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Building advice -Reply




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From:  Chip Atkinson [SMTP:chip-at-sophie.bolix-dot-com]
Sent:  Wednesday, March 18, 1998 5:19 AM
To:  Tesla List
Subject:  Re: Building advice -Reply



On Tue, 17 Mar 1998, Tesla List wrote:

> 
> ----------
> From:  Dale McLane [SMTP:Dale.McLane-at-epa.state.oh.us]
<SNIP>
> 1)  Dr. Hall's plans are contained in the "Surefire Tesla Coil Cookbook".  I have not bought the plans
> yet ($24.95 + S&H), and was wondering if anyone on the list had used the plans and built a coil
> according to his specs.  Uses an 8" PVC form, but don't know other details.  I visited his web site on
> the Tesla Ring at http://home.sprynet/halldo/teslaord.htm.  Any comments as to whether these are a
> good set of plans would be appreciated......?

Dale,

I couldn't find the URL, but no matter.  While no expert on tesla
coils (compared to others on the list), I'd say save your money for
other stuff.  From what I understand you have for a power supply, a
15kV neon transformer (even if it's a 60ma one), an 8" coil is a bit
big for starters.  I tried an 8" coil first with minimal results (no
streamers, just sparks to a grounded point), then a 5" coil that
didn't do anything at all, and finally a 3" coil that worked great.

For a 15/60 (15kV, 60ma) transformer you should start a bit smaller
such as 3" or 4" coil.  The other advantage of smaller is that it's
much cheaper.  No point in making beginner's mistakes on a $50 item
when you could do the same to a $10 item :-)

IIRC, the 3" coil that I built that finally worked for me was about
900 turns of 26ga. magnet wire on a 3" pvc pipe form.  I used a flat
primary of 4 ga. solid ground wire, a static gap made of 1" diameter x
3" long sections of rigid Cu pipe.  I can't remember what I used for a
cap, either door knob types or a home made rolled cap.  Whatever it
was, it worked well for me.  

You can try www.pupman-dot-com.  Look in the "How-to's" section under the
index.  There is a bunch of information that you may find useful for
starting out.  Note that the site isn't up all the time right now.
I'm working on that....



> 
> 2)  Also ran off a copy of the TCBA subject and volume index.  Plan on subscribing in the near future.
>  Any suggestions or comments as to which back issues would be helpful in building a "most bang for
> the buck" TC using a 15 kv neon trans. as a power supply would also be appreciated.

I don't know what the issue is, but if you can get the one with the
description of Gary Legel's 3" coil, that's the one I modeled my first
success after.
> 
> 3)  For the uninitiated i.e. myself, and others who may be in "lurk mode", 
> what is the function of a "Tank Circuit" ??

Well, that's the heart of the tesla coil.  The "Tank Circuit", so
named because of the capacitor acting as a big tank of electricity
that dumps into the circuit, is the primary circuit that excites the
secondary.  It consists of three basic components tied together with
heavy wire.  They are the capacitor, the spark gap, and the primary
coil.  

A capacitor and inductor in series will oscillate upon connection.  Of
course the oscillation dies out rather rapidly due to resistance, so
it is necessary to keep pouring energy into the circuit.  (In a tesla
coil the energy is taken out of the circuit by the secondary too
through electromagnetic coupling, but we can ignore that for now)

The spark gap serves as a switch that allows the transformer to charge
the capacitor.  As the capacitor charges, it gobbles up less
electricity and thus the voltage rises.  It's the same as if you have
a garden hose with a big hole in it.  The pressure between the faucet
and the hole is low, but as you plug up the hole the pressure rises.
Once the voltage is high enough, the spark gap starts conducting and
the tank circuit loop is closed.  At this point the circuit
oscillates.  The frequency of the oscillation is determined by the
amount of inductance and capacitance in the circuit.  The inductor
(primary coil) acts as an electrical flywheel, keeping the current
moving in one direction.  This reverse charges the capacitor and the
circuit can "swing" the other way.

The spark gap has to stop the conduction somewhat quickly because the
primary and secondary circuits never oscillate at the same frequency.
(Probably among other reasons, but this is the reason that I came up
with after considerable thought.) When the primary starts oscillating
(ringing) it induces a high voltage in the secondary (don't exactly
know why it's higher than the primary tho).  The primary and secondary
start off in the correct phase relation to each other, but with
different frequencies, they quickly become out of phase and "fight"
each other.  This is when the primary needs to get out of the picture
and the spark gap quench is what breaks the circuit.

> 
> 4)  It it possible to build a "tuneable variable frequency TC" that would work on certain frequencies, I
> realize that there probably harmonics all over the place, but........?

It probably is, but the thing about the TC is that the primary and
secondary are tuned to match frequencies as closely as possible.  

Corrections and polite flames "welcome".

Chip