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Re: [TCML] 6" Tesla coil plexiglass secondary - FOR SALE

Hi Don, Dave and all,
Although PVC's dielectric loss is higher than Plexiglas (PMMA), when used as a coil form the differences appear to be negligible. High-Q secondaries can be constructed using either material.
However, there IS a significant difference when PVC is used in the presence of corona. PVC easily forms conductive carbonized tracks (that look similar to Lichtenberg figures!) when the surface is subjected to any corona or surface arcing. The PVC breaks down, forming a semiconducting layer of chlorine ions on the surface of the PVC that leads to cumulative degradation and carbonized tracks. On the other hand, PMMA is very resistant to tracking, and is often preferred for high voltage designs for this reason.
I suspect that most coilers won't see problems with PVC as long as they imbed the secondary winding underneath one or more layers of polyurethane, polyester, or epoxy and properly shielding the top of the winding with a suitable topload. The higher dielectric constant and insulating properties of any of these coatings helps to prevent corona that might otherwise form in high E-field regions. In particular, using a high dielectric constant sealant will eliminate high E-field air-wire-PVC "triple points", preventing corona from forming and extending the life of the PVC coil form. 

Bert
--
Bert Hickman
Stoneridge Engineering
http://www.capturedlightning.com
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World's source for "Captured Lightning" Lichtenberg Figure sculptures,
magnetically "shrunken" coins, and scarce/out of print technical books
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dfroula@xxxxxxxxxxxxx wrote:

David,

At the time I got into TC work, there were enough cautionary notes
about PVC secondaries that I just decided to bypass the issue and go
with the plexiglass. The plexi secondaries have always worked quite
well for me, not to say others have not gotten perfectly acceptable
operation from PVC.

The one 6" PVC TC secondary I made was a nightmare to wind. I mounted
it on a lathe, but could never get it centered correctly, as the PVC
tube was so irregular in shape. The plexiglass extruded tubes I
bought from Midland Plastics were much more dimensionally consistent
and easily centered for winding.

Electrically, I have built VLF receiving loops by winding wire on
untreated pine, even on mounted doors with nails on each corner. They
worked fine, although receiving antennas are quite another thing from
a high-power Tesla coil.

I was never able to find much quantitative documentary evidence on
the benefits of one over the other. I just played it safe. In the
end, it seemed to end up being a "religious" issue.

It does look nice, though! I recall one coiler who wanted to fill his
plexiglass secondary with neon. I wonder how that worked out!!! ;-)

Best,

Don

--- On Wed, 1/16/13, drieben <drieben@xxxxxxx> wrote:


From: drieben <drieben@xxxxxxx> Subject: Re: [TCML] 6" Tesla coil
plexiglass secondary - FOR SALE To: "Tesla Coil Mailing List"
<tesla@xxxxxxxxxx> Date: Wednesday, January 16, 2013, 9:57 AM Hi
Don and Jon,

I'm thinking the "truth" here may lie somewhere in between your two
opposing viewpoints ;^) Don, although your position is most likely
correct in re- gards to the relatively high radio frequnecies
(10's or 100's of MHz, or more), I believe Jon's position is
probably more relavent for the relatively very low frequencies of
TC operation -10's or low 100's of KHz (that's 3 orders of
magnitude lower!) and you would be hardpressed to actually notice a
difference in the output of a PVC vs. a plexiglas formed secondary
coil without some pret- ty fancy instrumentation. My Green Monster
10 kVA pole pig driven, ARSG coil has a 12" gray PVC duct pipe
secondary form and it really rocks and I've never been able to
detect notable warmth of the form above ambient temperature after
running it. Of course, with the massive 12x56 topload, I'm running
around the high 50's KHz, which is ex- tremely LOW frequency
compared to the typical modern radio or television communication
frequen- cies, with a possible exception of AM bands - from the mid
500 KHz to 1.6 MHz range. I'm thinking that the SG losses as well
as the internal dielectric losses of the primary capacitor are far
more con- sequential, as this is the main place that one tends to
noticed heat significantly above ambient tempe- ratures. And if you
have a good, low loss dielectric capacitor, you won't notice
significant heating here, either.

BTW, "I" have experienced the electret effect with PVC formed
secondaries and I believe others in this forum have mentioned it in
the past as well.

David

----- Original Message ----- From: <dfroula@xxxxxxxxxxxxx> To:
"Tesla Coil Mailing List" <tesla@xxxxxxxxxx> Sent: Wednesday,
January 16, 2013 8:35 AM Subject: Re: [TCML] 6" Tesla coil
plexiglass secondary - FOR SALE



One interesting effect of using a plexiglass form for

the secondary is that the plexiglass seems hold a charge after the
coil is shut off (depending where in the charging cycle the coil is
shut off). The prevalent theory is that the plexiglass exhibits an
"electret" effect where the polymer structure of the material will
hold an electrical charge for some time. PVC forms I have used do
not seem to exhibit the effect.


A true electret is a sort of "permanent magnet", except

that the material retains a permanent electrical, rather than
magnetic charge. True electret polymers are used to construct
electret microphones, tha depend on the charge to produce an output
signal.


I have gotten minor zaps from a plexiglass secondary

hours after the coil has been turned off.


Don

--- On Tue, 1/15/13, Jon Danniken <danniken@xxxxx>

wrote:



From: Jon Danniken <danniken@xxxxx> Subject: Re: [TCML] 6"
Tesla coil plexiglass

secondary - FOR SALE

To: "Tesla Coil Mailing List" <tesla@xxxxxxxxxx> Date: Tuesday,
January 15, 2013, 8:42 PM On 01/15/2013 11:53 AM,
dfroula@xxxxxxxxxxxxx wrote:


Wound on low-loss plexiglass form. Far

superior to

lossy PVC coil form!

I don't think the difference in RF losses between

a

"low-loss plexiglass form" and a PVC form are even noticeable
in a tesla coil.

Is this just advertising hype, or do have actual

data from

real-world operation showing "far superior"

performance with

a "low-loss plexiglass" form?

Also, is this a special "low-loss" version of

plexiglass,

because I have yet to come across any type of

plexiglass

that specifies lower RF losses in any of the

plastics

dealers I have come across.

Thanks,

Jon

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