MOVED

see http://amasci.com/tesla/tc.html

















TESLA COIL HINTS                                   Bill Beaty 1994

I've built a couple small (under 2ft) coils, neither over 500Watt, and 
I've read quite a bit, so now I'm an expert, right?     :)


LONG, HOT DISCHARGES 

If you want to make long arcs, a sphere or oblate terminal is required for
the top of your coil.  You'll only get corona displays if using a bolt or
a metal rod as the main terminal.  No long arcs.

This occurs because whenever a large hollow sphere or toroidial electrode
is used, then during each pulse from the spark gap, the voltage field in
the space around the top of the secondary coil can build up to extreme
levels before the plasma finally ignites and creates a spark.  The large
sphere causes delayed sparking and higher energy.  A sphere has no sharp
points, so it can support a huge voltage BEFORE the spark eventually
starts.  When corona finally does break out on the sphere surface, the
e-field in the space around the terminal has already built up so high that
a branching spark-streamer will be driven by the ambient field to quite
long lengths.

The toroid terminal should be at least as large as the coil diameter, and
preferably should be 2-3 times as large.  Be aware that the toroid
terminal acts as a capacitor to ground, and significantly reduces the
secondary coil frequency, so you'll have to re-tune the primary when
adding or removing the toroid or switching to a different size.  Another
hint: place a small needle-discharger on the surface of your toroid.  
This will allow an underpowered coil to create fairly long arcs.  
Experiment with the size of this "trigger" device (sometimes a thumbtack
is best, sometimes a small nut works well.)


WATTS THE PROBLEM

For tesla coil spark length, voltage is less important than wattage. I've
heard it quoted that long lightning-like TC arcs require about 500 to
1000 Watt per foot of discharge when the coil is designed and tuned
properly. Be aware of this when designing the power supply section of a
big coil.  Those huge coils that spew 10ft lightning bolts are usually
being driven by 10,000W power pole transformers ("pole pigs"), with the
transformer's primary/secondary wired backwards to provide voltage
stepup.  And to improve your coil's performance, try to increase any of
the characteristics which lead to higher wattage.  Examples: use heavier
wire (especially in the whole primary coil/capacitor/sparkgap circuit),
adjust optimum power-coupling between primary and secondary, use heavy
gauge wire on the secondary, build a short and wide secondary coil, and
use higher supply-voltage during operation (try a DC system with a rotary
gap, or try using many neon transformers in parallel.)


COUPLING ERROR

Incorrect coupling between primary and secondary can be a source of low
performance or failure. Even when tuned perfectly, a coil won't work well
if over- or under-coupled.  The typical way to adjust the coupling is to
vastly overcouple the coil at the start, then reduce coupling until it
works best.  Do this by designing your primary to be wound quite close to
(or over) the secondary, but prevent arcing between primary and secondary
by using heavy plastic (etc.) as insulating shield between the coils.  
Place the primary against the base of the secondary (or over it, in the
case of a cylindrical primary), then raise the secondary up out of the
primary in stages while testing the coil for max output (and possibly
retuning each time.) For spiral primary designs, the spiral should be
fairly small with close turns, and initally positioned very close to the
lower end of the secondary.  A cone-shaped primary gives tighter coupling.  
If arcing between primary and secondary is a problem, use plastic
insulator plates or tubes between the coils.  If arcing between the turns
of the primary is problem, space the turns wider, caulk the edges, or
paint the edges with Urethane, remove burrs and sharp pointed splices 
from the wire, etc.


CAPACITOR/SUPPLY MISMATCH

The power supply transformer's current and the main resonator capacitor's
capacitance must be chosen for best match.  If the capacitor is too large,
and the transformer too small, the capacitor will not fully charge on each
half-cycle of the 60hz supply, and the total system wattage will be
reduced.  If the capacitor is too small, it will only draw a small current
from the power supply, and the actual wattage of the system will be lower
than the capability of the supply transformer.  Refer to a good TC design 
program for choices of capacitance versus transformer current.


