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Re: They do self-heal...



   True, the so called "self-healing" caps can take a beating and some
serious over-voltage before they give up, unlike any other cap in which
when you go much above the rated voltage it just arcs over or the
dielectric suffers punch trough and the capacitor dies. I have been
conducting my own (modest) experiments in the MMC field and would like
to share them with you all:
 I tested 5 capacitors with my recently acquired HVDC Laser power
supply (a 38-kilogram, half a meter wide, and some 30 cm tall and 30cm
deep stabilised DC power supply with kV and mA meters). The metering in
the power supply is very accurate, as is the stabilising function (It
can actually arc continuously at 7000V 160mA DC!).
 All the five capacitors I tested showed similar results. The
capacitors tested were all WIMA MKS 4 (K4), rated for 1uF at 400V_.
They are metallized polypropylene dielectric and they are encapsulated
in epoxy filled 3X1cm rectangles.
 Onto the results:
 Up to about 800volts nothing happens. In the 900 - 1000Volts range
there were a few (3 at most) little snaps, indicating that the
dielectric had punched trough and the arc blasted the thin metal layer
on the other side. Going to 1.2kV gave a few more snaps. Up to than the
capacitance hadn't changed significantly and the capacitors still
worked well. Than, turning the variac up to 1500V caused the capacitor
to emit a quick sequence of snaps (the dielectric was punching trough
all over) and than it shorted out. The cap that survived the longest
died at 1.66kV. The one that lasted the shortest went at 1480V.
 What does it mean? Well, obviously one cannot generalise from just 5 I
tested (well, they cost 2 bucks each at the rip-off place I get them
from so I can't just waste a couple hundred and do statistical analysis
on the batch), but I would be comfortable running those caps up to 1kV
per capacitor, so these are definitely an MMC cap candidate (anyone
ever tried them?).
 The capacitors are very well built and perform well. They are, in
fact, what I use for my (under development) 5000W double car ignition
coil driver. I tried using motor run caps (Mylar film dielectric?) but
performance was much worst.
 Now, one point that I feel I have to bring up is that the
micrometre-thin metal layer over the dielectric film is just thick
enough that a punch trough at a certain voltage will vaporise enough
metal around the punchtrough to prevent further arcing. This occurs at
0.00something Joules for my caps. Therefore, what would happen if I was
to string a bunch of them in parallel and overvolt them? Sure enough,
the metal layer would be blown away enough to considerably lower
capacitance, and, probably destroy the capacitor internally. Has anyone
experienced such problems with self-healing capacitors? It would
definitely be something to watch out for in higher power coils! Also, I
wonder if this very thin film doesnít considerably increase the
capacitorís ESR.


 Sam Barros.
Soon moving to Sao Paulo, Brazil.
===
 Sam Barros,
sambarros-at-yahoo-dot-com
ICQ#:15156975

 "The Less You Know, The Better You Sleep"...
 "Evolution Stops When Stupidity Is No Longer Fatal"
 "If At First You Don't Suceed, Increase The Amperage"
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