XPost: sci.electronics.basics, sci.electronics.design, sci.electronics.misc   
   From: pcdhSpamMeSenseless@electrooptical.net   
      
   On 9/6/2014 2:20 PM, Andrew Gabriel wrote:   
   > In article <540AE8AB.9020801@electrooptical.net>,   
   > 	Phil Hobbs  writes:   
   >> On 9/6/2014 2:43 AM, piglet wrote:   
   >>> On 06/09/2014 05:51, Fester Bestertester wrote:   
   >>>> AC voltage ballasts are superior to DC ballast.   
   >>>   
   >>> I would think the ebay sellers desciption is roughly   
   >>> right. In a DC arc the electrode material will "burn   
   >>> off" and migrate only in one direction. In AC arc the   
   >>> wear reverses each cycle and cancels out. This was   
   >>> known from carbon arc lights back 150years ago.   
   >>>   
   >>> I am surprised that any headlamp Xenon arcs are DC   
   >>> and if they were DC arc then lifetimes would be   
   >>> very short.   
   >>>   
   >>> piglet   
   >>>   
   >>   
   >> Normal arc lamps are asymmetrical--they run from DC, and fail quickly if   
   >> you put them in backwards. Dunno about HIDs.   
   >   
   > Yes, depends on the design of the lamp, and where exactly the   
   > useful light source is inside it.   
   >   
   > Light source can be the positive column (as with long tubes),   
   > cathode fall region (neon indicators), or an electrode surface   
   > (carbon arc).   
   >   
   > If the lamp is designed for DC, the electrodes could well be   
   > asymmetrical, designed to handle different operating temperatures,   
   > different thermionic emission properties, different wear rates,   
   > etc. An asymmetric DC lamp operated on AC would have shorter life.   
   >   
   > Also, any lamp operated on AC may have a larger light source,   
   > which if the optics were not designed for this may cause light   
   > spill out of the designed beam pattern and less intensity inside   
   > the designed beam pattern. It *may* be that some headlamps use   
   > DC to get a smaller more intense light source, which can be more   
   > accurately focused by beam optics into the required pattern, to   
   > achieve higher intensity in-beam and less spill outside the   
   > intended beam.   
   >   
   > The main point is the lamp should be operated on AC or DC, which-   
   > ever it was designed for (including the design of the lamp optics,   
   > headlamp reflectors and lenses).   
   >   
   > Electrode wear does not all end up on the other electrode. In   
   > many lamps none of it does (it ends up spluttered over the inside   
   > of the arc tube), but in some cases this can be avoided (e.g. the   
   > hollow cylindical electrodes of cold cathode signage tubes, where   
   > most of it is retained within the electrode itself).   
   >   
      
   Do you know any actual arc lamps that are symmetrical?  The heat load is   
   heavily concentrated on the anode.   
      
   Cheers   
      
   Phil Hobbs   
      
      
   --   
   Dr Philip C D Hobbs   
   Principal Consultant   
   ElectroOptical Innovations LLC   
   Optics, Electro-optics, Photonics, Analog Electronics   
      
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   Briarcliff Manor NY 10510   
      
   hobbs at electrooptical dot net   
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