how can a light bulb become a short circuit?

Discussion in 'Electronic Repair' started by dstromb, May 1, 2005.

  1. dstromb

    dstromb Guest

    I was doing a search of diybanter when trying to figure out how to fix
    touch lamp, and found this thread:

    http://www.diybanter.com/showthread.php?t=37812

    ... where someone claimed it's possible for a light bulb to become
    momentary short circuit while it's burning out. This sounds crazy t
    me, but in the lamp I'm looking at, the triac is shorted and a trac
    running between it and a wire that goes to the bulb is vaporized on th
    circuit board. Only thing I can think of to cause that would be a shor
    on the bulb side of the trace.

    So, on that basis I guess it must be possible for a light bulb t
    generate a short. But I can't understand it at all. Can anyone explai
    how it happens? A little arc as the filament opens would make sense
    but that's not the same thing.

    I've ordered a replacement triac and will repair the trace - I'
    guessing that will fix it, I don't find anything else that tests bad

    --
    dstromb
     
    dstromb, May 1, 2005
    #1
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  2. In article <>,
    dstromb <> wrote:
    > .. where someone claimed it's possible for a light bulb to become a
    > momentary short circuit while it's burning out. This sounds crazy to
    > me, but in the lamp I'm looking at, the triac is shorted and a trace
    > running between it and a wire that goes to the bulb is vaporized on the
    > circuit board. Only thing I can think of to cause that would be a short
    > on the bulb side of the trace.


    Yup - it's common enough. They often have internal fuses, but will still
    trip an MCB or blow up a dimmer - most of which have pretty inadequate
    triacs anyway.

    Think what happens is the filament supports are under spring tension. When
    the hot filament fails through burning out they move and short.

    --
    *I got a sweater for Christmas. I really wanted a screamer or a moaner*

    Dave Plowman London SW
    To e-mail, change noise into sound.
     
    Dave Plowman (News), May 1, 2005
    #2
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  3. "dstromb" <> schreef in bericht
    news:...
    >
    > I was doing a search of diybanter when trying to figure out how to fix a
    > touch lamp, and found this thread:
    >
    > http://www.diybanter.com/showthread.php?t=37812
    >
    > .. where someone claimed it's possible for a light bulb to become a
    > momentary short circuit while it's burning out. This sounds crazy to
    > me, but in the lamp I'm looking at, the triac is shorted and a trace
    > running between it and a wire that goes to the bulb is vaporized on the
    > circuit board. Only thing I can think of to cause that would be a short
    > on the bulb side of the trace.
    >
    > So, on that basis I guess it must be possible for a light bulb to
    > generate a short. But I can't understand it at all. Can anyone explain
    > how it happens? A little arc as the filament opens would make sense,
    > but that's not the same thing.
    >
    > I've ordered a replacement triac and will repair the trace - I'm
    > guessing that will fix it, I don't find anything else that tests bad.
    >
    >
    > --
    > dstromb


    Happens most of the time with bulbs mounted in some atmospheric lighting.
    Bulbs that do not hang from the ceiling when they go, may have flying around
    some pieces of the defective wire that temporarely makes a short. Quality of
    the bulb is also an issue here. Ever bought four lamps that had bulbs with
    build in reflecting mirrors. They lived pretty short and all of them blew a
    16A fuse when they went. The replacement bulbs never gave that problem. The
    internal wires were insulated in the glass over a much longer length. Maybe
    there were some other differences too.

    petrus bitbyter
     
    petrus bitbyter, May 1, 2005
    #3
  4. "petrus bitbyter" <> writes:

    > "dstromb" <> schreef in bericht
    > news:...
    > >
    > > I was doing a search of diybanter when trying to figure out how to fix a
    > > touch lamp, and found this thread:
    > >
    > > http://www.diybanter.com/showthread.php?t=37812
    > >
    > > .. where someone claimed it's possible for a light bulb to become a
    > > momentary short circuit while it's burning out. This sounds crazy to
    > > me, but in the lamp I'm looking at, the triac is shorted and a trace
    > > running between it and a wire that goes to the bulb is vaporized on the
    > > circuit board. Only thing I can think of to cause that would be a short
    > > on the bulb side of the trace.
    > >
    > > So, on that basis I guess it must be possible for a light bulb to
    > > generate a short. But I can't understand it at all. Can anyone explain
    > > how it happens? A little arc as the filament opens would make sense,
    > > but that's not the same thing.
    > >
    > > I've ordered a replacement triac and will repair the trace - I'm
    > > guessing that will fix it, I don't find anything else that tests bad.
    > >
    > >
    > > --
    > > dstromb

    >
    > Happens most of the time with bulbs mounted in some atmospheric lighting.
    > Bulbs that do not hang from the ceiling when they go, may have flying around
    > some pieces of the defective wire that temporarely makes a short. Quality of
    > the bulb is also an issue here. Ever bought four lamps that had bulbs with
    > build in reflecting mirrors. They lived pretty short and all of them blew a
    > 16A fuse when they went. The replacement bulbs never gave that problem. The
    > internal wires were insulated in the glass over a much longer length. Maybe
    > there were some other differences too.


    While the filament parts flying is one possibility, one that is also
    likely or more likely is described here:

    http://members.misty.com/don/bulb1.html#wbs

    --- sam | Sci.Electronics.Repair FAQ Mirror: http://repairfaq.ece.drexel.edu/
    Repair | Main Table of Contents: http://repairfaq.ece.drexel.edu/REPAIR/
    +Lasers | Sam's Laser FAQ: http://repairfaq.ece.drexel.edu/sam/lasersam.htm
    | Mirror Sites: http://repairfaq.ece.drexel.edu/REPAIR/F_mirror.html

    Note: These links are hopefully temporary until we can sort out the excessive
    traffic on Repairfaq.org.

    Important: Anything sent to the email address in the message header above is
    ignored unless my full name is included in the subject line. Or, you can
    contact me via the Feedback Form in the FAQs.
     
    Sam Goldwasser, May 1, 2005
    #4
  5. dstromb

    Guest

    On top of all the other possibilities mentioned, the wires and supports
    all emanate from a relatively small area. The hot and nuetral are not
    that far. If burning base down it is quite possible for a small part of
    the filament to break off and directly short those outer wires.

    JURB
     
    , May 1, 2005
    #5
  6. dstromb

    James Sweet Guest

    <> wrote in message
    news:...
    > On top of all the other possibilities mentioned, the wires and supports
    > all emanate from a relatively small area. The hot and nuetral are not
    > that far. If burning base down it is quite possible for a small part of
    > the filament to break off and directly short those outer wires.
    >
    > JURB
    >


    Possible yes, but in the vast majority of cases, an arc is struck in the
    argon fill gas, it melts the ends of the lead-in wires right off. An arc is
    low impedance which gets lower the hotter it gets, exactly the reason a
    discharge lamp requires a ballast.
     
    James Sweet, May 1, 2005
    #6
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