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Re: bulkheads / baffles in secondaries

Original poster: "Scott Hanson" <huil888@xxxxxxxxxxxx>

Bill -

I put a lot of work into my secondaries with epoxy
coating, etc and I've installed internal baffles in nearly all of them. My
preference for
baffles was triggered by the destruction of a small 2-1/2" diameter
secondary due to internal arcing long ago. In my opinion, to be an effective
arc stop the baffle must be a gas-tight seal to the ID of the coil form. I
use two baffles per coilform, around 2-6" inboard of the ends of the
windings, depending on coil diameter. Any use of solvent-based adhesives
will likely
result in trapped solvent vapor between the baffles (if you use two), and
although ignition is unlikely if the baffles are properly sealed I never
wanted to take the risk or have the trapped solvent slowly attack the inside
of the coilform.. Accordingly, I use electronic-grade or "neutral cure" RTV
silicone sealant. Examples of General Electric "neutral cure" silicones are
RTV 122, RTV 128, RTV 160, RTV 162, etc.   Most or the common "household"
grade RTV sealants release acetic acid vapor as they cure, with the cure
cycle being triggered by exposure to atmospheric moisture. I don't think its
a good idea to trap acetic acid between the baffles, which is why I use the
electronic grade RTV. The electronic grade RTV sealants release alcohol
vapor during their cure cycle. There are also some automotive grade RTV
sealants that don't use acetic acid cure systems, advertised as safe for
catalytic converters, but I don't have any experience with them.

Baffles can be installed either prior to, or after winding the coilform.

First, you need to cut the baffle so it is a near-perfect fit in the bore of
the secondary coilform. If you are using PVC pipe, you will be surprised to
see how much variation there is in the bore diameter and roundness of the
bore in a given piece of pipe. A slight interference fit is preferred, as it
allows the baffle to self-fixture itself at the desired location. I use a
router mounted in a router table along with a Craftsman "circle cutting" jig
to get perfectly circular disks.Once the baffles are fabricated, mark their
desired position in the ID of the coilform. Next, starting with the UPPER
baffle, apply a heavy bead of RTV sealant in the bore of the coilform about
1/2" ABOVE the desired location of the baffle. Then, slowly press or tap the
baffle into position until the locating marks are visible. This causes the
baffle to act as a squeegee, pressing the bead of sealant ahead of it and
forming a pretty uniform fillet of sealant. Allow the RTV sealant to cure 12
hours, then apply a second bead of sealant on the top side of the baffle.
Now you've got a nice fillet of sealant on both sides of the baffle, sealing
and securing it. For the second baffle, drill a small hole in the center,
then repeat the sealant application process for both sides of the baffle.
Finally, after the rest of the sealant has cured, inject a small amount of
sealant into the center hole in the bottom baffle to complete the baffle

Since I started using internal baffles, I haven't had any secondary
experience internal arcing, even with arc length 2X the secondary length.
Its likely that optimum toroid diameter, corona rings, and other
field-shaping aids at the upper end of a secondary will help reduce any
tendency toward internal arcing, but a correctly-installed
(hermetically-sealed) baffle provides an absolute barrier against internal
arc damage.

Scott Hanson

----- Original Message ----- From: "Tesla list" <tesla@xxxxxxxxxx>
To: <tesla@xxxxxxxxxx>
Sent: Tuesday, August 15, 2006 1:33 PM
Subject: bulkheads / baffles in secondaries

Original poster: Gomez Addams <gomezaddams@xxxxxxxxx>

This is addressed to those who have experience installing internal
baffles or bulkheads in secondaries, for the purpose of preventing
internal creepage sparks (internal racing arcs):

I have never used internal baffles before, and have never seemed to
need them on any of the secondaries I've built.  (although once,
someone building secondaries for me for commercial sale at a company
I was consulting for had racing arc problems, but I soon discovered
they had decided to paint the secondary form with black paint "for
looks" before winding it!)

But now I am attempting to build one of the most robust, good- looking,
and efficient / high-performing coils I've ever built,
taking into account all the new developments which have turned up
since I last built a coil, so I thought I'd go ahead and install
baffles just for the heck of it.  I seriously doubt they'll do any
harm, and if they'll make the secondary even more "mistake-resistant"
then I'm all for it.

So my main question is: do you typically install them while building
the form, with holes in the center of each baffle for your winder
shaft (assuming you use one), or do you wind the coil first, remove
the end plates, install solid baffles, and replace the end plates, or

Since I have a large enough lathe, I _could_ wind the form without a
shaft going all the way through, but I prefer to use a shaft to
provide support to the form.

Also, I'm thinking it would be smart to leave holes for ventilation
so things like glue fumes (after construction) and later, ozone, can
escape the interior, so perhaps baffles with center openings are best?


 - Bill Lemieux