Re: A 1/4 wave Tesla coil

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Original poster: "Bert Hickman by way of Terry Fritz <twftesla-at-uswest-dot-net>" <bert.hickman-at-aquila-dot-net>

Hi Skip,

Very interesting system - you've always liked large diameter resonators!
Sounds like your system is under powered, overcoupled, has too low a
resonator inductance, which is forcing you to use an inefficient
primary. Since you have a SRSG, the simplest change would be to go to a
larger tank cap (try going to a LTR cap) to increase the bang size.
However, before doing so, reduce the coupling, since you appear to be
near the limit for your system. Once you've got the system dialed in
again, you can then look at increasing the size of the toroid. I'd
increase the major diameter to at lest 35-40", and the minor to at least
10" (but you MAY need to add a breakout point). This should help drive
the secondary frequency down, and let you regain use of more primary
turns. However, you may need to drive it with more power to truly make
it sing. 

The root cause of the problem is due to the relatively low secondary
inductance (for a resonator of this size) of about 79 mH which is
forcing you to use a relatively small primary inductance in order to
stay in tune with your reasonably sized tank cap. To improve the
system's efficiency significantly, you'd need to increase the number of
secondary turns so that you can increase primary inductance, thereby
lowering gap losses.  

Good luck, Skip!

-- Bert -- 
-- 
Bert Hickman
Stoneridge Engineering
Email:    bert.hickman-at-aquila-dot-net
Web Site: http://www.teslamania-dot-com

Tesla list wrote:
> 
> Original poster: "Skip Greiner by way of Terry Fritz
<twftesla-at-uswest-dot-net>" <sgreiner-at-wwnet-dot-com>
> 
> Hi All
> 
> I thought that some of you might be interested in a true 1/4 wave coil.
> Following is a complete description. This was designed and built to be
1/2 half
> of a twin. The second secondary is now under construction and will be added
> soon.
> 
> Secondary:
> Skeleton form made of 12 polystyrene 1/2" rods and acrylic spacers
> Size: 14" x 30"
> Turns: 240
> Wire: #16 stranded PVC covered machine tool wire, 1/8" in diameter
> Wire length: 880'. This corresponds to about 280khz
> Free air resonance: 400khz
> Ctop: toroid 8" x 25" dryer duct with aluminum foil coating
> Ctop capacitance: ~22pf
> Secondary resonance with Ctop: ~280khz
> 
> Primary:
> Diameter: 19"
> 5 turns 3/8" copper tubing on 5/8" centers
> Capacitor: .021uf
> Primary resonance with cap: ~280khz
> 
> Primary/secondary system resonance: ~280khz
> System Q: over 150 but measurement limited by my instrumentation
> K: Not measured at this time but expected to be .21 or higher
> Best coupling is obtained with top of primary about 1/2" above bottom of
> secondary
> 
> Power: 15/120 NEST
> Break: SRSG, 1800 rpm and 120 BPS
> Contact spacing: 0.2" total, so far but still working up to larger spacing
> 
> Performance:
> Operating frequency: 280khz
> Spark length with no breakout tab: multiple arcs 48"(+)
> Spark length with breakout tab: single arcs 60"(+), very, very thick
> Interesting note: With no Cop, arcs are close to 42" from top terminal, but
> coil is obviously poorly tuned and much corona around top turns of secondary.
> 
> This coil operates at the design frequency. There is no indication that
any ion
> cloud adds capacitance to the system that might lower the operating
frequency.
> This coil was designed basically using Medhurst and Wheeler formulae. No
tuning
> was required after assembly except to phase the SRSG and to increase the
> coupling between primary and secondary.
> 
> Changing the primary turns only reduces the output length of sparks. Question
> to those on the list with experience: The output sparks are very thick
and fat
> but only reach 60"(+). The coupling is as close as possible, just before
racing
> sparks start, and changing the SRGS phasing only decreases the length. I
would
> appreciate any suggestions to increase the spark length. John F., I know you
> have done much in this area and I would appreciate your comments in
particular.
> Note also that the above indicated operating characteristics occur with about
> 100 volts input and additional voltage up to full line voltage seems to make
> little if any difference in spark length.
> 
> Skip Greiner