HID current type

Can anyone, please, help with technical question? Seems to me, miniature hid bulbs under 35W (even 32W) are all DC operated, and 35W and higher are AC... Is that right? Also are there any differences between DC and AC lamp structure and specifications?

Thanks a lot for any info, it will be helpful.

By no means the complete answer to your questions but, The Tactical HID 24 Watt originally was released in a DC out model. Many of us here on CPF had problems with very significant flickering of the bulb. