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Re: [TCML] 8" Secondary Woes

To the probable chagrine of some other list members, I am going to suggest that you could go ahead 
with the 9" dia. former and simply tie the bottom of your secondary to the innermost turn of your pri-
mary, effectively making them at the same RF ground potential. This will eliminate the tendency for
primary/secondary flashovers due to low clearance. Now it won't prevent the racing sparks on the
secondary surface if the secondary is too tightly coupled to the primary. This can only be addressed 
by proper elevation of secondary in relation to the plane of the primary spiral.
Now on to why I suggest simply tying the bottom of the secondary to the inside of the primary (auto-
transformer configuration). I'm sure I will get flamed for suggesting this because if the primary is tied 
to the RF ground and secondary and a spark happens to hit the primary while simultaneously striking 
a bystander, then the unlucky bystander will effectively be tied to the HV 50 or 60 hz AC in the primary 
circuit. Well, first off, I personally have no plans to place myself within "striking distance" of the output 
spark of a decent sized coil, anyway - RF grounded primary or not! I think we all can agree that placing 
one's self into the path of the output sparks of a medium to large Tesla coil without special training and 
equipment is NEVER a good idea!
Also, from personal experience with my Green Monster pole pig, RSG driven coil, common
RF grounding my secondary with my primary coil seems to prevent most downstream kickback
flashover headaches in my control panel circuitry. I've tried it both ways and when I ran it with
the primary coil "floating", I experienced terrible downstream flashovers in my control panel, in
spite of significant EMI filtering. Once I reconnected the inner primary turn to the common RF
ground with the bottom of the secondary, this problem disappeared. And I've been running my
big coil in this fashion for nearly 10 years.
David Rieben
From: Greg Peters <greg.j.peters@xxxxxxxxx>
To: Tesla Coil Mailing List <tesla@xxxxxxxxxx> 
Sent: Tuesday, September 3, 2013 9:29 AM
Subject: [TCML] 8" Secondary Woes

Hi all,

Well I've done something stupid. I wound a beautiful spiral primary with a
12" diameter inside turn on nice polycarbonate supports. I assumed that I
would be using an 8" (200mm) secondary and that 8" was a standard pipe
size. I was aiming for 2" between the primary and secondary. Never assume
anything! The nearest PVC water pipe OD is either 160mm (6") or 250mm (10").

8" OD tube is as rare as hens teeth in Australia. The only exception is 8"
acrylic tube which is difficult to cut and work with. I'm not a big fan of
acrylic, as I always seem to get my saw stuck, etc and I can see my
beautiful former being destroyed.

I have called nearly every pipe and plastics supplier in the country and I
am out of luck. No polycarbonate, styrene, PVC, polyethylene, polypropylene
or other plastic in 8" OD! I even called some of the gas pipeline suppliers
- I CAN get a length of blue 200mm LDPE pipe for about $500....not ideal
(expense wise).

I noticed that McMaster Carr has very cheap polycarbonate tubing in 8" OD,
and placed an order only to be informed that they will not ship to
Australia. There are a few other not so cheap US suppliers who may be able
to assist if necessary though they seem significantly more expensive.

Anyway, the closest thing I have found in Oz for a reasonable price ($200)
is a length of orange PVC electrical conduit at 225mm (9") OD.

So, my question to the list. Do I persist with trying to find an 8" tube,
possibly even from overseas, or settle on the 9" conduit and tolerate
having only 1.5" between my primary and secondary? I expect I will need to
lift the secondary quite a bit to prevent overcoupling and even flash overs
from the primary which will be running 24kV. Is anyone else running a
largeish coil with this kind of spacing?

This was meant to be a museum quality build and the idea of having to
significantly lift the secondary is frustrating, but realistically it's
looking like the only option! The coil has been painstakingly designed to
sit exactly in the middle of my shed space for maximum spark length in all
directions. Oh well, next time I will be more careful and assume nothing!


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