Failed: 300W Linear Halogen Bulb

Halogen lights are one of the least efficient lighting technologies and they really should be replaced wherever possible. Sure, they’re more efficient than the humble incandescent, but only by a percent or two, rather than the 85% or so that LED lights are. Whenever I see the chance to take one out of service permanently, I do it.

Of course, they were the favourite of many “atmospheric” lighting applications because they could be both brighter and crisper than incandescents and were nice and warm when dimmed. They also lasted longer than incandescent globes due to the effect of the halogen, which takes vaporized tungsten and “re-deposits” it on the filament allowing it to burn at higher temperatures with an acceptable lifetime.

We had a pair of 300W halogen uplighters. The first one had its tube blow taking out the dimmer as the filament somehow burned through the glass and touched the metallic earthed reflector. The second one was rarely used, and the tube only just went two weeks ago.

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When removing it from the luminaire, the ceramic had crumbled to bits and promptly fell off. The globe itself was covered internally with a lot of metal (likely tungsten) deposit, indicating some failure of the halogen cycle. This internal “metallized” coating would have likely increased envelope temperatures significantly, and decreased efficiency as it absorbed light on the way out of the bulb.

The cause? I had observed that the TRIAC dimmer was leaky, and as a result, in the “off” setting, it was actually running the globe at a very low power. Once I had spotted that, it was already too late – prolonged operation of halogen globes at low dimming levels tends to degrade their lifetime. The reason is that the halogen cycle which returns vaporized tungsten back to the filament only works above a certain temperature. Below this temperature, it doesn’t function, allowing the tungsten to evaporate and thin the filament which eventually breaks.

It appears to be part of the reason why I had been warned that halogen globes should be operated at the rated voltage, and no less than about 10% below the rated voltage. Dimming doesn’t immediately threaten the globe, such as trying to dim a non-dimmable CFL would, but it seems unwise. I have seen some long-used halogen globes fail with some envelope darkening from the inside, but not this significantly!

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The globe probably wasn’t that well made, or the fixture probably applied a bit too much compressive force on the tube, as it seems the glass did crack at one side where the lead connects to the filament. This doesn’t seem to have compromised the internal atmosphere, but shouldn’t happen.

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Placing a flash behind the globe allows us to see that the filament broke near the middle, and that allowed the stand-offs to move slightly out of position. There’s also another position near the left where the filament is a little uneven, suggesting that was another failure point which would have followed shortly.

Ultimately, given the leaky dimmer, there was no reason to keep this luminaire going with a halogen globe – so I just shoved an LED fixture on top of it in a rather inelegant way and called it a day. I’m sure the planet will thank me for that!

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2 Responses to Failed: 300W Linear Halogen Bulb

  1. Coops says:

    “The reason is that the halogen cycle which returns vaporized tungsten back to the filament only works above a certain temperature. Below this temperature, it doesn’t function, allowing the tungsten to evaporate and thin the filament which eventually breaks.”

    This is a common misconception, and widely regurgitated. Whilst kind-of technically correct, it is forgetting that the rate of tungsten evaporation is ALSO dependant on the temperature.

    Without the Halogen Cycle, the filament simply functions as a standard incandescent, and the normal rule of thumb is that dimming by 10% will double the life, dimming by 10% will quadruple the life, etc.

    There is never any situation where dimming of the Tungsten lamp will decrease it’s life. And when dimmed, the increase in life is so dramatic that the Halogen Cycle is not needed anyway.

    The photos in this blog show a VERY low quality lamp that has failed due to an air leak.

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