Another Polypropylene capacitor failure Senseo

[Dataforensics]

After a repair of faulty/low capacitance ones in an AV amp as discussed in blog forum, I had another failure.

Have a Senseo Quadrante pod coffee machine, and it developed a fault whereby it would suddenly turn off after a couple of seconds of operation. A bit of Googling pointed me to several Dutch sites mentioning a capacitor related service note. I could not find a circuit diagram, or the service note but after a struggle with dismantling found a PIC controller card with a DAIN 0.47K 275V X2 polypropylene capacitor, dated 11/09, so not that old.

When removed it reads around 97nF. replaced it with an EPCOS equivalent and now have coffee again.

I appreciate these capacitor types are 'self-healing' but in human terms it a bit like saying 'well he lived, but the arms and legs dropped off'

Tony

[SeanB]

Self healing just means it cannot go short circuit and catch fire............ Well, in most cases it will not. See Dave's aircon motor run unit, which was a dry film unit. The oil filled ones ( now filled with palm kernel oil in place of the good old PCB oil) do self heal as well, but fail open circuit rather than as a charred mass. Some I have opened that were down 50% on capacity had nearly half the foil blown off by the internal arcing. The foil ones melt into a solid mass, hopefully the fuse link inside operates.

[Dataforensics]

Hi,
Thanks for replying.
Me being too vague, I was not trying to query the physical mechanism, but the term 'self-healing'.
I see the result of healing as being made good or working as well in the past whereas with these capacitors its in my view more like a damage mitigation result, the capacitor is not 'healed'.
Possibly the term was made up by marketing rather than the engineers?

Tony

[T4P]

Depends on what they are getting at,
self-healing but where? Normally foil caps don't self heal and will develop holes in the dielectric after time and in the end short, that's what they mean by self-healing, it self-heals the holes that form after time but the sacrificial lamb here is the dielectric increasing it's permeability

[SeanB]

The term is used to denote the clearing of internal shorts by the metallisation of the surrounding foil being burnt off by the current that flows when a small portion of the dielectric breaks down from voltage stress. The thin dielectric is used to make the unit smaller for a defined voltage, so that you can have a 2.2uF capacitor rated at 400V in a casing the size of a matchbook rather than being a unit the size of a pack of 30 cigarettes. They use a thinner foil and thinner plastic film so that the capacitance is both higher and the internal resistance is as well, relying on having a short path from the foil to the edge bonds to keep the overall ESR low. If you use a thicker film then there is less chance that a weak spot will develop from voltage stress or from internal voids or degradation or locally thinner film after stretching, but the capacitance is much reduced.

Thus the self healing is really self fault clearing if the voltage is high enough and there is sufficient energy available to vapourise the foil.