HID ballasts: AC or DC?

Some of the vendors of automotive HID light conversion kits are saying that AC voltage ballasts are superior to DC ballast.
http://www.ebay.com/itm/151085113168?ssPageName=STRK:MESELX:IT&_trksid=p3984.m 1555.l2649
(Scroll down to the heading "AC vs. DC".)
Now that we know what the seller says, what's the science?
Cheers.
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On 06/09/2014 05:51, Fester Bestertester wrote:

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
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On 9/6/2014 2:43 AM, piglet wrote:

Normal arc lamps are asymmetrical--they run from DC, and fail quickly if you put them in backwards. Dunno about HIDs.
Cheers
Phil Hobbs
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"Backwards" includes running from AC I presume?
Thanks.
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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).
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On 9/6/2014 2:20 PM, Andrew Gabriel wrote:

Do you know any actual arc lamps that are symmetrical? The heat load is heavily concentrated on the anode.
Cheers
Phil Hobbs
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I don't have any headlamp arc lamps. I did a google search for images, and they all looked symmetrical, but I don't know how representitive those images were, or if they were intended for AC or DC operation, so it didn't tell me anything useful.
I have several other symmetrical arc lamps, but those are intended for AC operation, so that's not a surprise.
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Andrew Gabriel
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On 9/6/2014 3:48 PM, Andrew Gabriel wrote:

I did a bit of digging too, and it looks like (unsurprisingly) high pressure sodium flood lamps are usually symmetrical. Dunno about HIDs.
Cheers
Phil Hobbs
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Tesla (AC) is superior to Edison (DC). S*
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