Principles of Operation
HISTORY Though his accomplishments are
frequently overlooked in recent times, much of humanity relies on Croatian-American
Nikola Tesla's (1856-1943) most utilized invention- the alternating current
(AC) power distribution system. Tesla developed efficient transformers,
motors, generators, and lighting systems that use AC as we know it at 50
or 60 Hz. But he quickly saw the advantages of pushing the frequency
higher. In "Alternating Currents of High Potential and High Frequency"
(1891), Tesla described his work with frequencies in the range of several
thousand Hz to several million Hz. In the course of these studies,
Tesla invented the "coil" that bears his name: a resonant step-up transformer
that develops extremely high voltages with ease. Tesla generated
high-frequency oscillations by means of resonant LC "tank" circuits, which
were periodically energized and completed through the action of a spark
gap or rotary switch. He coupled secondary and tertiary resonators
to these oscillators to make a variety of Tesla coils. The largest
functional Tesla coil was built in Colorado Springs by Tesla. It
could reportedly produce sparks 130 feet long and transmit substantial
electrical energy over 25 miles without wires. Tesla's biographers
(see below) tell the stories of this impressive facility.
Nikola Tesla in a famous double exposure from Colorado Springs. The
Tesla coil shown is the world's largest functioning one to date.
The coil at left is actually a tertiary coil, or "magnifier," while the
primary and secondary coils are supported on the surrounding building and
Over the years, technology has evolved and so have Tesla coils.
Builders have found what makes the best coils, and although Tesla took
plenty of secrets to the grave, we can make a better Tesla coil out of
modern components than even Tesla himself could in his time. Additionally,
the vacuum tube and transistor have opened new courses of Tesla coil innovation.
There is still much to be learned about Tesla coils, particularly the theory
of operation. There are big differences in performance between tube
and transistor-driven coils and their disruptive counterparts that are
not fully understood.
Tesla coils have historically had no application in mainstream technology,
although the concepts of resonance and high-frequency power that Tesla
pioneered have a myriad of applications, from television and radio transmitters
to switching power supplies that are in the computer you are using now.
However, Tesla coils are perhaps the easiest way to get dazzling high-voltage
effects for the entertainment industry, and Tesla coils are frequently
used in that setting now. And due largely to the resources of the
internet, "Tesla coiling" has become a popular international hobby.
HOW DOES A TESLA COIL
WORK? In speaking of Tesla coils, we usually mean a resonant,
air-core transformer with magnetically coupled primary and secondary coils.
And specifically, we usually mean that we are looking for long sparks from
the secondary. Although the Tesla coil is a step-up transformer,
its function depends very little on the standard transformer idea of primary
/ secondary turns ratio equaling primary / secondary voltage ratio.
For one thing, much less than all of the primary magnetic flux passes through
the secondary in Tesla coils, making them a poor transformer by traditional
rules. Also, if you have seen a Tesla coil operate outside of resonance,
where it would be relying largely on transformer action alone, you know
how little this has to do with the big sparks.
LC resonance is the key to the Tesla coil-
pumping energy into the secondary system at its resonant frequency allows
for a buildup of energy there, developing high potentials and long sparks.
Depending on the way a coil is built, transmission-line or antenna phenomena
have something to do with operation as well as lumped LC resonance, and
there is still a good bit of disagreement about how much this contributes
(open quarter-wave transmission lines have a standing-wave pattern, with
a voltage antinode at the open end and a current antinode at the closed
end). Tesla came up with a quarter-wave formula for the amount of
wire to wind on the secondary, and that has been a standard rule in coil
building for many years. But the well-performing modern coils use
rather large terminal capacitance (big toroids) and it has been shown that
large current flows throughout the whole secondary, not just the base,
in these coils- and furthermore, the current into the toroid appears in
phase with that in the base lead. The quarter-wave idea would predict
low current at the top of the coil and large current at the bottom, and
also a phase difference between top and bottom. If you follow old
'40's and 50's directions, and wind a really long amount of wire onto the
secondary, and you don't use a topload, you may see a standing wave pattern
by holding a fluorescent lamp near the secondary. It will light up
every odd quarter wavelength at the antinodes, clearly showing an antenna
There has to be some sort of circuit to drive
the Tesla coil at resonance- you need a means of generating oscillations
at the right frequency. In most coils, the primary coil and the primary
capacitor are connected through a spark gap to make a disruptive oscillator.
