.

You can find the correct schematics on the archive at: 
	http://www.funet.fi/pub/sci/electrical

> 
> 4. should I wind a smaller coil - for example 2 inches by 10 inches with finer
> wire first??? it's actually harder of course to use finer wire.

You don't want finer wire - number #22 is recommended for your 4" form. 
The #24 you have is close enough.  I used #28 and I cannot tune the coil
because of it.  My next coil will used #22.

> 5. what kind of spark gap do you recommend?? I read that a rotary is not a
> good idea for neon sign xformer based coils, so a multiple break static gap
> maybe????

Yes.  The "Richard Quick" gap is great.  Consists of short lengths of
1.5" copper pipe mounted on the inside of a piece of 6" PVC.  This
allows a small fan to keep the gaps cool - also helps to quench the
sparks which improves performance.  You probably want 7 or more gaps of
0.028".  But don't put too many.  In the schematic I described, more
gaps increase the load on the transformer.

	Steve Falco