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Fluorescent lamp question

  1. Jan 30, 2010 #1


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    I apologize if the question is posed in a wrong forum.
    I've had a lamp similar to this one: http://upload.wikimedia.org/wikipedia/commons/4/42/Fluorescent_Lamp.jpg for about 2 years now. Ever since I bought it, when I turn the light off, every 30 to 50 s it emits flashes of light. One flash every 40 s approximately. It does this for more than 30 minutes after I turn off the light.
    I should mention that its color is white (though I notice it's a very bit blue). This phenomenon doesn't occur with my other similar lamp which is more yellow.
    I wanted to know what's going on.
    Thanks in advance.
  2. jcsd
  3. Jan 30, 2010 #2


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    You may have a faulty power switch.

    The light can't generate its own power, yet it takes power to produce these flashes. So it must be coming from the mains, even though it is supposed to be turned off.
  4. Jan 30, 2010 #3


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    Hmm strange. How do you explain (or have a guess) that after 1 hour this effect doesn't occur anymore? It always occur right after I turn the light off; after a certain time it disappear completely.

    By the way, what do you mean by "mains"? English is not my mother tongue and I feel this word as so many uses that I won't find the correct one.
  5. Jan 30, 2010 #4
    As vk6kro says, it sounds like your power switch is faulty. Not the power switch you push but the electrical switch. It's probably heat related.
    After the lamp has been off at least an hour, turn the lamp on for a few seconds, then shut it off. I don't think it will act the same as it would if you turned it on for more than a few minutes, then shut it off.
  6. Jan 30, 2010 #5


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    By the way, what do you mean by "mains"? English is not my mother tongue and I feel this word as so many uses that I won't find the correct one.

    "mains" just refers to the street electricity supply. Usually either 120 volts or 240 volts AC.

    Your English was very good in the original question and I would not have guessed that it wasn't your native language.
  7. Jan 31, 2010 #6
    I used to have a fluorescent desk-lamp in my bedroom on my desk. In the middle of the night the cat would rub against it and the lamp would light.
    So, I unplugged it so I could sleep
    That made no difference, the lamp would still light up when the cat walked by.

    Got rid of the lamp, kept the cat.
  8. Jan 31, 2010 #7


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    Ok thanks for the replies. I'm going to try the test of turning it off, wait an hour or more and turn it on a few seconds, then turn it off and see what happens.
    I didn't really understand where the problem is. If it was the mains, shouldn't all my lights in my appartment suffer (from?) the same problem?
    I'm having visit these days so I'll try the test as soon as I can, and let you know of course.
  9. Jan 31, 2010 #8


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    There seems to be a little bit of leakage across your switch when it is turned off.

    The first thing inside the fluorescent light will be a bridge rectifier followed by a capacitor.

    In this one light, the capacitor is charging up with the small current that is flowing through the switch. The other lights may have resistors across this capacitor to discharge the capacitor.

    This is called a bleeder resistor and it is usually put across high voltage capacitors in power supplies to avoid dangerous voltages remaining on capacitors when the power is removed.

    In this one light, this resistor may be absent or faulty, so the very small currents accumulate and after 2400 mains cycles there is enough voltage on the capacitor to operate the light briefly.

    If you have lights in parallel, replacing one of the other lights (as a test) with an incandescent light should stop this one flashing.
  10. Jan 31, 2010 #9
    My comment about the cat had a point.

    Some fluorescent bulb typess, in proximity of a suitable electric field, will flicker & even glow steadily. No wires need to be attached at all, although attachment of the un-switched neutral may aid the flicker.

    an exaggerated example

    If you have another wire running in your ceiling above & near the luminaire, then that may explain it. Especially if this is an old building with knob & tube wiring, or the wiring predates ground wires.

    If you have a nail or drywall screw through a wire, dating from construction (a typical home electrical fault), then the result will be a locally strong electric field, and yet will not trip the breaker of fuse due to the reasonable insulating quality of dry wood.

    Make sure the luminaire is properly grounded (measure wrt a known good ground), as a grounded metal luminaire will somewhat shield the fluorescent bulb. It is not uncommon for an amateur home electrician to end up with a reversed hot-neutral or a hot ground wire. All of these things will make your lamp flicker & glow.
    Last edited by a moderator: Apr 24, 2017
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