[R-390] Paper Capacitor Replacement II

[Curt]

"..basics are......pretty sound."

:)  I like it!



On 1/6/2012 2:54 PM, Randy and Sherry Guttery wrote:
> On 1/6/2012 1:28 PM, Barry wrote:
>> Can someone explain why ceramics are better in RF applications (such as bypass) than film-and-foil
> I can give you a short-form "reason"...  Ceramics are made
> by plating both sides of a ceramic wafer with conductive
> material - each of which form the plates of the capacitor.
> As ceramic is an excellent dielectric - it can remain thin
> and still have high withstand potential.  Being thin - a
> given capacitance requires much less "surface area" than a
> thicker dielectric - so the area of the plate is small -
> having little resistance. The leads attach directly - then
> the whole thing dipped in a coating to protect from
> contamination. Simple - low inductance and low resistance
>
> many poly caps are made by coating both sides of some
> flexible insulating material (which becomes the dielectric)
> with conductive material (which become the plates. The
> flexible material stacked with another insulator layer -
> then rolled into a "tube" to make it compact.  Now being in
> a coil - (rolled up) it now exhibits considerably more
> inductance than it would were it left flat. Also - having a
> large area - the resistance is also greater than a similar
> value capacitor that uses thinner dielectric (which would
> have smaller surface area).
>
> So ceramics have less inductive reactance losses at RF than
> most polys - however - polys tend to have better stability
> at audio.  Recall that ceramics use a thin wafer for a given
> capacitance - that thin wafer may distort at audio
> frequencies causing changes in capacitance. Ceramic's
> tendancy to react electrically to physical distortion is the
> principle behind ceramic phono cartridges.
>
> This is - of course -  "over simplified" - but the basics
> are pretty sound...
>