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Help with resistor

Discussion in 'General Electronics Discussion' started by dickie, Sep 26, 2013.

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  1. dickie


    Sep 26, 2013
    Hi guys

    I have a 12v 1.4 AH supply and a 4.5v bulb (3 cell) . Which resistor could i use to stop it blowing?

  2. Harald Kapp

    Harald Kapp Moderator Moderator

    Nov 17, 2011
    Impossible to say.
    You need to give the power rating of the bulb, too. Either tell us its wattage or the Amperes it uses at 4.5V.

    Whichever the outcome will be, you're going to loose a lot of power in that resistor (compared to what you use to light up the bulb). From the 12V source voltage 7.5V (12V-4.5V) will drop across the resistor. Since both the lamp and the resistor see the same current, the power in the resistor will be 7.5/4.5=1.7 times more than the power of the lamp.

    What exactly do you want to achieve? Chances are that there exist better possibilities than a resistor to achieve what you want.
  3. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    I would make a resistor out of two of these bulbs in series and place it in series with the bulb.

    (In other words, place three bulbs in series)
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