Re: My First Tesla Coil

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

Original poster: FutureT-at-aol-dot-com 

In a message dated 5/13/04 6:52:37 PM Eastern Daylight Time, 
tesla-at-pupman-dot-com writes:

> >
>
>   I've wanted to build one of these for several years now, but just never
>got around to it. But finally, I'm now ready to build my first coil!
>Although I have spent years in electronics (mostly repairing TV's etc), the
>principles involved in Tesla coils are still not completely within my
>grasp. That's why I'm asking for your help.


I'm glad to hear you're ready to build a coil.

>Consideration #1 : Number of coil turns / turns ratio
>
>   Most info I've collected from the web, suggests winding about 2-4
>thousand turns of 20-24 AWG magnet wire, for the secondary of the coil.


It's not often that I've heard that recommendation.  The consensus these
days is about 1000 to 1500 turns for small or medium coils.  Possibly less for
very large coils.

The

>primary should be about 20-25 turns of heavier (<14 AWG) copper wire. Is it
>theoretically possible, to use a turns radio of 1:100 (say, primary=10
>turns and secondary =1000 turns), to multiply the input voltage by 100x?
>(assuming 100% efficiency, obviously it will be much lower in real life).
>Secondly, how does one determine the proper number of turns?


Use a computer program such as Bart's JavaTC to calculate the number
of primary turns needed.  This will depend on the secondary design, the
toroid size, and capacitor value.  The primary and secondary are both
tuned circuits and must be tuned to about the same frequency.  Move
a tap wire on the primary to exactly tune the coil to give longest sparks.
Don't worry much about the turns ratio.  The key to long sparks
involves the use of both current and voltage in the sparks.  Many
factors go into all this.  There are issues such as wire losses and
gap losses to consider.

And finally,

>can the primary and secondary windings be isolated from each other by
>putting a larger-diameter piece of PVC pipe between them? (to prevent arcing)


This is not needed if the coil is built correctly.


>Consideration #2 : High Voltage Transformer
>
>   Since I have literally dozens of old MOT's in my parts pile, it goes
>without saying that this will be what I use. My first question is, are the
>older (bigger) ones better? Size might be a consideration depending on the
>enclosure I built this stuff into. And also, is it possible to use two
>transformers (primaries wired in parallel, secondaries in series) in order
>to get twice as much voltage to feed to the spark gap/capacitor? Since two
>MOT's would draw more than 15 amps from the wall, some kind of current
>limiting would be needed (custom-wound inductor?)


Yes, secondaries can be used in series.  Some folks have put even more
in series.  Others have built elaborate doubler or tripler circuits using
MOT's.  Current limiting can be used and can be accomplished using
other MOT's


>Consideration #3 : Tank Capacitor
>
>   It seems that everyone uses a tank cap comprised of dozens, or hundreds,
>of small HV caps to get the required value and voltage. I have a fair
>number of mylar caps (mostly from 70's and 80's color TV's) which may do
>the job, but I'm wondering if microwave oven capacitors would work. I have
>dozens of em', and most have values of 0.7 - 1.0 uF at up to 2400VAC -
>each! Only thing is, they have bleeder resistors (typically 10meg-ohms
>each) and might not withstand high operationg frequencies. And what about
>high-voltage ceramic capacitors, are these any good?


Any type of mylar cap will be lossy at the Tesla coil frequencies
and may heat up and be destroyed.
Sometimes they work OK for short runs but I don't like them.


>   Maybe that's enough questions for now. It will take some time to
>assemble all the components, and work out the design. I look forward to any
>advice you may have.


Have fun and be safe,
John