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Re: Neon transformers and HV

>         Although any number of Tesla Coils have been built by
>         experimenters using neon sign transformers - they are =not= a
>         good choice for this.  Many reasons.

Neon sign transformers are not necessarily perfect, but they are a 
very good choice for the beginning to intermediate (i.e. - non-pole pig) 
coiler and can be used for coils up to around 2-3kVA.  With proper RF 
protection, they are reliable, the voltages you can normally find are 
ideal, they are (relatively) inexpensive, they are current limited so 
quenching the gap is not a big problem, and you can gang up a bunch 
of them for higher currents.

> 
> TE>I do realize that the neon xfmrs are center ground, and have wired
> TE>them as such... Am I missing something obvious???
>
>         Yike!  You're siphoning off all your rf energy (or most of it).
>         There is nothing in the =primary= circuit of a Tesla Coil which
>         requires grounding.

The center-tap of the secondary winding on a neon sign transformer is 
tied to the case, which is normally grounded for safety.  That's just the 
way that they're made.  And grounding the case certainly does not 
siphon off "all" or even "most" of the RF energy.  If it's really that bad, I 
would love to see what my 4" coil could do without all of that horrible 
"siphoning" going on (I currently can get 20" sparks with a 15kV, 60mA 
neon sign transformer, and that's with a very mediocre primary AND 
secondary).

>         The center tap of a neon transformer is grounded to permit the
>         suppression of interference when they are used in sign service -
>         get it???
>         The interference is caused by unwanted =rf energy= incidental to
>         the operation of a neon tube.

What kind of RF energy does a neon sign generate?  There really isn't 
a whole lot of voltage across a lit neon sign, and I would think that it's 
pretty much 60Hz stuff.

>   In a Tesla Coil we =want= rf
>         energy.  Lots and lots of it.  Lots of good, wonderful,
>         free-flowing, warm and juicy rf energy. We do not want to
>         suppress any part of it. None. Nix. Nil. Nyent. Nada. Okay?

As before, grounding the case does not suppress any rf energy.  It's a 
safety measure. Okay?

>         What G.E. is telling you is "Rf-wise we didn't take the
>         proper precautions, so these capacitors are leaky as a Chinese
>         junk after a typhoon - better not use them on anything rougher
>         than damn-near pure dc."

What GE is really saying is that the caps were probably designed for 
DC applications and might have a high internal inductance, etc... that 
would make them not the best for pulse or AC applications.  I have 
some "DC" rated caps that I use that can hold a charge for a couple of 
days.  They may not be the best for Tesla coil work, but they're nothing 
like a "chinese junk after a typhoon".

>         The capacitors of choice for Tesla Coils should be mica
>         transmitting capacitors, ceramic, or glass.

The capacitor of choice would be a low-inductance polypropylene cap, 
either rolled or stacked.  These are easy to make and very hard to beat 
in performance.

>         Tune your Tesla Coil (primary and secondary) to the resonant
>         frequency this series-connected capacitor bank provides.

A capacitor does not "provide" a resonant frequency; the condition of 
resonance is determined by both the capacitance AND the inductance 
(actually, the capacitor by itself does have a resonant frequency 
because of the stray inductance, but this is inconsequential).  You 
normally tune your primary circuit by adjusting your primary inductance 
so that the resonant circuit formed by your primary capacitance AND 
primary inductance resonates at the frequency that your secondary 
coil/discharger resonates at.  You normally don't "tune" your 
secondary, other than by possibly changing dischargers.

>  What was good enough for Dr. Tesla should be good enough for you.

As Richard Quick said, Tesla did not have the "high-Q" materials that 
we have available today.  His primary capacitance at Colorado Springs 
was somewhere around .05 to .1 uF and he had to use a bunch of large, 
heavy, liquid-filled boxes and a bunch of bottles that were constantly 
breaking down.  You can go out and buy a 60kV polypropylene pulse 
rated cap today for $150 that takes up a small fraction of the space and 
gives much better performance.  Just because Tesla used gutta-percha 
covered wire and salt water caps doesn't mean that they are the best 
choices today.  What Tesla was able to accomplish was more of a 
tribute to his genius than to the qualities of the materials that he had to 
work with.  

Get with the program, guy...this is the nineties (1990's that is)!


Steven Roys (sroys-at-radiology.ab.umd.edu)