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Which is more efficient? 12V, 20W bulb or 6V, 25W

Discussion in 'Electronic Basics' started by Tibur Waltson, Jan 15, 2004.

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  1. Today's spotlight is so bright that when I shine a light at close range, for
    example looking for a black widow, I could be blinded temporarily. The
    spotlight uses a 6 volt lead-sealed battery and puts out 5000,000 candle
    power and uses a 25W bulb. Will the 12V-bulb compared to the 6V-bulb
    last two and a half times longer using 6V as a power source?

    12V-Bulb 1.67A, 7.2-Ohm, 20-Watt
    6V-Bulb 4.17A, 1.44-Ohm, 25-Watt
  2. Bob Masta

    Bob Masta Guest

    If you use a 12V bulb with a 6V battery it will last
    "forever". Bulb life is inversely proportional to
    some high power of the rated voltage (like the
    11th power or something), so even a slight
    reduction results in tremendous life increase.
    HOWEVER, you will get a lot less light output
    than simply half!

    Bob Masta

    D A Q A R T A
    Data AcQuisition And Real-Time Analysis
  3. happyhobit

    happyhobit Guest

    Your 'Subject line' is about efficiency but your question is battery life.
    Bob answered the battery life, let me address efficiency.

    A 6 volt lamp on 6-volts is more efficient than a 12-volt lamp on 6-volt
    because of the temperature it's running at. 10-percent less voltage results
    in 30-percent less candlepower.

    A Halogen lamp would be more efficient but would probably have a shorter

    If you include your 'application' into the equation (Is it more light than
    you NEED?) then a 12-volt 10 watt might be more 'efficient'.
  4. happyhobit

    happyhobit Guest

    I'm sorry I meant a 6-volt 10 watt not a 12-volt 10 watt.
  5. This would mean that a 4.7-volt 10 watt Halogen would be more efficient.
    If only I can figure out how to prevent the filament from shorting out.
    Thanks all.
  6. Ian Stirling

    Ian Stirling Guest

    You need to find a 6V 10, or 5W light.
  7. happyhobit

    happyhobit Guest

    Interesting idea. If you could pulse it, it might work. I know that an LED
    pulsed with a higher current appears brighter than one running on a steady
    but lower current.

    I was wrong when I said that the halogen lamps had a shorter life. Newer
    halogen lamps last longer than conventional vacuum-incandescent lamps.
  8. Ian Stirling

    Ian Stirling Guest

    Which arn't that conventional.
    Most lamps are inert gas filled.
  9. Vacuum lamps are rather common.

    Conventional incandescent lamps with a wattage near or over something
    like 10 watts per centimeter of visibly-aparent filament length usually
    get the traditional argon-nitrogen fill. The gas slows down filament
    evaporation, and that allows designing for a higher filament temperature
    that is better for radiating visible light.
    But if the power per unit of visibly-apparent filament length (or maybe
    length plus diameter) is less than something like 10 watts per centimeter,
    then a vacuum is better than the usual argon-nitrogen mixture. A gas fill
    in these lamps would conduct enough heat from the filament to reduce
    efficiency of producing visible light more than redesigning for a higher
    filament temperature (with argon-nitrogen) would increase the efficiency
    of producing visible light.

    As it turns out, heat conduction from a wire (or a rod) to a surrounding
    gas is usually nearly enough proportional to the length of the wire, but
    does not vary proportionately with the diameter. If you increase the
    diameter of a hot wire, you also increase the thickness of the "boundary
    layer" of hot gas around the wire, and that decreases the temperature
    gradient within the boundary layer, often (not always) nearly as much as
    the circumference of the wire is increased. So heat conduction by a
    gas from a long thin filament is more than from a shorter, thicker or
    wider (even if tightly coiled and wider) filament that has the same
    exposed surface area. This explains why 15 watt 120V incandescent lamps
    have a vacuum while 12V lamps of the same wattage have a gas fill.

    - Don Klipstein ()
  10. happyhobit

    happyhobit Guest

    Are you saying that there is no such thing as a 'conventional
    vacuum-incandescent lamp"? I don't personally know, or care, if there are
    more inert gas or vacuum incandescent lamps. That's not what I was saying.
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