TRANSFORMER PROTECTION

One problem you may run into: the RF from your coil's primary L/C section
goes backwards through the circuit, adds to the transformer's secondary
voltage and increases the peak voltage there.  Also, the high frequency
will tend to concentrate its voltage on fewer turns inside the supply
transformer, rather than spreading it over the entire internal coil as the
60Hz energy does.  This tends to exceed the transformer V-out rating and
can create internal arcs which will wreck a neon-sign transfomer.  You can
put a 'safety gap' sparkgap across the transformer output.  Disconnect the
coil circuit and adjust this gap's spacing so it is a bit larger than that
needed to start an arc.  The safety gap will then protect the transformer
against large voltage pulses by arcing and safely shorting them out.  But
if RF and pulses are a problem in your design, this gap will continually
fire.

A partial cure:  use a design which places the main spark gap across the
power supply and the main capacitor in series with the supply.  In normal
operation the voltage across the gap will then be lower than voltages
elsewhere.  A better cure: put a low-pass filter between power supply
transformer and the TC primary circuit, with series inductor and parallel
filter capacitor wired so that it filters out the RF energy coming FROM
the primary.  Use a high-inductance handwound aircore choke coil and a
high voltage capacitor.  Numerous TC project plans recommend an inductor
which is far too small to stop the hundreds-KHZ energy from getting back
to the transformer.  For the filter coil I used a couple of pounds of #28
wire wound as a 6"dia, 1/4" thick pancake between two plexi disks, with
the outer terminal going to the supply and inner terminal going to the
tesla coil.  For the filter cap I put a .001 capacitor directly across the
power supply transformer output, and added a small "safety gap" across the
cap for good measure.  Adding this filter circuit let me completely remove
the Variac I had been using to control the output, so I could then run the
coil wide open at 120V with no firing of the safety gap.


FINEWIRE TUBULAR COIL- BAD!

I've seen a number of old coil plans in magazines which direct you to
wind incredibly fine #40 wire upon a long, narrow 2"  form.  This is a
poor design, and is one of those infectious misconceptions that spread
from book to book.  The wire has too high a resistance, and it leads to
energy loss via wire heating.  A much more powerful coil can be had by
using thicker wire on a wider form.  A 5:1 ratio (or less)  of coil
length to dia is recommended.  If your coil is 3ft. tall, it should be
about 1ft. in diameter.  Tesla's original coils were shaped like barrels!  
My last coil was 2' tall, 5" diameter, wound with #24 magnet wire.  It
made 14" sparks from an 8" carved foam sphere terminal covered with epoxy
and AL foil. Its transformer was fairly tiny, so 14" was short for the
size of coil.

If you want a lower frequency of coil, don't use finer wire for more
turns.  Instead, make the secondary bigger in diameter. Some people have
explored the use of heavier wire at the ground end of the coil and fine
wire above. The energy wave near the base is composed of high current, low
voltage, while the wave at the top is high voltage, low current, so an
ideal secondary would have tapered wire, with thicker wire at the grounded
end. 


THE "REAL" TESLA COIL

Nikola Tesla's large coils were not the same as the "Tesla Coils" shown in
most project plans.  His coils were composed of three coils, not two.  The
first two coils form a large diameter air-core close-coupled stepdown
transformer.  The transformer primary is tuned with a large capacitor. 
The transformer secondary is a few turns and heavy wire on the same large
coil form as the primary.  The third coil is a large "extra coil" in place
of the usual teslacoil secondary.  The "extra coil" was self-resonant and,
rather than being driven by magnetic coupling, it was driven by having its
bottom wire directly connected to the heavy secondary winding of the
2-coil transformer. In typical project articles, the "transformer" section
is entirely missing, and the "extra coil"  is driven inductively.  Tesla's
3-coil system is known by the name TESLA MAGNIFIER, and a big one can put
out arcs approaching the megawatt level.  Check out the "plastic bucket" 
coil project in Radio Electronics magazine for an example of a magnifier
coil. 

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Are you aware of the Tesla Coil Builders Association?  The back issues
of their newsletter are an incredible library of Tesla material, and
are available from the publisher (see elsewhere on my WWW page)