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Factors on which wattage of electric bulb depends

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ericgibbs

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On what Factors does wattage of electric bulb depend on?

Ive seen 60 W bulb, 100 W bulb etc, what causes that?
Since P= IV. is it possible to Switch on the bulb using various values of I and V, where thier product reults in 60W /100 W ?

Its Ohms Law.

A 100Watt for 230V operation passes a current of 0.435Amps, so its Resistance is R = V/I = 230/0.435 = 528 Ohms when HOT.
 

Noggin

Member
Yes, you can lower the voltage to the bulb to get lower light output from it. This is often done with a variac (dimmer). You probably don't want to put 100 watts through a 60 watt bulb though.
 

ericgibbs

Well-Known Member
Most Helpful Member
Does this mean that the same bulb can be used in various voltages??? " to give the same output "

No you cannot get the same light output when using the same bulb on lower voltages.
A bulb [tungsten lamp] rated for 230V will work a 110V but the light output will be much lower.
Use ohms law for the 230V and 110V options then work out the power.:)
 

Noggin

Member
No you cannot get the same light output when using the same bulb on lower voltages.
A bulb [tungsten lamp] rated for 230V will work a 110V but the light output will be much lower.
Use ohms law for the 230V and 110V options then work out the power.:)

Oops, i misread his question and answered a slightly different one :)

Ohm's law isn't sufficient though. The resistance of the bulb will vary greatly with voltage across the filament.

then....my question is how do you know at what temp u hav to operate a bulb of 100W ? 230V/100V,,,, Full Confused

If a bulb is rated for 100w at 230v, then when you put 230v across the bulb it will dissipate 100w of power. If you lower the voltage, the current will decrease (obviously), the temperature will decrease (a little less obvious), and the resistance of the filament will decrease (a little less obvious too). This makes it difficult to know what voltage gives you what power.

Recently, I did a project in which I needed to be able to control the power to a 250w 110vAC bulb. My first assumption was that the bulb resistance was relatively constant and would introduce little error across the 110v range. I quickly learned that the assumption was WAAAY wrong. So I made a table in Excel and filled it in with various voltage and current measurements.

For instance:
0.41A @ 5.39v
0.51A @ 8.67v
0.55A @ 10.74v
0.93A @ 28.8v
1.20A @ 45.25v
1.55A @ 70.29v
2.09A @ 120v

I took over 30 measurements to build my table, that was just a few of them.

I was then able to calculate power (V * I) and resistance (I / V / V)

I then graphed Voltage vs Resistance and saw that it made a VERY nice curve

For the bulb I used, bulb resistance = 6.61 * voltage^0.454

Based on that information, you can calculate power vs. RMS voltage of a bulb. You just have to take the time to figure out the relationship.
 

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Noggin

Member
Length of the filament I suppose. Longer filament = higher resistance, lower current, lower wattage.
 

audioguru

Well-Known Member
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I think the thickness of the filament determines the wattage of a light bulb.
If the filament is short then it will be too hot and will burn out soon. If it is long then it will not be hot enough and will produce a dim red light.

You want a high power light bulb last as long as a less power one.
You want a low power light bulb to make white light, not dim red.
 

Noggin

Member
That makes sense too. I don't know which way it is. How confidant are you that it is the thickness of the filament? It doesn't matter much to me which way it is. The reason I said length was that it has more of an affect on surface area which I would assume is better for light output, and higher power bulbs don't tend to last nearly as long as lower power bulbs.
 

kpatz

New Member
It all boils down to the resistance of the filament when hot (lit). Lower wattage bulbs have higher filament resistance.

The color temperature of the bulb depends on the temperature of the filament surface, the hotter the whiter, which in turn is a function of the wattage and the surface area of the filament.

I think a combination of filament length and thickness is used to construct different wattage bulbs. Higher-watt bulbs usually have a shorter, straight (but coiled), thicker filament, while lower-watt bulbs have a longer thinner C-shaped filament held up with multiple supports.

You can drive a high-watt bulb at a lower voltage to reduce its wattage output, this is how light dimmers work. Drive them too low though and you'll get a red or orange light instead of white. You could also drive a lower-watt bulb at a higher voltage, and get more light/watt usage out of it, but the thinner filament will burn out faster.
 
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Mr RB

Well-Known Member
may be...:D , Thanks to all n Edison for inventing this Bulb!

He didn't. He was just a crooked rich New Yorker who was quick enough to get the first *US Patent* for light bulb stuff that just about everybody was experimenting with; electric light caused by passing current through filaments in evacuated and gas filled bulbs;
Invention of the Light Bulb: Davy, Swan and Edison - EnchantedLearning.com

And don't get me started on all those so called "documentaries" saying things like "thanks to the Wright brothers human beings could finally take to the skies" etc etc. At that time there were teams all over the world working on powered flight, it was a very popular topic with paper journals being mailed worldwide showing people's designs/successes/failures etc. It was really the invention of a system to make Aluminium cheap (before 1900 it was a rare earth metal!) that allowed lightweight Aluminium engines to be built that allowed humans (including the Wrights) to finally build flying machines.

What the Wright brothers really achieved was a massive publicity engine hyping up everything they were doing in the newpapers even long before they had any successes with their flying machine. It's also to be argued that their early flights were all in "ground effect" and they were not actually the first to achieve powered flight, but I'm not gonna go there... ;)
 
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