Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!usc!apple!olivea!orc!inews!cmdnfs!bhoughto From: bhoughto@cmdnfs.intel.com (Blair P. Houghton) Newsgroups: sci.electronics Subject: Re: Do Halogen lamps need ballasts? Message-ID: <1147@inews.intel.com> Date: 29 Nov 90 00:31:21 GMT References: <714@skipper.dfrf.nasa.gov> Sender: news@inews.intel.com Distribution: sci.electronics Organization: Intel Corp, Chandler, AZ Lines: 39 In article <714@skipper.dfrf.nasa.gov> rando@skipper.dfrf.nasa.gov (Randy Brumbaugh) writes: >A chemical process (the halogen cycle) is used to improve this >in these new fangled "halogen" lamps. The tungsten chemically >bonds with the halogen gas in the envelope. When the gas >contacts the hot filament, the chemical bond is broken, and >the tungsten is re-deposited in the filament. These lamps >won't last forever, because the tungsten isn't deposited >at the same spot it came off the filament. If a region starts to thin-out its resistivity increases, increasing its power dissipation, increasing its heat. The rate of deposition is a monotonic function of the temperature, so the hotter spots tend to _collect_ tungsten, fattening them. The halogen cycle not only prevents rapid loss of filament material, it repairs minor flaws in the filament and prevents hot-spots due to these flaws. Halogen lamps burn out (barring some extensive flaw) only when one of the non-tungsten parts fails (the filament supports weaken from temperature cycling, the way the filament would if it weren't for the halogen cycle). >Also, the quartz envelope shouldn't be touched by bare >hands -- oils on the skin will weaken the structure and may >cause bubbles or lamp failure. The oil causes a region of different thermal conductivity; when the heat is applied this will cause a region of different temperature which will cause a difference in expansion which will cause a stress that will rupture the envelope. The oil doesn't "weaken the structure." You can wipe it off. >Tungsten-halogen (T-H) lamps are now used almost exclusively >for stage and television lighting applicatuions, where they --Blair "Stage Nerds 'R' Us..."