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Re: More details for diagnosing coil problems. (fwd)
Original poster: List moderator <mod1@xxxxxxxxxx>
---------- Forwarded message ----------
Date: Tue, 12 Dec 2006 19:18:22 -0800 (PST)
From: G Hunter <dogbrain_39560@xxxxxxxxx>
To: Tesla list <tesla@xxxxxxxxxx>
Subject: Re: More details for diagnosing coil problems. (fwd)
I think the ball terminal should be very close to the
top of the secondary in order to supress top turn
corona. The sphere lacks the shielding properties of
the toroid, so best placement is probably with the
bottom of the ball nearly touching the uppermost
secondary turn. The goal is to get the very top
windings of the secondary in the "shadow" of your top
load. Besides, at 250VA input power, primary strikes
are not really much of a concern. Place a breakout
point (thumbtack, etc.) on the ball terminal, dim the
lights, and try every primary turn until you get best
spark. Fine tune by moving around on the best primary
turn. The "sweep spot" can be pretty small with some
systems. On my 6" coil, the sweet spot was only 1/3
Multi-gaps are low-Q and can be pretty lossy. Since
you're only playing with 250VA, a low loss, high-Q
single static gap may work better. A couple of
spherical brass drawer pulls set on L brackets comes
to mind. A single gap also lends itself to easy
adjustment. For longer runs, blow a muffin fan over
Good luck with it.
--- Tesla list <tesla@xxxxxxxxxx> wrote:
> Original poster: List moderator <mod1@xxxxxxxxxx>
> ---------- Forwarded message ----------
> Date: Tue, 12 Dec 2006 15:14:45 -0800
> From: john welch <jowelch@xxxxxxxxxxxx>
> To: Tesla list <tesla@xxxxxxxxxx>
> Subject: More details for diagnosing coil problems.
> Thanks to all the helpful people who have responded
> to my message. As
> requested, here are some more details on my coil:
> Again, the problem is that all I get are small (4
> in) bluish arcs coming
> from the top of the secondary.
> Primary Circuit:
> NST: 7500V, 250 VA - 30 mA
> Two .005uF Pulse Discharge CAps (rated at 10,000V)
> in parallel to get .01uF
> 7 inch radius flat coil made of 1/4 inch copper
> tubing, - 6 turns
> spark gap made of 7 pieces of 1/2 inch copper pipe.
> .09 inch total gap.
> RF line filter and 22.5 uF power factor capacitor
> Secondary Circuit:
> Coil: 3.5 in diam, 17 in height, 630 turns of 22 ga
> magnet wire. wound
> around pvc, not shellacqed
> 7in diam very smooth aluminum ball
> 4 in between top of coil and bottom of ball
> I measured the resonant frequency of both circuits
> with a scope to be
> about 460 kHz.
> I have heard that the ball needs to be well
> separated from the top of
> the secondary coil so as to not get arcs to the
> secondary coil, and
> someone here suggested that the ball needs to be
> close to the top of the
> coil, so I am confused and would love clarification
> on that point. One
> of the students suggested lowering the ball just on
> a hunch and I said I
> thought it should be raised.
> Another question I'm unclear on - Do I need to
> provide a grounded
> electrode for the coil to arc to, or should I expect
> to see streamers
> just going into free air? I thought the latter was
> the case, but am not
> quite sure. When I provide a grounded electrode, it
> will arc to it for
> about 4 inches. Without electrode, I get about four
> 4 inch bluish arcs
> coming from top of coil.
> For grounding I tried just connecting to a big metal
> table below the TC.
> Seems I get better results with the outlet ground. I
> know sticking a
> loose wire in the outlet is funky, but I'm just
> doing it to test the
> coil. When it works and I understand it, I'm going
> to refurbish it all
> in a more permanent fashion.
> Thanks again for your help.
> John Welch
> Cabrillo College Physics Dept.
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