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Car Battery and 120vac 100 watt bulb

  1. Jan 15, 2009 #1
    Can a 12 volt car battery illuminate a 120vac 100 watt bulb ? I not sure if a bulb that is rated for 120 volts can still be powered with a battery pushing 12 volts.
    I think the 100 watt bulb only needs 1 amp to be illuminated with the 120 volts.

    I'm still trying to figure out the basics.
  2. jcsd
  3. Jan 15, 2009 #2
    The filament will not be nearly as hot at 12 volts as it is at 120 V so the light will appear much dimmer and more reddish.
  4. Jan 15, 2009 #3


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    The light output is proportional to Current4 and so roughly also voltage4
    If you use only 1/10 the voltage - you are only going to get 1/10000 the light - you would barely see a glow form a 100W bulb.
  5. Jan 15, 2009 #4
    The resistance of a lamp at full voltage can be 10 times of the cold resistance so simple Ohms Law does not apply.

    I have tried a 240 v 60 W bulb on 24 V and there is no light at all. On 30 V it glows dimly.
  6. Jan 15, 2009 #5
    Thanks for your replies. My thinking was wrong. I was thinking that the light wouldn't illuminate even dimly. I thought since the bulb was rated as 120 volts, that the filament must have been designed so that 12 volts couldn't generate enough fiction/resistance to produce the light. But I guess ohmns law says that any amount of volts (no matter how small) must produces some amount of power ( no matter how small). So I guess a 100 watt bulb can produce a dim light with a small amount of power ( maybe 1 watt of power).
  7. Jan 15, 2009 #6


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    Yes it will produce some power.
    The problem is that the power dissapation is is proportional to I2, so (assuming the same resistance) 1/10 the voltage produces 1/100 the power.
    But the light emitted is temperature4, so slightly less electrical power means a lower temperature means MUCH less light

    On the other hand a 100W bulb run from a 12V car battery will last for years!
  8. Jan 15, 2009 #7
    The resistance of a 240 V 60 W is calculated as 960 Ohms.

    Rounded off figures......

    Cold it is 70 Ohms.

    At 12 V it is 170 Ohms.

    At 24 V it is 280 Ohms.

    So cold - full voltage the resistance increases by 14 times.

    1/10 - full voltage the resistance increases by 3.5 times.

    So at 1/10 Voltage the input wattage is 2 Watts or 1/30 of the full voltage Wattage not 1/100th as you would expect with a constant resistance.

    But there in very little light output at 2 Watts. There is a VERY feint glow in a completely darkened room. Milliwatt of light or less.

    "lower temperature means MUCH less light" .... Yes indeed.
    Last edited: Jan 15, 2009
  9. Jan 15, 2009 #8


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    I hadn't appreciated the resistance drops that much.
    For calculations about lamp output you normally use V3.5 (less than 4 to account for resistance) but thats at near specification voltage.

    I just tried it with 12V and a 60W bulb (all my 100W were replaced by CF) and you can see the filament dimly glowing orange but it wouldn't illuminate anything.
    Your eye has an a amazing range of sensitivity.
  10. Jan 15, 2009 #9
    Ten times seems to be a figure quoted a lot. I heave found 14 times (cold/hot) and somebody has measured a bulb at 23 times..


    Years ago a work colleague couldn't understand why fuses were blowing feeding a wardrobe strip-light. The switch on current was at least ten times the normal current.
  11. Jan 16, 2009 #10


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    In my experience it's 15x less resistance at room temperature. At any rate, we're all in the same ballpark.

    Not only is the radiated power less, it is shifted farther into the IR at these lower voltages.
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