Re: [TCML] how much does secondary diameter affect efficiency?

[Gary Lau <glau1024@xxxxxxxxx>]

Hi James,

My experience has been, without exception, that bigger is better.  This is
for both top load and secondary size.  My 15/60 coil uses a 6" secondary,
and I think if you were to use a 4" secondary, you would very quickly have
problems with top load to primary arcs, unless you had a very large
secondary height-to-diameter ratio.  While that goes against conventional
wisdom, I'm not sure that anyone has ever cited a definitive reason that is
bad.  But I'd stick with the 5:1 or less ratio anyway.

Having the larger 6" diameter forces you to have a higher secondary
inductance, and to use the same primary cap, forces you to use a comparably
higher primary inductance.  A higher primary inductance results in lower
primary current, and that results in lower spark gap losses.  The higher
inductances results in lower operating frequencies, and that results in
lower skin effect losses in the primary.

Using my very small 4/20 NST originally used a 1.9" x 9.5" diameter
secondary and a 3" x 6" top load, Fres=500KHz.  I up-sized the secondary to
4.2" x 9.97" with a 3.5" x 10.75" top load, lowering Fres to 221KHz.
Performance increased considerably, both in going to the larger top load,
and further still with the larger secondary (a 2-step upgrade).

I found a cut-off piece of 6" PVC at a construction site.  That's probably
the best way to go - just be sure to ask if it's an active construction
site.  Mine had been abandoned for many years.

Regards, Gary

On Tue, Feb 28, 2012 at 4:48 PM, James Hutton
<b-u-r-t-o-n-boy@xxxxxxxxxxx>wrote:

>
> Hello, I'm about to begin building a 15kv 120ma coil.I read somewhere that
> with powers over 1.5kw, you should be using atleast 6inch PVC.I have lot's
> of spare 4inch PVC, but I can't seem to find any 6inch at any local
> hardware stores. (except for 10ft long 6inch sewage pipes for $70)So I'm
> just wondering if it is absolutely necessary to go bigger than 4 inch when
> using 1.8kw, and how much it would affect the efficiency of the coil if I
> use 4 inch instead of 6.(also, If I do use 4 inch, how long should my
> windings be?)
> thanks!
>
> -James Hutton
> _______________________________________________
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