Re: 10 Questions to help me construct my first TC

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

Original poster: "Terry Fritz" <twftesla-at-qwest-dot-net>

Hi Steve,

At 03:08 PM 12/15/2002 +0000, you wrote:
>I've been following this thread with some interest. The answers people 
>have been giving seem to describe something a bit like my own first coil 
>that I built a couple of weeks ago. Except mine is a bit taller, the 
>secondary form is only 3.5", the topload is 12" overall diameter, and it 
>has a 10kV/25mA NST instead of a 7.5/30. Anyway, I hope you don't mind if 
>I ask a couple questions of my own:
>
>1. Most folk agreed on 1.5 to 2x LTR, for the tank cap. But someone 
>recommended resonant, to "squeeze the most power out". I chose resonant 
>too, for that reason, but now I have some doubts. How much more power do 
>you really get from a resonant power supply, and is there more danger of 
>blowing the NST?

The theoretical factor for LTR static cap size is pi / 2 times the resonant 
cap size or 1.57.  There is no great power advantage between resonant or 
LTR systems.  Too many other factors are involved to say one has more power 
than the other.  It does seem LTR systems produce a bit longer arcs, but 
that could be due to LTR systems being used by more advanced coilers 
too.  Sync gap LTR systems do have a big advantage due to their ability to 
finely suck power out of the transformer with precise timing and inductive 
kick effects.

There is no doubt that LTR systems are far safer for the NSTs!!  A gap 
failure with a resonant system "will" blow the NST or cap (protection gaps 
help alot).  LTR systems actually reduce voltage on a gap failure.  Since 
LTR systems and protection circuits have become popular, NST failures have 
fallen dramatically.  NST failures with LTR systems and protections 
circuits that are "properly built and adjusted" are almost unheard of.


>2. I noticed you were talking a much bigger topload than what I have just 
>now. That combination of pizza pans and flex duct is going to be like 
>15"-18" in overall diameter. I also saw it said that the O.D. of the 
>topload should be the same as the wound length of the secondary, which for 
>my rig would be 20". I'm wondering if the bigger topload would help me get 
>longer sparks:

I am not sure you can really make a top load "too big" unless you can't 
tune it or something.  Even the few folks that have managed to make a top 
load so big that it does not breakout, just have to add a little point to 
it to make it arc.  Too small top loads tend to breakout early or have many 
streamers that remove power from the system before maximum arc length can 
be achieved.  However, multiple streamers are cool too so it depends on 
what you like.  "Very small" top loads breakout way too soon and can 
dramatically decrease streamers.  If you want the longest streamer, then 
you want a big top load the breaks out late and just supports one 
streamer.  It's a trial and error thing and one can keep sticking extra 
stuff on top of the coil to adjust it.  In general, I would say to make the 
top load as big as you reasonably can with the materials you have and 
declare it to be "perfect" ;-))

>I'm just getting 17.5" (between a breakout point and another grounded 
>point) and Terry Fritz's formula suggests I should expect 24".

That's John Freau's formula not mine ;-)

1.7 x SQRT(10000 x 0.025) = 27 inches.  (I see your NST is really 27.7mA  ;-))


>Spec and picture of my coil is at:
>http://homepages.strath.ac.uk/~cnbp111/tcspec.html

Oh!  Real specs and pictures :-)))  I just never expect that ;-))

Your page seems to be in the UK, so use 50Hz numbers and all instead of 
60Hz.  I probably knew this before, but I see so many coils, I just forget...

The resonant cap size is:

10000 / 0.0277 = 1 / (2 x pi x 50 x C)  where C = 8.817nF

The LRT size is theoretically pi/2 times that, but everyone uses a factor 
of 1.5 (there is a little loss) so we get a value of 13.2nF for an LTR 
cap.  Your 9.4nF cap is between these two which is fine really.  However, 
be sure to use safety gaps since it can resonant to high voltage if the 
main gap fails.  Personally, I would add caps to get too 13.2 nF, but I 
have boxes of extra caps lying around here too ;-)

The top load looks like it might be a little small.  However, it is nice 
and smooth which helps.  If you are getting multiple streamers, you may 
want to look into the "dryer duct" toploads.

The spark gap may be a problem.  The coins and the small arc areas my tend 
to rob the system of power.  You may want to consider going to copper pipe 
sections glued to a board something like:

http://hot-streamer-dot-com/TeslaCoils/Misc/terrygap.jpg

About 0.010 inch spacing seems to work well and you don't need nearly as 
many tubes.  The advantage here is that the arc has a very big surface area 
that lowers the losses.

I would start with making safety gaps if you don't have them already (your 
present gap could be used for that).

Then I would get some copper tubes and just glue them to a board for a new 
gap.  I think that would get you to 21"  A bigger top load and adding more 
primary capacitance should get you to 25".

A sync rotary or triggered gap and sync LTR cap size would get you to 27+ 
inches.  But work on that gap first and play with the top load size.  Those 
two will make the most difference.

Cheers,

         Terry



>Thanks for your advice
>
>Steve C.
>