Fluke or fire hazard?

March 8, 2009 3:51 AM

My halogen bulb just got melty. Should I throw out the fixture?

I've had a 500 watt halogen work lamp that I've used daily for about 6 months. This evening I noticed that the light output dimmed significantly and when I turned it off I saw that the bulb had gone from a smooth glass tube to a lumpy melted tube. I don't keep it near any flammables or anywhere it would retain extra heat. It still puts out light, but probably 1/4 of what it did. I know these things get dangerously hot, but glass-meltingly hot seems ridiculous.

Do I have a fire hazard here? Should I replace the bulb or the whole thing?

posted by Ookseer to home & garden (7 comments total)

I've found every halogen lamp I've used to be a fire hazard. I eventually gave up on them.

If you're going to replace the bulb, be aware that any oil - like a fingerprint - or other detritus on the bulb can cause it to heat "irregularly" (unevenly), causing the bulb to fail prematurely and perhaps spectacularly.
posted by Dunwitty at 3:59 AM on March 8

My guess is that some oils got on the bulb, possibly from improper handling during installation. You're never supposed to touch those halogen bulbs with your bare hands at all, as skin oils contaminate the quartz glass and cause premature failure. I think the lumpiness is the premature failure in progress. I would replace the bulb and keep the lamp.
posted by jon1270 at 4:00 AM on March 8

jinx...
posted by jon1270 at 4:00 AM on March 8

I've had this happen with a couple of halogen lights. What seems to happen with mine is the filament breaks, but lands on the support wire in such a way that the support wire bridges the gap. The result was dim output from a filament much too close to the glass envelope, causing it to shatter within a few minutes. The fact that it goes dim instead of just burning out completely is my clue to replace the bulb pronto unless I want to be dealing with yet another broken bulb. I think this is entirely a problem with these type of bulbs (and maybe even specific brands) rather than anything wrong with the fixture itself.
posted by FishBike at 7:17 AM on March 8

That's the most common failure mode for quartz envelope bulbs. It usually happens because of some microcontaminant on the bulb (Like skin oils - above), or dust. Get a replacement, be sure the bulb is clean before starting it up the first time (Cleaning out the lamp and giving the bulb a wipedown with a lint-free cloth soaked in rubbing alcohol is a good idea, but not touching the bulb is required.

Halogen lamps aren't normally fire hazards(You're doing the right things, it seems), but they're pretty inefficient, and DO throw out alot of heat. Not so bad for the winter, but if you're going to be using this lamp in the summer and running air conditioning, it might be a good idea to replace it with a fluroscent fixture?
posted by Orb2069 at 7:42 AM on March 8

Nope, nothing wrong with the fixture as you describe it. That's how halogen tubes like that fail.

Filament bulbs are simple and kind of beautiful in their simplicity- they are just a piece of wire that you run electricity through. A fixture can't "overdrive" a bulb and cause it to fail. It's one of the basics of electricity- the voltage is the "push", the amps are the "pull" and the watts used is the balance of the two. When you first turn the light on, the voltage pushes through the filament. It's the same force that pushes the power through the cord when you plug something in- it sees voltage on one side, and none on the other, and it wants to pass that voltage along. Anyway, the filament is just a wire, and all it wants to do is conduct the electricity from one side to the other. If it was a thick wire, it would try to pass as much power through itself as it could- you'd have the same effect as sticking a coathanger into the outlet. It would pass tons of power (or "draw" amps) and would either get hot and melt, or trip the circuit breaker. (A safety device that prevents so much power from going through the wires in your walls that they would get hot.) But I digress. Now, the filament is skinny, so it resists that voltage pushing through. It's also made of a type of metal that happens to not melt very well. Because of that resistance, it gets hot. And because it doesn't melt very well, it can get really hot- so hot it glows very brightly. It is absorbing some of the energy as its passing through the wire and turning it into heat and light. It also happens that as it gets hotter, its resistance increases. It quickly gets into an equilibrium between the voltage pushing through, and the power it can pull through. At that point, your bulb is absorbing 500 watts of power and converting it to light and heat.

My point is, that's why you can put a 250 watt bulb in a fixture, or a 10 watt bulb in a fixture, and the 10 watt bulb won't immediately explode. The bulb is self limiting- it will only use the power it wants to. (This all assumes we're talking about the same voltage- if you give a 110 volt bulb 220 volts, it will burn out immediately. If you put a 220 bulb in a 110 socket, it will do nothing, or be very dim and orange looking.) So your fixture isn't overheating the bulb or anything like that.

The advice to make sure you don't touch the bulb is correct- I even go so far as to clean the bulbs before I install them. Clean paper towel and windex.

One thing to look out for, however, with halogen tube bulbs like this, is to make sure the contact are clean and uncorroded. It there is corrosion, this creates resistance and heat at the connectors, and a safety issue. Evidence of this is burnt spots at the ceramic tips of the bulbs when the bulb burns out. This also usually damages the fixture. But if the contacts on the bulb and on the fixture are nice shiny metal, replace that bulb and bathe in the glorious heat...
posted by gjc at 9:48 AM on March 8

I'm aware of the "don't touch the bulb". This one got the alcohol and microfiber cloth treatment before being installed.

Heat is the primary feature of this light. I'm in San Francisco, no one has AC and many of us run the heat in the summer. My heating bill has actually gone down since I've started using the light, mostly due to crappy location of my heater.

Now that the thing has cooled down and I can get a close look at he bulb it looks like something fantastic happened to it, the interior of the bulb is completely covered in silver as if the filament just vaporized. The glass case melted very neatly into links of little 1/2 inch sausages. I'll see what a higher quality bulb will do next time.

The contacts are a bit dingy, I'll give them a polish before I replace the bulb.
posted by Ookseer at 1:26 PM on March 8