[vcf-midatlantic] The RIFA Cap Says: PFFFFFT!

[Jonathan Gevaryahu]

On 12/14/2015 1:07 PM, Systems Glitch via vcf-midatlantic wrote:
> Just had one blow in an old ECL monitor, releasing tons of stinky capacitor smoke. Had to take the monitor outside to let it finish smoking.
>
> I know these are "X2 safety rated" devices, but what does that really mean? "Keep two handy cos they fail a lot?" Seems to me that not blowing up would be a great safety feature, something you wouldn't relegate to cheap rice paper in epoxy.
>
> Thanks,
> Jonathan
>
The RIFA/X2 caps like PME271M (see 
http://www.farnell.com/datasheets/1943741.pdf ) are metallized paper 
capacitors coated in a non-flammable wadding of some sort, the whole 
thing coated in epoxy in a potting shell. From what I've been told, the 
reason they fail is that the thermal expansion coefficient of the 
paper/wadding is not the same as the epoxy, causing the epoxy to 
eventually develop cracks. The cracks allow moisture inside, which 
eventually causes the capacitor to fail. (I.e. momentarily short and the 
fail/explode open; RIFA capacitors are designed to fail open.)

Newer RIFA caps have a fully surrounding  plastic 'potting shield' 
casing outside the epoxy portion to try to reduce moisture impregnation, 
I believe.
I'm not sure how well it works.


-- 
Jonathan Gevaryahu
jgevaryahu at gmail.com
jgevaryahu at hotmail.com