Re: D.C. Cox TCBA Article
Tedd Payne <paynet-at-interaccess-dot-com> said:
>I've just read the new issue of TCBA NEWS*, and there was an article of
>interest to those like myself who plan to build a smallish neon
>transformer-powered coil. In short, Cox compared (1) a "conventional"
>circuit with its spark gaps in series to the neon transformer secondary
>winding, and (2) a "recommended" circuit with its spark gaps in parallel to
>the neon transformer secondary.
>1. Do you agree that the "recommended" circuit (fig. D) is better than the
>"conventional" circuit (fig. A)?
I did a series of experiments three nights ago (one day BEFORE my
TCBA News came), examining the RF kickback using the two circuit
configurations. For those who don't have their hands on the article,
circuit A places the capacitor across the power transformer, while circuit D
places the spark gap across the transformer. I agree with D.C. Cox that
placing the spark gap across the transformer greatly reduces the kickback to
the transformer. I did see differences in performance with the two
configurations, which I think is due to transformer core saturation (power
arc effects) with the spark gap across the transformer. By adjusting my
spark gap, I was able to achieve equivalent performance with either
configuration. I am investigating this further.
>2. Do you agree that the torroid and air-core chokes described are
>appropriate & adequate?
D.C. Cox recommends placing two chokes in series on each side of the
line between the transformer and spark gap. One is a multi-millihenry iron
powder or ferrite choke, discussed extensively in many earlier postings. He
also adds an air core choke (40 turns 18 AWG on a 1.5 inch diameter PVC
pipe). He claims this blocks VHF and UHF RF, which forms in the short stubs
between the transformer and spark gap. I have suspected this for some time,
but have not been able to measure it due to equipment limitations. Anyone
care to place a nice expensive VHF 'scope near an operating tesla coil?
BTW, I have also seen this done by Richard Hull on a pole pig in one of his
video tapes. Comments Richard?
>3. Do you agree that additional "safety gaps" (and "capacitors to
>ground") are not needed in the "recommended" circuit?
Mr. Cox states this with the caveat that the system be in reasonable
tune. If you are off resonance, there could be substantial kickback. I had
a metal pan fall off the top of an operating coil once, resulting in a
number of rapid safety gap firings before I could power it down. I also had
a choke arc across once. I suggest leaving in the safety gap, in case a
choke shorts or the tune changes substantially. I don't know about the
>4. Do you agree that fewer than 5 gaps in series should be used for a 30mA
>transformer, and that "overquenching" is a concern? If less than 5, how
>many? (Cox's article was unclear about this)
This is highly dependent on the spacing between the gaps, and the
amount of air flow through the gaps. I have found that 5 is good for a 15
kV 30 mA. neon. I think Mr. Cox is implying that the total gap distance
must not be too great, in order to guarantee protection of the transformer.
Hopefully, he will discuss this further in a later article, as he hinted.
Mark S. Rzeszotarski, Ph.D.