Looks like we agreed on the danger of voids inside dielectric materials for
capacitors and such. These are not the things that you pick up in the ARRL
Handbook I'll bet. Having been there, designed with Kapton* film, and then
experienced repeated failures, I learned my lesson about it. Sometimes good
old AIR is the best dielectric of all, just takes so much darn space to
make a capacitor of hundreds of pF or more.
>Date: Tue, 16 Feb 1999 11:11:36 -0700
>From: "Richard W. Ehrhorn" <firstname.lastname@example.org>
>Subject: RE: [AMPS] coil form plastics
>In a similar vein, the very common transformer insulation Nomex* (DuPont,
>nylon-based I believe) also is available mica-filled, which has better
>characteristics for some applications - (high temperature?) - but don't
>remember whether the difference is physical stability or dielectric
>strength/corona resistance or both.
Nomex* polyaramid fiber is laminated into sheets which are great insulation
for 60 Hz transformers. As a matter of fact, it is sold into that market
for class H high temperature insulation, allowing transformers to run
hotter, with less iron and copper, and cheaper. Unfortunately, this makes
them 'hot potatoes' to deal with in a chassis, sources of extra heat to
deal with. And they have poorer regulation, with smaller wire diameter,
more copper loss.
Remember the last solar storm experience in the 80's when some major power
grid failures in Canada caused blackouts in the Northeast USA? They had
some BIG transformers fail, that were already running hot, due to the
induced DC current in the long transmission lines from the geomagnetic
effects. Somewhere (IEEE I think) I read that the old heavy iron
transformers survived better, due to the headroom in their design. Better
insulation might have helped, but I think the saturated iron caused the
temperature to soar beyond reason. I am getting off the subject....
I did an experiment with Nomex* at 90 Mhz years back, with high field RF
exposure. Wanted to see if it would heat up. It is such as good insulator
(used in fireproof suits) that it didn't feel warm to the touch. But over
about 10 mintues, I heard a pop, and opened the oven. The block had blown
apart, and from the center oozed a black gunk resembling tar - much stink.
Apparently, it was trapped moisture inside which was heating up via RF
rotation of the bound water / dipole moment (H2O is great for this, in
microwave ovens). But the Nomex* insulated it, so the heat built up and
POW. It was very wierd.
Needless to say, I haven't used Nomex* in RF, but suspect it isn't very
good, being also hydroscopic. Maybe someone can look up the loss factor, to
see if it is comparable with Delrin*, nylon, plexiglass, or what.
* trademarks of E. I. DuPont de Nemours and Co.
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