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Secondary Coil
The
secondary coil receives energy from the primary.
The secondary is built to be a resonant inductor with the top load and
surroundings as a capacitor. This is one area where pretty much
every Tesla coil has something in
common. It is generally a cylindrical form (sometimes conical,
and rarely flat) on
which a thin copper wire is wound. The coil form is commonly made
of
PVC pipe although an infinite number of other materials may be
used.
After the wire is wound on, the coil is usually coated with
something to prevent corona losses, reduce secondary strikes and
racing sparks,
and this
coating also keeps the wire from unwinding
(polyurethane, lacquer, shellac, epoxy and some homemade
concoctions as well, but
I use oil based gloss polyurethane).
The wire is usually magnet wire (that red wire on electromagnets, like
in motors or transformers, it comes in other colors too) it is coated
with some kind of enamel to
keep it from shorting, but to allow maximum thinness (this is why there
is no rubber) for maximum turns per inch so maximum magnetic field can
penetrate. Usually between 750 and 1500 turns are
recommended, in anything from 28 to 18 AWG wire (some Tesla coil
diagrams and plans recommend using much thinner wire, but from my
research and experience, I believe slightly thicker is better, because
although you get less turns and therefore less inductance (less voltage
output), you have
much less resistance, so more current gets through, hence
arcs are more sustainable, lending to bigger discharges, this also
prevents racing sparks and other nasty problems). The
bottom of the coil is
directly connected to the RF ground, which is a thick wire going to the
ground, literally, mine first RF ground was
a
metal support beam in my basement,
which was not exactly an ideal ground and resulted in lower
performance, but it worked. Most people use a copper pipe
or actual ground rod pounded into the dirt outside which is what I use
now (making the
ground wet increases
conductivity, so performance would increase, imagine putting it in a
creek, It would work great, and you could have fish for dinner!) the
top of the primary is connected to some type of top load
(toroid,
sphere, etc.) to add capacitance to the circuit, and give your arcs a
place to generate from (note, a larger capacitance lowers the resonant
frequency, and the peak voltage, but delays the time to arc breakout,
allowing more energy to be stored in the secondary, allowing for bigger
sparks). My 3-in secondary is limited to about 2
ft arcs, but my 6-incher has put out about 5 ft arcs, so the size makes
a difference (as a bigger coil has a bigger inductance, and current
carrying capability)!
Below are some secondary pictures, including the making of the 6-incher. I rotated the tube while my friend Nathan tensioned and fed the wire onto the form.
Below are some secondary pictures, including the making of the 6-incher. I rotated the tube while my friend Nathan tensioned and fed the wire onto the form.
