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OT?: Lightbulb life, curiousity

Discussion in 'Electronic Basics' started by [email protected], May 28, 2005.

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  1. Guest

    I don't know if these are the right places to ask this, but it's the best
    I could find on the server. Sorry if it's OT:

    I was just thinking today what influences a lightbulb's life more.
    They're rated in hours, but I know for a fact they burn out sooner if you
    turn them on and off more. Kinda like a HDD, but to way less extent.
    But, how far does that extend.. a HDD I've heard lasts longest if it's
    on/off 1x/day, assuming you're going to sleep and/or not use it for at
    least 10-12hrs. I wonder how it works for lightbulbs, say 40watt. I
    assume it would matter more with more wattage because there'd be more
    diff. between hot and cold states. So, if you're going to leave the light
    off for 2hrs, should you not and leave it on instead? You know, what's
    the min. time you'd have to leave it off for before you actually cost life
    due to on/off even though you're saving hrs. Hey, you think about these
    types of thing while you're sitting on the can! So anybody know?
  2. Mick Sharpe

    Mick Sharpe Guest

    When you switch on an incandescent lamp, some the tungsten that the filament
    is made of evaporates and remains in a gaseous state until the lamp is
    switched off, when the tungsten gas will condense out. Unfortunately, since
    the envelope of the bulb offers a much greater surface area than the
    filament, the gas condenses onto the bulb, where it remains. As you switch
    the lamp on and off, metal slowly migrates from the filament to the bulb
    envelope, turning it black. Eventually, the filament becomes too thin to
    accomodate the supplied current and the lamp 'blows'.

    As to your optimisation problem, this should probably be addressed to
    sci.maths where I am sure it will cause a good deal of head scratching. :)
  3. tesseract

    tesseract Guest

    Mick Sharpe writes:

    "When you switch on an incandescent lamp, some the tungsten that the
    is made of evaporates and remains in a gaseous state until the lamp is
    switched off, when the tungsten gas will condense out. Unfortunately,
    the envelope of the bulb offers a much greater surface area than the
    filament, the gas condenses onto the bulb, where it remains. As you
    the lamp on and off, metal slowly migrates from the filament to the
    envelope, turning it black. Eventually, the filament becomes too thin
    accomodate the supplied current and the lamp 'blows'. "

    Quite true, and there are additional factors. Heating and cooling add
    stress. If the
    filament is a coil, it is an inductor, and AC self inductance will
    cause it to flex, it possibly
    being less flexible when it is cool. There may be additional factors
    Fluorescent bulbs are now fairly cheap, and power leds are just over
    the horizon,
    available but pricey. Both convert more of the electrical energy into
    light rather than heat.
    I know that some exceptional incandescents that were never turned off
    lasted over twenty years.

  4. w_tom

    w_tom Guest

    Power cycling has virtually no effect on light bulb life
    expectancy. Factors that determine life expectancy are
    filament temperature (ie voltage) and vibration (when filament
    is hot). If power cycling adversely affected light bulb life,
    then orange lights on traffic signals - flashing all night -
    would be the first to fail. Reality is completely different.
    Hours of operation - be it the red, green, or orange-
    determine light bulb life expectancy. Power cycling only
    affects life expectancy when the human uses feeling rather
    than first learn facts.

    No science fact says power cycling is destructive, as
    claimed. But so many will believe without first demanding the
    numbers. Show me? Junk science concepts such as 'power
    cycling damage' can't provide those numbers. Why? For some,
    feelings are good enough to proclaim a fact. No wonder so
    many people believe and worship Rush Limbaugh.

    Take a 120 volt light bulb. Increase the line voltage to
    128 volts. What happens to bulb life expectancy? Bulb lasts
    only 50% of its predicted life. Notice new here. The
    industry benchmark for such information is the IES Lighting
    Handbook. If power cycling was so destructive, then those
    making such claims can cite that fact from the IES.

