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Re: bulkheads / baffles in secondaries

Original poster: "David Rieben" <drieben@xxxxxxxxxxxxxxx>

Hi Scott, all,

Up until now, I've kind of been "lurking" on this
particular thread but I suppose now I'll go ahead
and throw my hat in the ring ;^) The only time that
I ever experienced internal arcing in a coil form was
with the first Information Unlimited BTC-3 coil kit
that I ordered, nearly 20 years ago. Of course I was
an "infant" in Tesla coil knwoledge and know how in
the mid to late 1980's and didn't have the internet or
know about Harry Goldman's TCML. I didn't really
understand the concept of high voltage creepage over
the smooth surfaces of materials that are otherwise
good insulators at this time, either. Of course as time
went on and I started getting "smarter" I started instal-
ing "creepage discs" inside the secondaries of my coils
but til this day, I'm still not totally convinced that the in-
stallation of creepage disc is absolutely critical to non-failure
operation of the secondary coil as many attest. On my
most recently completed "Green Monster Tesla coil",
http://www.teslauniverse.com/members/drieben/
I simply cut (2) 1/2" thick disc from a poly cutting board
and stick each one in each end of the secondary bore
about 6" from the end. I did have a hole for the shaft
to pass through while winding and coating the coil and
I ended up siliconing a piece of plexiglass over the holes
when finished winding and coating the coil. I guess what
I'm trying to say is that I've never made it a precision ma-
chining operation as far as the disc making a perfect air-
tight fit to the inside of the coil form. I simply cut a short
piece sample of the secondary coil form and placed the
circle over the poly cutting board and draw a circle with
a magic marker. I then simply cut the circle out with a jig-
saw. It was a close enough cut that I could push the circle
into the bore the depth that I desire and then I simply apply
a bead of plain ole clear GE silicone "Window and Door"
rubber sealant (yep, the acetic acid based stuff that smells
like vinegar while curing) and just on the exterior side of the
disc. My secondary coil is but 49 1/2" long but I drive it with
a 15 kVA pig and a .1 uFd pulse cap with a 12 x 56 toroid
and get 11 to 13 ft arcs from it without ever the hint of any inter-
nal tracking or flashover, yet, anyway ;^)) If I forget to wipe
down the exterior of the secondary coil before firing the coil
I do get an occasional racing spark, which promptly reminds
me to wipe it down ;^)

I also remember reading in one of the old TCML newsletters
where Richard Hull was talking about a maggy that he built where
the "third" coil was only like 10" x 24" long without ANY internal
creepage discs and was spewing out 10 or 11 footers with no
problem. I'm not sure the third output coil of a magnifier would
fall into the same category as the secondary coil of a classic 2-
coil Tesla coil? in this situation, but I thought that this was mentioning.

David Rieben

PS - My Green Monster webpage hasn't been updated in nearly
a year, so it's pretty out-of-date, but the major coil components
(primary, secondary, capacitor, toroid) are still the same.


----- Original Message ----- From: "Tesla list" <tesla@xxxxxxxxxx>
To: <tesla@xxxxxxxxxx>
Sent: Wednesday, August 16, 2006 2:05 PM
Subject: 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
silicone
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
installation.

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.

Regards,
Scott Hanson