[ARC5] Disc Capacitors

[Brian Clarke]

Hello Wayne,

The formula for capacitance is C = kA/t, where k is the dielectric 
coefficient of the insulation material between the plates, A is the common 
area of the plates and t is the distance between the plates. So, the 
capacitance is approximately proportional to the diameter squared. The 
Voltage rating is determined by the dielectric strength of the particular 
ceramic chosen and the thickness of the ceramic wafers between the plates.

The manufacturer will then dip the finished capacitor is some epoxy or other 
ceramic, which will add to the thickness and the diameter.

The easiest way to test Voltage rating is to use an ionisation tester to 
discover that minimum Voltage at which breakdown occurs; then for safety, 
run at no more than say half the breakdown Voltage.

Unless you have access to a tomography suite and a chemical analysis lab, 
there is no easy way to answer your question.

73 de Brian, VK2GCE.

On Sunday, March 03, 2013 8:37 AM, Wayne said:


>I have some .01 mfd caps that are not marked as to max voltage capability. 
>They are pretty good size, at least a half inch in diameter, maybe more.  I 
>also have a bunch of .01 mfd caps that are marked "50V" and are no more 
>than 3/16 inch in diameter.   I need some that are good for at least 500V.
>
> Does anyone know a good rule of thumb for diameter versus voltage relative 
> to disk caps?  Bigger means higher voltage; that follows logically, since 
> you need more insulation, but what kind of a estimate can we make based on 
> size?
>
> Thanks.
>
> Wayne