More details for diagnosing coil problems. (fwd)

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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.