Re: Tesla coil DESIGN requirements

On Sat, 23 Dec 1995, lbliao wrote:

> Thank you so much for your kind reply, but I have many questions, and their
> answers would help a lot.

Sorry to have taken so long to reply but I've been on vacation.  I am 
also taking the liberty of posting this to the Tesla coil builder's 
list and I would like to continue this thread there, since there are 
people who know much more than I do (both theoretical and practical), and 
would probably have more to contribute.

Please note that most of what I have said has been more empirical than 
theoretical, and has been compiled from years of experimentation by 
Richard Hull, D.C. Cox, Bill Wysock, Robert Golka, Richard Quick, and 
others.  Essentially, the rationale is that these methods are those that 
these dedicated experimenters have found to build efficient, high 
powered Tesla coil systems that can easily produce discharges that are 2 
or more times the secondary length (I think Richard Hull and the Tesla 
Coil Builders of Richmond currently has the record with discharges about 
7.5 times the secondary length for one of their magnifier systems).  To 
attempt to rigorously explain the workings of a Tesla coil would probably 
be a good doctoral thesis, and in fact I think one of the mailing list 
members is doing (has done?) just that.

> > 
> > Secondary:
> > 
> > Thin walled pipe plastic (lexan, plexiglass, polypropylene, PVC, etc...)  
> > PVC tubing should be heated to drive out moisture and sealed with non-water 
> > based urethane or two-part epoxy/acrylic/etc sealant.  The tubing can be 
> > hermetically sealed at both ends with plexiglass disks epoxied on - the 
> > wire should NEVER enter the coil form.
> OK what is the rationale for this? If the rationale is that sparks/leakage
> doesnot take place due to moisture on the inside surface of tube, what about
> the outside surface, where the coil is wound?

The field inside the coil should be fairly homogenous as long as no part 
of the conductor is allowed inside of the coil form to distort the field.  
When you start getting discharges much longer than the secondary, any 
wire inside the coil form would generally arc to ground inside the coil.  
Richard Quick seals his coils at the end, Richard Hull doesn't.  Both 
have obtained excellent results with their respective methods.  The 
urethane is used to seal the PVC since it is hygroscopic.  The discharge 
terminal is very important in a high-powerd system, and 1 - acts as a 
terminal capacitance and brings the frequency down, 2 - gives a higher 
voltage before a discharge breaks out, and 3 - acts as an electrostatic 
shield and shapes the field to protect the secondary coil winding from 

> >  The winding should be close wound 
> > with about 1000 turns of enamel covered (magnet) wire.  A 4" dia. coil 
> > should be wound with 24 gauge wire and have a winding length 20" long, a 
> > 6" diameter coil would have a winding length of about 24" and be wound 
> > with 22 gauge wire.  Larger diameter coils (8"+) work well with winding 
> > aspect ratios around 3:1, and the wire size should be chosen so that you 
> > have about 900-1000 turns.  
> What is the rationale for this aspect ratio rules for secondary? I agree
> about the number of turns, but can you explain the aspect ratio rationale
> from the field distribution of the primary? 

Experimentation and observation.

> Is the primary outside or 
> inside the secondary? I have seen tesla coils in which the outside winding
> is primary, but secondary is hidden. WHere is it?

The primary is generally outside the secondary.  I have seen coils where 
a flat pancake primary is next to a flat pancake secondary, but I have 
not seen a coil where the primary is inside the secondary or the 
secondary is hidden.

> > Primary:
> > 
> > A flat spiral or inverted cone primary transfers energy efficiently and 
> > is preferable to a helical primary.
> Why? Reason? Rationale?

Empirical observations.

> >  The inside turn of the primary 
> > should be 3 to 4 inches larger in diameter than the outside diameter of 
> > the secondary coil base.
> Why?

To help prevent spark-over from the primary to the secondary.

> >  The primary should be wound with a conductor 
> > that offers a lot of surface area - figure on using 3/8" diameter 
> > soft-wall copper refrigerator tubing or copper ground strap.
> Is this because of skin effect? Litz type wire?

Yes, the skin effect plays a large part in the choice of primary 
conductor, and I have also read that the current distribution on the 
primary is not homogenous, but is greater on the side of the primary 
closest to the secondary.  Some people have also investigated the use of 
Litz wire, and have posted their findings on the list.  What they 
have found is that Litz wire has a much higher RF impedence in a Tesla 
coil circuit, and a solid conductor is a much better choice.

> >  The 
> > complete primary should have 10-15 turns and the inter-turn spacing 
> > should be 1/4" to 3/8".  
> Is this to decrease the interwinding capacitance to get high frequency
> resonant oscillations, on every break? What is the human body resistance?
> what is the formula of skin depth? what is the human skin thickness for
> safe current conduction? That should give the min. freqency required.

The distributed capacitance in the primary is not high enough to be of 
concern.  The spacing is to prevent inter-turn spark-over.  The primary 
system frequency is determined  by the secondary that you are using since 
it's a lot easier to change the number of turns in the primary than to 
change the secondary.

> > Larger 
> > diameter coils generally work better with flat pancake primaries.
> Why?

Empirical observations.  It has been reported that it's easier to find 
critical coupling with a flat primary at high powers.

> >  Hard 
> > plastic is used for the coil supports/forms, but wood supports can be used 
> > with plastic "combs" to hold the windings in place.
> WHat are combs and how can wood, a bad, moist, insulator sustain?

"Comb" are just radially positioned plastic stand-offs used to hold the 
primary windings in place.  The primary is supported by the plastic, and 
does not touch the wood.

> > Discharge terminal:
> > 
> > A well-designed coil can take what would seem to be a disproportionately 
> > large discharge terminal.  This brings the resonant frequency down and 
> > increases the efficiency of the system.
> How does that happen? what is the theory or basic idea behind this statement?

The discharge terminal acts as a capacitor to ground and brings the 
resonant frequency down.  A larger terminal acts as a larger capacitor 
and lowers the frequency more than a small terminal.  A lager terminal 
will generally have a larger radius of curvature, and will allow a higher 
voltage to be obtained before the discharge breaks out.  A larger 
terminal capacitance will also place more of the secondary capacitance at 
the top of the coil, making the distributed capacitance proportionately 
less, but what exactly that would do I'm not sure.