[R-390] Paper Capacitor Replacement II

[Barry]

That's just the level I was looking for.

I'm familiar with the construction of both types and knew that the ceramics were physically smaller, but I was unaware that the wound layers of a poly or paper cap introduced significant inductance or resistance - at least enough to be a factor at HF frequencies.

Thanks!

Barry - N4BUQ


 On Fri 06/01/12  1:54 PM , Randy and Sherry Guttery comcents at bellsouth.net sent:
> 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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