Size and weight of portable coils

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "by way of Terry Fritz <twftesla-at-qwest-dot-net>" <FutureT-at-aol-dot-com>

Hello all,

Often it is desired to build a portable coil which can be taken
to various places and events.  Often too, a coil of reasonable
power and spark length can be heavy and bulky.  I think it 
would be useful to consider how a coil can be made compact
and light, yet powerful.

Many components are heavy such as rotary gaps, blower
motors, large spun toroids, large NSTs, etc.  To keep the
weight and size down, here are some possible approaches.

Let's say for instance we want a 65" spark.  This demands
that the secondary be about 26" tall to prevent coil breakdown
at 120 bps.  It is likely that the sparks will have a tendency to
arc down and strike the primary however.  One therefore 
wonders if it may be better to make the secondary taller to
eliminate the need for a small toroid under the main toroid
to keep the sparks away from the primary.  Certainly this
approach is simpler.  I estimate the secondary should be
about 28" tall.  The form should be a lightweight, thinwall
plastic.  It will also help for both size and weight to make
the secondary narrow at maybe 5" or so, perhaps 4".
Next consider the toroid.  A corregated one
is light, but doesn't give such nice moving sparks, so a spun
one may be better.  The spun one can be removed for
transportation so weight is not so much of an issue here.
I estimate that a 6" x 21" toroid is large enough.
The weight of the secondary wire on the form can be quite 
heavy if thick wire is used.  Thinner wire will give better or
at least as good results and is a lot lighter and cheaper.
For the base of the coil, a lightweight wood such as pine
can be used.  For the gap, a very small rotary can be used,
or perhaps a triggered gap, although the blower used to cool
the gap may add to the weight, and give no weight advantage
overall.  The primary can be made from 1/4" copper tubing which
is not that heavy, or possibly aluminum tubing.  The primary
can be supported by a ribbed structure for light weight.  If an
NST is used for power, it can be unpotted to reduce the weight,
and to increase reliability.  Also some shunts can be removed
to give the NST a little more zip.  Safety gaps only can be used to
protect the NST since they can be built lightly and compactly.
In a properly built sync gap or triggered gap coil, the safety
gaps will never fire.  The capacitor can be an MMC for light
weight.  I like to make the secondary so it simply plugs into
the base.  There is a brass contact at the bottom of the secondary
and in the base.  Contact is via gravity.  I use this method for
my TT-42 coil with excellent results.  Casters can be mounted
to the coil's base, to allow it to be wheeled through
large buildings instead of carried.  Special carrying cases can
be made to protect the toroid and secondary during transport.

John Freau