Why does a lightbulb get suddenly much brighter shortly before it blows?


Why does a lightbulb get suddenly much brighter shortly before it blows?

In: Physics

The filament gets thinner over time continually. It reaches a point where the material gets super excited, ie too much power being pushed through for the remaining material, gives off more light yet overheats as it does so.

Two possibilities:

If it blows when first turned on, which light bulbs often do, the filament is cold. When the filament is cold it’s resistance is lower, that means, as per Ohm’s law (Current equals Voltage divided by Resistance), the current flow will be much higher and it vaporises the filament almost instantly.

That will result in a much brighter flash than you’d expect from the brightness the bulb normally outputs.

If it blows when it’s hot, the filament fails, and there’ll be an electrical arc can be formed between the ends of the filament. This electric arc will be much brighter than the light from the filament, and as the ends burn away the arc will widen getting brighter. Arcs behave in the way that as they get hotter, their resistance falls, so again the output will get brighter and brighter until the filament burns away or the breaker trips. All this happens in an instant, in both cases, so it just appears like a super bright flash before the light disappears.

As an incandescent bulb is used, it is slowly getting thinner and thinner. It is literally evaporating atoms due to the high heat. (This is one of the reasons that a coil shape is used. It allows some of the boiled-off atoms to re-deposit.)

As the filament gets thinner, its electrical resistance increases, which increases how hot/bright it is. This effect is not really noticeable at first, but at the end of life it becomes important.

Some spots are going to thin out faster than others due to random variation in manufacturing, grain size, etc. Those areas are going to get a bit hotter and thus thin out a bit faster. This creates a “positive feedback” effect. A spot gets thinner so it gets hotter so it gets thinner faster so it gets hotter etc. etc.

Eventually some spot gets very thin and things come to a rapid conclusion. The feedback effect runs very fast, and the very thin spot doesn’t just evaporate atoms, it boils. It turns to a small spot of liquid, and then a plasma in a brief flash of light as current arcs through the plasma. And then it’s gone, leaving a gap in the filament and the bulb no longer conducts electricity.

There can also be a secondary effect in tungsten filaments where the tungsten atoms migrate within the filament, causing it to take on an odd, chunky shape with thick and thin areas as some of the grains grow at the expense of others. But I can’t for the life of me remember what it’s called right now.