How They Work
The halogen gas in the bulb basically recycles tungsten atoms that boil off the white-hot, tungsten metal filament. This process gives halogen bulbs their long life -- but it only works at extremely high temperatures. If a 120-volt halogen bulb is dimmed by more than 40 percent, to less than 72 volts, the process stops and the wayward tungsten atoms instead stick to the glass walls of the bulb. In a relatively short time, the filament weakens and breaks.
Some halogen bulb makers say you can minimize adverse effects on halogen bulb life caused by dimming. They recommend keeping the light level above 60 percent of full brightness, and returning the bulb to full brightness for at least one minute before turning it off. Halogen bulbs may be of higher wattage than standard incandescent bulbs, so your dimmer should be rated for the wattage of the halogen bulbs it controls.