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Why Do Light Bulbs Fail Early?

Aug 1, 1998 12:00 PM, Edited by Robert J. Lawrie

I read the answers to the questions about fast burnout of light bulbs in the Oct. 1997 issue. Although all of the answers are helpful, I believe I have another answer to solve the problem.

Standard incandescent lamps are manufactured with a coil filament. The lamp could be energized when the voltage sine wave is near zero, but changing at the most rapid point of the sine wave. At this point, the observed laws of electromagnetics dictate a large transient current will exist in the filament. The filament heats quickly and experiences transient physical forces. As a result, the filament pops—even if it is a new bulb! The contributing factors of ambient temperature and manufacturing tolerances add to the chance point of switching to cause premature failure. A good solution to this problem is to install a dimmer and bring the lamp on “slowly.” —L.D.E.

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I don’t agree to several points about the short life of light bulbs in the Oct. 1997 issue. First, in the second paragraph, a sentence states: “They are less likely to take variations of voltage.” Shouldn’t that be “more likely?” The 130V bulbs tend to be high-quality bulbs, whereas you can purchase 120V bulbs from good to trash. This could account for any improved life span of the 130V bulbs.

Second, the idea that vibration can harm a hot filament vexes me. Isn’t it true forensic engineers use filament damage as a way of telling which bulbs were burning during plane crashes and the like? The theory is: If the lamp is on during the crash, the filament will normally not break because the heat created allows the filament to stretch. Would this eliminate vibration in any situation where bulbs burn constantly? —T.F.

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