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Newbie:Current and voltage draw through light bulb

Discussion in 'Electronic Basics' started by Michael, Jan 31, 2007.

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  1. Michael

    Michael Guest

    I am fairly new to this and have several (simple) question:

    I have a krypton flashlight bulb (markings on bulb say 4.8v .5A). The
    original flashlight that the bulb was removed from used 4 AA batteries.

    1. Wouldn't the 6v from the batteries be 'pushing' the bulb a bit and
    reducing its life?

    2. If I want to wall-wart power this bulb, what voltage wall wart would be
    recommended?

    3. Since most wall warts (unregulated) supply higher voltage than the
    sticker says, what method
    should I use to bring down the voltage to the recommended voltage
    (regulator, resistor, etc)
    (I remember once trying to do something similar to this project and using a
    resistor. Even
    though the resistor was a high wattage resistor it got REALLY hot)

    4. I think this question probably will have the same answer as #3, but if I
    want to dim the bulb what
    method would be recommended?

    Thanks for you assistance
     
  2. John Fields

    John Fields Guest

    ---
    For alkalines, yes, but at 1/2 an amp the internal resistance of the
    cells would drop the voltage somewhat, and as the batteries were
    drained the voltage would also drop. For NiCd and NiMH
    rechargeables I believe their "flat" voltage region is around
    1.2V/cell, so that would work perfectly.
    ---
    ---
    4.8V @ 0.5A, but since that's not a readily available value I'd get
    one rated at 5V @ 0.5A and drop the voltage to the lamp with a
    resistor:


    Vin - Vlamp 5.0V - 3.8V
    R = ------------- = ------------- = 2.4 ohms
    Ilamp 0.5A

    The resistor would need to dissipate:


    P = IE = Ilamp * Vin - Vlamp = 0.5A * 1.2V = 0.6 watts


    Which means you should use a resistor capable of dissipating at
    least 1 watt; a higher wattage allowing the resistor to run cooler.
    ---
    ---
    Most wall-warts put out their rated voltage when they're fully
    loaded, so in your case, since your load has an odd voltage
    requirement you'd want to pick a wall-wart with a slightly higher
    voltage but which is rated for the same output current as your load,
    then drop the extra voltage with a series resistor, as illustrated
    above.
     
  3. When you draw .5 A from the batteries you wind up with 4.8 V (not strictly
    true but it works).
    4.5 V.
    Use a multi voltage unit, run at 3 V.
     
  4. jasen

    jasen Guest

    5V ones are fairly common, use one of them.
    5V ones are usually regulated :), or you could use a 4,5V unregulated one
    that'd probably be close enough.
    to drop 0.2V at 0.5A you need a ideally a 0.4 ohm resistor, 0.39 will
    probably be will close enough, actually 5V would be close enough.

    usually it's just easier to buy a nightlight,

    Bye.
    Jasen
     
  5. Michael

    Michael Guest


    Thanks
     
  6. Michael

    Michael Guest

    Thanks
     
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