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Re: Capacitors
Tesla List wrote:
>
> >From julian-at-kbss.bt.co.ukFri Sep 6 22:00:49 1996
> Date: Fri, 6 Sep 96 11:38:52 BST
> From: Julian Green <julian-at-kbss.bt.co.uk>
> To: tesla-at-pupman-dot-com
> Subject: Capacitors
>
> >The color of the spark always tells me a lot about a persons capacitor.
> > purple or purple-violet tells me the fellow is a novice and his caps are
> >at the very bottom of their performance curve. Brilliant pretty blue
> >sparks are better but indicate high loss in the tank capacitor is still
> >taking place. Rich blue-white sparks indicate only satisfactory
> >operation. Snow white arc channels only fringed in brilliant blue
> >indicate top performance operation and snyergistic tank operation and
> >energy transfer. With enough power input, to over come losses, Mylar
> >and mica can give quite impressive sparks. Both will heat rapidly and
> >only the Micas stand much chance of survival if long runs are the norm.
>
> Thank you Richard for this posting, you explain a symptom that I am getting,
> namely the purple-violet sparks of the novice.
>
> I must admit I thought that I had built a good cap having made it from foil,
> poly sheet and oil, using the extended foil ends to form the terminals. I
> understand that the advantage of the extend foil ends allows terminal
> connection down one side of the cap plate thus keeping self inductance low.
>
> My question is how do you make a good cap? Or is the bad performance due
> to some other factor? How much does conductor size of the primary tank
> circuit make a difference? Size of toroid (14" on a 7" coil), operating
> frequency (220KhZ). What I do know is that the sparks are not white.
>
> My plan is to make as many mistakes on my current coil in order to learn
> the most from the experiance. Work has just started on a 10" coil which
> I hope to fire at 10KVA.
>
> Any comments most welcome.
>
> Julian Green
Julian,
As long as you've terminated your capacitor(s) in a manner which gives
you a low resistance and inductance connection to the foil layers, you
should get good solid performance out of the LDPE dielectric. Your tank
capacitor is probably just fine. Instead, you may simply need a larger
diameter toroid to lower the secondary ringdown frequency, coupled with
more primary inductance and possibly more tank capacitance. Here are
some things to look at:
While you didn't provide very many details about your coil, the
operating frequency of 220 kHz seems to be high for a 7" coil.
Generally, the lower the operating frequency, the "hotter" the output. A
7" coil should be topped with a toroid at least 21"-32" in diameter for
best performance. A larger outer diameter increases the capacitance,
while a larger torus diameter will increase the voltage buildup before
breakout. [You can ALWAYS get the coil to break out, even with a very
large toroid, by simply taping a "bump" of aluminum foil tape onto the
outer portion of the toroid]. With a larger toroid, and more primary
inductance, your 7" coil should ideally be running at 125 - 150 kHz.
Assuming you can tune your primary tank circuit by tapping out or adding
more turns on your primary (15 - 20 turns MAX), you can live with the
same size primary cap. While you didn't provide any details on your
primary - as long as you're using 1/4" or larger diameter copper wire or
tubing, and heavy wire for the rest of the tank circuit, you should be
OK.
Worst case, you may need to build or buy another cap to connect in
parallel. You also need to make sure you have a solid low resistance RF
ground going to the base of your coil - a poor ground will rob
substantial power from your sparks. This will become critical when you
migrate to a 10" 10 KW coil! You may also want to try increasing the
coupling between your primary and secondary. A low coupling coefficient
(k) produces high coil voltages but also makes for thin, wimpy sparks.
Increase k by lowering your secondary vs primary, or by raising the
primary vs the secondary. Carefully test any new position at low, then
medium power levels in a dark room, watching for signs of inter-turn
flashovers on the secondary, or flashover between a portion of the
secondary winding to the strikerail/primary. If all seems OK, then try
again at high power.
Optimum coupling is the point just before the secondary shows evidence
of breaking down (like seeing a "Christmas tree" shaped blue haze or
actual flashovers from the top of the winding to the strikerail, or
sporadic flashovers between various points on the secondary). While
"tweaking" the coupling, you may also need to lower the toroid height a
little at this point (and perhaps retuning again). Note that increasing
the coupling may also mean you may need to go to a better quenching gap.
A vacuum gap (a series of spaced copper tubes with high velocity air
sucked through the gaps) or an airblast gap may be necessary if you are
running over 2 KVA power levels. Now's the time to begin looking at
these, since you'll need them anyway in conjuction with the rotary for
your 10 KW beast.
Finally, you didn't indicate what voltage/current you were driving the
coil from. In the final analysis, there is NO substitute for power!
Increasing the supply current permits more rapid recharging of the tank
capacitor, larger values of tank capacitance, and more gap firings per
AC half-cycle. More rapid sparking along the same ionized trails
intensifies the "average" power dissipated in the arc channel, and also
adds to overall streamer length. Coupled with the other changes above,
your purple-violet sparks will turn into loud white fire!
Sorry for the long-winded answer to your simple cap question! =;^)
Safe coilin' to ya!
-- Bert --