What makes an incandescent light bulb run out?

Question

What is or are, the most likely and common ways - the weak links in the chain (including I assume poor electricity wiring / configured housing) - that cause a traditional incandescent light bulb to blow?

And how much does manufacturing / materials quality of the bulb make a difference in any of those factors? (In other words, should an expensively, state-of-the-art constructed bulb in terms of materials purity and construction robustness, generally last a lot longer than the average bulb on the market?)

I understand it is a fairly simple lighting technology, so what complications cause them to not last longer than they generally do? Is 'blowing' the only way that an incandescent can ever expire?

Answer 1

Most light bulbs have a number of service hours designed in. This is achieved fairly accurately and on purpose during manufacturing. Consumer grade light bulbs burn out faster and the awful truth is that in that way the manufacturer ensures it can keep producing the light bulbs and make money. The main cause is the tungsten filament slowly evaporating until it gets too thin to carry the current. The trick during manufacturing is to etch the filament to a carefully designed thickness so the lifetime is reduced programmed.

The origins of programmed life light bulbs began with the Phoebus Cartel in 1924.

There are also light bulbs with a special stronger filament (basically they skip the 'etch-cycle'), these are for use in high reliable applications and last longer. They are commonly used in places that are hard to reach and are more expensive. Not sure about the proper name of these light bulbs. Because of the extended life time these bulbs are more expensive as a manufacturer has only limited room for producing spares. (What need is there for spares if the bulb doesn't fail?)

Where Australia and EU banned the consumer grade bulbs, the strengthened filament is not banned because of its specialized application. The latter type is just not available in your regular shop around the corner.

Answer 2

I'd say that an incandescent bulb is quite forgiving about power quality. They generally blow up because tungsten, that is, the material the filament is made of, slowly evaporates until the filament breaks. So what really comes into play is the filament manufacturing, how it's held in place, if its thickness is regular or not, if the tungsten is pure enough, and so on.

A filament breaks because it eventually becomes a little thinner at a point. That point heats more, causing more tungsten to evaporate, making it even thinner... That's good old nasty positive reaction. Thermal shock can do the job too, that's why you tell your children to stop disco-switching the lights.

Interestingly enough since the filament usually just breaks in one point, it's possible to try to give a broken lamp some afterlife time. If you have the broken bulb plugged in\$^1\$ and shake it, it's possible for the two filament ends to touch: the point heats a lot and the filament might weld itself and give you ten or so more hours of life.

\$^{(1)}\$ Please, just don't try that if you are not sure it's safe.

Answer 3

Most of the above data is incorrect. Tungsten was tested as a lamp filament about 1900 and found to be useless due to sag, a coiled filament was impossible. In about 1915, GE found by adding some other materials, they could produce NON SAG Tungsten which is still in use.

However, non sag tungsten is subject to DC etching such that the development of surface roughness changes the emissivity such that after 1000 hours operation, the lumens have decreased 30 percent. The filament is cooler and lamp life increases. The DC etching continues but much slower. There is no cure, the only solution in the case of automobile headlamps is to replace the bulbs. Most people do not realize their headlamps are below legal limits.

Another incandescent lamp problem that is not what people think, is halogen lamps. Halogen does nothing for an incandescent lamp except allow the use of a smaller bulb by keeping the bulb free of condensed tungsten, which in turn allows a higher fill gas pressure. Double the fill pressure, lamp live doubles. Result, higher lumens per watt with the same lamp life.