Subject: PRIMARY COIL
From: richard.quick-at-slug-dot-org (Richard Quick)
Date: Sat, 11 Mar 1995 22:25:00 GMT
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Quoting Mark Conway:
MC> Hi Richard,
MC> I've just had a mammoth session cutting out all the bits of
MC> plastic for my flat plate cap - next weekend I should be
MC> able to add the foil and hopefully complete the cap.
Flat stack plate capacitors are fairly easy to make... but they
are extremely tedious, with hours upon hours of time required for
a good construction. I am sure you have already found this out.
The advantage though is that these caps are the best design for
high performance Tesla coils. Given the lack of thick plastic
sheet there in New Zealand, you really don't have much of a
MC> After hearing of Ed Sonderman's success with his coil I
MC> really want to finish off my one. I just hope that my main
MC> cap will hold up - I am trying to keep everything really
MC> clean during the construction and I figure that it should
MC> theoretically be able to withstand up to 26000 volts. If it
MC> doesn't stand up to the high voltage its going to be really
A spot of good news for you perhaps... Ed Harris posted a polymer
reference which stated that the thinner the dielectric, the
higher the breakdown strength per mil! This may be yet another
reason that the flat plate caps work so well.
MC> After the cap all I will need to complete are the
MC> suppression caps and the primary coil. I have the soft
MC> copper tubing for the coil. I notice from your postings that
MC> you recommend making a "comb" out of plexiglas for the
MC> copper tubing to sit above the wooden support. I see in the
MC> photo from Richard Hulls guide to the Colorado notes that
MC> Richard Hull also uses this method but I remember that in
MC> your video, the primary coil you use is on wooden supports
MC> which you say were heavily dried and polyurethaned. Is using
MC> such a primary coil almost as good as using a high Q plastic
MC> support? The reason that I ask is that I will be able to
MC> make the primary coil support a bit faster if I can just use
MC> wood alone.
Plastic supports for the primary coil are preferable: but you can
"kiln" dry the pre-cut wood parts in an oven set to low temper-
ature; let the parts dry for 24 hours, and them assemble the
dried parts with a water free two-part epoxy. Once assembled,
the coil form will then require about five or six heavy coats
of polyurethane to prevent moisture reabsorbtion.
The reason I state this (and have done it for a couple of my
older primary coil forms) is that the primary conductor is
generally not in intimate contact with large areas of the coil
form. Certainly the construction is very different from a
secondary coil where plastic forms are a must.
High tank circuit Q's and sharper tuning will be achieved with
all plastic coil forms. Remember, even PVC is better than wood in
the long run. My large primary is a combination of wood, with
plastic "comb" supports, and then later I decided it is worth-
while to spend the money (and time) to build primary coil forms
using all plastic parts. The main reason is that you cannot come
close to removing all of the moisture from wood.
... If all else fails... Throw another megavolt across it!