Testing Fluorescent Tubes for Cathode Emission

Discussion in 'Electronic Repair' started by Martin McCormick, May 13, 2005.

  1. Many fluorescent light fixtures use two tubes in series per
    ballast. If one tube stops working, both are dark. I have a bunch of
    4-foot 40-Watt tubes ranging in condition from probably good to most
    likely bad. It occurred to me that it should be possible to test the
    cathode emission on each tube by applying the 3-volt heater voltage to
    the cathodes at each end and then determining what voltage it takes to
    cause the gas to ionize. If one feeds the cathodes by either a
    by filer transformer or two separate filament-type transformers, then
    there should be no common connection between the ends of the tube.
    One could use a current-limited AC source or even a current-limited DC
    source to apply voltage between the ends of the tube. A good tube
    should break down at around 175 to 200 volts while a bad tube will
    need a higher voltage due to reduced cathode emission.

    One should be able to vary the high voltage source so that the
    voltage can be increased until breakdown occurs.

    If the current limiting is substantial such as allowing only a
    milliamp or two at 200 volts, then the tube would probably glow weakly
    but the idea here is to determine breakdown voltage.

    Does anybody see any reason why this shouldn't be a valid test method?

    Thanks.
    --

    Martin McCormick WB5AGZ Stillwater, OK
    Information Technology Division Network Operations Group
     
    Martin McCormick, May 13, 2005
    #1
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  2. Martin McCormick

    Guest

    Martin McCormick:
    .... gosh.... simple... I just install the suspect fluorescent tube
    into an operating, good two tube fixture. If the tube works in place
    of a good one that I just removed then I keep it, otherwise it is
    trash. After "testing" I usually only pair up used tubes with used
    tubes and new tubes with new tubes.
    electricitym
    ..
    ..
    ..
     
    , May 13, 2005
    #2
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  3. First check for continuity of the filaments. Often, the open and that's
    a simple test! :)

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    (Martin McCormick) writes:

    > Many fluorescent light fixtures use two tubes in series per
    > ballast. If one tube stops working, both are dark. I have a bunch of
    > 4-foot 40-Watt tubes ranging in condition from probably good to most
    > likely bad. It occurred to me that it should be possible to test the
    > cathode emission on each tube by applying the 3-volt heater voltage to
    > the cathodes at each end and then determining what voltage it takes to
    > cause the gas to ionize. If one feeds the cathodes by either a
    > by filer transformer or two separate filament-type transformers, then
    > there should be no common connection between the ends of the tube.
    > One could use a current-limited AC source or even a current-limited DC
    > source to apply voltage between the ends of the tube. A good tube
    > should break down at around 175 to 200 volts while a bad tube will
    > need a higher voltage due to reduced cathode emission.
    >
    > One should be able to vary the high voltage source so that the
    > voltage can be increased until breakdown occurs.
    >
    > If the current limiting is substantial such as allowing only a
    > milliamp or two at 200 volts, then the tube would probably glow weakly
    > but the idea here is to determine breakdown voltage.
    >
    > Does anybody see any reason why this shouldn't be a valid test method?
    >
    > Thanks.
    > --
    >
    > Martin McCormick WB5AGZ Stillwater, OK
    > Information Technology Division Network Operations Group
     
    Sam Goldwasser, May 13, 2005
    #3
  4. In article <>,
    Sam Goldwasser <> wrote:
    >First check for continuity of the filaments. Often, the open and that's
    >a simple test! :)


    Yup. Open filaments are a show stopper all right.

    These tubes all have good filaments but are old enough that
    some, probably most of the cathodes, are depleted from use.
    --

    Martin McCormick WB5AGZ Stillwater, OK
    Information Technology Division Network Operations Group
     
    Martin McCormick, May 13, 2005
    #4
  5. Martin McCormick

    sofie Guest

    Martin:
    If they are depleted from use then you don't need to any more of a test than
    trying each one in with a good tube in a working fixture. If they light up
    then keep them around.
    As electricitym indicate in his reply post you should probably pair them up
    with other used tubes. I usually replace them both at a time with two new
    tubes in my double tube fixtures so I always end up with an odd amount of
    good, used tubes that can be put into an easy access garage or shop two tube
    fixture with just one bad tube.
    --
    Best Regards,
    Daniel Sofie
    Electronics Supply & Repair
    - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -



    "Martin McCormick" <> wrote in message
    news:d62u1m$e85$...
    > In article <>,
    > Sam Goldwasser <> wrote:
    > >First check for continuity of the filaments. Often, the open and that's
    > >a simple test! :)

