X-capacitor failure modes?

Klaus Kragelund <klauskvik@hotmail.com> wrote:
 
> Well, this PSU was on ground, testing the modules to be used for space
> flight. The SCR was to protect the space modules, not the PSU. 
 
> Cheers
 
> Klaus

Thanks. That makes a lot more sense. Much easier to change the fuse!

JK
 

 
> Check out Panasonic's design of X class PU and PE caps.
 http://www.digikey.com/product-search/en?pv13=41&pv1292=25&pv1292=27&FV=fff40002%2Cfff80010%2Cfffc0009%2Cfffc000a%2C142798%2C142799&mnonly=0&newproducts=0&ColumnSort=0&page=1&chp=0&quantity=0&ptm=0&fid=0&pageSize=500

Also see the video of the segmented film being shorted by arcs in action to see how shorting is localized to minimize healing required (the isolated segments connected with tiny fuse links are not shown)

Phil Allison wrote:

so having series pair solves the problem.
> 
> A damaged X cap might lose one of the pair and so the capacitance goes up.
Wow, that explains a lot!  And, although the capacitance goes up, the
voltage rating goes down.  Next time you apply power, the other side
of the cap is just about sure to blow!

Jon

On 23/09/2013 21:25, Jon Elson wrote:
> Phil Allison wrote:
>
> so having series pair solves the problem.
>>
>> A damaged X cap might lose one of the pair and so the capacitance goes up.
> Wow, that explains a lot!  And, although the capacitance goes up, the
> voltage rating goes down.  Next time you apply power, the other side
> of the cap is just about sure to blow!
>

Sounds like a ping-pong failure mechanism to me!

-- 
Mike Perkins
Video Solutions Ltd
www.videosolutions.ltd.uk

Mike Perkins wrote:
> On 23/09/2013 21:25, Jon Elson wrote:
>> Phil Allison wrote:
>>
>> so having series pair solves the problem.
>>>
>>> A damaged X cap might lose one of the pair and so the capacitance
>>> goes up.
>> Wow, that explains a lot!  And, although the capacitance goes up, the
>> voltage rating goes down.  Next time you apply power, the other side
>> of the cap is just about sure to blow!
>>
> 
> Sounds like a ping-pong failure mechanism to me!
> 

Mine sounded more like a Thompson sub machine gun. Rat-tat-tat ..
rat-tat ... rat-tat-tat-tat ... Got me out of my chair prontissimo.

-- 
Regards, Joerg

http://www.analogconsultants.com/

There is an other failure mode: total loss of capacity due to internal cracks. Not noticeable when used in a filter, but when used as a voltage dropper in a power supply. 

"Wim Ton"
>
> There is an other failure mode:
> total loss of capacity due to internal cracks.

 **  ????


> Not noticeable when used in a filter, but
>  when used as a voltage dropper in a power supply.


** I have seen that kind of failure and attributed it to one of the pigtail 
terminations going open.

 If you look at how it is done, it's a wonder it ever works.


...   Phil

X rated caps in plastic are designed to be self healing if possible and fail open.

"Anthony Stewart"

>X rated caps in plastic are designed to be self healing if possible and 
>fail open.


** This idiot just makes stuff up.

The vast majority of metallised film caps are "self healing"  and class X 
caps typically burn out at the end of their useful life due to too much 
internal damage.



...  Phil 

In article <ba6k80Fqo6tU1@mid.individual.net>,
 Joerg <invalid@invalid.invalid> wrote:

> Folks,
> 
> This afternoon I took the old HP-4191A apart. Yesterday in the middle of
> some measurements it went pop-pop-pop, loud, like firecrackers. I ran
> over and turned it off, upon which the popping stopped. An immense
> amount of light-gray smoke wafted out and it had an odd stench to it.
> The stench lingered for hours.
> 
> I can't find any source and upon powering it up the analyzer worked
> fine. That was before I changed anything. As if it had repaired itself.
> The only thing I could see is a crack in the plastic of the X-capacitor.
> Snipped the thing out, measured, has almost 0.28uF capacitance (27% more
> than stated) and no leakage current. There is another X-cap and two
> Y-caps inside a canned IEC receptacle but those can't be the culprits
> because that is on the line side of the power switch.
> 
> Is it possible that an X-cap self-heals to the point where you can
> barely see a thing, yet release a serious plume of smoke? It is a Rifa
> GPF-series film cap 0.22uF/250VAC with all kinds of agency logos on there.

Those foil capacitors can blow bits of confetti out the sides and still 
work fairly well afterwards.  The metal is so thin that it vaporizes 
more than the insulator.  You can call it self-healing but I wouldn't 
trust it to last long with the innards exposed to air.