Re: More details for diagnosing coil problems. (fwd)

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Original poster: List moderator <mod1@xxxxxxxxxx>



---------- Forwarded message ----------
Date: Tue, 12 Dec 2006 18:43:49 -0800
From: Barton B. Anderson <bartb@xxxxxxxxxxxxxxxx>
To: Tesla list <tesla@xxxxxxxxxx>
Subject: Re: More details for diagnosing coil problems. (fwd)

Hi John,

Thanks for your update. Right off the top, I can tell you that the NST 
is very small power. That in itself is major problem with getting longer 
sparks. Small coils are really sensitive to height above ground or when 
placing on a metal plate such as the metal table you mentioned. The coil 
will always need to be retuned when changing height or say throwing a 
counterpoise under the coil. For example, if your coil "is" tuned well, 
and you decided to throw a counterpoise under it, you would immediately 
find that spark length would decrease. Retuning is required. The coils 
are very sensitive to external objects. The large the coil is, the less 
the ground height affects the coil.

Regarding the topload ball. You'll probably want to position it directly 
on top of the coil. If you don't, you will experience corona off the top 
of the secondary. The topload should "shield" the secondary from corona 
and possible arcing. A ball is the "last topload" that needs to have 
significant space between the coil and topload. Due to the spherical 
shape and dependent on the topload size, the topload may or may not 
sufficiently shield the top of the secondary. For toroid shapes, the 
shielding is far less a problem. The toroid is basically a horizontal 
plate with a ring of some size at it's outer edge. The horizontal flat 
plate area performs the shielding with ease, so the topload can be 
placed higher above the secondary without the corona problems. A sphere 
is just geometrically difficult unless the sphere is significant in size.

The coil will arc to air or to a grounded object. If the power is so low 
that a nice long arc is just rare or non-existent, then a grounded 
object near the topload can be used.

If you want to increase spark length significantly, upgrade the NST. Try 
to find something in the 12kV to 15kV range and between 30mA and 60mA. 
This is really what the coil needs (and lower the top terminal). When 
the top terminal is lowered, remember to retune.

Take care,
Bart

Tesla list 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
>
>******************************
>John Welch
>
>Cabrillo College Physics Dept.
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