    >
    > Yup. Open filaments are a show stopper all right.
    >
    > These tubes all have good filaments but are old enough that
    > some, probably most of the cathodes, are depleted from use.
    > --
    >
    > Martin McCormick WB5AGZ Stillwater, OK
    > Information Technology Division Network Operations Group
     
    sofie, May 13, 2005
    #5
  6. Martin McCormick

    James Sweet Guest

    "Martin McCormick" <> wrote in message
    news:d62bi1$dko$...
    >
    > Many fluorescent light fixtures use two tubes in series per
    > ballast. If one tube stops working, both are dark. I have a bunch of
    > 4-foot 40-Watt tubes ranging in condition from probably good to most
    > likely bad. It occurred to me that it should be possible to test the
    > cathode emission on each tube by applying the 3-volt heater voltage to
    > the cathodes at each end and then determining what voltage it takes to
    > cause the gas to ionize. If one feeds the cathodes by either a
    > by filer transformer or two separate filament-type transformers, then
    > there should be no common connection between the ends of the tube.
    > One could use a current-limited AC source or even a current-limited DC
    > source to apply voltage between the ends of the tube. A good tube
    > should break down at around 175 to 200 volts while a bad tube will
    > need a higher voltage due to reduced cathode emission.
    >
    > One should be able to vary the high voltage source so that the
    > voltage can be increased until breakdown occurs.
    >
    > If the current limiting is substantial such as allowing only a
    > milliamp or two at 200 volts, then the tube would probably glow weakly
    > but the idea here is to determine breakdown voltage.
    >
    > Does anybody see any reason why this shouldn't be a valid test method?
    >
    > Thanks.
    > --
    >



    The phosphors normally wear out first, when the lumen output drops below
    about 70% of new the tube is shot. Just pop each tube into a fixture with a
    fairly new tube and toss out any that don't come up to reasonably close to
    full brightness after a couple minutes.
     
    James Sweet, May 15, 2005
    #6
  7. Martin McCormick

    James Sweet Guest

    "Martin McCormick" <> wrote in message
    news:d62u1m$e85$...
    > In article <>,
    > Sam Goldwasser <> wrote:
    > >First check for continuity of the filaments. Often, the open and that's
    > >a simple test! :)

    >
    > Yup. Open filaments are a show stopper all right.
    >



    Not always, I have some electronic ballasts that can fire up a tube with an
    open cathode just fine. I used one in the ozonator in my hot tub to squeeze
    more life out of a $60(!) bulb that had both cathodes blown open when the
    original choke shorted.
     
    James Sweet, May 15, 2005
    #7
  8. Martin McCormick

    mike Guest

    Martin McCormick wrote:
    > Many fluorescent light fixtures use two tubes in series per
    > ballast. If one tube stops working, both are dark. I have a bunch of
    > 4-foot 40-Watt tubes ranging in condition from probably good to most
    > likely bad. It occurred to me that it should be possible to test the
    > cathode emission on each tube by applying the 3-volt heater voltage to
    > the cathodes at each end and then determining what voltage it takes to
    > cause the gas to ionize. If one feeds the cathodes by either a
    > by filer transformer or two separate filament-type transformers, then
    > there should be no common connection between the ends of the tube.
    > One could use a current-limited AC source or even a current-limited DC
    > source to apply voltage between the ends of the tube. A good tube
    > should break down at around 175 to 200 volts while a bad tube will
    > need a higher voltage due to reduced cathode emission.
    >
    > One should be able to vary the high voltage source so that the
    > voltage can be increased until breakdown occurs.
    >
    > If the current limiting is substantial such as allowing only a
    > milliamp or two at 200 volts, then the tube would probably glow weakly
    > but the idea here is to determine breakdown voltage.
    >
    > Does anybody see any reason why this shouldn't be a valid test method?
    >
    > Thanks.


    Ok, but back to the original question.
    What about testing of tubes?
    I've tried to test laptop backlight bulbs with a transistor curve tracer
    without much success.
    A 5" tube from a battery-powered fluorescent lamp breaks down at about
    500VDC. But there seems to be little correlation between breakdown
    and goodness of the bulb. Methinks different mechanisms are in play.
    mike

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    mike, May 15, 2005
    #8
  9. Martin McCormick

    James Sweet Guest


    >
    > Ok, but back to the original question.
    > What about testing of tubes?
    > I've tried to test laptop backlight bulbs with a transistor curve tracer
    > without much success.
    > A 5" tube from a battery-powered fluorescent lamp breaks down at about
    > 500VDC. But there seems to be little correlation between breakdown
    > and goodness of the bulb. Methinks different mechanisms are in play.
    > mike
    >


    You're comparing apples to oranges. A laptop backlight tube is a cold
    cathode fluorescent, more like a neon tube. The breakdown voltage is related
    to gas fill and pressure, but the "goodness" of the tube is dependent on the
    emissive coating on the electrodes and phosphor condition as well. The best,
    and only reliable way to tell the condition of a fluorescent tube is to fire
    it up on a known good ballast rated to operate that tube. You can tell more
    from the brightness of the tube than anything else since they depreciate
    gradually with use and will usually get down below 50% before they fail to
    light at all.
     
    James Sweet, May 15, 2005
    #9
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