    Those who want honest facts will appreciate why filament
    vaporizes. The relationship between line voltage and life
    expectancy is an inverse exponential to the power of 13. How
    do you know which is accurate? Only one cites both a source
    AND the numbers. Power cycling is destructive only when myths
    and personal feelings replace science.
  5. w_tom

    w_tom Guest

    All good and nice as long as one doesn't first ask questions
    such as why? Assumed is that a bulb lasted due to no power
    cycling - using reasoning in direct violation of well proven
    industry science.

    Take a 120 volt light bulb rated for 1500 hours. Illuminate
    it with 85 volts. How long will it last? About 15 years -
    running continuously. Longer if power cycled. So why did
    that bulb last 20 years? One simply wildly speculated - no
    power cycling - and then declard that speculation as fact.
    Speculated in direct contradiction to numbers and facts
    published generations ago.

    Which has more credibility? One who speculates only because
    of an observation? Or one who learns how light bulbs fail AND
    learns the well proven industry numbers? Junk science is
    characterized by no numbers and incomplete speculation. Junk
    science says power cycling damages light bulbs. If true, then
    where are industry citations that support that claim?
    Industry makes no such claim. Some people just know because
    they saw something and then speculated. At best, that
    speculation does not even qualify as a theory. However, they
    then push that specualtion as if it were fact.

    No numbers further demonstrates that power cycling damage is
    a myth. Filament temperature, as defined by voltage,
    determines a bulb's life expectancy. Numbers provided in the
    other post.
  6. w_tom

    w_tom Guest

    Using Mick Sharpe's reasoning, we then ask how to extend the
    life of an incandescent bulb. Light bulbs on 60 Hz
    electricity power cycle 120 times per second. Therefore if we
    power the bulb with DC electricity, then no power cycling.
    Therefore the bulb must last longer because some the tungsten
    no longer evaporates and remains in a gaseous state until the
    lamp is switched off 120 times per second.

    Then we apply experimental evidence. A bulb at 120 VDC
    lasts same as a bulb at 120 VAC RMS. Why? Voltage and hours
    of operation - not power cycling - determine bulb life

    Next, someone will claim that the speed of power on affects
    bulb life expectancy. Somehow they just know even without
    science based facts, citation, or numbers. No wonder the
    president could lie about weapons of mass destruction and so
    many blindly believed him. Feeling is now sufficient to
    declare something as fact.
  7. John Fields

    John Fields Guest

    Well, considering that the filament _does_ have a thermal time
    constant in excess of 8.3ms leads me to believe that being fed with AC
    VS DC can't _really_ be likened to power cycling, where the filament
    would be expected to be powered on until it reached its normal
    operating temperature then powered down and allowed to cool to near
    ambient before the cycle repeated.
    That's not totally true, in that local hot spots on the filament tend
    to boil off tungsten more quickly at those locations, causing the
    filament to "neck down" there and provide high-resistance spots where,
    when the filament is turned on, most of the voltage applied to the
    cold filament will be dropped, causing that spot to vaporize and the
    filament to separate. I'm sure you've noticed that incandescent lamps
    almost always fail when they're turned on, precisely for that reason.

    So, if the lamp were to be operated continuously, the total number of
    lamp-on hours would be greater than if the lamp were operated
  8. Mick Sharpe

    Mick Sharpe Guest

    Many thanks for your most courteous correction. I sometimes wonder what I'm
    doing here answering honest questions as well as I can :)
  9. spudnuty

    spudnuty Guest

    I've actually have a carbon filament bulb that I got from Kennedy
    Webster in Chicago. Its been burning every night from 6PM to 7PM for
    about 7 years now in my front hall . It burns very orange perhaps
    ~22,000º K and it is VERY sensitive to shock and vibration even when
    they're cold. These are often used in antique fixtures and it's
    suprised me how long it's lasted. I always thought tungsten was
    They're most famously used in the Monadnock building in Chicago.
  10. w_tom

    w_tom Guest

    Being politically correct is akin to lying for a self
    serving gain. I am not politically correct; have no interest
    in being so corrupt. Mick Sharpe obviously was not insulted
    if the objective is accurate facts.

