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Dying fluorescent lamp flashes

Discussion in 'Electronic Basics' started by Bob Masta, Jan 1, 2005.

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  1. Bob Masta

    Bob Masta Guest

    Hello, all.
    I have an old compact fluorescent lamp that has
    served admirably for many years. I finally had to
    take it out of service because it started flashing.
    The lamp is the old non-electronic ballast type. The
    ballast appears to be nothing more than a big inductor
    in series with the tube. The lamp lights OK at first,
    then after several seconds (maybe as much as 30)
    it starts flashing: The tube goes dark, then the
    heaters on the ends glow, there is a blue flash
    visible from the end where there is no phosphor,
    the lamp lights... and a second or two later the
    whole thing repeats.

    So, any idea what causes this? When the tube is
    lit, it appears to be about normal, or maybe with
    a bit of extra flicker. My best guess is that
    "something" has changed in the gas, either leaked
    out or plated out on a surface, etc, such that it
    will only conduct properly when warm. So it starts
    OK, but after the heaters cool down it can no longer
    support the discharge. But what exactly is that
    "something" that has changed?


    Bob Masta

    D A Q A R T A
    Data AcQuisition And Real-Time Analysis
  2. The heaters (filaments, electrodes) do not support an arc as well as
    they used to.

    The filaments are coated with a thermionically emissive material. Usage
    causes wear and starting also causes wear on this material. It chemically
    degrades, evapotates, and/or sputters (molecules of the material get
    dislodged by positive ions crashing against the negative electrode).

    You reach a point when there is so little left that you need extra heat
    to get it to sustain the arc.

    Often in fluorescent lamps, the arc does some working from the filament
    material itself in addition to or instead of the thermionic coating before
    the lamp completely dies. When that happens, some filament material
    usually sputters and noticeably blackens the end of the tubing where that

    - Don Klipstein ()
  3. Bob Masta submitted this idea :
    try replacing the starter. thats a small metal can with 2 terminals on
    it.. it removes with a slight twist and pull action. They're cheap.
    look at the ends of the tupe. Are they dull or going black? bad tube.
  4. Electron-emitting material on the heaters has worn out from use
    and/or from cold starts. This is a normal inevitable failure mode
    of hot-cathode fluorescent lamps in general, as well as at least most high
    pressure mercury vapor lamps and sodium vapor lamps of either high or low

    The heaters are normally kept hot enough to thermionically emit
    electrons at an adequate rate by heating from the ends of the arc in the
    tube. This fails when the thermionically emissive coating is worn away.

    - Don Klipstein ()
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