What is a Tesla coil?
2003
A Tesla coil is device to develop very
high voltages pioneered by Nicola 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 0.01 uF at 3 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.
Further developments - another mini coil
2004
(click to enlarge)
This is a small coil running on half of an NST giving 6 kV at 30 mA .
The other half of the NST had been burnt out and increased the current
draw mildly. The spark gap used 3 segments of an 11 segment RQ (Richard Quick) gap
with 1/2 inch pipe close but not touching. If made specifically for this
coil then 4 or 5 copper pipes could have been bolted on to a flat
section of acrylic or other insulator as the geometry of the cylindrical
setup is trickier. The primary is a cylindrical 15 turns of PVC coated
wire and the secondary is about 11 inches with 260 turns of 20 G (0.031
inch = 0.81 mm) enameled wire on a 4 inch PVC pipe. In the photo on the
left you can see corona at the top of the pink primary winding. The
spacing between primary and secondary is also from the same 4 inch PVC
with two sections cut longitudinally to allow them to snap on to the
secondary. This close coupling probably gives this more 'transformer'
action than 'resonant' action. The capacitor is a series connection of 13 x 100 nF 3 kV
ceramics giving 7.7 nF at a nominal 39 kV. There is no toroid and the
secondary earth goes to the NST and mains earth. This is about as
quick, easy and safe as they come for 2 inch sparks.
(click to enlarge)
Above, for kids of all ages.
I probably run this coil morefor demos than any other as it is quick, easy and
safe.
Tesla on a car
2005
For a bit of silliness, I put the
mini Tesla coil above on my car.
My Honda Civic at the time was 20 years old (which makes it younger than
the Space Shuttle). To run my stereo I have had a 300 W inverter in
there for about 15 years running a 100 W/ch hifi amp and 8 ohm speakers
which used to take up all the parcel space and back seat as well. The
inverter has been utterly reliable and in the meantime I am on to my
third amplifier and third set of speakers.
(click to enlarge)
I simply plugged the Tesla coil in to the 300 W and put it on the roof.
It is perfectly capable of being driven around but is rather
underwhelming for that. As a precaution I earthed the car aerial but I
probably didn't need to as I have run a car radio only a foot or two
away from 4 foot sparks. I suspect they have very good static
suppression. The Honda died later that year (unrelated to the TC)
and as a mark of respect and mourning I have not bought a replacement
but use my bicycle instead.
(click to enlarge)
Of course, since 2005 I have done a lot of
TC
on a car stuff which is vastly more spectacular. If you don't see
anything else on my site, see this.
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)
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 38kHz with the 90nF 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.
