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Re: Building a new coil


From: 	Malcolm Watts[SMTP:MALCOLM-at-directorate.wnp.ac.nz]
Sent: 	Monday, June 23, 1997 9:09 PM
To: 	tesla-at-pupman-dot-com
Subject: 	Re: Building a new coil

Hi Rod,

> 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.

Way to go :) I guess she won't have too many problems 
attracting/repelling the blokes in a few years.
 
> I've done a few rough calculations, plus using a few rules of thumb, I've
> come up with this design -
> 
> Secondary:
> ----------
> 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.

I get about 570kHz unloaded.

> (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)

My (educated) guess on that toroid above that coil would be about 10pF
additional C which would make about 18.5pF total so I'd say it would 
resonate about 380 - 390kHz all up.

> Primary:
> --------
> 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)

Sounds about right.
 
> 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.

Increase I'd think. 

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

Yeeeehaaa!!  I guess you really mean 12kV :)  I think the cap might 
be a bit on the large side for that transformer but if I've got my 
figures right, you'd still be able to tune the finished machine with 
around half that using the same primary coil. 

> 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).

At that distance you'd require about 2 max I'd think. I stand to be 
corrected. 
> 
> -----< 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
> shield.
> 
> 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
> re-energized. 
> 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
> place).
> 
> 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! :-)

Sounds nice overall. Best of luck!
Malcolm