Unexpected aging effect seen in light-bulb filaments

Article By : John Dunn

If you examine a light bulb filament closely, you might see an unexpected aging effect that might have been averted with the inclusion of mechanical filament support.

I have occasionally read that incandescent light bulb filaments undergo metal electromigration that eventually causes the filament metal to part company with itself and open, blowing out the bulb. Let’s call this the common wisdom.

We have a ceiling light fixture that was installed 12 years ago. It uses three 40-watt bulbs, which were installed fresh and new at that time, but one of those bulbs finally blew out. I was up on the ladder with a fresh light bulb in hand when I noticed something about the two remaining bulbs that were still working.

I could see through the frosted glass that the two remaining filaments had sagged under the influence of gravity; this is not an electromigration effect. With AC excitation, that phenomenon does not dominate. These two filaments have simply stretched.

photo of two light bulbs with deformed filamentsFigure 1 These deformed filaments sagged under the influence of gravity.

Presumably, these two bulbs are ready to blow out too, but exactly when they will is difficult to say. It reminds me of reading how some astronomers say that the blue giant star Betelgeuse is about to go nova, but nobody is quite sure of when. Maybe that will happen tomorrow or in a hundred thousand years. Again, we don’t know. In the meantime though, I have my own two little Betelgeuses.

Looking more closely at light bulb structure may shed light on the issue (no pun intended).

diagram of light bulb structure showing filament supportFigure 2 The typical incandescent light bulb structure shows a filament support.

The innards of a typical incandescent light bulb will look a lot like Figure 2. Only one filament support is shown, but sometimes two such supports are used. However, such a support may simply have been omitted as with my two Betelgeuse lamps.

Although the two bulbs have done pretty well to have stayed functional for as long as they have, the inclusion of filament support would have helped mitigate the sagging, the filament drooping, and would thus have extended the bulb service life.

Extended by how much? I have no idea. However, by leaving out the filament support(s) and thus shortening the anticipated bulb service life, people will have to buy replacement bulbs more often, don’t you think?

This article was originally published on EDN.

John Dunn is an electronics consultant, and a graduate of The Polytechnic Institute of Brooklyn (BSEE) and of New York University (MSEE).

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