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X-capacitor failure modes?

Discussion in 'Electronic Design' started by Joerg, Sep 21, 2013.

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  1. Joerg

    Joerg Guest

    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.
     
  2. Klaus Bahner

    Klaus Bahner Guest

    I've once had a fluorescent lamp which produced popping sounds for some
    time and also smelled a bit of burned plastics. Was hard to localize
    where it came from hence it took some time to find the culprit.

    It turned out to be an X-capacitor, which also had a crack in the
    housing. Had to look carefully, because besides the rather unimpressive
    crack, it looked just fine.
    No idea. I didn't made any measurements on it, just replaced it. Thus I
    don't know whether it self-healed or not.

    Regards,
    Klaus
     
  3. It's possible.. the old Philips 630VDC (I think the AC rating was
    250VAC on those) Polyester caps from that era were explicitly claimed
    to be self-healing. I don't think I ever accidentally tested it, but I
    would have expected less drama (but if it formed an arc track
    internally, then an arc might have become self-sustaining). I would
    have expected more leakage, but maybe it burned back.

    Did it smell more like burning polyester than burning transformer?

    If I had it, I would stick it on my "classic" Hi-pot tester and see
    where it breaks over.

    I guess you could dissect it and see what the insides look like,


    Best regards,
    Spehro Pefhany
     
  4. Joerg

    Joerg Guest

    Judging by the smell I was almost sure it was the transformer. But it's
    encapsulated and the only place smoke could come out is a small hole
    where all its wires tunnel out. But there was not smoke residue anywhere.

    The X-cap sits right in the open, so that is one of the very few parts
    that could spew stuff without leaving traces.

    Maybe I'll do that, I kept it. But first my wife wants me to barbecue
    chicken wings, bratwurst and peppers. And when I pulled that analyzer
    out of the rack she remarked "Don't put that back in before cleaning
    back there!". It's running for 2h now, sans problems, with a new X-cap.
    While I cleaned the lab bench, rack and all that.
     
  5. Joerg

    Joerg Guest

    Ah, ok, so it could indeed be the X-cap then. I was impressed how much
    smoke it let off. The whole room (size of a bedroom) was full of smoke.

    [...]
     
  6. They are Self-Healing to the point that they don't pose a safety risk.
    <http://www.kemet.com/kemet/web/home...ege Presentations/$file/EvoxRifaRFIandSMD.pdf>

    Cheers
     
  7. Jasen Betts

    Jasen Betts Guest

    They're supposed to self heal

    Smoke has low density, you can fit a lot of smoke precursor into a small space.

    if it's got a crack it should definately be replaced, why not do some
    destructive testing and see if it can withstand the test voltage
    required and if it makes the right sort of wrong smell.
     
  8. Some smoke is ok, It is the flame and sustained smoking that throw red
    flags for UL. X caps fail open, so they should not smoke for long.

    If Joerg left the power on long enough, he probably would not have
    seen flames or more smoke.

    I would have had a 'Oh sh... moment and turned the power off too" ;D

    Cheers
     
  9. Presumably his immediate objective was to interrupt the proximate
    cause of the popping and smoking rather than to fully test those
    ancient capacitors to UL standards. ;-)


    Best regards,
    Spehro Pefhany
     
  10. Phil Allison

    Phil Allison Guest

    "Joerg"
    ** Yep.

    ** I have been in the room twice when one of them blew itself up - RIFA
    brand both times.

    The first one was years ago inside a fan heater and the second in a portable
    TV late last year.

    Filled the room with yucky smelling smoke and set off the alarm.



    .... Phil
     
  11. legg

    legg Guest

    Never heard of Rifa GPF.

    In fact, if it's Rifa, and manufactured since 1980, it's part type
    will begin with the letter P, even after their amalgamation with Evox.

    Metalized paper PM (or PZ), metalized polyester PH.

    Perhaps you've mistaken the brand?

    The 4191A was first released in 1980.

