Re: first coil experiments (fwd)

To: tesla-at-pupman-dot-com
Subject: Re: first coil experiments (fwd)
From: Tesla List <mod1-at-pupman-dot-com>
Date: Sat, 1 Mar 1997 14:50:59 -0700 (MST)
Approved: mod1-at-poodle.pupman-dot-com


---------- Forwarded message ----------
Date: Fri, 28 Feb 1997 13:17:05 -0500 (EST)
From: Esondrmn-at-aol-dot-com
To: tesla-at-pupman-dot-com
Subject: Re: first coil experiments (fwd)

In a message dated 97-02-28 04:41:42 EST, you write:

<< 
 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 used a neon sign Xformer, and an acrylic 4" tube on which I wound about 
 45.5" of #24 magnet wire (that's 2100 turns according to my calculations)-
it 
 seemed happiest with a 5 to 10 turn primary, at which point I could draw a 4

 or 5 inch spark from the top of the secondary, but it would not arc into
free 
 air.   I'm using a 15KV neon sign transformer (30 ma, GE model 9tcx520at, a 
 ,001 microfarad mica capacitor (cornell dubiller type PL?254-46, rated at 
 25KV, 25 amps at 5,000 kcs, 18 amps at 1,000 kcs, 11 amps at 300 kcs).  This

 is a fairly large aluminum enclosed capacitor with a 4" insulator and a
large 
 terminal on it - don't know what it came out of, I got it at a swap meet for
a 
 few dollars and cleaned it up.
 
 so I have some questions:
 
 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)??
 
 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??
 
 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???
 
 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.
 
 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????
  >>

William,

It's hard to know where to start here.  Your coil is not working well because
it is way out of tune.  This is why I always recommend to design, calculate,
design, calculate - etc. until you are reasonably sure what you are planning
to build will work.  You can save a lot of time, money and disappointment by
investing the appropriate design time on the front end of a project like
this.  Enough of the lecture, here are some answers:

I did some calculations on your coil and come up with the following:
Secondary inductance is 35 mh.  H/D ratio is 11.4  number of turns is about
2043.  This will self resonate (with no toroid or topload) at about 292 khz.
 Making some assumptions about your primary ( 10 turns of # 10 wire with 6.0"
I.D. and .30" spacing between turns) it's inductance would be about 30
microheneries.  This primary and secondary combination want a .0126 mfd
capacitor - and that is tapped at turn 10, with no toroid.

Cutting the secondary down to 22.0" winding length would be a good start.
 This would yield 16 mh, about 988 turns and H/D ratio of 5.5:1.  This would
be a better start.  You want between 900 and 1000 turns with a H/D ratio of
about 5.5 maximum.  With the same primary, this system will still need a .006
mfd capacitor and again, that is tapped at turn 10 with no toroid.   You want
to design the system so it tunes at about 80 or 90% of the full primary with
a small toroid to allow room for tuning and larger toroids (which will drop
the frequency and require more primary turns).

I would definitely ground the center tap of the sign transformer to your main
RF ground, along with the bottom of the secondary and the center gap of the
safety gap.  I would float the primary.  A good spark gap for this coil would
be the cylindrical Richard Quick static gap.  You can find instructions for
this at the ftp site.  I would suggest making a rolled poly cap which you
also can find at the ftp site:
http://ftp.funet.fi/ftp/pub/sci/electrical/tesla

Good luck and don't give up.

Ed Sonderman