RE: APPROPRIATE CAP...?
Subject: RE: APPROPRIATE CAP...?
From: richard.quick-at-slug-dot-org (Richard Quick)
Date: Wed, 1 Feb 1995 23:19:00 GMT
>Received: from uustar.starnet-dot-net by csn-dot-net with SMTP id AA03159 (5.65c/IDA-1.4.4 for <tesla-at-grendel.objinc-dot-com>); Wed, 1 Feb 1995 17:58:29 -0700
* Carbons Sent to: klineda-at-univscvm.csd.scarolina.edu
> From: Dan <KLINEDA-at-UNIVSCVM.CSD.SCAROLINA.EDU>
> Subject: Re: APPROPRIATE CAP...?
> To: Nikola Tesla aka Chip Atkinson <tesla-at-grendel.objinc-dot-com>
Dan> Thanks for the great reply!
I try, though I have posted a couple of second rate replies this
week which I was later set straight on |-)
Your deductions about capacitor Q, peak power, secondary spark color,
and nitrogen ionization states was right on the money BTW.
RQ> If the coil had been properly capped, with no holes drilled
RQ> and the wire never entering into the coil form, it would have
RQ> held up without breaking down or burning up.
Dan> You are correct! The coil DID have holes drilled in it, and the
Dan> wire DID go inside the form! Can you explain to me the reasoning
Dan> behind not doing that? *Every coil* I have ever built eventually
Dan> burned. I always thought it was just poor materials, too much
Dan> power or something. What is it?
When you drill holes, or introduce wire inside the coil form you com-
promise the electrical (or more properly, the dielectric breakdown)
strength of the coil. Now if you have really poor efficienies, as most
new coilers do, it won't matter. But after the beginner learns to tune
and couple properly; the spark will find it easier to arc score the
inside of the coil form to reach a low potential, than to exceed 1/2 -
3/4 of the winding length to terminate in the air. The inside of the
coil becomes the path of least resistance. At first I tried to baffle
the inside of the coil, but after trapped solvent vapor caused a newly
constructed coil to explode rather seriously (I caught my 20 x 5 com-
mercial toriod on the way to the floor while the tank circuit was still
firing) I went for outside help. Richard Hull told me: "never, never,
NEVER drill holes or introduce the wire in the secondary coil form".
When I flipped the switch and threw juice into the very next coil I
built after receiving this advice, a giant spark reached out and
punctured a florescent tube 60 inches away from the discharge term-
inal (I was using a cylinder spark gap). Every florescent tube in
the entire shop lit up, and that particular fixure suffered a ballast
failure shortly afterwards... I never looked back. BTW, Richard Hull
also said that when you get to the point that the "classic" coil con-
struction does not hold up, you have graduated to advanced coiling.
Congratulations are in order.
Dan> A couple more questions...:)
Dan> I see a post now and then about using a variac to tune-up at lower
Dan> voltages. Could you please explain the necessity and the procedure?
Well a variac (as I am sure you know, but others may not) is simply
a variable transformer. It is placed in the low voltage supply circuit
before the HV step-up xfmr that charges the tank circuit capacitors.
Generally speaking; you get longer component life (all components),
smoother (and all around safer) coil operation; when you can fire a
coil at low voltage and gradually increase the power. This goes for
the coil that is in perfect "peaked" tune, but even more so for the
coil that has been recently completed, altered or adjusted. As input
power levels increase, so does the need for primary voltage control.
If you throw the works into a coil that is newly completed (and/or out
of tune) and your input power is over 1.5 kVA, grit your teeth. There
is a good chance that something could fail "in a hurry".
Dan> On the small coil that burned, (up above), I had two different
Dan> sized toroids. (No, not at the same time. :) The smaller toroid
Dan> put out sparks continuously These were about 12 inches long. The
Dan> larger toroid put out single discharges repetitively, one following
Dan> another. These were the 16 inch, double forked discharges. Any
Dan> reasons for the difference? The long sparks were trying to get to
Dan> the spark-gap and/or the capacitor bank. Any way to avoid this?
Dan> Thanks a lot! Dan <klineda-at-univscvm.csd.scarolina.edu>
First, it is OK to stack a couple (or more) toroids directly atop
one another, or even separated or raised with stovepipe sections,
salad bowls etc.. I do this kind of stuff all the time when I am
A well tuned coil can easily overpower a small discharger or toriod.
When the toriod is overpowered, many spark discharges (streamers) will
break out at the same time. Now assume for a moment that there is no
increase in input power; toroid changes, coupling adjustments and
retuning are allowed. As the toriod gets larger, the number of simul-
taneous streamers decrease, as the length and power of the streamers
increase. Also, as the toriod gets larger, the coupling can be tight-
ened considerably. The longest possible spark is obtained when the
toriod is increased in size to the point that only one streamer is
issuing out. At this point all of the 1/4 wave system energy is
apparent in this one spark. Even with no increase in input power;
the tightened coupling, and increased efficiency of the well grounded
coil with large toriod is such that 50% longer sparks are easily
obtained. The current peak delivered in these longer sparks will
be impressive. Mark Graalman will tell you this is a function of
the increased capacitance (or isotropic capacitance) of the larger
discharger allowing more current flow through the coil.
To prevent these sparks from striking essential tank circuit
components: proper toriod height adjustments and some strike
shielding will be necessary. I posted a two or three page paper
about this a couple weeks ago. To recap briefly: as the toroid
get larger, it may be safely raised up. This helps prevent down-
ward strikes. Properly grounded strike shields (hardware cloth,
metal pipe "rails", or scraps of sheet metal with connections to
system ground) may be, and should be, used to protect trouble
spots from secondary strikes. As power levels increase it is
important to prevent direct high current strikes to the primary.
... If all else fails... Throw another megavolt across it!
___ Blue Wave/QWK v2.12