Light bulb question- running a 120 volt bulb at 30 volts- watts reduced by 75%?

[KYErik]

I am looking for a spare bulb for an old Bell and Howell 8mm movie projector. It takes an 80 watt bulb that runs on 30 volts (supplied by the projector). The correct bulb is about $55.

I found a 250 watt, 120 volt film projector bulb at a yard sale for $1 and bought it. Will this bulb use 62.5 watts if I run it on 30 volts instead of 120 volts? Will it last as long?

 

[CADplans]

The bulb will last forever,,, but, it will give off a little glow, if any light.

 

[k0ua]

You might have to strike a match to see the glow.. Well, it might not be quite that dim, but it will be very very dim. And it will last a very very very long time.

 

[MossRoad]

From:
Watts/Volts/Amps/Ohms conversion calculator

My guess is this....

The original 30v 80w bulb would be about 11.25 ohms resistance and pull about 2.6666 amps.

If you have a 250w bulb rated for 120v it should have about 57.6 ohms of resistance.

So, if you put that 57.6 ohm bulb in a 30v circuit, it'll draw about 0.521 amps and only put out about 15.625 watts.

It'll be very dim. That's my guess.

Go to that link and plug in some numbers. Its kind of fun.

 

[CalG]

E=I^2R

It's the exponent ^2 that tells you the entire story.

 

[k0ua]

From:
Watts/Volts/Amps/Ohms conversion calculator

My guess is this....

The original 30v 80w bulb would be about 11.25 ohms resistance and pull about 2.6666 amps.

If you have a 250w bulb rated for 120v it should have about 57.6 ohms of resistance.

So, if you put that 57.6 ohm bulb in a 30v circuit, it'll draw about 0.521 amps and only put out about 15.625 watts.

It'll be very dim. That's my guess.

Go to that link and plug in some numbers. Its kind of fun.

But, it is hard to calculate the resistance of a light bulb because it is dynamic. In other words as the light bulb heats up its resistance changes considerably as more current flows thru the filament until equilibrium is reached. So your working voltage, current and resistance's are likely correct for a stabilized bulb. BUT your low voltage will not swing the resistance as much. So it is kinda hard to figure. Perhaps someone with more practical real world light bulb experience could weigh in on this OR get this, we could do actual experiments of measuring lumens output, at the two voltages and current measurements, OR we could just screw the thing in the socket and see what happens... Yeah.!

 

[MossRoad]

Quick! To the lab!!!

 

[CalG]

But, it is hard to calculate the resistance of a light bulb because it is dynamic. In other words as the light bulb heats up its resistance changes considerably as more current flows thru the filament until equilibrium is reached. So your working voltage, current and resistance's are likely correct for a stabilized bulb. BUT your low voltage will not swing the resistance as much. So it is kinda hard to figure. Perhaps someone with more practical real world light bulb experience could weigh in on this OR get this, we could do actual experiments of measuring lumens output, at the two voltages and current measurements, OR we could just screw the thing in the socket and see what happens... Yeah.!

For a dollar, I would just screw it in. It's not going to burn up unless it is defective from the git go.

 

[MossRoad]

I have some lightbulbs in my kitchen in a 4 bulb fixture. We've been in the house about 21 years. Those lights are the first on and last off every day. One finally burned out a few years ago. I had a hard time reading it, but I finally figured out they are 130v bulbs. They give off a nice yellow color, not harsh white. Since there's 4 of them, its plenty to light up our little two-butt kitchen.

 

[joefromga]

When you heat up steel to make it white hot and glow do you think that heating it half the temperature would make it glow half as much?

That's doubtful and the filament inside the bulb only gives off light at a certain temperature and is not linear lumens:wattage input.

Still a fun experiment. Keep us posted.