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Homework Help: Light Bulb Ballast (Please help)

  1. Dec 2, 2009 #1
    The room temperature resistance of the tungsten filament in an ordinary incandescent light bulb is about 1/15th of the hot resistance exhibited during the normal operation at 117 VAC. Therefore, when you throw the switch to turn on the light bulb there is a huge surge of current in excess of the steady-state operating current until the filament heats up. This can cause the light bulb to "blow out."
    So you need an inductor to not allow current change too suddenly and a capacitor to prevent arcing.

    Design an LC circuit to protect the tungsten filament of an ordinary 75-Watt incandescent light bulb from over the current-transients greater than 50% of the normal operating current at the moment of turn-on!

    So can anyone help or give me a head start how to make this circuit!
  2. jcsd
  3. Dec 2, 2009 #2


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    Staff: Mentor

    Welcome to the PF. Adding the inductor in series will limit the inrush current. Can you explain how that works, and show us the equation relating the voltage across an inductor to the current through it?

    And where do you think they are talking about an arc forming? Why would there be an arc, and when would it occur?
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