    Fact remains that incandescent life expectancy is a function
    of filament temperature (voltage) and other environmental
    factors such as mechanical shock when filament is hot. This
    assumes a statistically average bulb; making manufacturing
    defects (ie hot spots created by a production defect)
    irrelevant. The underlying point is why some promote myths
    not based upon science.

    Again the lessons of history are cited. Many even believe
    lying politicians who don't provide numbers - be it Saddam's
    weapons of mass destruction, or people of German ancestry
    persecuted in Checkoslavakia. Either way, its called
    propaganda. How do politicians, et al get away with lies? No
    numbers. No responsible sources. And people who just blindly
    believe these lies because it is the first thing they are
    told. Beliefs based upon emotion rather than first demanding
    supporting numbers and facts. Again, it's called
    propaganda. A glaring symptom of propaganda is no numbers. If
    they provided numbers, then their lies could be easily

    No insults posted or intended. Just a blunt fact about
    light bulb life expectancy and similar examples from history.
  11. Guest

    And where are your numbers ?
    troll ?
  12. Mick Sharpe

    Mick Sharpe Guest

    It is your bluntness, Sir, that is the problem
  13. tesseract

    tesseract Guest

    I wish that people who require numbers of others would post a few of
    their own.
    A little tolerance of others and a less superior tone would be
    Try posting political posts on soc.politics, where they belong.
    Remember that most come on groups for an interesting discussion, which
    differing opinions, and not to function as the therapist for someone
    wishing to make some
    obscure correlation between filament life and the truthfulness of
    Let's try for more light and less heat.

  14. Mick Sharpe

    Mick Sharpe Guest

    Amen to that!

    Tungsten halogen, or cold cathode? :)
  15. John Fields

    John Fields Guest

    Your relegation of the hot spots to irrelevance because they don't fit
    into your preconceived notion of how a "statistically average"
    filament should behave is disingenuous and is being used as a point
    from which you can segue into a political diatribe. The _fact_ is
    that it's precisely the anomalies in the filament which lead to the
    creation of hot spots, i.e. its inhomogeneity, which causes turn-on
    failure. As you've been told several times, this is _not_ the venue
    where your political spew is welcome, so take it somewhere else.
  16. Mick Sharpe

    Mick Sharpe Guest

    I'm interested to know when deposition of the filament metal on the bulb
    envelope actually occurs, assuming that it is filament metal that turns the
    bulb black. Is it whilst the lamp is switched on and in a pretty much
    equilibrium state, or is it during turn-on when the lamp is warming up, or
    turn-off when it is cooling down, as I have been led to believe? Anyone know
    or have an opinion?
  17. m II

    m II Guest

    Funny how he sounds identical to Gimminy Boob and John P* Bengi, those fountains
    of misinformation.

  18. John Fields

    John Fields Guest

    A conventional (non - halogen) incandescent lamp is _never_ in a state
    of equilibrium when it's operating because there is no mechanism in
    place to return the evaporated tungsten to the filament. Instead,
    since the envelope is cooler than the filament, the tungsten
    evaporated from the filament condenses on the inner wall of the
    envelope, blackening it. As a consequence of the evaporation of the
    tungsten, the filament narrows because of the metal it's lost.
    Because of impurities in the tungsten some spots on the filament "boil
    off" more metal than others, causing those spots to narrow more than
    others, eventually causing a failure to occur at the weakest spot.

    In order for an incandescent lamp to run more nearly in equilibrium
    (that is, for the rate of metal being returned to the filament to
    equal the amount of metal leaving the filament), a halogen is used in
    the fill gas. For more detail, Google "halogen cycle" or go here:
  19. Mick Sharpe

    Mick Sharpe Guest

    If metal is evaporated continuously while the lamp is in operation, may I
    conclude that the idea that power cycling affects lamp life is an old wives
    tale, at least as far as evaporative metal transfer is concerned?

    Thanks for the link, BTW
  20. Pig Bladder

    Pig Bladder Guest

    Less Fililng!
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