RE: new coiler with new coil, is my frequency ok?

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

Original poster: "Lau, Gary by way of Terry Fritz <teslalist-at-qwest-dot-net>" <Gary.Lau-at-hp-dot-com>

Hi Ryan:

Several things about your coil seem at odds with what is generally done.

The ratio of the length to diameter of your secondary coil, called the 
aspect ratio, is much higher that what is generally done.  For small-ish 
coils, a ratio of 5:1 or maybe 6:1 is typical, but 16:1 seems much too high.

You didn't say what you were using for a power supply.  That would to a 
degree influence what size your secondary coil should be.  If you had a 
power supply capable of powering a coil to generate 7 foot sparks, you 
would want a much longer secondary than 16".  But if your power supply 
could only power a coil to generate 4" sparks, then anything longer than 
about 2" would probably suffice.

A 1" secondary is also on the small side.  Unless you're constrained to 
building something deliberately diminutive in size, a larger diameter will 
give you higher inductance and lower operating frequency, with better 
efficiency.  Again, it depends on the power supply selection.

The spark gap electrodes should be blunt, rounded, and something that will 
not get hot at the point of arcing.  Sharpening the screws is exactly what 
you don't want.  In addition, blowing air across the electrodes will in 
most cases significantly improve performance.  This helps keep the 
electrodes cool, and it removes ionized air from the gap to keep the 
breakdown voltage consistent.  Take a look at the numerous Tesla web sites 
at how others make spark gaps, or look at mine at www.laushaus-dot-com/tesla.

1.54MHz is very high compared to most coils, but again, it you're making a 
very small or low powered coil, it may be appropriate.  But in general, 
losses are lower with lower frequencies.  80KHz to 500KHz is more typical.

If your toroid is made of a disk of sheet metal, then it's a disk, not a 
toroid.  Any flat piece of sheet metal will have an edge that will generate 
corona regardless of how its filed or sanded.  It will work, but 
poorly.  While it may bring down the secondary frequency, the top load 
needs to be free of edges that will generate corona, and an approximation 
to a true toroid or sphere is needed.  Again, peruse the many Tesla sites 
and see what others use for top loads.

Regards, Gary Lau
MA, USA

============================================================

Original poster: "Ryan Molecke by way of Terry Fritz <teslalist-at-qwest-dot-net>" 
<ryan-at-molecke-dot-com>

Hi all,

     I have been avidly reading all postings due to my new enthusiasm with
everything Tesla!
I'm an EECE student at UNM here in Albuquerque.


      My first attempt at a coil seems to run ok and generate somewhat of a
field near the toroid, but no breakout yet. I'm using a basic variac and
simple tank capacitor/spark gap circuit (just for starters). My secondary is
very small gauge copper wire wound tightly - 1200-1500 turns geusstimate, on
16" or so of approx 1" diameter PVC.
      I'm getting a rather high resonant frequency on the secondary, about
1.54 Mhz, and I have my 2.7 mF capacitor/primary tuned to the same frequency
(it is about 10 turns of heavier copper wire with about 2.5" diameter,
helical 4"). My spark gap is very crude at this point. Is 1.54 Mhz too high
of a frequency for a simple spark gap using two sharpened screws?
      My toroid is a flat circular sheet with rounded toroid-like edge, about
5" diameter of thin sheet metal. This disk alone brought the resonant freq.
of the socondary assembly down a good .5 Mhz. I'm going to try atatching a
nail to see if I get a point corona, maybe my toroid has too much surface
area?

Please, suggestions! criticisms! advice!

Ryan Molecke
Reason-at-eece.unm.edu