[R-390] Info on Capacitors

[2002tii]

Dave wrote:

>I read what I could find easily on self-healing, but the difficulty I'm
>having is this: These things are sold and used in commercial gear where
>failure to meet specs leads to liability lawsuits in a very large hurry.  If
>they aren't suitable at more than a fraction of the rated DC working
>voltage, how do the cap mfrs. stay in business?

Different applications can tolerate carrying the occasional (or not 
so occasional) clearing current more gracefully than others.  It's 
not a matter of not meeting spec -- manufacturers are very up-front 
about self-healing, and the applications in which careful designers 
use metallized film capacitors are ones that are able to withstand 
the abuse.  (Not so careful designers just use them because they are 
cheaper and smaller.  I've seen more than a few military and 
commercial designs that had serious reliability problems right from 
the start due to the unfortunate selection of metallized film capacitors.)

"Self healing" is a real case study in marketing -- turning a 
disadvantage (heck, call it what it is -- a recurring failure mode) 
into a "feature."  Hey, look -- our capacitors FIX 
THEMSELVES!!  Yeah, great.  Better to just buy capacitors that don't 
have a recurring failure mode in the first place.

Best regards,

Don


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