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More info on HB Caps (was Re: Re: Reducing Self Resonance In Rolled Caps).
Hi Ruud,
P.S: Ross O.: If your interested, the post is yours
;o)) as I haven´t seen this topic mentioned on any
of the websites and it might take me a while to
post it on mine.
Original Poster: "DaxisWeb user" <""-at-mail.daxis.nl>
snip (warning longer post)
>Hi Malcolm, Reinhard and the rest of you C^4's,
>Ok, ok, I'am convinced. But it's difficult (for me) to
>ignore the tremendous supply off plastic for free or
>very cheap. Could the extended rolled cap design
>be improved by applying silicon to reduce creep arcing
>over the plates?
Hmm, my C4 times are over ;o)) Back to the subject:
Your problem isn´t air ENTERING the cap, but rather air
being entrapped in (any) homebrew construction. The
silicon will only make matters worse, not better, esp. if it
contains acetic acid, which ionizes rather easily. To prevent
ionization, you will need to keep the volts per mil VERY low,
which makes for a somewhat bulky cap. Ionization is the
death of every cap. This internal ionization is called partial
discharges, which leads to a phenomenon called treeing,
(because it looks like a tree branch), which weakens the
plastic (it changes the molecule structure) and leads to
premature cap failure. Actually, there are two additional
types of treeing. One being thermal treeing and the other
being water treeing. Both, however, MUST be avoided
at all cost, because they send your cap to HV heaven over
a (short) period of time.
Partial discharges are local corona discharges that occur to some
extent in just about any HV isolation material. They occur because
of non uniform material (thickness), minute dirt particles, entrapped
air and so called "Lunker areas" (very, very small hollow areas).
Corona and the free radicals (O3, NOx) it produces, will split up
the long polymer chains, that the plastic is made of. This
structural damage to the plastic is called treeing. The insulation
voltage (volts/mil) within these zones is sharply reduced. It takes
a while for this to happen, but it DOES happen. As soon as the
V/mil has been "eaten" low enough, the dielectrica will be punctured,
of course, leading to cap failure. Elevated temperatures will increase
the rate of destruction. This might explain, why even commercial
PS caps (very lossy at RF frequencies) will die very quickly in HV/
RF environments. I´m not quite sure how the self healing aspects of
some commercial caps will counteract the phenomenon of partial
discharges, but no homemade PE cap has any self healing
capability at all.
Any kind of disturbance (i.e. non uniformity) in the E-Field (sharp
metal edges for example) will also increase the production of
partial discharges (->corona formation). Important to know is that
this phenomenon does NOT lead to instant failure, but rather, it
accumulates over time and then leads to failure. I believe this to
be one of the major reasons why commercial pulse caps are built
with a very low volt per mil rating (lots of series caps). And now it
should make sense why coilers like Bert Hickman (72-96 mils)
or Ed Sonderman (90 mils) suggest such "gigantic" mil numbers
for homebrew caps. Oil DOES NOT totally prevent the formation
of corona!! Of course a pulse cap always has a limited lifetime,
simply because it is a highly stressed component.
Interestingly enough, mica caps are totally insensitive to partial
discharges. However, mica caps are VERY sensitive to voltage
spikes exceeding their rated limit, so they really aren´t any
better in coiling usage :o) (In answer to a post from the other
day).
However, if you can get clear (not black or colored!!) plastic
(PE or PP only) for free, go ahead and try it. If you consider all
the work involved and that rolled caps are usually not worth (or
able) to be repaired, I still think the MMC is the way to go. ;o))
The MMC has all the bonuses you could ask for, like uniform
thickness and virgin material that has never seen a human hand
(fat, dirt inclusion, etc) which is wound tightly under constant
pressure and in a vacuum or rarified atmosphere. Something a
homebrewer will never be able to copy. As a matter of fact the
material used in commercial (MMC, too ;o}) cap is so thin, that
it would tear, if handled with your bare hands. I don´t think you
will notice a difference between a normal and an extended foil
design. Your (anyone's) homebrewing abilities are not high
enough (i.e: your cap WILL have losses) to make use of the
difference. What I want to say, is the lower ESR of an extended
foil cap will be "eaten" up by other internal losses. Secondly, the
extended foil design reduces your useable square area of the foil
by quite a bit. Further more, the volts per mil go DOWN (not up!)
as material thickness increases. In other words, let us say your
material has 500V per mil dielectric strength. A 100 mil thick
sheet will NOT have a dielectric strength of 50kV. If you are
lucky, you may be able to get away with 1/4 of that, but not much
more. Lastly, two more things: First the material thickness is
NOT constant. Whip out your micrometer and measure the plastic
in several places. The 200µ sheeting, I used, was NEVER 200µ
thick, but more like 140-160µ. Secondly, look at the plastic with
a microscope. You´d be surprised to see what kind of voids and
dirt inclusions it has right from the roll. Convinced ya ?? ;o))
Coiler greets from Germany,
Reinhard
P.S: You asked about low voltage coiling a while ago. I think I
can qualify for this. My 8" coil is presently being fed by a single
7.5kV NST modified to give me 1275VA and I am getting 63"
arcs using a KISS flat static gap, with NO additional cooling.
Peak primary current is roughly around 600A at present. If you
are interested in pictures and details, mail me off list. Remember,
it is NOT the primary (HV) voltage that powers the coil, but
rather the xformer´s capability to charge the cap, which is why
I view the HT PSU of a TC as being a current, rather than a
voltage, source. There is also no direct relationship between
coil output voltage and spark length. In other words, a coil with
a lower output voltage can indeed produce longer sparks than
a coil with a higher output voltage. The secret lies in the topload
and the coil´s ability to charge it. Once the discharge has been
initiated, your coil must be capable of suppling enough current
(not voltage) to keep the plasma channel hot and ionized.