[R-390] Paper Capacitor Replacement II

[Randy and Sherry Guttery]

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...

-- 
randy guttery

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