Does anyone understand Carbon-filament lamps?

[lctorana]

Hi CPF experts,

I am interested in obtaining and playing with an old carbon-filament lamp. Has anyone had any hands-on experience with these?

Do they dislike being overdriven? Probably.
Being underdriven?

To maximise the life, should I run it at exactly the design voltage, or somewhat lower?

I know they are physically robust, as the British Navy found them more robust than tungsten filaments in WWI battleships, but are they electrically robust, too? Is a soft-start circuit necessary?

All three questions are probably related, but I'd welcome any carbon-filament discussion or experiences.

 

[LukeA]

I don't know anything about carbon filaments, but it strikes me that the British Navy probably did not use soft-start circuits in 1914.

 

[broadgage]

Carbon filament lamps are obsolete for general lighting, but can still be obtained for special applications.
The efficiency is very low, a 60 watt carbon filament lamp has about the same output as a 25 watt tungsten lamp, or a 5 watt CFL.

Like any filament lamp, overvolting will substantialy increase the light output, though at the cost of a much reduced lifetime.

Undervolting will greatly increase the life, though the already poor efficiency will be made still worse.

Carbon filament are even more sensitive to voltage variations then tungsten lamps, this is because carbon has a negative temperature coefficient, that is the hotter the filament the lower the resistance.

A carbon lamp overvolted by 10% will draw roughly 15% more current.
A tungsten lamp overvolted by 10% will only draw about 5% more current.

Carbon filament lamps, as noted above are very vibration resistant, and used to be used in line powered inspection lamps, on board ships, and on vibrating machinery.

The filament of a carbon lamp is much shorter, for a given voltage than that of a tungsten lamp, the filament of which generally has to be tightly coiled to fit in the bulb.

 

[lctorana]

Thank you!

I knew someone would know.

It's only to play with, it's only a toy, so I don't care about lumens/watt, but what you say about the I/V characteristic is fascinating.

So while neither filament is ohmic, tungsten tends toward current regulation (I knew that) and carbon tends toward voltage regulation (I didn't know that).

Pictures of carbon bulbs show enormously long loopy filaments, so the length comment is counter-intuitive, but I do know that tungsten filements are coiled (or coiled-coiled) and are much longer than they seem.

Undervolting will greatly prolong the life - nice to know - but be really dim - that's OK too. Oh, and I did know they are obsolete, btw, by about 80 years. That's why I'm so chuffed to find a source.

I will definitely buy some now. Can't wait.

 

[broadgage]

Carbon filament lamps started to be replaced by tungsten about 80 years ago, as you point out, however they were still in use quite recently for some applications.
London ungerground used them for lighting signs and notices in tunnels, until at least the 1970s, owing to theire vibration resitance.
Several London newspapers used them for lights on high speed printing machinery, for the same reason.

Many, many years a special voltage regulator circuit was used for dynamos, when great accuracy of output voltage was required.
This used two tungsten lamps and two carbon lamps all of the same nominal rating, wired in a Wheatstone bridge arrangement, as the dynamo output altered even very slightly, the voltage distribution accross the lamps would alter significantly, and this voltage was used to alter the field current, or the steam supply.

The main uses of carbon lamps today are as low power radiant heaters for pets or livestock, and for general lighting when an old fashioned look is required.

 

[lctorana]

...a special voltage regulator circuit was used for dynamos, when great accuracy of output voltage was required.
This used two tungsten lamps and two carbon lamps all of the same nominal rating, wired in a Wheatstone bridge arrangement, as the dynamo output altered even very slightly, the voltage distribution accross the lamps would alter significantly, and this voltage was used to alter the field current, or the steam supply.

That is just brilliant.

I love genius.

 

[B@rt]

Here's one that has been going for over 100 years...

 

[Juggernaut]

This bulb http://www.bulbtown.com/30T14_4_E26_RADIO_120_240V_RADIO_PERMA_GLOW_p/l4080.htm
are Carbon filament type and are said to operate from 120 volts to 240! I would think that the rated bulb life applies to the higher voltage, if that was the case running at 120 it should last forever.

 

[StarHalo]

You can make your own carbon filaments using carbon powder and cotton string (which is how they were originally made), but the hard part is getting a vacuum in the bulb.

 

[lctorana]

You can make your own carbon filaments using carbon powder and cotton string (which is how they were originally made), but the hard part is getting a vacuum in the bulb.

I'm sure MacGuyver would find a way somehow.

(Oh, and btw, I got excited about nothing. The lamps were advertised but discontinued. Drat.)

 

[Juggernaut]

I'm sure MacGuyver would find a way somehow.

(Oh, and btw, I got excited about nothing. The lamps were advertised but discontinued. Drat.)

Oh that sucks:thumbsdow. Though there is still a large selection at my previous link I posted:thumbsup:, though I'm not sure if they are what your looking for:shrug:.