[sdiy] capacitor voltage ratings

[Roy J. Tellason]

On Friday 20 May 2005 08:06 pm, John Henson wrote:
> To an extent this is true.
> You can (and probably should) replace 16V caps with 25V ones if your
> voltage rails are + & - 12V or 15V.

I think that anybody who uses 16V caps with a 15V supply is really asking for 
trouble!  Even with a 12V rail it's still not that much margin...

> The downside is that the capacitance of electrolytics is dependent on the
> voltage across them.

Why is that a downside?

> If memory serves, electrolytics only come close to their specified
> capacitance if the voltage across them is within 75 or 80% of their
> specified voltage.

Yeah,  but what your memory isn't telling you is this:  Electrolytics get up 
to being able to deal with any given voltage by a process called "forming", 
in which the actual dielectric (not the electrolyte,  but an oxide layer on 
one of the plates) gets thicker over time with applied voltage.  The thicker 
it gets the _lower_ the capacitance gets.  So if you use a cap at 
significantly less than its rated voltage,  you end up with a thinner layer 
and therefore _higher_ capacitance.  I don't see how this would be a 
downside.  :-)

> Thus, space considerations aside, putting 100V or higher Caps across synth
> type power supplies is largely self defeating.

Space considerations would probably be a determining factor,  all right.  But 
then,  I think that most of what folks are building here is going to be 
limited by the amount of stuff one wants on the front panel,  and that the 
innards are all going to be fairly compact in comparison.