DIY Doctor

Porch lighting getting too hot

Postby griffda » Sat Dec 22, 2007 7:59 pm

I am having problems with my porch light.
The fixing has been okay for several years but recently started to get very hot.
I decided to change the fitting to a 3 lamp spotlight and now the whole unit gets very hot.
The light seems okay otherwise and the wiring appears correct.
Is there any reason this should start to happen?
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Postby john9159 » Sun Dec 23, 2007 4:58 pm

Loose wiring due to terminals not being tightened may lead to overheating.
Light fittings usually specify a maximum lamp rating; this should not be exceeded to get "extra" light.
Has the porch got adequate ventillation? if it's small and sealed against draughts and has no ventillation things get a bit warm as lamps need oxygen (oddly enough).
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Postby kuzz » Thu Dec 27, 2007 10:28 am

A normal light bulb is made up of a fairly large, thin, frosted glass envelope. Inside the glass is a gas such as argon and/or nitrogen. At the center of the lamp is a tungsten filament. Electricity heats this filament up to about 4,500 degrees F (2,500 degrees Celsius). Just like any hot metal, the tungsten gets "white hot" at that heat and emits a great deal of visible light in a process called incandescence. If the tungsten was burning in the presence of oxygen it would last about one second. I think John must come from the days of gas lamps, because the bulb is a sealed unit.

If your light is wired correctly with no loose connections, then it may be hot due to the heat from the bulbs especially if it is a halogen type as a halogen lamp also uses a tungsten filament, but it is encased inside a much smaller quartz envelope. Because the envelope is so close to the filament, it would melt if it were made from glass. The gas inside the envelope is also different -- it consists of a gas from the halogen group. These gases have a very interesting property: They combine with tungsten vapor. If the temperature is high enough, the halogen gas will combine with tungsten atoms as they evaporate and redeposit them on the filament. This recycling process lets the filament last a lot longer. In addition, it is now possible to run the filament hotter, meaning you get more light per unit of energy. You still get a lot of heat, though; and because the quartz envelope is so close to the filament, it is extremely hot compared to a normal light bulb. Isn't cut and paste marvelous.
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