Re: TESLA COIL

From: Mark Fergerson (nunya_at_biz.ness)
Date: 01/30/05 

Date: Sun, 30 Jan 2005 09:23:56 -0700

Blackbeard wrote:
> My kid is building a tesla coil for his science fair project. I'm a
> plumber, not an electronics guy. Right now, he has put together a
> primary coil (made from 1/4" OD copper tubine) and he has a secondary
> coil made from (approx. 1500 winds of 24-gauge wire around a piece of
> 4" PVC pipe). He made his toroid out of semi-rigid as the plans
> suggest.
>
> Now he's getting held up on the capacitors and spark gap. His plans
> call for 1-1/2 copper to make the spark gap. I can't see any point in
> using 1-1/2" copper (mostly because I have 3/4" immediately
> available). Will it make any difference if we use 3/4" copper for the
> spark gap instead of 1-1/2" copper. The plans call for 7 pieces of
> 1-1/2 copper (2" in length). You put them in series and the spark arcs
> from one piece of copper to the next. Seems to me it would do the
> exact same thing with 3/4" copper. But 3/4" copper has less copper
> (thus (I assume) less resistance). Will that make a difference or can
> we go with the 3/4"?

   The curvature of the pipe's surface partly determines when the gaps
"fire" (what voltage they can get up to before they arc over). The
greater the curvature (smaller diameter), the smaller the voltage. This
is ordinarily not an issue, but if your primary circuit isn't running at
a high enough voltage it won't arc at all.

   But mainly, the pipes are going to get hot just from carrying all
that current. The more thermal mass (more copper), the better.

   Duplicating somebody else's plans allows you to duplicate their
results. Change something and you get different results, and you might
not be able to tell what's going on (or what will happen); change too
many things and you're almost guaranteed to let the magic smoke out of
something with no clue what went wrong, leading to tossing the thing
aside in favor of video games (assuming you don't burn the house down). ;>)

> Also, the plans call for a home-made capacitor made by rolling a piece
> of aluminum flashing and piece of plastic or rubber. They say to roll
> it tight. But they give no directions on how to hook this thing up. I
> have TWO posts on the high voltage transformer (15,000 volts).

   Oh, dear. Rolled caps are multiple disasters waiting to happen. "Beer
bottle" caps are simpler and more tolerant of, um, errors in execution.

   Hooking the thing up should be fairly obvious; do the plans you're
working from not include a wiring diagram?

> I assume the sequence I need to follow is
> 1. Connect both posts from the transformer to the home-made capacitor
> (how?)
> 2. Run a wire from the capacitor to the spark gap (those pieces of
> copper pipe..again, how?)
> 3. Run a wire from the spark gap to the primary coil

   Where'd he find the plans you're using? I don't expect you to draw a
diagram, but if you could refer us to a webpage...

> 4. How does the secondary coil get power? One person told us that the
> secondary coil should just be grounded and that the 1500 turns would
> generate a magnetic field and generate energy (ie, the lightening that
> hops of the toroid)

   "One person" doesn't quite know what he's talking about; all the
energy comes out of the wall plug and is transferred (minus losses)
through the system from gap, to primary circuit, to secondary circuit,
to ground.

   Ordinary transformers consist of coils that transfer energy very
efficiently through their magnetic fields, and are designed to work at
line frequency (60 Hertz here). Tesla coils aren't quite like ordinary
transformers; they're coupled resonators and you can design them to work
pretty much at your frequency of choice, typically a few thousand Hertz.

   Put way too simply, when the gap fires the primary tank circuit
(inductor and capacitor) "rings" at its L-C frequency like a struck bell
except it generates an oscillating magnetic field instead of radiating
sound, and the field induces oscillating current in the secondary (it
"rings up"). The toroid forms one plate of a capacitor, and the ground
the other (which is why you ground the bottom of the secondary; "one
person" got that right). The secondary current charges up the toroid's
capacitance. When the voltage on the toroid exceeds the breakdown value
for air, the "lightning hops off the toroid".

   I said "way too simply" because there are subtleties like the fact
that the secondary will not resonate at the exact same frequency as the
primary. Being able to point out and explain those subtleties will get
your son a better grade, and teach him a few things.

   Also, the voltage gain in Tesla coils is _not_ directly proportional
to the secondary/primary turns ratio of the coils like in ordinary
non-resonant transformers, but rather to the ratio of the primary
circuit/secondary circuit capacitances. The secondary needs lots of
turns to get enough inductance to resonate with the miniscule
capacitance of the toroid. You want lots of voltage gain, so you want a
large primary cap, which means the primary coil needs few turns to
resonate at (nearly) the same frequency as the secondary.

   If the toroid doesn't "break out" into feathery streamers or an
outright blazing arc, all the energy stored in the secondary capacitance
sloshes back into the primary circuit (because the secondary current
generates its own magnetic field, inducing current back into the
primary), which can do Very Bad Things to the primary circuit
components, and maybe feed back into the house wiring (depending on your
actual circuit). It's a good idea to deliberately put a "breakout point"
on the toroid to prevent this. Just solder a thumbtack to the toroid
point-out, and don't allow anything you value (like your son) to get
between it and the ground.

> Our plans are pretty vague. Any experienced advice would be very
> helpful.

   Go to:

http://www.pupman.com/

   and look through the FAQs on the left. Definitely read the Safety Sheet:

http://www.pupman.com/safety.htm

   to keep the fun from overriding prudence. We can't learn from our
mistakes if we don't survive them.

   If your son seems to be "bitten by the Tesla bug", he might want to
subscribe to the pupman mailing list. Lots of advice available from
helpful, friendly coilers; some with experience, some with degrees, and
many with both. If that would be too time-consuming for this project,
check out the Tesla webring; many pre-tested circuits out there to use
as guidelines.

   Mark L. Fergerson

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