Any personal experiences with epoxies that you found to be harder than
I cannot suggest a harder epoxy but you pressed a sensitive button on
me
so here I go.
Many years ago (1991, to be precise) I opted to epoxy-fill my first
5kV
coils I did for my then employer in Cologne, Germany. I used no
multiplier,
straight flyback @ 5kV; about 1000 windings on an RM8 core, winding
and insulation layers being an art of their own. I located some very
liquid
epoxy meant for that purpose, then the whole module went filled, using
vacuum to make sure there were no cavities. Everything worked fine,
the filling
was perfect - I got asked how come the space between the *windinds*
was
not filled (0.05 wire, mylar foil between each layer) by my then
employer...
Some years later I had founded TGI in Bulgaria and did the first HV
source
making the coil more or less the same. However, I used off-the-shelf
epoxy;
it did not get as hard (although it was by far not as liquid before
hardening),
and after some warmup it began to conduct just enough to make the
thing
noisy... I wasted more than one coil (wound with a lot of work) until
I
got what was going on, I spent days if not weeks on that nightmare...
Eventually I learned I needed no filling at all, just a few drops of
melted
silicon at the right spots did the job (still does) quite well.
And on another occasion I had a guy from a detector repairshop in
Sofia
use the same effect trying to cheat on me... The HPGe gamma detectors
are very sensitive things, the front FET is cooled to -90C or so for
lowest
noise. The bias is a few kV (3.5 in that case), and the HV input is
filtered
through a 1Gohm/0.47uF group. Well, he had had the detector in his
hands
to "check it" for me and had put a stripe of such epoxid along the
resistor
between its pins... (The resistor is a rectangle, say 20x5mm, 1mm
thick).
After some warmup - the preamp consumes not so little, they have not
changed its design for >20 years - the detector begins to behave like
when it needs repair. Well he did not get it for repair because I
looked
and discovered what he had done and cleaned the mess up -and the
detector
worked fine. A few years later he got the same detector in his hands
directly from customers and did the same, this time he had added a
stripe
across the capacitor, though, and had scratched the paint of the
resistor between the pins. Mind you, I had told him I knew what he had
done the first time and he had done it again. I guess the epoxy must
have
had braindamaging effect as well.... (and I had refused to believe
other
people telling me he was sabotaging detectors before I got burned, the
epoxy must have worked on my brain as well - perhaps while dealing
with my coils...
.
Dimiter
