Re: Rolled foil caps

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Charles Hobson by way of Terry Fritz <twftesla-at-uswest-dot-net>" <charles.a.hobson-at-btinternet-dot-com>

Chris Dobson wrote:

Dear List,

Being a glutton for punishment, I like making rolled foil and poly caps. I'm
currently making a bank of nine units to try and make them last a little
longer
by applying the MMC approach to them. My question is this : when I've
finished
a cap, I put the DMM across it to get a ball-park for its value, and then
drop
it in the case and fill with oil : as the oil soaks in, the value rises by a
couple of nF, but it seems to peak out, then drop a little - - - - what's
going
on there ?  :O\

Regards

Chris

Hello Chris,

Transformer oil has a dielectric relative permittivity of about 2.4 with
respect to air.  . (ref. Table on page 22, "Variable Capacitors and
Trimmers" by G. W. A. Dummer.)  If your Swiss rolled capacitor had no air
pockets inside, you wouldn't have experienced capacitive changes as you did.
The oil  evidently replaced air pockets in your capacitor and caused the
increase in capacitance. I hope this is helpful

Chuck