    Interesting thing about the older Rifa parts, was that their outer
    layer was transparent - an overstress or corona can often be seen, as
    well as heard, but usually only in locations that are subject to the
    attentions of overly zealous test technicians with manually adjustible
    hipot equipment.

    RL
     
  12. Joerg

    Joerg Guest

    Yep. The smoking was quite intense, had to open the windows and leave
    the lab area to avoid coughing. Plus the odor was really nasty.
     
  13. Joerg

    Joerg Guest


    Maybe it's PME. The label says this:

    Rifa GPF
    40/085/56
    PME 271 M 622

    Then also "CM1" rotated 90 degrees on the right. No Evox mention on there.

    No, says Rifa on there and has the logo.

    The service manual is from 1980 but I believe the machine itself was
    built around 1985. It's a good system. Ok, had its quirks. A big
    electrolytic in it gave up once. Then they had screwed up the backup
    battery charger but since I fixed that it doesn't lose calibration data
    anymore. The plastic component clamp levers for the 16092A probe head
    are wimpy and, predictably, one snapped in half. Still usable but one
    has to have strong fingernails. Other than that it has served me well.

    This cap developed a crack across one of the flat sides. Considering the
    loud pops and the amount of smoke I'd have expected a total meltdown but
    other than the crack I can't see any damage.
     
  14. Joerg

    Joerg Guest

    It is rated 250VAC and I gave it 300VAC from my "Australia test setup".
    No popping, no smell. But who knows, the suspect area might have already
    vaporized itself.
     
  15. http://fire.nist.gov/bfrlpubs/fire86/PDF/f86012.pdf

    Some numbers on time to incapacitation and time to death in there.



    Best regards,
    Spehro Pefhany
     
  16. Joerg

    Joerg Guest

    The two Labradors were smart and immediately left the area. They must
    have read it :)

    Such burning stuff can be dangerous, it has killed many travelers at
    Duesseldorf airport, via smoke inhalation.
     
  17. Phil Hobbs

    Phil Hobbs Guest

    I had a very nice hand-wired Krohn-Hite filter croak itself a year or so
    back--the clamp on one of the 'computer-grade' filter caps was loose,
    and the cap worked its way down through the chassis till it shorted to
    the bottom of the case. (No fish paper there, for some silly reason.)

    This resulted in clouds of transformer smoke filling the lab, which was
    a shame, because it was a really nice box and the transformer was an
    irreplaceable custom job. It even had completely separate power
    supplies for the two channels, to prevent crosstalk.

    Good thing I'm nuts about turning all the lab power off when I'm not
    there--it could have happened any time at all.

    Cheers

    Phil Hobbs



    --
    Dr Philip C D Hobbs
    Principal Consultant
    ElectroOptical Innovations LLC
    Optics, Electro-optics, Photonics, Analog Electronics

    160 North State Road #203
    Briarcliff Manor NY 10510 USA
    +1 845 480 2058

    hobbs at electrooptical dot net
    http://electrooptical.net
     
  18. Joerg

    Joerg Guest

    Still, watch those X-caps in them boat anchors where you now have so
    many of. I have seen these golden-clear plastic caps in many of them. My
    lab has pretty good line spike filtering and the cap ran at less than
    50% of its rated voltage yet it decided to blow. Problem is, things such
    as this impedance analyzer can be unattended for long periods of time
    when you run an experiment. Same with a spectrum analyzer when it runs a
    baseline scan over a 100MHz swath with 1Hz BW. You don't want to come
    back from lunch only to see trucks with flashing blue lights surrounding
    your building because a fire alarm went off.
     
  19. X class caps have improved a lot with fusible links built into thin metal foil pattern so that less heat damage to vaporizing toxic plastic with only fractional loss of capacitance with many fuse links in the entire rolled metalized layer.

    Check out Panasonic's design of X class PU and PE caps.

    I don't know the MSDS of your part, but smoke from molten PVC wire insulation caused more lethal risk than the radiation at Three Mile Island.
     
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