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Re: First Light!!! Now, how to tune it?



Original poster: "Terry Fritz" <twftesla-at-uswest-dot-net>

Hi Bill,

At 05:09 PM 5/12/2001 -0400, you wrote:
>Well, I just fired up my first coil for the first time, and it worked! 
>I was running it outdoors (my only option with my low ceilings), in
>daylight, so it's hard to say how long the streamers were.  Perhaps a
>foot or so, which is far less than I should be able to get with three
>15/60 NST's.  It was exciting to see it work though.  I can't wait to
>see it at night.

Congratualtions on getting it going!!  The sparks look much much better at
night!!  You have 2700 Watts of power so your streamers should be around 7
feet (John's formula) when all is just right. 

>
>Here are the specs:
>
>Power Supply: Three 15/60 NST's, with Terry's protection filter on the
>high voltage side, and redundant line filters and a 50 amp variac on the
>120V side.

Perfect!

>
>Capacitor: A Maxwell 30 kV, 0.06 uF cap (model # 37321).  I know, it's a
>bit large even for LTR.

15/180 resonant size is 31.83nF and LTR is 45nF.  With 60nF I think you
will hit a 60Hz charge BPS which probably is not too bad.  You should use
input line fuses to each transformer.  With such a large cap, the shunts
may saturate and it could draw a lot of current.  Maybe a 15 amp fuse per
NST.  However, this is far less a problem with a static gap design.  You
should consider getting one more 15/60 NST :-))  You may also want to add
about 150uF of power factor correction to each NST.  That will drop the
current to each NST about 6 amps for a total of 18 amps less current draw
which is a big deal in this case.

>
>Static Spark Gap:  My own invention, sort of. 9 copper pipes mounted on
>two PVC rails, but one rail slides, with a compression spring on one
>end, and a bolt on the other end.  Turning the bolt causes the one rail
>to slide, which adjusts the spacing between all 9 copper pipes
>simultaneously.  This is all mounted on a blower box.  So it's a typical
>copper pipe static gap, with a one knob gap adjustment feature. The gap
>can be continuously adjusted from a closed position to wider than would
>ever be used.
>
>Secondary: 1000 turns of 22 awg, 6.5" (closer to 6 5/8) diameter, 27.5"
>winding length.

The most important number here is the inductance.

L = (N x R)^2 / (9 x R + 10 x H)

L = inductance of coil in microhenrys (ÁH)
N = number of turns
R = radius of coil in inches (Measure from the center of the coil to the
middle of the wire.)
H = height of coil in inches (1000 x 3.25)^2 / (9 x 3.25 + 10 x 27.5) =
34.716mH

>
>Primary: Flat spiral, 19.5 turns of 1/4" copper tube, 1/2" spacing on
>centers.  Inside diameter 9.5".  It's  way oversized - I had it tapped
>at about 5 1/2 turns.  (I was originally planning on less power and a
>smaller cap)

Big primaries are great since they can tune anything you want!

>
>Toroid: 6" aluminum dryer duct wrapped around an 8" diameter disk
>(actually two aluminum pie
>pans).  Used a thumbtack as a breakout point, but there were streamers
>coming out all over (is that a good sign?).

6 inch cord with 14 inch center to center spacing...

>
>Ground: Currently only a 32" segment of copper clad iron ground rod.

That's fine.

>
>All interconnects are 1/4" copper tubing, except I use welders cable to
>my primary tap clip (the welders cable is both heavy and very flexible),
>and 15kv rated high voltage wire from the power supply to either side of
>the spark gap.
>
>Misc: Strike rail, and plexiglass cover over primary.

I assume the strike rail is split so that it does not act like a shorted turn.

>
>So how to go about tuning?  (I don't have access to any fancy equipment)

We can calculate it...

>
>Coupling? I didn't even think to adjust the coupling during the first
>run.  Currently, the bottom winding of the secondary is about 1" above
>the level of the primary.

We can calculate that too with MandK or ACME.  The coupling is 0.119 and
would be 0.146 if the primary was level with the secondary.  This is fine
either way as long as you don't get racing arcs from over coupling.

>
>Tap point?  I got 5.8 out of the JHCTES program, but only used a guess
>of 15pf on the toroid.

E-Tesla6 will tell you the secondary resonant frequency which is key in all
this.  I had to guess a little at a few dimensions but it should be pretty
close.  Looks like about 28pF (top load adds 16pF) and 160kHz.  I get 5.88
turns on the primary which is just what JHCTES says even though the
programs I used are different (but we use all the same equations one way or
another).  You may want to go to 6 turns to add a bit for streamer loading.

