Building a new coil

From: 	Rodney Davies[SMTP:rgd872-at-anu.edu.au]
Sent: 	Monday, June 23, 1997 9:39 PM
To: 	tesla-at-pupman-dot-com
Subject: 	Building a new coil

Hi all,

I'm designing a new small TC for my neice to build for her high-school
science project.

I've done a few rough calculations, plus using a few rules of thumb, I've
come up with this design -

Sealed 4" diam PVC pipe, 22" long.
740 turns of 22 guage enamel coated copper wire closewound.
Winding length of 20".
(I take it that #22 wire is 0.67 millimetres thick?).

Estimated self resonant frequency (unloaded): 317kHz.
(Frequency should drop to a lower frequency with top toroid.
 BTW, does anyone know how to do a rough calc of a secondary res.
 freq. with a known capacitance of a toroid and height above
 top secondary turn?)

Top Terminal: 12" x 3" toroid. (approx 12.95pF)

10 turns of 1/4" copper pipe.          _    _
Spacing of 1/4" between windings ---> (_)--(_)
                                          |----- 1/4" gap.

Archimedes flat Pancake coil configuration, with first turn spaced at 
1" from base of secondary.
+ Strike Rail (Probably not required, but a good idea).

Width: 4.75"
Average Radius: 5.375"
Calculated Inductance: 30.33uH (at 10 turns)

Capacitor Value: 0.01uF 20KVDC (Big cylindrical type, not poly, but
hasn't failed me yet).

Hence will require an approx. inductance of 25.2uH to resonate at 317kHz,
so I'll probably tap somewhere between 9-10 turns.
Mind you, that will decrease with the the added secondary capacitance.

Neon Transformer: 120KV 30mA Centre Tapped on secondary.

Spark Gap: RQ Quenched static type gap with 10 gaps spaced at approx
           1/8" each. Able to tap at any gap number desired (ie 1->10).

-----< end Specs >-----

Well, I estimate I should get sparks from anywhere between 12" - 20", I'm
hoping to atleast obtain the coil winding length in spark length.

Will an average power level of 300VA be adequate? Well, I think so...

I'll have this built and finished by around the middle of July..

I'm building a nice box to put the primary components in, with a lid to
have the primary and secondary on top, covered with a large clear perspex
box to prevent careless fingers getting hurt, or I can think of worse...
I'll most likely have it lined with chicken-wire mesh for a faraday

Since it's for a school project and has to go on display, I'm rigging up a
timer circuit such that the coil can only be energized for say, at most 10
seconds, which will then require 60 seconds (or more) before being
Also, for safety sake, I'm also building an interlock such that if the top
plastic shield is removed, the power will be immediately disconnected. The
coil will not function unless the interlock is set (ie the shield is in

I'll also have sufficient ventelation (small exhaust brush-type fan)for
the enclosure to remove heat and unwanted gases (ie ozone). 

But, as my first TC teacher told me many years ago, if *YOU* don't build
the coil yourself, then you've learnt nothing...so, I'm making my neice
(being the very hands-on, contructive and practicle person that she is)
will be building it under my guidance. I'll only do the fiddly safety bits
myself to save time and ensure correct operation.

...anyway, your comments and ideas are welcome...as always! :-)

Thanks All!