My 8" coil details. was: Re: was low voltage trans.

Hi Mike, all,

> Original Poster: CHURCHMON-at-aol-dot-com
>  Hi there Rienhard and all, could you post the rest of you,re specs on
> the 7,500 volt 8" coil that is producing 63" sparks.I too like the
>idea of using low power and utilizing the "biggest bang for the volt

Sure, no problem. As I havenīt been coiling lately (no time ;o{ ), the
specs havenīt changed, so Iīll just copy and paste the info that I
have posted earlier throughout the year. You can also visit my website
at: http://rwbuchner.future.easyspace-dot-com/ This is the main page,
so YOU decide what you wanna look at ;o) While the spark length
info is totally outdated, youīll get the basic construction idea. As
soon as I find the time (sic), I will update the site.

XTC-1 (pronounced "Ecstasy", but means eXperimental Tesla Coil #1)

Modified 75mA NST. German NSTs are of low voltage design.
However, they can give loads of current. They are of open
frame design with no potting (the insulation is dipped in
HV varnish, however). I maxed out a 75mA NST to around
350mA. That is an increase of 467%!!! At this stage the
xformer does get pretty warm, but it survived many Jacobīs
Ladder experiments (see my page). At the 170mA, I am
running now, it stays absolutely cold, even though this is
a 226% increase in current over itīs faceplate value.

PowerClass:  1-2kVA
#: 1 (at this time; planned are 6 modified NSTs for 5-6kVA)
InputVolts:  230V / 50Hz
InputCurrentA:  7.5A
OutputVoltage:  7500V rms
OutputCurrent:  Original: 75mA. / Now:170mA
Limiting:  Internal (modified) magnetic shunt limiting

SaftyGap:  0.75" (wide, I know ;o)
BypassCapType:  NONE
ChokeType: NONE

Main Gap:
Multigap flat static gap (not the TCBOR cylindrical type). This gap uses
(against all advice) two gaps made of aluminum. This results in a
measurable increase in spark length. The exact reason is not quite
clear. Most probable is the reasoning that the hefty ion migration
causes the spark to be blown out. This gap does NOT use any kind of
forced cooling. I find this unnecessary due to two things: One being the
"relatively" low current and the other being that I use a high number of
gaps for the 7500V. A static gap will really thrive on the highest
possible number of single gaps (in series). The only real limit is the
xformer voltage, but use as many as you can. It WILL improve
performance. This gap will be replaced by a homemade 8 electrode
200 bps synchronous rotary. For my reasoning behind the 200bps
SYNC RSG, please see my posts on the Pupman List (in the
archives) or search for Richie Burnettīs posts.

Gap: 9 gaps a 1.1mm each ; two of which are made from Al.

BPS: Unsure. Definitely more than 100bps. A good guesstimate
would be 300-400 bps. A static gap is NOT set to a specific firing
rate, so this will vary with the gap setting and the capacitance of
the primary cap and the current capability of the xformer.

Primary Cap:
Type: MMC (of course ;o)
ĩF at this stage: 67nF (for 15kV comparison: 16.75nF cap)
Kv: 12kV DC; run at 10.6kVpeak ACV

Geometry:  Flat
InnerDiameter: 11.8"
WireDiameter: 3/8" (0.375") soft copper tubing
TurnSpacing: 3/8"
Total # of Turns: 10
Inductance: at tap below: 23.44ĩH
BestTap: Turn 5.8-6.0

Width: 7.88"
Total length:106 cm
Chosen winding length:83.65 cm=32.93"
h/d ratio: 4.1825

0.85 mm = AWG "19.5"
i.d:0.85mm = 0.033465"
o.d.:0.908mm = 0.035748"

Wire spool weight before/after winding: 22lbs/12lbs = 10lbs of wire
The secondary is considerably heavier due to the "gallons" of epoxy
and varnish that I used to embed the wire.

Turns: 920+1.25 space wound over 12cm = 4.72"
Aprox. wire length: (579m=1898 ft)
DC resistance= 17.0 ohms
Calcīd inductance: 36.0547 mH
Measured 35.6 mH
Cself=14.75 pF
Bare secondary Q (measured) = 196

FresLoaded =127khz (at this stage)

Type: Dual Toroid (I NEED MORE! and a greater ROC)
Width: T1 = 20" / T2 = 23"
Height: 4.33" = 21.70pF // 5.31" = 25.19pF
Ct(total) ~ 28.865pF


Spark length data from start of project till now:
0.) 5nF (a try at an oil-less) rolled poly design & 562VA NST. This made
for 8-10" sparks.
1.) 15nF MMC & 562VA unmodified NST. This got me around 30" sparks
2.) 25nF MMC & 562VA unmodified NST. This got me around 41" sparks
3.) 31nF MMC & 900VA unmodified NST (shunt turned out for 120mA).
This got me around 50" sparks. I never achieved a single breakout
(always multiple, even when the 50" connected to a grounded rod).
4.) 67nF MMC & 1257VA modified NST. With a little tweaking I get 63".

