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Discussion in 'Misc Electronics' started by Allan Adler, Oct 10, 2003.

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  1. Allan Adler

    Allan Adler Guest

    I have some naive questions about light bulbs that maybe someone here
    knows how to answer.

    (1) What is the theoretical voltage-current relationship for a light bulb?
    I realize this depends on aspects of its construction, including the
    material the filament is made of.
    (2) The resistance of a light bulb apparently increases with voltage but
    not linearly. What is the theoretical voltage-resistance relationship
    of a light bulb?
    (3) What mechanisms explain the relationship among voltage, current and
    resistance of a light bulb and how do I compute them? For example,
    how do the temperature and the work function of the filament and
    the resulting electron cloud around the filament behave and how do
    I compute their effect, if any, on current and resistance.

    These questions derive from my continuing attempts to read Kloeffler's
    book, Electron Tubes. I realized that the problems I was having with it
    were all my own fault, caused by plunging into the chapter I was interested
    in instead of reading the book carefully from the beginning. Often I can
    get away with it, but Kloeffler's book is written a lot more carefully than
    I realized and even the parts that I considered too trivial to read contain
    some information that is necessary for understanding the conventions of rest
    of the book. So, now I'm starting to appreciate the book. In particular,
    it is nice to see that he starts with light bulbs to illustrate his
    graphic techniques. I'll refer to a light bulb as a "unode". But he has
    no theoretical discussion of the characteristics of unodes and I would like
    to fill that lacuna.

    Ignorantly,
    Allan Adler


    ****************************************************************************
    * *
    * Disclaimer: I am a guest and *not* a member of the MIT Artificial *
    * Intelligence Lab. My actions and comments do not reflect *
    * in any way on MIT. Moreover, I am nowhere near the Boston *
    * metropolitan area. *
    * *
    ****************************************************************************
     
  2. Boris Mohar

    Boris Mohar Guest

     
  3. The resistivity of tungsten (the material normally used for lamp
    filaments), as for most metals, increases with temperatures. This
    change in resistance is a property of the material, and not something
    that the light bulb designers specifically designed.

    I suspect that tungsten is used for lamp filaments because it has a
    very high melting point, and a fairly high resistivity. This means
    that the filament can be heated white-hot without fear of it melting,
    and you don't need a long wire, or very thin wire, to make a useful
    lamp.
    Ohm's Law.
    As I recall, the filament or cathode in a vacuum tube is specially
    treated to make it emit useful quantities of electrons. The filament
    of a light bulb will not be so treated, because it doesn't need an
    electron cloud around it.
    A light bulb is not a "unode" (whatever that may be). A light bulb
    has very little in common with a vacuum tube.
     
  4. JeffM

    JeffM Guest

    A light bulb is not a "unode" (whatever that may be).
    At various points in the evolution of incandescant lights,
    the envelopes have been evacuated
    rather than filled with (relatively) inert gas.

    For historical perspective,
    the Edison Effect was discovered by Thomas Alva
    when he was messing around with light bulbs.

    This was long before John Ambrose Fleming & Lee deForest put it to use.
     
  5. JeffM

    JeffM Guest

    A light bulb is not a "unode" (whatever that may be).
    At various points in the evolution of incandescant lights,
    the envelopes have been evacuated
    rather than filled with (relatively) inert gas.

    For historical perspective,
    the Edison Effect was discovered by Thomas Alva
    when he was messing around with light bulbs.

    This was long before John Ambrose Fleming & Lee deForest put it to use.
     
  6. Indeed. For that matter, a light bulb does not even enclose a vacuum.

    Harry C.
     
  7. Some do. Roghly on an average, it is bgetter to use a vacuum than to
    use the usual argon-nitrogen mixture if the wattage is less than 10 watts
    per centimeter of apparent filament length (or maybe length plus diameter,
    with these dimensions being the aparent-"visible" overall dimensions).
    With near or over 10 watts per centimeter, light bulbs usually get a gas
    fill.

    Some examples among 120V ones in the USA:

    Tubular refrigerator bulbs of 25 and 40 watts have a vacuum, and the (not
    especially easy to find) 60 watt one has a gas fill.

    "A19" "regular" bulbs with the coiled-coil filament about 2-2.5 cm long
    normally get a gas fill from 25 watts on up. 120V bulbs with the
    multi-supported C-shaped filament 40 watts and up get a gas fill. 120V
    bulbs 15 watts and less generally have a vacuum.

    The reasoning: A gas fill reduces filament evaporation by having gas
    atoms "bounce" evaporated tungsten atoms back onto the filament. This
    permits the filament to operate at a higher temperature for a given life
    expectancy. (Note that gas filled ones have a trend of producing whiter
    light than vacuum ones.) The higher temperature makes the filament's
    radiation more in the visible and less in the infrared (although still
    much more infrared than visible).
    The drawback of a gas fill is that it conducts heat from the filament -
    and this means energy going in that is not radiated at all. Heat
    conduction is nearly enough proportional to filament visibly apparent
    length but surprisingly independent of visible apparent diameter, since a
    wider filament has a thicker "boundary layer" of gas around it and the
    thicker boundary layer has a lower temperature gradient to nearly cancel
    the greater circumference. Thinner filaments in a gas fill have almost as
    much heat conducted from them (per unit length) as thicker ones, which
    makes gas heat conduction a higher percentage of input power with the
    thinner filaments. This is why filaments with more wattage per
    centimeter of apparent length do better with a gas fill and ones with
    lower wattage per centimeter of apparent length do better with a vacuum.

    - Don Klipstein (, http://www.misty.com/~don/index.html)
     
  8. Can you tell us how two light bulbs compare: the U.S. 120VAC light
    bulb compared to the Euro 240VAC light bulb. I assume that the U.S.
    filament is shorter and heavier than the Euro. So is it more
    efficient?

    I have an old Tensor lamp that uses the #91 (IIRC) light for autos.
    Thing was always needing a lamp (maybe because I used it a lot!), but
    they were readily available at the store. In that case, it didn't
    seem as if the heavier filament was helping it last longer.

    This weekend I bought a light bulb from the auto store. It's a
    regular tubular light for a car dome light, but it has a blue coating
    on it to make the radiated light blue. It draws more than .83A at
    12V, so it eats up more than 10W, and gets hot (I'm surprised the blue
    coating doesn't melt). Yet it puts out surprisingly little blue
    light, in fact I'd say a dozen blue LEDs would easily outshine it.

    --
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  9. actual_current = rated_current * ((applied_voltage / rated_voltage) ^ 0.55)
     
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