neon indicator lamp failure

Discussion in 'Lighting' started by AgX, Aug 26, 2012.

  1. AgX

    AgX Member

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    On fleamarket found electronic flashlights (solo or in-camera) I repeatedly found samples that flash but nevertheless show no glow ("loaded" sign) at their indicator neon lamp. And I waited for some minutes to exclude cases were the circuit permits flashing before the break-through current of the lamp is reached.

    Typically these types of lamps go for many thousands of hours.

    Any ideas about the cause of such failures aside of a broken connection?
    (Another cause might be a too high current through the lamp, and thus destroying it. Though I got no idea how such a current could arise.)
     
  2. Prof_Pixel

    Prof_Pixel Member

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    Neon bulbs don't last forever. In addition, there is typically a RC (Resistor and Capacitor) circuit used to make to lamp flash (blink); this could easily go bad.
     
  3. Gerald C Koch

    Gerald C Koch Member

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    I have seen this symptom many times but it is not the neon bulb that is failing but the main capacitor that powers the flash. Neon bulbs have a long useful life, far longer than that of capacitors. When the neon bulb no longer lights or is slow to light it indicates that the capaacitor is no longer capable of holding a full charge. The unit may still flash but not at maximum output. The condition will continue to get worse until the unit no longer will flash.
     
  4. Leigh B

    Leigh B Member

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    Yeah... What he said ^^^.

    Capacitor failure is extremely common, almost guaranteed if the flash is more than ten years old.

    It's an easy fix (replace the capacitor), but may not be worth the cost depending on the size and quality of the unit.

    - Leigh
     
  5. AgX

    AgX Member

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    Gerald, good point in drawing my attention to the main capacitator.

    My idea though was/is that in case the main capacitator won't have it's full capacity any longer, the voltage will finally still rise up to the charging voltage. The lamp will glow (though no longer indicating the fully or rather sufficiently loaded state of the main capacitator).

    However, in case that loss of capacity is due to a "electrical leakage" of the capacitator, I guess there might be a situation where at the capacitator is a lower voltage, still sufficient to flash the flash bulb but not to make the neon bulb glow.

    This all makes me realize I did not yet consider sufficiently the issues of capacitators...
     
  6. Bill Burk

    Bill Burk Subscriber

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    Yeah, what they both said ^^^.

    The issue is that you get up to 80% full charge, maybe, just enough to flash. But not the right amount of light to use the chart for proper exposure.
     
  7. Leigh B

    Leigh B Member

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    The problem is not loss of capacitance, it's leakage.

    Leakage will prevent the capacitor from charging to its full voltage.

    The neon indicator is a voltage comparator circuit. It comes on when the voltage across the capacitor rises above 250 volts (for example).
    If the voltage can only rise to 240 volts, the neon indicator will never light, but the flash may still fire at reduced brightness.

    This is the way flash systems work. I was a warranty repair station for Novatron strobes, so I'm reasonably familiar with the technology.

    - Leigh
     
  8. mr rusty

    mr rusty Subscriber

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    May I just had, don't mess with trying to repair flashes if you don't really know what you are doing. I didn't and did. I had a "little knowledge" that discharging the capaitor might be a good idea. The bang when the screwdriver shunted the terminals was something else and scared the s***s out of me. Decided I wouldn't do that again!
     
  9. Steve Smith

    Steve Smith Subscriber

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    Capacitors like to be discharged slowly with a resistor, not suddenly with a screwdriver. However, in practice, discharging the capacitor into the flash tube is closer to the screwdriver method than the resistor method!


    Steve.
     
  10. Steve Smith

    Steve Smith Subscriber

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    It probably harms the screwdriver more than the capacitor.

    EDIT: How did that become two posts? I thought I just added to the first one!


    Steve.
     
  11. E. von Hoegh

    E. von Hoegh Member

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    If you let the flash charge and run for a while without flashing it, the capacitor might re-form and the flash begin to function normally.

    Sometimes the capacitors will come back to life, more often they don't. But it's worth a try. I have an old Honeywell from the early 60s, if I let it sit without use, the capacitor loses it's form. Charge the capacitor for a while, and the flash functions normally; I use it as a slave.
     
  12. AgX

    AgX Member

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    Well, I dit exactly the same with two screwdrivers when I still was at school. I learned my lesson...

    But you are right: Everybody be warned fiddling around with high-voltage capacitators!!
     
  13. Leigh B

    Leigh B Member

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    Safety is always a paramount concern when dealing with high-voltage equipment.

    Even a small capacitor in a camera-mounted flash can store enough charge to kill a person.

    If you're not familiar with proper methods for servicing high-voltage equipment, don't try it.

    - Leigh
     
  14. Chan Tran

    Chan Tran Member

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    To discharge the capacitor I use a low end Fluke DMM (model 114) and in their low end models they have what is called a Low Z voltage mode which the impedance of the meter is only 2 KOhms vs the normal 10 MOhms impedance. It would discharge the capacitor safely, reasonably fast and also tells you what the voltage is.