light bulb burn out

Discussion in 'General Electronics Chat' started by mjellis315, Jan 10, 2012.

  1. mjellis315

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    Jan 10, 2012
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    Why does a incandescent bulb brown out when run under voltage?
     
  2. thatoneguy

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    Feb 19, 2009
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    When the filament of a light bulb heats up, the resistance of the filament increases, which limits the current allowed to pass.

    When the filament isn't at full voltage, too much current is allowed through, and it acts like a fuse.

    You'll notice that light bulbs tend to burn out right when you turn them on, this is due to the filament being weakened/made fragile by long on times, so when it's cold, the transition to hot not only stresses it, but the current through it will also aid in the filament failing.
     
  3. wmodavis

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    Oct 23, 2010
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    What exactly do you mean by "BROWN OUT"?

    Oneguy gave some pretty good information, but is that what you were asking about? It just isn't clear enough to me to provide anything without a definition.

    What I think of from the term you used doesn't quite fit the context of the rest of your question.

    Wikipedia says a brownout is "drop in voltage in an electrical power supply".

    So that would make your question "Why does a incandescent bulb 'drop in voltage' when run under voltage? "

    Do you mean to ask "Why does a incandescent bulb become dim when run under voltage?"

    ---WORDS ARE EVERYTHING---
     
  4. thatoneguy

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    My post assumed he meant "burn out".

    Though glowing dimly falls under the same behavior for the characteristics of bulb filaments.
     
  5. praondevou

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    I never managed to burn a light bulb with undervoltage. And I let them run for a long time.
    Can somebody give an example when this is supposed to happen? At what voltage percentage?
     
  6. thatoneguy

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    At 5%-10%, before the filament is warm enough to glow.

    Lights have been underpowered to give a dim light and have lasted 100 years nearly continually lit, but they were operating at "very dim glow"
     
  7. Ron H

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    Can you provide a link to a corroborative statement? I can't imagine how a light bulb filament would survive for months when white-hot, but burn out when it was not even glowing.
     
  8. praondevou

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    Ok, I connected 10VDC to a 120VAC/100W light bulb. I have 260mA. Does DC count or does it have to be AC?

    I can leave it connected overnight and tell you what happens.

    EDIT: 1h later, no didn't burn.
     
    Last edited: Jan 10, 2012
  9. strantor

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    I read something somewhere about a chemical in the bulb that prevents the filament from burning out. At the proper intensity this chemical clings to the filament & below the proper intensity it clings to the bulb, allowing the filament to burn out. I probably butchered that explanation to the point it's not even correct, but that's how I remember it.
     
  10. Ron H

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    I suspect that you read about a Christmas light shunt.
    [​IMG]
     
    strantor likes this.
  11. hgmjr

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    Jan 28, 2005
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    I have my own theory about why incandescent bulbs eventually burn out.

    I believe the primary factor is the compromising of the vacuum that was originally established in the bulb at the time of manufacture. By compomise, I mean that the virtually oxygen-free environment within the glass becomes contaminated with oxygen. It is the presense of a small amount of oxygen inside the bulb that weakens the filament so that it eventually fails. The source of the oxygen are weak spots in the glass. The glass itself is very thin as can be confirmed from observing a broken bulb.

    I believe that during the bulbs installation into the socket, a small amount of skin oil is deposited on the outer surface of the glass. This skin oil is a weak acid and thus it tends to etch the glass where it has been deposited. In the presense of the heat given off by the bulb when it is on, the etching process is accelerated.

    Those familiar with halogen bulbs will recall that they have explicit warnings about handling the bulbs during installation. The bulb manufacturer even provides a small piece of material for use during installation to avoid touching the bulb during installation. The reason this caution is required is that halogen bulbs reach far greater temperatures and thus they would fail sooner than an ordinary bulb due to this etching effect.

    I have experimented with cleaning bulbs with light detergent and water and then installing the bulbs being careful not to touch them with my hands. I use a paper towel to handle the bulb. It has been my observation that bulbs installed in this way have consistently out lasted bulbs that were installed by hand.

    The amount and concentration of acid present on the skin varies widely over time so the influence on the bulbs life due to this effect is very unpredictable.

    Anybody can easily perform this experiment for themselves. I usually use a sharpie pen to write on the metal base of the bulb the date I installed it so that I can see how long the bulb lasted.

    hgmjr
     
  12. BillB3857

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    Light bulbs burn out when they can't hold any more dark.. Check out "Dark Sucker Theory" for a full explanation.
     
  13. thatoneguy

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    Wiki on Light bulbs, in lifetime section
     
  14. Ron H

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    Sorry. The statement I intended to question was the one above. I don't have any issues with your other comments.
    I don't see any corroboration of the above statement in the link you posted.
     
  15. thatoneguy

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    That was something I read somewhere when using banks of lights to discharge, it could be incorrect, or I forgot context.
     
  16. R!f@@

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    +1 ...... :D
     
  17. PaulEE

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    A light bulb is manufactured.

    A chemically inert gas is injected. Enough that the following occurs, but not enough that all heat from the filament is conducted away:

    This gas prevents atoms of tungsten (or whatever the filament is made of) from flying off and disappearing; rather, more often than not, the tungsten bounces back onto the filament.

    The filament is initially uniform upon manufacture. Eventually, it becomes non-symmetrical from the gas shielding thing going on above. Turning the bulb on and off introduce thermal stresses, but not as many as you'd think. Metal conducts both heat and electricity pretty well. Since the gas and vacuum is the only thing around the filament, there isn't so much a sharp temperature gradient, but eventually, even the small stresses that there are, along with physical vibration, break the filament.

    You could bury a light bulb in the ground and come back 200 years later with the bulb still intact. Skin residue will not eat away the glass. And, if you had the acid that COULD eat glass near you, I would stay very, very far away from you.

    Applying a voltage to a filament causes a current to flow. The current flowing causes power dissipation. The resistance is a square of the function of current of the bulb. Burning a light bulb out by applying "lower than rated power" sounds a bit far-fetched in the light cast by Ohm's Law...right? By the way, Wein-Bridge oscillators employ this very characteristic of light bulbs in regulating, very well, the amplitude of the output, by regulating the gain of the circuit, with the bulb in the feedback loop. I'm not 100% on the square-law relation, but it's an exponent greater or equal to two.

    Eventually, since a perfect vacuum does not exist, and since oxygen is inevitably in the bulb to some extent, the filament will oxidize over time near the seal of the base as well as from residual oxygen in the bulb.

    Halogen lamps get very hot. Skin oils on the surface of the glass cause uneven heat loss between the glass and air surrounding it. This is sort-of like putting the glass in a small vice and cranking on it...until...CRACK. Pressure in a halogen bulb is much higher than a regular bulb also, which causes this to occur more readily.

    ;)
     
    Last edited: Jan 11, 2012
  18. R!f@@

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    +2 :D

    Sheeeesh !!..every one is a professor..

    I dunno what or who to believe. Both makes sense.

    Careful Mate..you are now on thin Ice.
     
  19. PaulEE

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    Dec 23, 2011
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    Ha. Thin ice. How do oils on your hands magically eat glass? Hydrofluoric acid eats glass. Hand oils lubricate small watch gears.

    :p
     
  20. R!f@@

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    Careful Mate. Play nice, cause I like the way you talk.

    We need good minds.

    And please do not tread on thin Ice.
     
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