I would say in the cases 1-3, I maxed out the spark length vs. input VA
pretty well. In the last case (current design stage), I need a much
larger toroid and the ~600A primary current are pretty much the limit
for my unblown static gap. I also tried increasing / decreasing
coupling, but it didnīt help in making the sparks any longer (making
them shorter was easy enough though :oÞ). Best spark length was
achieved, when the primary (!) was ~4.8" ABOVE the lower most
secondary winding. Going up to 5" ABOVE didnīt change it too much.
I didnīt go above 5" as I have seen racing sparks in earlier setups
(i.e: in case 3) appear above 5". Decreasing the primary height to
4" started reducing spark length. Primary tapping is semi-critical.
Moving the tap more than (+/-) 1/4 turn will let the output drop like
a rock. None of the above MMCs are mains reso caps, although
case #3 does come pretty close to it. Optimized, I think I can get
around 65-70" from this 1275VA setup.

I use zinc coated (for long life) T-iron. My RF-ground is composed
of 4 pieces of 5mm thick x 45mm wide T-iron, each about 4 ft long.
These are pounded into the ground about 4 ft apart (length of the
T-iron is the width of the spacing in between). T-iron has two real
advantages: It is nearly impossible to bend. The "T" form is VERY
strong and iron is stronger than copper. You can smack away at it
with a hammer and it WILL go into the ground straight. It wonīt be
affected by rocks (unless there the size of a large brick). My
backyard contains a lot of smaller rocks and sand. If you use your
good old angle grinder to sharpen the bottom of the T-iron to a mildly
sharp point, it will almost slip into the ground just by looking at
it. (okay, okay, I admit: not quite) The second advantage of T-iron
is the surface area is much, much larger than a piece of round rod
(which is why I never understood why coilers use rods at all). 3 of
the four T-iron pieces are about 1ft underneath the sod level and
the last one is about 4" above ground level. I painted it red for
visibility and it has a sticker on it that says: "RF-ground" Do
not remove". You wouldnīt be able to anyway. Once the thing is in
the ground, there is almost no way you will get it back out ;o)).
A brass screw (plus a little bit of grease to prevent corrosion),
fastened with a normal lockwasher and nut makes my connection
terminal. I use a wing nut on top of this to connect my grounding
wire (35mm^2 = AWG 0), which has a big  soldering lug on it.
Forget about using a single ultra long piece of rod. It is much
easier, and from an RF standpoint actually better, to use several
pieces of shorter rod (or T-iron for that matter :ö). It only took
me about 2 hrs. total to make the whole RF ground, including getting
everything together, the work, replacing the grass and cleaning up.
My method: no sweating, no cursing, almost no digging and it makes
a pretty good grounding system, or at least my coil seems to think
so. (:o)).

Misc Info:
Pri current: ( for the current design stage)
= V*SQRT(C/L) = 10606V*SQRT(67*10^-9F/23.44*10^-6H) = 567A
With my present MMC design, this works out to ~43A per cap
string. The peak primary current will be above the 1.25kA level
in my final design. My MMC stays ICE cold, even though I ran
the coil for a total of 1.75 hrs (a few minutes at a time, cool
down time was low, as the coil was off only during adjustments)
at the 567A stage.

Primary Joules: 0.5*7500V*sqrt(2)*(67*10^-9) = 3.77J.
The final design will up this to around 12-13J

The spark length in the final stage should be at LEAST 120" (10ft),
although I have done some calculations showing me the output might
be closer to 12ft.

This coil was designed (with a "low" primary voltage of 7500Vrms or
10606Vpeak) to clear up some of the myths in coiling, such as the
"need" for high voltage xformers (i.e. >>9kV). It makes ABSOLUTELY
NO DIFFERENCE, if you run a coil with low voltage, high current (and
a big primary) cap or if you go the more conventional way of high
voltage, low current and a small primary cap. The ideal voltage range
lies between 6-16kV. Going any lower will result in gap problems and
going much higher than 16kV (all values are rms!!) will result in
corona problems. The low voltage design does have one disadvantage
(for high power coils): you need a spark gap design, that will handle
high currents. However, the low voltage approach does have an
advantage: As the voltage is low and the current high, this kind of
xformer makes a VERY stiff psu (power supply unit), which is an
important consideration. esp. when designing a high breakrate coil.

I think secondary preparation is of outmost importance, if you want to
dabble with high input power and high coupling factors. My PVC form
was sanded, force dried, coated inside and out with PU varnish and
allowed to harden. Then the wire was wound on the former. I used a
homebuilt winder to aid construction. The wire was wound under constant
tension. I also made sure that the wire was not kinked and there was
only minimal space (0 space is impossible) between the turns. The
whole form was coated with a slow drying, 24hr. hardening time)
epoxy resin, sanded, coated with a 2nd coat of resin, sanded once
again, coated with PU varnish, sanded and given a final coat of PU
varnish. This makes the coil and former glass hard and very smooth.
The electrical properties of such a coil construction are very good
(you can REALLY stress it). As a matter of fact, during a run, my
primary coil tilted and I got hefty flash-over between primary and
the secondary coil. I shut down and examined the coil. My coil had
survived without the slightest mark. I am 100% positive, that if I had
not finished the coil the way I did, it would have been trashed during
this experience. My primary is ~5" ABOVE the lowermost secondary
turn, so coupling is very high (actual value must still be measured). If
the wire was not "embedded" in epoxy and PU varnish, I am pretty
sure I could NOT couple this high. I also have a "screw on" RF ground
connection. I do NOT like the RQ/TCBOR RF ground connection

Coiler greets from Germany,