The circuit produces a damped RF wave at the resonant frequency whenever
the gap fires. More recently, vacuum tube and transistor oscillators
have been applied to do the job. Tesla said that the spark gap was
a necessary evil and in his day it was. They are noisy, make ultraviolet
light, eat electrodes, and frequently involve moving parts. Tube and transistor
circuits don't have this element. These circuits take feedback from
the primary tank circuit in the proper phase and amplify it to generate
continuous undamped oscillations. But they don't give the same performance
as their more traditional spark-gap cousins- in particular, they have much
poorer efficiency in making long sparks. The reasons why are not
too clear to me. Maybe it has to do with driving wave shape and pulse
frequency, or with primary impedance differences between tube / transistor
coils and disruptive coils. Recent developments to improve efficiency
of tube coils include "sputter" or "staccato" modes -terms are due to J.
Freau- which are pulsed modes of operation where the oscillator operates
for a short time and is then off for an interval.
WHAT IS MY EXPERIENCE WITH
TESLA COILS? I have had a keen interest in Tesla coils ever since
I read a book that showed one of Tesla's conical coils roaring away at
full bore in New York. The book gave a simple (and misleading) definition
of the apparatus- "air-core transformer." I started right then to
make air-core transformers by removing the iron cores from small AC transformers,
and at age 10 I got in trouble for actually plugging one of these coils
into the wall socket (big black burns on the hardwood floor). The secret
to big sparks is resonance, and I had to order another book ("Tesla Coil"
by George Trinkaus) to find that out. Actown Electrocoil donated
a neon-sign transformer, and I began my real coiling career with that.
I used my early coils to make x-ray photos, do Kirilian photography, get
DQ'd for safety at a technology fair, and cause endless parental worry.
Christmas lists in those days usually consisted of about one item: "distribution
transformer." Fat chance! But my dad was willing to take me
to see the great Richard Hull of Richmond, VA. What really struck my fancy
at Hull's lab was a model of Duane Bylund's solid-state coil. Hull
gave me a discharge tube filled with neon and told me to walk with it away
from the silent coil- I got about twelve feet away before the discharge
went out. The silence of that machine, and its ability to toss out
a thick popping 8" streamer, and its radiation, were impressive. So, I
started down the tube coil route at that point. The first was a ~30
MHz lashup using a TV sweep tube. The coil could only make 1" of
flame. Mark Rezsotarski's coil is similar but is designed much better.
I went with the high frequencies, since these required the smallest parts
and budget. I had an 810 coil for a while that made 6" sparks, but
I had not learned that you don't push the secondary leads through the form
to secure them- and so I burned up many a secondary from the inside doing
this. More recently I have used 811's and 833's with decent success.
I have dabbled with FET drivers (modified from switching power supplies)
and gotten some nice sparks- but I have found that these things have a
life span of under a minute! I have an ongoing project for UNC physics-
a dual 833-C tube coil. My current 833-A coil is detailed below,
as well as an 811-A coil that worked pretty well for under $20 in parts
FIRST- proper electrical safety procedure
is assumed, not necessarily indicated, in the following
sections, and Carl Willis is not responsible
for any carnage or damage resulting from the use of his information
or materials. Note: it can be dangerous,
not to mention frustrating, to jump right into Tesla coiling
as a rank beginner in electronics...make sure
your capabilities are commensurate with the demands
of a project, especially with respect to working
safely, before you start.
for my 833-A coil page
for my 811-A coil page
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