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Re: New Coil

In a message dated 97-02-06 00:53:09 EST, you write:

<< Greetings Coilers.
 
 Just putting the finishing touches on my new (and first) coil and I thought
 I might share the design with you and see what you think. I was never very
 good with math (got held back a year in Electrical Engineering because of
 calculus) so I adopted the build it and it shall work principle. Always
 been a maker not a designer :)
 
 Primary CCT
 
 5 KV 30 mA Neon XFormer
 salt water capacitors (bottle type) ~ 4.3 nF
 Air quench type spark gap using 1" x 1.5" round brass electrodes and/or a
 commercial plasma p/s spark gap (3 series electrodes mounted in 1.5" square
 brass bases on a porcelain mount)
 1\8 copper tubing inside 3/8 (OD) PTVC (?) tubing close wound in a spiral
 with a 30* rise. Inside ~3" diameter outside ~10.5" diameter.
 Primary tapped from 4th  to 12th turns on alternating sides for tuning
 strike rail 1\2"above top coil mounted on top of primary coil form
 
 Secondary CCT
 
 2" x 13" schedule40 PVC pipe
 ~450 turns #24 magnet wire close wound over 11" of form
 top and bottom of coil connections are 2 1\4" x 1\8" copper disks
 insulating inside form is 1\2" insulboard (to prevent arcing between
 copper)
 torrid is 9" x 3" Styrofoam covered with aluminum ducting tape and has a 5"
 x 1\8 copper disk in the middle
 
 All connections are made permanent where possible (solder and/or brazing)
 and all wiring is made with 50 Ohm coax (used for industrial plasma Tank
 Ccts)
 
 One of the questions I have is concerning the Tank Cct. There seem to be a
 number of ways to connect the components. I am connecting the capacitors
 across the HV XFormer and the spark gap and coil in series with this. What
 are the disadvantages \ advantages with this setup. Is there a
 configuration that would be more effective ? 
 
 Also I have noticed a lot of static gaps use a combination of two or more
 in series. What are the advantages \ reasons for doing this. Would I be
 best to do this ... use one ... the other ? I will probably try all the
 combinations in time, but some input that would make the first run a
 success would be appreciated.
 
 Any comments as to things I might have overlooked \ forgotten would be
 greatly appreciated. If there are any industrious mathematicians out there
 I would be interested in the particulars on this coil, but don't lose any
 sleep ... I didn't :) TIA for any and all comments suggestions and (ahem)
 flames regarding this work of (hack hack) art.
 
 		Tim 
 
 ******** Tim Maleske *********************************** >>


Tim,

I've done some calculations on your coil.  Here is what I came up with:

Secondary:  I calculate 494 turns using #24 wire at 46.3 turns per inch, 11"
winding length at 97% factor for spacing (close wound but allowing for some
variations in density).  This comes out to 2.1 mh which is quite low for a
Tesla coil secondary.  You really want all the inductance you can get, even
in a small coil.  For a small coil like this you would have been better off
with smaller dia. wire, like #28.  You also want somewhere between 900 and
1000 turns - and to keep the height to diameter aspect ratio under 5:1.
 Tall, candlestick looking coils work well for tube driven coils but not too
good for the traditional spark gap coil.  This secondary will resonate at 1.7
mhz unloaded and about 935 khz with your toroid.

Primary:  I understand you have 1/8" copper tubing inside a plastic tube
which equals 3/8" dia overall, closewound.  If this is incorrect please let
me know.  The inductance of this primary should be about 28 micro heneries.
 With a .0043 mfd capacitor, this will resonate at 458 khz.

As you can see, you will not be able to tune this coil.  Using the primary
and secondary that you have, you want a .0013 mfd capacitor.

I have this plugged into a spreadsheet.  Let me know if you want me to send
it to you.

For tank circuit connections, it is best to place the spark gap across the
output of the transformer.  This helps to limit the tank circuit voltage that
is impressed back across the transformer - i.e. it wil live longer.  You
might also want to place some 3,000 ohm 10 or 20 watt resistors in series
witht he transformer outputs, and some inductors would not hurt.  You also
should build a three terminal safety gap with the center terminal connected
to your transformer, secondary base connection and tied to a good RF ground -
not your house mains ground.  The other two terminals go right across the
transformer outputs.  Set the gaps to just barely fire when the coil is
running.

For spark gaps, you want a series of small gaps so the spark discharges will
quench quickly.  Your 5kv transformer is a little low for a Tesla coil so
your gaps will need to be small, maybe .010" and use four or five gaps in
series, you will need to experiment here.  Start with just a couple and
increase it as you gain experience.  Keep in mind, the larger the total gap,
the higher voltage the caps will charge to (longer sparks off the top) and
the more stress the transformer will see.

Good luck and have fun,  Ed Sonderman