>
>Static gap width?  What's the basic idea here?  I noticed with the gap
>set narrow, it seemed to fire much more steadily, while when wider it
>had a rather chaotic sound.

With your neat adjustable gap, I would set it for the best spark output.
The filter and it's safety gaps along with your very high cap value will
eliminate the chance of the main gap being too wide and hurting anything.
For tuning, I would set it small just to get things tuned up first.

>
>Most helpful would be a methodical way to adjust the tap point and gap
>width for optimal performance.  Can I adjust the spark gap first?  How
>to know when it's right?  And then how picky will it be about the tap
>point?  Do I need to wait until dark to tune it, or should I be able to
>see the streamers in daylight?

Set the gap small with a breakout point on the toroid.  You want a small
firing voltage just to get it all set up.  Best in the dark or low light.
You can then experiment a bit to find the best point and it is set.  You
may want to add 1/4 turn since the long streamer loading will drop the
resonant frequency a bit from the calculated 5.8 but experiment for the
best spot.  6 turns will probably be real close.

>
>Toroid?  Does it need to be bigger?  How big?

It does strike me as small for 2700 Watts input.  You will get smaller
multiple streamers instead of a longer single streamer.  But that is cool
too so what ever you like.

>
>Ground?  Would it make a big difference to have better grounding?

Assuming your soil is a little damp, I think your ground is pretty good.

>
>Any further help would be most appreciated, and thanks to the group for
>helping me get this far.  This thing has been many months in the making,
>and my wife will be quite happy when I'm done working on it.

Now you can spend all your time playing with it :-)))

E-Tesla6 calculations....
=============================================
 E-TESLA  V6.11C  November 21, 2000 
  
 Grid Size = 300.000000
 Scale Multiplier = 1.000000
 Terminated Coil
 Ceiling Distance = 100.000000
 Wall Distance = 50.000000
 Primary Height (Inner Turn) = 5.000000
 Primary Inner Diameter = 9.500000
 Primary Outer Diameter = 28.500000
 Primary Height (Outer Turn) = 5.000000
 Strike Rail Height = 7.000000
 Strike Rail Diameter = 28.500000
 Secondary Base Height = 6.000000
 Secondary Diameter = 6.500000
 Secondary Length = 27.500000
 Secondary Inductance (mH) = 34.715999
 Terminal-1 Height (center) = 40.000000
 Terminal-1 Cord Diameter = 6.000000
 Terminal-1 C-C Diameter (toroid only)= 14.000000
 Terminal-2 Height (center) = 40.000000
 Terminal-2 Cord Diameter = 0.000000
 Terminal-2 C-C Diameter (toroid only)= 0.000000
 Top Voltage = 100.000000
 Pass     Ccalc (pF)          Change            Fcalc (Hz)
  1000   C=1.98594       Err%=198493.609375       F=606139.56
  2000   C=6.58309       Err%=231.485565       F=332920.34
  3000   C=11.34899       Err%=72.396004       F=253557.66
....................
 49000   C=27.97376       Err%=0.007423       F=161502.72
 50000   C=27.97563       Err%=0.006654       F=161497.34
=============================================================

MandK for coupling...
=============================================================
 Mutual Inductance Program V2.0
 Copyright (c) 1998 by Mark S. Rzeszotarski, Ph.D.

 Flat Spiral Primary Coil Geometry
 Primary coil inside diameter (inches)=      9.500
 Primary coil outside diameter (inches)=     15.500
 Number of primary coil turns =      6.000
 Wire diameter (inches)=      .2500

 Solenoidal secondary diameter (inches)=     6.500
 Secondary height (inches)=     27.500
 Number of secondary coil turns =  1000.000
 Secondary coil wire diameter (inches)=      .0254

 Calculational results are as follows:
 Primary coil inductance in microhenries:           17.11
 Secondary coil inductance in microhenries:        34849.29
 Secondary coil distributed capacitance in picofarads:     12.27
 (Medhurst formula, assumes one end of secondary is grounded)

 Position is the secondary coil bottom wire position in inches
 above the bottom wire of the primary coil.
 A negative value means the bottom wire of the
 secondary is below the bottom wire of the primary.
 A positive value means that the bottom wire of the
 secondary is above the bottom wire of the primary coil.
 M = Computed mutual inductance in microhenries
 K = Computed Coefficient of Coupling
           Position           M                K
              .00          112.59            .146
              .50          101.97            .132
             1.00           91.75            .119
             1.50           82.22            .106
             2.00           73.52            .095
             2.50           65.71            .085
             3.00           58.74            .076
========================================================


Cheers,

	Terry


>
>	Thanks,
>	Bill Vanyo
>