Designing Tesla Coils

First I am going to repeat my coil winding guidelines that 
were posted here previously, but I am including additional
"rule of thumb" coil construction tips throughout the 
imported text. :::

The first step in winding a coil is to select a coil form. The
coil form should be a low loss material (we are talking RF
losses) like polyethylene, polystyrene, or polypropylene, Lexan,
or Plexiglas (acrylic): but a common material is PVC, which is
high loss. Thin wall tubing is best regardless of material.

So ask yourself: "How large a coil do I want?"
This is the first step to building a good Tesla Coil.  

Ratios of coil height to width are important. Small coils { coil-
forms 4" and under in diameter } go with aspect ratios  4.5 ->
5.0 to 1 ;; coil forms 5-6" in diameter go with 4.5 -> 4.0 : 1
aspect ratio ;; so the range here is between around 5:1 - 4:1.
Larger diameter coil forms ( 8+" diameter) have aspect ratios
around 3:1. Now we are talking about the actual winding length
here, so allow an extra inch or so of coil form on each end.
Determine the length required and cut the ends square.

These aspect ratio parameters are designed to allow the maximum
coupling between primary and secondary while providing a very
high inductance in the secondary coil (inductance per unit
volume), AND allowing the finished coil to maintain good
electrical strength.

The form must be sanded smooth of surface imperfections, dried
thoroughly, and if PVC is used, it must be sealed. A good sealer
is polyurethane, another is two part epoxy paint. By sealing the
surface of the PVC before you wind on wire you can negate the
excessive losses in PVC plastic coil forms. If necessary the coil
form may be sanded after the sealer had dried.

The coil should be wound with good quality magnet wire. I use
double Formvar enamel coated magnet wire. Magnet wire gives you
maximum inductance. A coil should have over 900 turns, but not
too much over 1000 turns. There is a little leeway here. Select a
gauge of wire which will allow the aspect ratio and number of
turns to fall within this range. THE WIRE MUST BE CLOSE WOUND,

I plug the ends of the coil form and run a dowel through a center
hole so that it will spin. I set up the wire spool on one end of
a pair of sawhorses, and the coil form on the other end. I wind
the wire on by hand, making sure the windings are tight, smooth,
and even. I use a dab of hot glue or tape to hold the first turns
in place, and make sure to leave a tail of wire at either end.

Once the coil is wound, it is sealed to prevent corona leakage
and breakdown. I use the same sealers mentioned above. Coats of
sealer are applied until there are no ridges and valleys in the
wire. In other words the coats must build up until the wire is
completely imbedded in sealer.

The wound, sealed, coil is capped at both ends with plexiglas
plates glued down with epoxy. I cut circles out of plexi sheet
that is about the same thickness as the coil form. I rough up the
surface around the edges to give the epoxy a bite. One small hole
may be drilled into the bottom end plate to allow the air
pressure to equalize, but under no circumstances should any other
holes be drilled. The wire is never allowed inside the form.


The primary design that works best with these type of limited (or
low) aspect ratio, high inductance, secondaries is the inverted
cone (or "saucer") shaped coil. On larger secondary coils with
insufficient air terminal capacitance (read "too small of a
toroid") the aspect ratio of the primary coil winding may have to
be lowered until the coil becomes a flat pancake; wound much like
the grooves in a phonograph record.

Generally I figure that for a six inch coil that I will need a
primary with about a 30 degree angle of slope from the inside
turn to the outside turn; though as I mentioned larger coils may
require scaling down the these numbers, or adjustments to the
secondary coil by top loading with very large toriod dischargers. 

I regardless of the secondary coil form diameter, I will make the
diameter of the inside turn of the primary three to four inches
larger than the outside diameter of the secondary coil base. This
distance is usually enough to prevent flashover, but if it is
not, poly plastic may be used to insulate.

How many turns of the primary? LOTS!

I don't wind primary coils anymore that have less than 15 turns
of primary conductor available for tapping. I space my turns
closely, and cram inductance into the primary coil as well.

Then I use my math (or Tesla Coil calculator program) to figure
out how small of a capacitor and power supply I need.

I do not like John Couture's JHCTES design program for producing
a modern, highly efficient, Tesla Coil. The design "rules of
thumb" that I am giving (worked out and tested experimentally)
requires a large top capacitance, or toriod discharger, to
prevent excessive corona losses and premature electrical
breakdown of the secondary coil. JHCTES will kick out most
secondary coils designed by my method because Couture's software
cannot take into account the high value of "C" the discharger
adds on top of the secondary "L". This large discharger is
essential to efficiency and durability in the design. Sparks
produced from secondaries wound and fired according to this 
"rule of thumb" method may exceed the winding length of the
secondary coil by a factor of four, or more, without breakdown
or excessive corona losses.

___ Blue Wave/QWK v2.12