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Semi-Rotten Spark Length
Hi All,
Iīve got a problem. Maybe someone has some ideas.
First of all, here is my complete coil data:
Xformer:
-----------
7500V modified NST (with a movable shunt).
(originally 75mA)
I shortend the shunt.
Now: 170mA (measured)
Input voltage: 230-260V
(depending on setup)
Primary Cap:
-----------------
MMC: 67nF
Caps per String = 12
Strings = 13
Single Cap= 62nF-at-1000Vdc
Input Joules = 0.5*7500V*sqrt(2)*(67*10^-9) = 3.77J
(for 15kV comparison this would be a 16.75nF cap)
Toroids:
-----------
4.33" x 20" = 21.70pF
5.31" x 23" = 25.19pF
The toroids are stacked directly on top of each other.
{Ct= k1*C(T1) + k2*C(T2) where K1 ~0.7-0.8 (for the
toroid closest to the top of the secondary coil) and k2
is 0.4 - 0.6 depending upon the spacing between the
two toroids.}
so:
Ct1 = 0.75*21.70pf= 16.27pF
Ct2= .5*25.19 = 12.59
=> CtTotal = 28.86pF
Primary Coil:
----------------
Flat primary
Total dia: 27.17"
Inner dia: 11.8"
Width of one side = 18cm = 7.09"
Center to center = 24.2cm = 9.5276"
Tapped at turn 5.8-6.0 (6.3 would be optimum, claims my homebrew proggy)
Pri induc = 23.44ĩH
Pri current = V*SQRT(C/L) = 10606*SQRT(67*10^-9/23.44*10^-6) =
567A (43A per cap string).
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).
My secondary coil specs:
-------------------------------
Wire spool weight before/after winding: 22lbs/12lbs
Wire specs:
i.d:0.85mm = 0.033465"
o.d.:0.908mm = 0.035748"
AWG 19-20 (19.5)
Secondary former:
------------------------
Diameter: 20 cm = 7.874"
Total length:106 cm
Chosen winding length:83.65 cm = 32.93"
h/d ratio: 4.1825
Turns:920 (+1.25 turns space wound over a length of 12cm = 4.72")
12 cm = heigth of toroid above actual coil winding
Aprox wire length: (579m = 1898 ft)
DC resistance= 17.0 ohms
Calcīd inductance: 36.047 mH
Measured 35.6 mH
Cself=14.75 pF
CTotal ~43.55pF
FresFree=218khz
Fresloaded = 127khz
Bare secondary Q (measured) = 196
Spark length data from start of project till now:
-----------------------------------------------------------
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 (I just turned the shunt out for 120mA).
This got me around 50" sparks.
Up to then I was really happy with my coil and itīs performance, but
letīs look at (being the latest setup) case #:
4.) 67nF MMC & 1257VA modified NST
(I shortend the shunt for 170mA output current).
This setup only gets me around 57" sparks. Only once did I get a 63" spark.
NOTE: Spark length is measured "point to point". I do NOT "straighten out"
the spark. Doing so, would give me longer spark lengths, but the way I
measure the length is more realistic, I think.
Now, why the **!-at-##-at- is my spark output in case number 4 so rotten?? I
almost doubled the input Joules compared to setup number 3. I increased
the xformer current by a factor of 1.4. I would have expected to get at
least around 65-70" continuous sparks, but no go. The only thing I could
finally get the coil to do, was to break out a single streamer in case #4,
using a blunt nail. Without the nail output spark length was only slightly (I
didnīt actually measure it, I just eyeballed it) less than the 57", but the
sparks swirled all around the toroid, making measurement difficult. I
tried various additional "toploads" as a discharge electrode. 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" 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 heigth 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 MMC are mains reso caps, although case #3 does come pretty
close to it.
Any clues, tips, or comments are more than welcome, because I am
totally STUMPED on this one.
Coiler greets from germany,
Reinhard