Re: gEDA-user: bypass caps

[John Doty <jpd@xxxxxxxxxxxxx>]

On Aug 21, 2006, at 1:26 PM, Stuart Brorson wrote:

> > Also remember that NASA recommendations tend to be super  
> > conservative.
>
> I would normally agree, except that NASA's 60% rating suggests using a
> 10V cap to hold off 6V, which seems too low to me.  A 2X derating  
> would suggest at least 12V to stand off 6V, whereas my 3X derating
> guideline says at least 18V rated caps.  I don't know why NASA
> suggests only 60% derating.

Remember that chips are easier to kill with overvoltage than caps,  
and standard operating voltages for chips are usually >60% of maximum  
ratings. A lot depends on what you're doing with it though. Capacitor  
voltage ratings for AC and high current pulse ratings are often much  
lower than DC. But a bypass cap tends to lead a rather sheltered life.

> > Also don't derate *too* much: I've gotten my wrist slapped by a  
> > NASA R&QA guy for using 50V ceramic bypasses in 5V circuits.
>
> 10X derating is waaaay over the top, don't you think?  But if the caps
> come in the same footprint as 15V caps, say, then why not?

Well, that was my reasoning. But the NASA R&QA guy didn't like it  
because of this self-healing theory...

John Doty              Noqsi Aerospace, Ltd.
jpd@xxxxxxxxxxxxx




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