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Light Bulb Burn-Outs

September 26, 2011   

How Light Bulbs Burn Out

Due to the high temperature that a tungsten filament is operated at, some of the tungsten evaporates during use. Furthermore, since no light bulb is perfect, the filament does not evaporate evenly. Some spots will suffer greater evaporation and become thinner than the rest of the filament.These thin spots cause problems. Their electrical resistance is greater than that of average parts of the filament. Since the current is equal in all parts of the filament, more heat is generated where the filament is thinner. The thin parts also have less surface area to radiate heat away with. This “double whammy” causes the thin spots to have a higher temperature. Now that the thin spots are hotter, they evaporate more quickly.It becomes apparant that as soon as a part of the filament becomes significantly thinner than the rest of it, this situation compounds itself at increasing speed until a thin part of the filament either melts or becomes weak and breaks.

Why bulbs often burn out when you turn them on

Many people wonder what goes on when you turn on a light. It is often annoying that a weak, aging light bulb will not burn out until the next time you turn it on.The answer here is with those thin spots in the filament. Since they have less mass than the less-evaporated parts of the filament, they heat up more quickly. Part of the problem is the fact that tungsten, like most metals, has less resistance when it is cool and more resistance when it is hot. This explains the current surge that light bulbs draw when they are first turned on. When the thin spots have reached the temperature that they would be running at, the thicker, heavier parts of the filament have not yet reached their final temperature. This means that the filament’s resistance is still a bit low and excessive current is still flowing. This causes the thinner parts of the filament to get even hotter while the rest of the filament is still warming up. This means that the thin spots, which run too hot anyway, get even hotter when the thicker parts of the filament have not yet fully warmed up. This is why weak, aging bulbs can’t survive being turned on.

Why burnouts can be so spectacular

When the filament breaks, an arc sometimes forms. Since the current flowing through the arc is also flowing through the filament at this time, there is a voltage gradient across the two pieces of the filament. This voltage gradient often causes this arc to expand until it is across the entire filament. Now, consider a slightly nasty characteristic of most electric arcs. If you increase the current going through an arc, it gets hotter, which makes it more conductive. Obviously, this could make things a bit unstable, since the more conductive arc would draw even more current. The arc easily becomes conductive enough that it draws a few hundred amps of current. At this point, the arc often melts the parts of the filament that the ends of the arc are on, and the arc glows with a very bright light blue flash. Most household light bulbs have a built-in fuse, consisting of a thin region in one of the internal wires. The extreme current drawn by a burnout arc often blows this built-in fuse. If not for this fuse, people would frequently suffer blown fuses or tripped circuit breakers from light bulbs burning out. Although the light bulb’s internal fuse will generally protect household fuses and circuit breakers, it may fail to protect the more delicate electronics often found in light dimmers and electronic switching devices from the current surges drawn by “burnout arcs”.

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