What is a Tesla coil?
2003
A Tesla coil is a high voltage device
pioneered by Nikola Tesla over 100 years ago. In its simplest form it is
an air cored transformer driven by discharging a high voltage capacitor
through a few of the primary turns. This sets up a high frequency
resonance in the secondary coil. This resonance occurs between the
inductance of the tall secondary coil and the capacitance of a rounded
metal object (typically a donut shaped toroid) and a very high voltage
results in streamers directly into the air or sparks to nearby objects.
(click to enlarge)
From the Tesla
coil sparks page. A Tesla coil such as this one with sparks of
8 feet equates to about 500,000 Volts. It is very hard to measure
this voltage directly. See calculations of voltage in
Tesla coil 6 inch. The
largest spark
that I am aware of at 325 ft was from a 5 MV Marx generator, not a Tesla
coil. Interestingly, real lightning is only estimated to be
10 - 120 MV . Lightning length appears to be determined by
cosmic ray shower activity and is vastly extended in length due to
relativistic electrons initiated by the cosmic shower.
A simplified circuit diagram of my latest coil is
shown below and also includes various filter circuitry and safety spark
gaps to prevent the very high voltages from wreaking havoc with the
lower voltage side. It includes 4 microwave oven transformers (MOT's)
driven by a variac. These supply about 8 kV to the primary coil,
capacitor and spark gap. This circuit is resonant at about 100 kHz and
drives the secondary coil and toroid which are resonant at the same
frequency.
(click to enlarge)
My first Tesla Coil
'discovery'
1972
My first TC was 'discovered' while running a spark spectroscopy project
(above) in high school 35 years ago. This used about 10 kV from
multivibrator excited dual ignition coils through 10 turns of an air
cored radio coil to quench it. The other end of the 130 turn coil
developed a corona visible in the dark spectroscopy room.
It was
a truly amazing sight.
(click to enlarge)
The left photo shows the spark spectroscopy
setup in use (but not in operation as a Tesla coil). The right
photo
shows the original high voltage setup with the spectroscope shown open
and the HV diode, air cored coil and cap made of 6 x 0.0033 uF 3 kV
ceramics in series to give 550 pF. The spark gap with clips to hold
the metal being examined spectroscopically is also shown.
Somewhat remarkably, this was an inadvertent Tesla coil complete with
spark gap, tank cap and air-cored coil AND it worked unintentionally.
(click to enlarge)
Above shows a reconstruction which took about an hour to make in May
2005. It uses the same size cardboard former as the old one 45x100 mm
which had to be reinforced with PVC. (Toilet rolls are not as strong
in 2005 as 1972) The wire gauge is 0.40 mm (prev 0.6 mm) and is wound
to 130 turns with multiple taps between 10 and 20 turns. The caps were
6 x 0.01 3 kV = 1600 pF (prev 550 pF) and power supply was my SIDAC
driven twin ignition coil setup. Spark gap was about 2mm as anything
higher gave racing arcs!
Spark length was only 1 cm but corona was visible with dark adapted
vision.
My first proper
Tesla Coil
(circa 1975)
(click to enlarge)
I later developed this idea with an old transformer
(above) from a dump which in retrospect was an old unpotted NST (neon
sign transformer). I used a single static gap with a 12 inch ferrite
cored coil of 11 primary and 100 turns secondary giving 2 inch sparks
with 26 small ceramic capacitors with 2 strings of 13 x 10 nF at 2.5
kV each. It ran nicely for about 20 years! Years after I made it I
heard that Tesla had beaten me to that discovery by quite a few years!
This photo is a mock up with most of the original parts.
Developing my 4 inch coil
2003
(click
to enlarge)
This is my neat 2001 dual NST supply (above), now has half of one of the
NST's defunct having overstressed it on my ARSG
(asynchronous rotary spark gap). It used two old NST's bought for A$40
from a local neon place and rated at 12 kV 30 mA each. It uses two power
factor correction capacitors of 15 uF each. I used a remote power switch
driving a relay.
(click
to enlarge)
This is how NOT to make 20 nF 30 kV capacitors. I used 200 x 10 nF (=
0.01 uF) 3 kV ceramics. Even wired as 10 nF 60 kV, I would blow a
capacitor every 30 seconds or so. They appeared to be substantially less
well rated than the long lasting ceramics in my first TC. There were no
equalizing resistors in either setup. It was later sawn in half for a
voltage multiplier shown later.
(click to enlarge)
The sparks grew bigger with successive improvements up to about 20
inches. Having gained a lot of experience and having read widely on the
internet increased my desire for more performance.
Time to move up: 'bigger is better'. (Remember this was 2003). Head off
to see the later 4 inch, 6 inch and 18 inch coils from the menu at the
top of the page.
Multilayer Tesla coil
2005
This still uncompleted unusual Tesla project is a spark gap driven air
cored resonant transformer, i.e. Tesla coil. This is a 500 meter roll of
2.5mm multicore electrical wire. It is probably about 1000 turns, i.e.
similar to my current 4 inch coil. It has been placed on top of my
primary and is resonant at 38 kHz with the 90 nF MMC tank cap. No real
attempt to tune or to optimize coupling.
Shown here with 3 inch sparks and a lot of inter turn corona and
breakdown.
(click to enlarge)
My plan is to optimize this to get the highest possible spark length to
coil length ratio.
I plan to do this by using the same wire as above but in a multi helical
coil arrangement. Using a single winding of insulated wire (the PVC will
hold off a much higher voltage) and a double spacer between each helix
(where the wire goes from outside in to start the next helix I hope to
maintain a high standoff voltage between turns plus a higher voltage
between helices.
With everything under oil I hope to be able to prevent racing arcs and
allow reasonably high coupling. There will be high quenching with the
1400 bps ARSG and reasonable power of up to 5 kVA from the 4 MOT supply.
I would aim for a height of one foot for the coil with 3 inch internal
diameter and ?10 inch external. I would hope for 600 turns plus. Coil DC
resistance is 3.5 ohms.
It will be mounted in an acrylic container filled with oil and will
probably need large insulating rings to prevent surface tracking and a
large insulated base to avoid primary to secondary strikes.
(click to enlarge)
I now have the 60 square spacers and the square housing to contain these
under oil, fixed to a large square base. The wire is ready and a toroid
is available. Now just have to undertake the complicated task of winding
the multiple pancake coils.
