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Fluorescent light capacitor function and failure mode

Discussion in 'Electronic Repair' started by John J. Lee, Nov 13, 2003.

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  1. John J. Lee

    John J. Lee Guest

    I'm trying to fix a fluorescent kitchen light. It was blowing the
    downstairs lighting circuit fuse every time the wall switch was turned
    on, even when the fluorescent tube was removed.

    The breakage seems to have been caused by me leaving something cooking
    on the kitchen grill for ages (never again!), causing the room to
    become very hot (condensation on the window, so maybe some in the
    light, too), and maybe some grease / carbon to settle on surfaces in
    the room (though I can't actually see or feel any). I'm pretty sure
    the light was switched on when I left it. There was no fire,
    thankfully.

    The light has a standard old-fashioned circuit with just a ballast
    (just an inductor?) and a "starter" (bimetallic strip thingy).
    There's also a big electrolytic capacitor, connected directly across
    the mains. So, with the fluorescent tube removed, the circuit should
    just be the capacitor connected across the mains.

    If I disconnect the capacitor from one side of the connecting block
    that it's wired to, switching it on at the wall no longer blows the
    fuse (and there's no short across the mains terminals with the
    capacitor connected). So it's the capacitor that's at fault, not some
    grease or something shorting things out.

    First, is the capacitor just there to smooth the voltage, to reduce
    flicker? Can I disconnect it and switch it all on again, to see if
    anything else is broken?

    Second, when I measure the (DC) resistance of the capacitor with a
    multimeter, I get 500 kOhms or so. Anybody care to guess what
    happened to it to cause it to start breaking down at 250V? Was it my
    fault, or was it just a coincidence that it started blowing fuses just
    as I left the grill on?

    TIA


    John
     
  2. Should be.
    Are you sure it's electrolytic?

    Should work without it if it's just across the AC input. There are oftem
    small uF caps for RFI filtering.
    If the cap is all that's bad, could certainly be a conincidence.

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  3. James Sweet

    James Sweet Guest


    Sounds like the capacitor is there for power factor correction, I'm assuming
    you're in the UK or somewhere else with 240v mains, over here most ballasts
    are the rapid start type and the power factor correction capacitor is
    internal.
     

  4. I've never seen a fixture with a large electrolytic across the power
    line. Are you from the UK since you use the word "mains" for the
    power line? Any capacitor is probably just to filter out
    high-frequency noise from the lamp arc, 50 or 60 Hz and many, many
    harmonics. The capacitor is probably breaking down when exposed to
    the full line voltage, but not under the low voltage from the ohmmeter
    section of the multimeter. Almost any capacitor rated at twice the
    line voltage will reduce the "noise going back onto the power line.
    It is most evident when listening to AM radio. If you can use the
    radios in your house with the light on, without the capacitor, then it
    probably is ok to continue to use it without interfering with your
    neighbors radio reception. But if you are in an apartment house, you
    may cause interference to your neighbors radio due to the ac power
    lines radiating the noise.

    H. R. (Bob) Hofmann
     
  5. James Sweet

    James Sweet Guest


    The capacitor has nothing to do with filtering noise, it's to raise the
    power factor. Your diagnosis that it's breaking down under the high voltage
    is likely correct though.
     
  6. John J. Lee

    John J. Lee Guest

    Yes, I'm in the UK.

    [...]
    I see. I had assumed it was to prevent flicker, but it doesn't
    flicker with the capacitor out, so I guess not.

    Yeah, of course -- I was wondering why, though. Perhaps condensation
    formed between the contacts and heated up the casing, damaging the
    internals, but if so I dunno what the physical mechanism of the
    breakdown is.

    Thanks for your help, everyone.


    John
     
  7. Guest

    Guest Guest

    : I've never seen a fixture with a large electrolytic across the power
    : line. Are you from the UK since you use the word "mains" for the
    : power line? Any capacitor is probably just to filter out
    : high-frequency noise from the lamp arc, 50 or 60 Hz and many, many
    : harmonics. The capacitor is probably breaking down when exposed to
    : the full line voltage, but not under the low voltage from the ohmmeter
    : section of the multimeter. Almost any capacitor rated at twice the
    : line voltage will reduce the "noise going back onto the power line.
    : It is most evident when listening to AM radio. If you can use the
    : radios in your house with the light on, without the capacitor, then it
    : probably is ok to continue to use it without interfering with your
    : neighbors radio reception. But if you are in an apartment house, you
    : may cause interference to your neighbors radio due to the ac power
    : lines radiating the noise.

    : H. R. (Bob) Hofmann


    The cap might be there for power factor correction. But on a single
    fixture? I doubt it unless local code specifies so.
     
  8. James Sweet

    James Sweet Guest

    See this link
    http://www.ambercaps.com/lighting/power_factor_correction_concepts.htm

    Virtually all rapid start ballasts in north america have a power factor
    correction cap, but it's traditionally sealed within the ballast casing.
    These can be identified by the lable saying High Power Factor, next time
    you're in a hardware store have a close look at a typical F40T12 rapid start
    magnetic ballast. In the UK the mains voltage is high enough that most
    fluorescent fixtures use a simple choke ballast, it's more efficient than
    the autotransformer ballasts we have here, but you need more than 120v to
    reliably start tubes longer than about 2 feet. Choke ballasts here generally
    don't have a capacitor (never seen one that did) likely because small tubes
    are rarely used more than a few at a time.
     
  9. James Sweet

    James Sweet Guest

    I should add that older preheat ballasts had this as well, hence the reason
    that they contained PCB's, which were used in the capacitor.
     
  10. John J. Lee

    John J. Lee Guest

    Ack! I guess this mean this capacitor sitting beside me might well
    contain PCBs. Luckily, it seems to be intact (but I'll certainly keep
    it in an outdoor cupboard now, just in case). Will my local dump know
    what to do with it, do you think (I'm in the UK)?

    The light is certainly an old one, not the instant start variety, and
    for all I know, it might be 35 years old. It's about 13 cm long,
    excluding contacts, and this is what's printed on it:

    AME C.22451
    I.I.C. CAPACITORS LTD
    5.0 MFD +- 10%
    250V RMS WKG
    50 C/S -25 TO +50 C
    INTERNAL RESISTOR
    FUSE FITTED


    I presume that's *milli* farads, since the thing is so big?

    Actually, the "fuse fitted" bit has me confused: why was it causing
    shorts if it has a fuse?!


    John
     
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