Re: Cap Size

Tesla List wrote:
> >From ulflyer-at-juno-dot-comSat Jul 27 20:47:16 1996
> Date: Sat, 27 Jul 1996 14:15:53 PST
> From: Stephen G Stanton <ulflyer-at-juno-dot-com>
> To: tesla-at-pupman-dot-com
> Subject: Cap Size
> Hi all:
> I've got my coil back in action with four banked 15kv neons at 30 ma
> apiece.  I'm using my old plate-glass-and-foil 90 lb cap which, when I
> first designed it, was 0.02 + or -.  The gap puts out a mighty roar
> indeed, and judging from the temperature of the line cord (from a
> microwave oven), it's drawing in the neighborhood of 18 to 20 amps.
> I believe I have the coil in resonance.  I don't really have a toroid;
> rather, it's two 10" salad bowls tackwelded together at the bead.  I get
> a wonderful writhing display of corona off the bowls, especially at the
> bead, and can draw out about a 14" arc at will.  Despite my best
> efforts, I can't get more, and yet, from what I am reading, with the 120
> ma or so going in, I should be getting much longer discharges.
> I was in the process of building my own a-la the 5 gallon bucket of oil
> and hundreds of baggies and foil, but I opted instead to go for the
> ready made variety -- seemed more cost-effective to me.  I took the
> plunge with the group purchase, but when it fell apart, I had second
> thoughts.  Now I'm wondering if I should go for the commercial cap to
> see if it will improve the discharge.  Am I on the right track, or is
> this a matter of not having the right toroid, or perhaps cleaning up the
> corona leakage?  Or, is it the gap?  I'm only using a simple series gap
> with 1/4-20 brass bolts, which seems to work nicely, but nowhere near as
> complex as the series gaps I've seen posted by Richard Quick.  As I open
> the gap, the discharge increases, but then I get ominous green flashes
> from the cap.  By past experience, this presages a punchthrough of the
> plate glass and then -- dismantle and cut and paste to fit around the
> shattered parts!
> Any advice at this point would be much appreciated.   I very much enjoy
> the postings on this net, and of course am enthralled by watching my
> coil do its thing in darkness!
> Thanks again.
> Stephen


You didn't say anything about your coil's diameter, wire size/length, 
primary coil construction, or type of RF ground you have on the base of 
your secondary. Assuming a fairly heavy gauge primary, reasonable aspect 
ratio secondary, 800-1000 turns of #20 - 24 AWG, and reasonably good RF 
ground, the areas to address are the gap, toroid terminal, and the tank 
capacitor (I put these in the order of increasing cost to fix...). 

At the input power levels you are running, some form of active quenching 
is necessary for your gap. Converting to an air blast (best) or 
multiple-electrode vacuum gap (very good) will result in significantly 
better performance. Just blowing compressed air or the discharge from a 
Shop Vac on your existing gap should help somewhat. 

Note that 0.02 uF and the transformer bank of 15 KV 120 MA is VERY close 
to hitting 60 Hz resonance between the inductance of the transformers and 
your tank capacitor. Don't open your gap too widely, since it may 
be the only thing preventing voltage over-stress on your neons. I have a 
similar setup using 0.02 uF homemade polyethylene plate caps - when I hit 
20% on the variac, primary current suddenly jumps to about 12 A, and my 
gaps begin firing. I have no doubt that if I widenned the gaps beyond 
their present setting (about 0.5" total) I could easly blow the 
transformers. The outputs hit 15 KV with only about 25 Volts of input due 
to resonant rise (transformer inductance and tank cap "Q" of about 5...).

On your coil, the maximum voltage at the output terminal is being limited 
by the relatively small radius discharge points at the seams of your 
discharge terminal. Energy is being prematurely "bled off" before 
building to the desired level. A relatively inexpensive (but very high 
performance) toroid can be easily constructed from a pair of pie pans or 
pizza pans and 1 or 2 pieces of flexible aluminum dryer duct available at 
most large Hardware or Building Supply outlets. This material is fairly 
rigid, and comes in pieces about 3 feet long which can stretch to about 8 
feet in length.

A small piece of rolled cardboard or sheetmetal can be used (on the 
inside of the duct) to help hold the ends while they are joined with 
aluminum duct tape.  Carefully smooth out all wrinkles in the aluminum 
duct tape to remove any sharper areas. You do NOT need to cover the whole 
surface with foil, just the joint(s) where the ends come together. I 
mount a 2" PVC end cap on the pie pan and a matching one on the top of 
the secondary. By using various lengths of 2" PVC pipe, the toriod can be 
securely held but easily adjusted height-wise. By going to larger 
diameter material (I went from 4" to 6" to an 8" x 32" toroid now), much 
higher voltages can be reached before "breakout". A larger toroid will 
lower your operating frequency and reduce the dielectric and skin effect 
losses in your system. You may need to increase the inductance of your 
primary to be in tune at the lower frequency. Total cost for a 
better performing toroid shouldn't exceed $25 (US).

You didn't provide much information on how you terminated the plates on 
your glass plate capacitor. As you probably know, glass is a fairly lossy 
dielectric material. If you combine this with relatively high resistance 
terminations on the capacitor plates, you will not achieve maximum energy 
storage and transfer to the secondary. Punch-through is a symptom of high 
dielectric loss. It typically occurs where the glass has been locally 
heated to the point where it begins to become conductive. Molten glass is 
a surprisingly good conductor! Since this is a "runaway" process, it can 
occur very rapidly, and will typically start where there is a small 
imbedded air bubble or other imperfection in the glass. Going to a 
carefully constructed home-made polyethylene plate or rolled-type cap 
(immersed in transformer oil) or a commercial polypropylene pulse cap 
typically makes for a MUCH hotter coil output. You can also run longer 
since the dielectric heating in the caps is MUCH lower. 

Finally, it is amazing how having a GOOD RF ground improves performance. 
When I was firing my 6" coil, going from 1 to 2 ground rods added another 
6-12" of spark length at the same input power level. Going to three rods 
became necessary when going to the 10" coil with the 32" toroid. 

Good luck, and safe coilin' to ya!

-- Bert --