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Old 05-19-2020, 09:43 PM
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Halogen bulb with blown filament getting extremely hot


I had a 75W halogen spotlight fail, and did not change it immediately. It's on the same switch as some other lights, so the circuit was live for about an hour before I went to change the bulb. The entire fitting was extremely hot - I almost burnt my fingers. Far hotter than it would be if the bulb were lit and operating normally. One of the power wires has a white casing, and it was discolored for 6 inches from where it enters the fitting.

I think this was the last non-LED bulb in my house, would have been a bummer if it started a fire. But I'm mystified about what happened here. No sign of anything weird like rodents chewing things, and something like that would surely just blow the circuit breaker. It must be the bulb itself. Either it failed and then somehow starting generating massive amounts of heat, or the filament failure was a result of it somehow generating heat and the heating continued after the filament failure.

Any ideas? How does an incandescent bulb with a blown filament generate huge amounts of heat?
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Old 05-19-2020, 09:50 PM
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Halogens are super hot...sounds like it did some damage but burned out before it could burn your house down.

"A 300 watt tubular halogen bulb operated at full power quickly reaches a temperature of about 540 C (1,004 F), while a 500 watt regular incandescent bulb operates at only 180 C (356 F) and a 75 watt regular incandescent at only 130 C (266 F).[10]"

https://en.wikipedia.org/wiki/Halogen_lamp
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Old 05-19-2020, 10:32 PM
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I think it was hotter than the normal operating temperature even for a halogen bulb. But even if I'm mistaken, how was it still heating up long after it stopped emitting light?

Last edited by Riemann; 05-19-2020 at 10:33 PM.
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Old 05-20-2020, 12:07 AM
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You said it 'failed'; I presume you mean it stopped generating usable light. I believe that can happen without the electrical circuit being completely interrupted. Then the fixture was still conducting electricity, without producing light. So the electricity had to be going into somewhere, like into heat. Good thing you removed it.
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Old 05-20-2020, 12:47 AM
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Yes, I realize that I misstated matters when I said the filament blew. I don't actually know that, since you can't see the filament in a halogen bulb, and I haven't attempted to dissect it. All I know is that it stopped producing light. And obviously it must still have been conducting electricity through some path in order to heat up.

Maybe this was something highly unusual, but it seems really bad that there is a failure mode in which it gets even hotter. I mean, I guess anything designed for a halogen bulb is designed to tolerate a lot of heat, but when it's getting so hot that the white casing on the power wire turns brown that doesn't seem good.

Last edited by Riemann; 05-20-2020 at 12:49 AM.
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Old 05-20-2020, 04:07 AM
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I used to have some halogen up-lighters in my sitting room. Whenever one failed it would always trip the circuit-breaker on the consumer unit in the garage and all the downstairs lights would go off. A right PITA. They soon got swapped for LEDs.

Last edited by bob++; 05-20-2020 at 04:07 AM.
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Old 05-20-2020, 08:16 AM
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Quote:
Originally Posted by Riemann View Post
Yes, I realize that I misstated matters when I said the filament blew. I don't actually know that, since you can't see the filament in a halogen bulb, and I haven't attempted to dissect it. All I know is that it stopped producing light. And obviously it must still have been conducting electricity through some path in order to heat up.

Maybe this was something highly unusual, but it seems really bad that there is a failure mode in which it gets even hotter. I mean, I guess anything designed for a halogen bulb is designed to tolerate a lot of heat, but when it's getting so hot that the white casing on the power wire turns brown that doesn't seem good.
Yeah, white to brown...

Long ago I moved into an apartment and picked up a couple of torchiere halogen lights. IIRC the bulbs were 300W. I'd flip them on and it was like bringing the sun inside. But I started getting high electric bills. The electric company actually sent someone (at my request) to figure out what was sucking so much and you guessed it. And as Bob++ notes, they'll overload a circuit and trip breakers so if your breakers fail...day-um!

Poking around I found this Aussie on youtube. Did you have canisters around the light (I hope)?

https://www.youtube.com/watch?v=tpQcUW7NfvI
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Old 05-20-2020, 08:57 PM
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I suppose it's possible that the bulb shorted out somewhere internal to the globe, but if it was my house, I'd be thinking that

(a) overheating had caused the socket to melt, and all the energy was going into a bad copper connection, or
(b) overheating had caused the socket to carbonize, and it was shorting out through the burned-out socket.
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Old 05-20-2020, 10:10 PM
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Originally Posted by lobotomyboy63 View Post
Did you have canisters around the light (I hope)?
Yes, fortunately all the workmanship on my house seems to be really good.

But I'm kind of shell shocked at how close I may have been to a fire. I had no idea halogen bulbs could be so dangerous.
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