120V vs. 240V on Light Bulbs

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jeff48356

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Consider a 100W light bulb designed to operate on 120V, which outputs 1530 lumens and has a life of 1500 hours.

If 240V were to be applied to this bulb,

1) How long would it last for?
2) How many lumens would it output?
3) How many watts of power would it consume?

What is the formula to calculate this given any light bulb's ratings?
 
Ohms Law

R=E?/P

R=120*120/100

R=14400/100

R=144 ohms


Now apply 240v instead.

P=E?/R

P=240*240/144

P=57600/144

P=400 watts.

I don't know how to calculate lumens, but my guess it would also follow the inverse-square laws (6120 lumens).

How long will it last? I'd say about 1-4 seconds.
 
jeff48356 said:
Consider a 100W light bulb designed to operate on 120V, which outputs 1530 lumens and has a life of 1500 hours.

If 240V were to be applied to this bulb,

1) How long would it last for?
2) How many lumens would it output?
3) How many watts of power would it consume?
Click to expand...



1) A fraction of a second
2) 0
3) 0
 
In this situation, Ohm's law does not apply with constant R, because the temperature of the filament changes so drastically.

A _very rough_ approximation for incandescent lamps is as follows:

current scales with applied voltage ^ 0.5
power scales with applied voltage ^ 1.5
efficiency scales with applied voltage ^ 2
light output scales with applied voltage ^ 3.5
life scales as applied voltage ^ -13

(Here is another reference with slightly different numbers)
http://www.walamp.com/Product_Pdfs/isl297b.pdf

These approximations fall apart when something happens such as the filament melting. And even if the filament doesn't melt, they will fall apart when operating at such a large difference from nominal. But assuming that they hold:

The expected life would be 10 minutes
The expected light output would be about 17000 lumen
The expected power consumption would be about 280W

-Jon
 
poof!

poof!

in this case would the proper term be "flashbulb" ?
 
winnie said:
The expected light output would be about 17000 lumen
Click to expand...

Fabulous! If you do the math, that gives a luminous efficacy of 60.7 lumens per watt. I now have a way to use incandescent lamping outside like the Architects want and still meet LEED minimum of 60! :D

I'll just have to start forwarding the hate mail frm the Maintenance staff to them too. :wink:
 
Not dangerous if you stand far away! I made up a service for a friend's brother's double-wide. POCO didn't connect the ground (or would it be common, neutral, thats another thread?) to the center tap, that was wrapped with white tape for at least a foot. Anyway, he turned on the main, all of the lights that were in the on position exploded. (his description) He called me and said I messed up....... anyway.......... I went over and checked it. I looked up at the weather head and said oh oh. I pulled the fuse at the transformer, pulled the meter and swapped the wires. Called POCO and told them what I had done and to put a new seal on. I got scolded by POCO of course...........
 
jeff48356 said:
Is this true no matter the wattage of the bulb?
Click to expand...

I expect that most bulbs will simply melt their filament before they get to a nice steady state run condition...but some high wattage, low color temperature bulbs might actually run.

How the bulb 'starts' can also make a significant difference. When an incandescent starts, the filament resistance is very low, and normally about 10x full current will flow for an instant. If you start a 120V at 240V, you get about a 20x inrush, which will blow any weak spot in the filament. By ramping up the voltage (say with a variac), the filament is already hot and current is more limited.

jeff48356 said:
I never tried this before. Is it a dangerous experiment?
Click to expand...

That would surely depend upon how you set the experiment up. There is a very real chance that the filament would melt and create an arc, and that arc could dissipate significant power, making the bulb explode.

-Jon
 
How bout' the opposite?

How bout' the opposite?

I know this seems obvious, but in cold northern Michigan, we used to install two or more 120V light bulbs in "series" with 120V supply. This was used in outdoor well pits to keep pipes from freezing. Bulbs were of course, dimmer than normal, but would last for years due to lower amperage flow through them. Still gave out enough heat for the purpose. Ohm's law does apply here as resistances in a series circuit are accumulative. (added together)

FYI

Dick K.:smile:
 
jeff48356 said:
Consider a 100W light bulb designed to operate on 120V, which outputs 1530 lumens and has a life of 1500 hours.

If 240V were to be applied to this bulb,

1) How long would it last for?
2) How many lumens would it output?
3) How many watts of power would it consume?

What is the formula to calculate this given any light bulb's ratings?
Click to expand...

1A. a mircosecond or two then blow up smithens [ try that on 277 v circuit it will be far much worst ]

2A 15000 + Lumines it will be very white colour before it will flashover to bleu colour then you know the bulb ready to explode due the arc flashover inside the bulb.

3A 200+ watts and good chance it can trip the breaker as well [ see above comment ] when the arc start the current will ramp up like crazy if the bulb interal fuse dont catch it the breaker will catch it next.


for some of the " old " school electricians if you recall rembering the ole style 277 v indscent light bulbs they always exploded when they burn out ??

[ this is true with higher wattage types]


Merci, Marc
 
rkrieger said:
Ohm's law does apply here as resistances in a series circuit are accumulative. (added together)
Click to expand...

I use some 220V bulbs in work lights at 120V as 'heat lamps' :)

Apparently some professional chocolate melting equipment uses ordinary light bulbs as the heat source, and there is always the 'easy bake oven' :)

A note about lamps in series. If you have a single lamp, you can use Ohm's law to calculate its resistance from the voltage and power rating; so a 120V, 240W lamp would have a resistance of 120^2 / 240 = 60 Ohms

If you take two of these lamps and put them in series and operate them at 120V, the series resistance will be _less_ than 120 Ohms. Resistances in series add up, but because of the different operating conditions of the filaments, the resistance of each bulb will be less than 60 Ohms. At room temperature with a cold filament, these lamps would have a resistance of between 3 and 12 Ohms.


-Jon
 
Ohm's Law

Ohm's Law

Jon: In my simple mind, I used to call the two different bulb resistances as "static" (no current flowing in filament) and "dynamic" (current flowing in filament). The resistance and therefore the current does indeed change as the filament takes on different properties while being heated up by current. You will get very inaccurate calculations if you attempt to determine current flow using Ohm's Law by measuring filament resistance with bulb "off".

I wonder if using listed wattage as one of the two needed values in Ohm's Law to calculate resistance is super accurate either. Do all bulbs rated at 100W each consume exactly 100W or is it "close enough" for the calculation??

Guess I have more "spare" time today to ponder this............:smile:

Thanks, RLK
 
Tungsten filament incandescent lamps are not pure resistors so Ohms' law is not directly applicable. When they are cold (i.e. have not been turned on) they have an extremely low resistance which increases (and self limits the current) as they get warmer. This is why lamps normally "burn out" when they are first turned on.

I believe the old carbon filament lamps had relatively constant reistances.

Edit for explanation: Ohm's Law is not directly applicable beacuse a 100W light bulb may or may not draw 100W at proper operting temperature, but it definelty does not when it is first turned on.
 
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rkrieger said:
Do all bulbs rated at 100W each consume exactly 100W or is it "close enough" for the calculation??
Click to expand...
Well, there are bulb manufacturing and material tolerances, and then there's supply-voltage variations.
 
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