Re: Started winding today....

Tesla List wrote:
> Original Poster: David Trimmell <davidt-at-pond-dot-net>
> Hello,  I must say that I have lost several rolled polly caps :>(   I once
> used (3) layers of 30 mill polly for one that I expected to work -at-15KVAC,
> But My Resonator chowed on it for lunch. After rebuilding it I have derated
> it to 9KVRMS!  At that level It seams quite reliable (yes, this was using a
> very respected transformer oil, Shell diala). Not to be a pessimist, but
> Tesla Coils will push beyond the envelope of rated values! So I would
> suggest keeping DC breakdown values an order of magnitude from your
> expected operation point. That is a significant exaggeration, but where do
> you draw the line of reliability? I would suggest 90 mills of polly for
> 10KV, Minimum. It can be done at even 40 or 60 mills, but only with
> commercially available equipment-materials, IMHO.
> Regards,
> David Trimmell


The real secret is in splitting up the voltage stress among
equally-sized capacitors connected in series. These caps fail when oil
breakdown is induced at the very edges of the capacitor plates - this is
where the e-fields are the strongest. These stresses are sufficient to
induce ionization breakdown of the mineral oil itself, even though your
poly thickness may be quite substantial. Under the applied stress, the
oil will self-generate gaseous breakdown products (typically hydrogen
and methane gas) which promote further ionization, and the destructive
UV and ionization byproducts quickly attack the LDPE dielectric. 

Using equally sized series-connected caps divides the voltage stress and
reduces the localized e-fields below the point where the oil breaks
down. Commercial capacitor manufacturers understand this principle quite
well, and they will typically construct their AC and pulse caps so that
each individual capacitor section sees no more than 3-5kVRMS. And each
individual capacitor must use at least 2 layers of dielectric to prevent
a single defect from breaking down the dielectric layer. If you design
your caps so that each section "sees" no more than 5 kVRMS and use
multiple layers of thinner material in each capacitor module, you should
be able to run with 15 kVRMS on 80-90 mils of total LDPE with no

Safe coilin' to you!

-- Bert --