Author Topic: How long will it last? (Read 24861 times)

Scott · « **on:** June 21, 2000, 09:08:00 pm »

I have a British bulb and socket out in my garage as a night light of sorts. Since it's only being used at half voltage, it's dim, but good as a night light. How long will it last,do you think? It's been in use five or six years,12 hours a day(it's on a timer). I think,at half voltage,it's life would be almost as long as shelf, since the filament is nowhere near as hot as it would be at full voltage. (US 110v Englad/Europe 230-240v). Just wondered. Scott

James · « **Reply #1 on:** June 21, 2000, 10:00:00 pm »

Hi Scott!

A quick calculation with the filament design equations predicts that a standard 240V British bulb being run on 120V supply will have a life of 8.8 million hours so this one would last well into the next millennium! However assuming that it is a 100W bulb, it will in fact only be drawing 34W, and will give less than 10% of the light you'd get if you ran it at the full 240V.

To demonstrate just how inefficient it is to under-run lamps, a 120V 34W lamp would give you about 400 lumens whereas this British bulb is only radiating 130 lumens while consuming the same amount of electricity.

Ed Covington · « **Reply #2 on:** June 21, 2000, 11:31:00 pm »

James, in general your use of the "design" equations is only valid so long as the failure mechanism doesn't change. In the case of a lamp being operated at half voltage the failure mechanism probably is different; it probably goes from evaporation to surface migration. For example, sub-miniature lamps fail by surface migration of tungsten atoms and then failure is by breakage because of weakness. Sorry to get so technical. In general your use of design equations would give a correct answer if the two voltages were 110 and 120, say.

Ed Covington

Bob Masters · « **Reply #3 on:** June 22, 2000, 08:26:00 pm »

Woah guys ! I haven't heard bulb discussions like that since I worked at Sylvania Automotive and Miniature Lighting (or Valeo-Sylvania now )
How interesting indeed !

Bob Masters · « **Reply #4 on:** June 22, 2000, 08:32:00 pm »

Sorry I forgot to ask, but is that an old bulb you're using in your garage ?

James · « **Reply #5 on:** June 25, 2000, 06:46:00 pm »

Thanks for the reply Ed, I know you shouldn't use those equations above 110% rated volts where other failure mechanisms due to overheating and different chemical transport reactions may occur, but didn't realise surface migration was so significant at the low temperatures. Now that you mention it I do seem to recall reading a paper you wrote on that subject many years ago.

Is surface migration what leads to the filament notching in miniature lamps as well, or is that simply because the wire in them is so thin that it begins to take the shape of individual crystals in the wire?

Scott · « **Reply #6 on:** June 25, 2000, 08:39:00 pm »

No, it's not old. A friend of mine went to England a few years back and brought me several British sockets,plugs and bulbs. Also, I have a custom,hand painted"stained glass" British bulb from a girl who runs a rockhound/pottery/crafts shop in England.(I got them free,in exchange for a geode and some other mineral samples she was looking for.) Also got some British candy, and a can of prawns..Scott

Ed Covington · « **Reply #7 on:** June 25, 2000, 10:58:00 pm »

Scott and James, I apologize for leading this discussion into technical waters but since we're there I'll say a few more words. Lamps of the household variety usually fail due to evaporation, leading to the over-heating in a region. If the lamp wasn't processed properly it might arc out but we exclude that scenario. Subminiature lamps operate at quite low temperatures (2000 degrees Kelvin) and at that temperature only about one tungsten monolayer will evaporate in a period of 1000 hours. Obviously then, evaporation does not lead to filament failure. Instead, surface migration takes over and , yes, individual crystals begin to form and "facetting" takes place. Notching occurs when a subminiaature lamp is operated on direct current (we assume alternating current for household bulbs). Notching leads to short life. In high pressure halogen lamps volume diffusion can enter in and large spherical holes in the wire result. So, enough of this. I think the real answer to the original question is that we don't know how to calculate the life of the English lamp operated at half voltage.

Dave · « **Reply #8 on:** June 26, 2000, 03:42:00 pm »

Oh, this discussion reminds me of something...a few years ago the local Kmart got a batch of household light bulbs that were claimed to last 100 years...made in USA by

...they were inside frosted bulbs in six packs, 60 and 100 watts. These bulbs contained a diode mounted somewhere inside the base that converted AC into pulsating DC current. It claimed this helps make the filament run cooler, extending life. But the truth is that DC causes uneven filament wear, like the "notching" Ed explained. Also remember that 120 volt filaments are much longer and thinner than low voltage filaments, so they are much more prone to damage caused by vibration and shock. Since the filaments in those 100 yr light bulbs are not evaporating evenly, that will cause weak spots in the filament, which ulimately causes the filament to break. In fact the store used some of the bulbs in a back room, and none lasted more than a few years, although the bulbs showed very little blackening. Overall, this is a total rip off.

Scott · « **Reply #9 on:** June 28, 2000, 11:01:00 pm »

In junior high school shop class(1976) I built a lamp for an aunt of mine that had a nightlight in its base,nothing more than two ordinary candelabra sockets wired in series. The bulbs I put in it in '76(not new bulbs,at that)are still working-nonstop since '76,with the exception of the occassional power failure.So,these bulbs have lasted a long time at half-voltage.