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AC Incandescent Lightbulb, DC Current?

sw-uk

New Member
I've often wondered would a traditional incandescent 40w, 60w or even a 100w lightbulb illuminate with 240V of DC current instead of AC, say using a DC step up circuit?
 

crutschow

Well-Known Member
Most Helpful Member
I've often wondered would a traditional incandescent 40w, 60w or even a 100w lightbulb illuminate with 240V of DC current instead of AC, say using a DC step up circuit?
As long as it's the rated voltage, it makes little difference whether it's AC or DC to an incandescent bulb, since it's just a resistor.
DC step up from what?
 

sw-uk

New Member
crutschow: I'm wanting to build some kind of voltage step up circuit, using battery input from 1.5v-12V, wondering if a load of capacitors in parallel would work?
 

Ramussons

Active Member
I've often wondered would a traditional incandescent 40w, 60w or even a 100w light bulb illuminate with 240V of DC current instead of AC, say using a DC step up circuit?
A 240 Volt incandescent bulb will be brighter at 240 V AC than 240 V DC. Guess why?
 

Nigel Goodwin

Super Moderator
Most Helpful Member
I've often wondered would a traditional incandescent 40w, 60w or even a 100w lightbulb illuminate with 240V of DC current instead of AC, say using a DC step up circuit?
An incandescent bulb isn't AC, it makes no difference if you feed it DC or not. But why would you be using incandescent bulbs anyway?, particularly from a lower voltage DC source.
 

Diver300

Well-Known Member
Most Helpful Member
No idea, and I don't believe it would.
There might be some very small effect. The filaments on small incandescent bulbs heat up and cool down fast enough that there is some modulation of the light when they are run on 50 Hz mains. It's nothing like as severe as CFL or LEDs that can effectively turn on and off 100 times a second, but there is some modulation.

It could be that the average efficiency is slightly more if the filament temperature is varying, because the efficiency changes a lot for small changes in temperature. Also there could be slightly more power on average on AC.

However, I don't think that either possible effect would be noticeable.
 

Nigel Goodwin

Super Moderator
Most Helpful Member
There might be some very small effect. The filaments on small incandescent bulbs heat up and cool down fast enough that there is some modulation of the light when they are run on 50 Hz mains.

Certainly a well known (and well used) effect, but not for giving a brightness difference.

It was widely used for strobe discs for setting the speed of record players - you used to get paper discs that could be stuck on a 45 record, which had all the different speeds included.

And obviously, the 'posher' decks often had strobe patterns round the edge of the platter, usually lit by a faster responding neon bulb.

There wouldn't be 'slightly more power' on AC, as that's what RMS is all about :D
 

Diver300

Well-Known Member
Most Helpful Member
There wouldn't be 'slightly more power' on AC, as that's what RMS is all about :D
I know that the RMS voltage would be the same, but if the light output is varying at 100 Hz, and the filament temperature must be changing at 100 Hz as well. The resistance of the filament changes quite rapidly with temperature, so the resistance won't be constant over each half cycle.

It then gets really complicated to calculate. With all of those changes being non-linear, it's quite possible that the average power goes up very slightly with AC, but it's also possible that it goes down.

I wouldn't know how to do the calculations. However I would be surprised if there would be a change that could be easily measured.
 

Nigel Goodwin

Super Moderator
Most Helpful Member
It then gets really complicated to calculate. With all of those changes being non-linear, it's quite possible that the average power goes up very slightly with AC, but it's also possible that it goes down.
It's also possible that it averages out exactly the same, and that the average figure is what is quoted in the spec :D

Certainly there's no reason to imagine any visible brightness difference between AC or DC.

For those old enough to remember?, didn't we used to measure AC power by comparing the relative brightness of two lamps, one fed from adjustable DC - you simply adjusted the brightness to be the same, and measured the DC power, to give you the correct(ish) RMS power of the other lamp.
 

danadak

Well-Known Member
Most Helpful Member
Web reference -

That's a lot of questions, but let me answer some of them. An
incandescent light source is very efficient at converting electricity
into heat to raise the temperature of the filament. It is inefficient
at converting that heat into visible light because it creates a
thermal distribution of wavelengths that contains most of its power
outside the visible range, particularly in the infrared. The hotter
you run it the better, but if it runs too hot, the filament doesn't
last long. You can do a little better with a halogen bulb, which
prolongs the life of the filament and allows you to run a little
hotter.

It is slightly more efficient to use DC since the temperature of the
filament will always be at its highest point, but this is a small
effect since the thermal time constant of the filament is typically
long compared to half a cycle of AC. You can safely ignore the 60 Hz
radiative losses that you are worried about since these will be tiny
compared to the total power. If those losses were important, you could
always use two filaments in close proximity, with the currents flowing
in opposite directions to drastically reduce the 60 Hz radiated field.

Then there is skin effect -


And effects due to thermal mass in AC environment.



Regards, Dana.
 
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Nigel Goodwin

Super Moderator
Most Helpful Member
Web reference -



Then there is skin effect -


And effects due to thermal mass in AC environment.



Regards, Dana.
Not really an issue at 50/60Hz, as made clear in the page you linked to - where it states skin effect is 8.5mm at 60Hz - if a bulb filament is thicker than 17mm I think we've got problems :D
 

danadak

Well-Known Member
Most Helpful Member
Not really an issue at 50/60Hz, as made clear in the page you linked to - where it states skin effect is 8.5mm at 60Hz - if a bulb filament is thicker than 17mm I think we've got problems :D

Of course that would depend on length of run from power source to filament. But
like neutrino concentration in the Copper I agree maybe inconsequential. For short
to moderate runs.

Regards, Dana.
 

Nigel Goodwin

Super Moderator
Most Helpful Member
Of course that would depend on length of run from power source to filament. But
like neutrino concentration in the Copper I agree maybe inconsequential. For short
to moderate runs.

Regards, Dana.
Inconsequential for anything inside a light bulb :D

For that matter, not even restricted to short to moderate runs anyway.
 

danadak

Well-Known Member
Most Helpful Member
Consequential for long runs though :)

A factor under certain circumstances. Secondary effects become important
at extremes.
 

Nigel Goodwin

Super Moderator
Most Helpful Member
Consequential for long runs though :)

A factor under certain circumstances. Secondary effects become important
at extremes.
I suppose if you were running a cable to the moon it might be 'consequential' :D

Depends on your definition of 'consequential' I suppose?. But is it something you've just 'made up' anyway?, google has little to say about it, and no mention of cables.
 

danadak

Well-Known Member
Most Helpful Member
1648135057629.png


Ignore 2

But is it something you've just 'made up' anyway?
No, physical facts....


Unlike this -
An incandescent bulb isn't AC, it makes no difference if you feed it DC or not.

And of course we have not discussed lifetime effects AC vs DC....
 
Last edited:

AnalogKid

Well-Known Member
Most Helpful Member
For those old enough to remember?, didn't we used to measure AC power by comparing the relative brightness of two lamps, one fed from adjustable DC - you simply adjusted the brightness to be the same, and measured the DC power, to give you the correct(ish) RMS power of the other lamp.
Did that in EE101 or EE102 lab. This was the only time those particular DC supplies were used. There was a pile of them that looked like they were built out of WWII surplus. Variac, bridge, choke, cap. Nothing fancy, did the job. Taught choke-input filters in the same lab.

ak
 

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