Jumping in with both feet?
From: Bert Hickman [SMTP:bert.hickman-at-aquila-dot-com]
Sent: Wednesday, January 28, 1998 9:10 AM
To: Tesla List
Subject: Re: Jumping in with both feet?
Welcome aboard, Steve! My comments are interspersed below.
Tesla List wrote:
> From: SBJohnston-at-aol-dot-com [SMTP:SBJohnston-at-aol-dot-com]
> Sent: Tuesday, January 27, 1998 3:48 PM
> To: tesla-at-pupman-dot-com
> Subject: Jumping in with both feet?
> Hello, Tesla-listers!
> Please allow me to introduce myself: Steve Johnston, in York, Pennsylvania.
> I'm 34, with 21 years as a ham radio experimenter and 15 years as a
> Broadcast Engineer (so I'm not a newbie to high voltage/current RF systems).
> I've had a passing interest in Tesla coils for years, but never took the
> time to fully study the details and build a system. Over the past week,
> however, my interest has intensified greatly as I was fortunate enough to
> come upon a nicely-crafted TC whose owner had died (hopefully his passing was
> not related to TC experiments!).
We certainly hope not as well! :^)
> This coil stands about 5 feet high not counting a top discharge terminal.
> The primary is made of 11 turns of 3/8" Copper tubing, spread about 12"
> high and 26" in diameter. The tubing is air-wound, held separated by plastic
> (nylon?) spacers. The secondary is wound with about #22 wire around a
> cylinder of 1/2" wide varnished wooden strips spaced about 1/2" apart. It
> consists of two sections in series, each 2' long, about 3 inches between the
> two sections, for an overall winding length of 4'. It is 16" in diameter
> with about 13 turns per inch. The secondary sits about 2/3 of the way into
> the primary -- perhaps by design to set the coupling between windings?
Sounds like a very nicely crafted coil! The helical primary would have
about 86 uH, and about 5" of clearance all around to the secondary.
Total winding lerngth of 48" versus 16" diameter is right in the
ballpark. The space-wound secondary coil reduces its inductance, and
raises it's natural frequency. It's not clear why he constructed it in 2
parts with a 3" gap in the center. Perhaps he wanted to have modular
building blocks to test different coil systems(?). Discounting the 3"
gap and using a 48" winding length, it looks like he's got about 580
turns, 41.7 mH, and a self resonant frequency (less top terminal) of
about 160 kHz. Because of the relatively small wire diameter, and the
reduced inductance due to space winding, the Q will be relatively low
(of the order of 40-45 or so), which will reduce its potential
performance a bit. The relative position of the secondary versus the
primary was to control the coefficient of coupling.
> There is a dome-shaped metal cover about 8 " tall and 6" in diameter that is
> probably intended to be a top discharge terminal. It could connect to the
> center contact bolt on the top of the secondary with a short length of solid
This terminal is too small to adequately protect the wires at the top of
the coil from breaking out, and adds relatively little top capacitance
(of the order of 6-7 pF) versus the secondary's self capacitance of
about 24 pF. However, the power source may not be capable of effectively
driving a coil this size with a larger topload. When the top terminal is
installed, the operating frequency of the secondary would be around 140
kHz. You may want to make a simple small toroid from 3-4" aluminum dryer
duct - check the archives for details.
> The "exciter" (if that is an appropriate term?) is built into a metal cabinet
> and consists of a Variac driving a 15 KV 30 ma neon sign xfmr (half the
> winding is used, centertap to one end), a small series inductor, and four
> caps in parallel. The caps are of unusual physical design (to me) but not
> homemade -- the four together measure about 0.02 uF. The spark gap is also
> commercially-made with circular, D-shaped electrodes with cooling fins behind
> -- the gap is easily adjusted.
You didn't mention anything about taps on the primary, so it sounds like
he was using the entire primary winding. When combined with the tank
capacitance of about 0.02 uF, the primary tank would have a center
frequency of about 121 kHz. The overall resonant frequency of the
secondary and top terminal combined would be about 140 kHz, so it sounds
like the system was roughly in tune - providing a tap to the primary
should permit you the tweak the tuning a bit for improved performance.
If the primary is only being driven off half of the neon (7.5 kV at 30
mA), the system sounds like it's severely underpowered. Also, at this
input voltage and current, the neon transformer is capable of driving a
0.011 uF tank cap optimally - the 0.02 uF cap sounds like its a bit too
large to permit rapid recharging after each gap firing. It would be
better driven by half of a 15 kV 60 mA trannie. It also sounds like he
may have been constrained by the voltage handling capability of his tank
caps. A coil this size can handle much more input power, with
commensurate increases in spark length. Doubling the tank cap voltage
(gap firing voltage) quadruples the coil's energy level and dramatically
increases output performance. You will probably need to replace the tank
caps with more robust ones.
The single spark gap is also a weak area, since it will not provide very
good quenching. This will further limit the coil's performance, since
secondary energy can transfer back to the primary tank circuit if the
gap does not extinguish at the proper time. For testing purpose, you
should be able to set the gap to 0.100". Because of the underpowered
power source, you may not be able to set the gap very much wider and
have consistent firing.
> Any opinions as to the capabilities of this rig? Would it be meaningful to
> temporarily short the spark gap and attempt to grid-dip the primary tank and
> see where it resonates? Does the secondary self-resonate with distributed
> capacitance? To what spacing should I set the gap when I start testing?
> Am I asking too many questions? I guess I'm way too excited and eager to get
> Thanks for any opinions and information!
> Steve Johnston
If you boosted the tank cap voltage capability, converted the gap so
that it consisted of multiple, air cooled, gaps, drove the coil from a
larger voltage and current source (such as a pair of 12 kV or 15 KV 60
MA transformers in parallel, using full output voltage), and added a
bigger topload on the secondary to protect the top and increase the
breakout voltage, the coil itself should be capable of providing 5-6
foot streamers. You'll also need to tie the base of the secondary to a
good RF ground - one 8' copper ground rod in damp earth should do it to
start, going to three or more if you appreciably increase the power
level. In its present configuration, you may be able to get only 12"
sparks or less, since it's basically very underpowered for its physical
size. You may want to check the Tesla List archives for more detailed
information of capacitor, toroid, and gap construction techniques. It
certainly sounds like you've got a nice working coil to start with - the
rest is now up to you!
Safe coilin' to you, Steve!
-- Bert --