Why do halogen lights get dim shortly before they burn out? I have one of those 5000W portable work lights. I noticed the other day the light seemed dimmer than usual. Yesterday it got so dim I got more light out of a standard 60W lightbulb. A few minutes later it died.
This puzzles me. Although the bulb contains a gas, it still has a filament. On standard bulbs the filament either works, or burns out. How can the filament on a halogen bulb put out less light, and get dim before it dies? I dont understand this?
No, I did not have a power outage or voltage drop. My power tools and trouble light worked fine.
The process of burning out, for any incandescent bulb, is a breakdown of the filament. Halogens run hotter by redepositing the filament back onto the filament. But eventually, the redeposits fail to keep one part of the filament thick enough. IOW the bulb eventually cannot conduct enough current due to a thin part of that filament. Remember, as filament gets hotter, then so does its resistance. Also remember that as current decreases, the light output decreases by something (if I remember) around an exponential factor of
IOW as that > Why do halogen lights get dim shortly before they burn out?
I think more usually 3-4, but maybe different for failing filaments since their temperature is often uneven.
When the filament has thinned by about 1 percent, there is usually a fatal thin spot. Light output from the filament by that time is usually only a few percent. More severe loss is usually from the bulb being darkened by condensed tungsten vapor.
I have seen a funny phenomenon with linear halogen lamps:
The filament breaks and an arc forms in the gap, especially if the filament breaks (as usual) when it is cold and the current flowing through it is higher. In most other bulbs, the arc usually expands and its ends move to the ends of the filament, and then the arc does not have any filament resistance limiting its current so it "blows up" into "The Bright Blue Flash".
But in a linear halogen lamp, at least in part from the higher gas pressure and maybe from the presence of the halogen, you need a higher voltage gradient than is across the filament to make the arc expand along the filament.
So the filament breaks and you get this little arc in the gap. At first, light output is reduced noticeably but not really dim. The filament current is reduced (due to the voltage drop of the arc) making most of the filament much dimmer, but the ends of the filament where the arc is are glowing blazing bright from the heat of the arc. Afterwards, the inside bulb surface around the arc may get darkened from tungsten vapor depositing on the inside bulb surface faster than the halogen cycle can clean the tungsten away. You get a dark spot in the bulb around the arc. You get a further reduction in light output.
One sure sign of a filament break with an arc sitting in it is if you turn the lamp off, you get no light at all when you turn it back on.
I did once see a "stable burnout arc" in a 60 watt non-halogen lightbulb. I believe the arc there failed to "blow up" because the filament broke when it was hot, and higher current makes arcs hotter and more conductive and more able to expand along the filament. This bulb was a hallway light in an apartment building and only loses power during power failures. And its filament was the multi-supported C-shaped one of a style that I think is C9, rather than the shorter coiled-coil filament in higher lumen shorter life "standard" lightbulbs.
If a halogen bulb runs dim but can be turned on again after being turned off, then maybe it is being discolored from something going wrong with the halogen cycle - a less common failure mode, maybe from a crack in the bulb. Also check for bad contact - a common problem with linear halogens - the contacts of the bulb or its socket may be corroded.
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