Re: gEDA-user: bypass caps

[John Doty <jpd@xxxxxxxxxxxxx>]

On Aug 21, 2006, at 12:52 PM, Stuart Brorson wrote:

> I just located my copy of NASA practice no PD-ED-1201, which covers
> electronic part derating.  Table I in that doc calls out derating caps
> by 60%.  It does not distinguish between electrolytics, ceramics, or
> other types of cap.  You may be able to locate that doc by Googling
> (TM) for it.
>
> Therefore, my 3X derating is conservative.  FWIW, 3X is not my
> invention, but rather a derating I leared somewhere along the line.
> In any event,  I'd rather stick with a conservative derating than have
> to fix burned boards!

Also remember that NASA recommendations tend to be super  
conservative. 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. There is a belief that ceramics can be self healing if not  
derated *too* much. I flew the caps anyway, hundreds of them, and  
they have been fine for nearly six years in orbit. With NASA, you  
need to do a lot of research to figure out which recommendations are  
rationally based in real experience, which are 40 year old  
superstition, and which are the result of empire building by  
ambitious bureaucrats.

For tantalums, some manufacturers publish data on expected life  
versus stress. I believe Kemet is one. The data suggest much longer  
life at low stress (voltage, ripple current).

John Doty              Noqsi Aerospace, Ltd.
jpd@xxxxxxxxxxxxx




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