Re: Tesla coil grounding and other questions

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: Bert Hickman <bert.hickman-at-aquila-dot-net> 

Hello Joe,

You may wish to do some research on current Tesla Coil designs
before going too far on rebuilding your coil. I remember when this article 
first came out (I was in high school at the time). I was playing around 
with a 2 kW VTTC coil using three VT4C/211A's in parallel. Never lost the 
"bug"... :^)

I had thought of scanning in this particular Popular Electronics article 
and putting it up on my web site, but I decided against it because it had 
so many problems and fundamental design flaws that severely limited its 
performance, and I feared that it would mislead folks. Unfortunately, some 
of the flaws were outright dangerous to to the continued survival of your 
12 kV 30 MA Neon Sign Transformer (NST) or the coil's operator and 
observers. The specific design problems include the following:

1. The glass plate tank cap was connected directly across output of the
NST. We (now) know that the spark gap should be connected across the NST
and the capacitor should be in series with the primary winding (i.e.,
swap the gap and the tank cap location on the schematic). This minimizes
the degree of RF voltage that can "back up" into the fragile windings of 
the NST.

2. The spark gap should have more robust electrodes and air cooling to
prevent overheating and improve quenching (which improves coil
performance). The recommended spark gap of 1" is much too far and creates a 
high risk that the NST will be overvolted and quickly destroyed.

3. There was no provision for tuning the coil (via primary tapping) or
to easily change the primary-secondary coupling. As a result the
original coil appeared to be grossly out of tune and a poor performer. The 
cover shot for the magazine shows weak corona off the top of the coil 
that's perhaps 2" long at best. Current designs using the same transformer 
can achieve 3-4 feet

4. There was no provision to connect an RF ground to the system. Instead, 
one of the HV outputs of the 12 kV NST was directly connected to the base 
of the secondary (very dangerous!). This raised the potential of the 
secondary to 6000 volts at 60 Hz! It then channeled the RF current from the 
base of the secondary into the HV input of the NST!

5. The glass plate capacitor is quite fragile, very lossy, and it
generates huge volumes of ozone. An up-to-date MMC capacitor (a
series/parallel combination of low cost polypropylene foil snubber
capacitors) will be virtually maintenance free, cheaper, self healing,
and will deliver much better performance. Some doorknob caps could also
work, but the type you'll want to use are designed for pulse duty, and 
sometimes appear on eBay as ceramic "laser caps".

6. The design has no topload capacitance (toroid). The original design was 
such a poor performer that no toroid was needed. However, if the above 
design flaws are corrected, corona will break out from along the top of the 
winding, and this will char the top of the coil form.

7. Finally, there are no RF filters or any safety gaps to help protect the 
NST from conducted RF or accidental excessive voltage conditions.

Continue reading the Pupman or T-2 lists and please explore the archives. 
You'll rapidly come up to speed as far as Tesla Coil technology for this 
century and avoid many potential pitfalls. And I'm sure you'll get 
excellent help and suggestions from many of the great coilers on both lists.

Welcome aboard and good luck!

-- Bert --
-- 
---
>Original poster: JOSEPH CACCIATORE <jocatch-at-us.ibm-dot-com>
>Hello. I am trying to resurrect my old tesla coil I made in high-school 30 
>years ago with plans that came from the July 1964 Popular Electronics 
>magazine. I still have the secondary coil and 12kv, 30ma NST which I want 
>to use. The secondary was built pretty close to the article, 4.75" 
>diameter, 34.625" long using #26 (or was it #28) wire. The NST is center 
>tap to the case.
>First question is grounding. The 120 vac for the NST of course is not 
>grounded (only 2 leads to the outlet). The design I have shows the bottom 
>of the secondary connected to one side of the primary tesla coil in an 
>auto-transformer arrangement. That is how the original design worked and I 
>was able to get 2 or 3" sparks from it.
>But reading on the net I see some places which say ground the bottom of 
>the secondary to the case of the NST only and some show the bottom going 
>to earth ground only. Which is correct? If I want to run it in a house or 
>display like at school, there won't be an earth ground to begin with.
>Also, if you have seen the PE issue, they had the spark gap in series with 
>the primary but all circuits I see on the net today shows the capacitor in 
>series. Is that better?
>For the capacitors, I am using thick window glass and aluminum foil, each 
>glass is about 18"x18". I am using 2 of them.
>Lastly, since I have about 75' of 15kv wire I want to make the primary 
>using the wire in a helical coil arrangement (I don't have copper tubing 
>nor anyway to bent it nor a mount for it). I assume I will need all the 
>wire since my basic calculations show I would need 52 turns for the 
>primary. Since 52 would require more wire than I got, could I go with a 
>harmonic and use 26 turns for the primary?
>Thanks for any input you guys may have. I got this beautiful secondary 
>coil I made and NST and I really would like to make it work.
>JC
>
>.