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Re: Smoking the Neons!

>>From bert.hickman-at-aquila-dot-comWed Oct 23 21:24:53 1996
>Date: Wed, 23 Oct 1996 07:15:59 -0700
>From: Bert Hickman <bert.hickman-at-aquila-dot-com>
>To: tesla-at-pupman-dot-com
>Subject: Re: Smoking the Neons!
>
>Tesla List wrote:
>> 
>> >> Subject: Smoking the Neons!
>> >Subject: Re: Smoking the Neons!
>> >> Subject: Smoking the Neons!
>> 
>> >From rwstephens-at-ptbo.igs-dot-netTue Oct 22 21:50:02 1996
>> Date: Tue, 22 Oct 1996 22:05:12 -0500
>> From: "Robert W. Stephens" <rwstephens-at-ptbo.igs-dot-net>
>> To: tesla-at-pupman-dot-com
>> Subject: Re: Smoking the Neons!
>> 
>>
><SNIP>
>I previously posted in part: 
>
>> >You also should add series inductors, caps to ground, and safety gaps to
>> >ground for reducing RF coupling to the transformers, and to provide a
>> >path to ground for any secondary "hits" that may couple into the primary
>> >circuit.
>> >
>> >Safe coilin' to ya!
>> 
>> >-- Bert --
>> 
>> Bert,
>> 
>> I do not concur with the advice to use RF bypass capacitors to
>> ground from the terminals of a small shunted transformeer such as a
>> neon.  Any capacitor large enough to be a good RF bypass (low
>> reactance) at a few hundred kilohertz will also be so large as to compete bigtime
>> with the system capacitor for the meagre charging power available
>> from the high impedance secondary winding of the transformer..  My experience
>> using merely measly 500 pF doorknobs on a 10kV -at- 23 mA funace transformer
>> is a failed transformer in about 45 seconds of operation into nothing else but
>> the bypass caps! I did this twice, the first time with 500 pF an they
>> second time with only 250 pF.  Both times I heard sizzling inside as they
>> failed. This has been discussed on the list before.  It seems that
>> the voltage does not distribute itself evenly along the secondary
>> winding and resonant effects caused by external capacitance loads can
>> cause crest points within the winding space and a hot spot followed
>> by insulation failure.
>> 
>> rwstephens
>
>Robert,
>
>I don't have firsthand experience with oil-burner transformers, other
>than noticing that they're not very large physically. I was assuming
>that Jerry was using two 9-30's or 9-60's with one or two 500 pF
>doorknobs in series tied to each transformer leg to RF ground. When you
>connect the rest of the tank/gap tied to the outputs, I'd think the tank
>cap would swamp out the impact of the smaller doorknobs at 60 Hz. I do
>remember there was _lots_ of earlier discussion on the merit/type/size
>of the HV chokes, bypass caps. I also remember discussions about the
>proposed neon failure mode being inter-turn corona/arcing caused by RF
>getting back into the secondaries. 
>
>After doing some analysis, I subcribe to the theory that what we're
>primarily attempting to block is not the hundred-kHz tank circuit RF,
>but the higher frequency MHz stuff coming from the parasitic circuitry
>surrounding the gap when it fires. At the multi-Mhz level, the smaller
>caps and chokes should be effective. The high-power, lower frequency
>stuff will, for the most part, be shunted by the gap when it fires.
>[This will probably "spark" some discussion again...]
>
>Do you think the resonant effects and crest points mentioned in your
>post are from higher harmonics of 60 Hz, or from residual RF getting
>back into the secondaries? I wouldn't expect to see 60 Hz resonance with
>the small bypass values you describe...
>
>Personal experience: 
>After going through a number of 15-30's I changed over to 15-60's, 500
>PF at 40 KV on each leg, safety gaps, and 5 mH chokes on each leg . I've
>had no subsequent failures for 3 months (knock on wood...). However, I
>suspect a major reason is that the 60 MA trannies are built better, have
>thicker secondary wire, and aren't as fragile as 30's. My neons, as
>Richard has said, are probably eventually doomed...
>
>My next source will be the 19.9 KV pig - fragile it will not be... :^)
>
>Safe coilin' to ya, Robert!
>
>
>-- Bert --

Bert,

You may be right that the tank circuit cap would 'swamp out' the 
effects of the 500 pF bypass caps.  In my experience cited I did not 
have a functional Tesla coil circuit following the furnace xfmer and 
bypass caps which were being tested solo.

Your comment about a bypass cap being too small to resonate at 60 Hz and 
perhaps causing a problem at a harmonic has merit I think.  I just responded 
to Dave Huffman's detailed post with spice analysis, schematics and pictures
suggesting that one might use a high voltage probe and o-scope and 
see what the waveform on the output terminal of a neon looks like 
with no bypass cap, and with a bypass cap.  Perhaps some sinewave 
distortion indicating enhanced harmonic content, and possibly then 
being an indicator of something going on at harmonics inside the 
secondary winding could be observed.

It is also probably appropriate to assume that 60 Ma neons are much 
more robust than 23 mA furnace transformers.  It is a good thing that 
oil filled pole pigs appear to be indestructable. 

Thanks for your thoughts as always.

Regards,
rwstephens