A couple of questions

 * Original msg to: Walt-at-rendition-dot-com
 * Carbons sent to: usa-tesla-at-usa-dot-net

Quoting Walt Donovan <walt-at-rendition-dot-com>:

> The problem was, the neon-sign transformers kept burning out, 
> and I finally ran out of the ones I had salvaged from torn-down 
> buildings. Does anyone have any ideas why they burned out? 

Neon sign transformers are simply not built for use as Tesla coil 
power supplies. The tank circuit dishes out enough abuse in the 
form of RF leakage and high voltage kickback that an unprotected 
neon has a very finite life span.

The neon transformer requires protective circuitry for long life 
as a Tesla coil power supply. The first line of defense is a safety 
gap with an RF grounded center post that is placed across the high-
voltage bushings of the neon power supply. This provides a low 
impedance path to ground for over-voltages that are typical of kick-
back from the Tesla tank circuit. The activity at the safety gap also 
gives the coil builder an indication of the tune.

The next line of defense is a bypass capacitance between each HV 
bushing and ground. The bypass capacitance need not be large, but 
it must be rated with plenty of voltage safety margin. Typically a 
capacitance of .0008 microfarads, or less, between each HV bushing 
and the RF ground is more than sufficient. A few dozen picofarads is 
a good starting point. Too large a value of bypass capacitance can 
cause problems. The best dielectrics for this bypass capacitance are 
the ones with the highest RF dissipation factors; glass, PVC, or DC 
rated barium titanate dielectrics are ideal in this application. The 
higher the RF dissipation factor of the bypass capacitor the more RF 
bleedover is converted to harmless heat. The voltage rating on these 
capacitors should start around 50-60 KVDC.

Between the bypass capacitors and the tank circuit you need a couple 
of fairly heavy RF chokes. My experience is that the best chokes for 
this application are large ferrite core toroids, say about two inches 
in diameter weighing around 1/4 - 1/3 of a pound each, wound with 
around 15-20 turns of some fairly thin PVC jacketed stranded wire.

Here is a wiring diagram of a protective circuit for a neon power 

    ||O            |             |        RF1
    ||O         ------- BC1      |                                  
---O||O         -------          |
   O||O            |             *
   O||O--GRND------|-----GRND----* SG                             
   O||O            |             * 
---O||O         -------          |
    ||O         ------- BC2      |                              
    ||O            |             |        RF2                 


        X1 = Is the step up xfmr (neon or neons) with grounded center tap
      GRND = Dedicated RF Ground for the HV/RF wiring and secondary coil
       BC1 = Bypass Capacitance
       BC2 = Bypass Capacitance
        SG = Safety Gap w/grounded center post
       RF1 = Ferrite Toroid Radio Frequency Choke
       RF2 = Ferrite Toroid Radio Frequency Choke

This circuit should be set up such that the values of BC1 & BC2 are
matched or at least close in value. The same would also go for the 
values of the inductance RF1 & RF2. 

How it works is like this: the RF chokes reduce the level of RF "hash" 
or bleedover present in the 60 cycle feed lines. This RF will gradually
deteriorate the delicate HV windings on the neon sign transformer. I
have made relative measurements of these types of RF chokes, the simple
instructions I have provided will produce a a choke that will work very
well in this application. I sometimes purchase these chokes surplus and
strip the old windings off of them, then wrap the cores in a few layers 
of friction tape for a little insulation, and finally wind the core with 
thinner wire.  

The bypass capacitors are placed between the chokes and the HV bushings
on the transformer as shown in the diagram above. The level of RF in
this section of the line will already be lower due to the presence of 
the chokes, but the remaining RF is bypassed to ground or converted to
heat by the capacitor dielectric. The result is an acceptable level of
RF on the HV bushing itself.

None of this circuitry is going to stop a HV kickback originating from 
the tank circuit. Such kickbacks are quite powerful, megawatt peak 
powers being possible in a high performance "desk top" coil these days. 
Kickbacks are seeking low potientals. Without a safety gap across the 
HV bushings of the neon, the kickback will seek the transformer core 
through the winding itself. The safety gap places a lower impedance 
pathway to ground which effectively shunts powerful kickbacks that 
would otherwise destroy a neon sign transformer.

Richard Quick
... If all else fails... Throw another megavolt across it!
___ Blue Wave/QWK v2.12