
#1
Jan1509, 11:23 AM

P: 5

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. Thanks. 



#2
Jan1509, 11:37 AM

P: 1,784

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.




#3
Jan1509, 12:15 PM

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P: 8,961

The light output is proportional to Current^{4} and so roughly also voltage^{4}
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. 



#4
Jan1509, 12:37 PM

P: 359

Car Battery and 120vac 100 watt bulb
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. 



#5
Jan1509, 12:53 PM

P: 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).




#6
Jan1509, 01:27 PM

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Yes it will produce some power.
The problem is that the power dissapation is is proportional to I^{2}, so (assuming the same resistance) 1/10 the voltage produces 1/100 the power. But the light emitted is temperature^{4}, 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! 



#7
Jan1509, 02:15 PM

P: 359

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. 



#8
Jan1509, 02:32 PM

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I hadn't appreciated the resistance drops that much.
For calculations about lamp output you normally use V^{3.5} (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. 



#9
Jan1509, 03:01 PM

P: 359

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..
http://www.newscientist.com/article/...rstlight.html Years ago a work colleague couldn't understand why fuses were blowing feeding a wardrobe striplight. The switch on current was at least ten times the normal current. 


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