I would like to share a little information from my past experience of experimenting with AC ballast and DC ballast and how it effect the lamp life and tear on the lamp.
On a lot of DC lamps that iv worked with are only designed to run DC only, and some Lamps are partially compatible with DC, and some just cant even run it right. One must ask them self why would they want to use DC? Well one is that it reduces flickering. Electricity flows differently or in one direction. The big thing about AC vs DC, is there is no Impedance wave form. There is just the positive electrode and a negative electrode, which can effect the amount of light on one electrode, and resulting in heating up on one end but less on the other. So you can notice that a lot of times one end of the electrode usually the negative end, because of Positive Mercury Ions, will run to the negative side and cause it get hotter and brighter on that side. Fluorescent is known for this. To correct this situation, Special Iron ballast with special reverse switch will reverse the polarity on the same electrode so the Positive Mercury Ions will attract to the other side and equally distribute the mercury Ions.
Mercury vapor lamps can run DC, but it might only run reliably if the Tip end of the electrode is negative and the and the base is positive. The starting electrode will do better when its got the positive side. But what could happen is if the nearby main electrode is a positive side it could cause a thin film of metal that could build up and short the starting electrode to the nearby main positive electrode.
Metal halide and Sodium vapor lamp should never get DC due to the hot metal vapors which will leach in the quartz and cause wear on the Quartz, due to the DC electrical field. What I was told, is that the Negative Main electrode will not release as much Metal vapor since most of the Ions are Positive, and will condense on the electrode rather than the nearby arc tube. Asymmetrical electrodes with a short arc length will run DC, but those lamp are usually XBO, and HBO lamps ect..
Low pressure sodium, is usually the same, the Sodium Ions will drift the negative side of the electrode and experience almost Instance destructive electrolysis problems.
Hope they better explain to you guys the problem with DC HID lamps.
« Last Edit: November 25, 2008, 12:32:19 PM by Nineaclock »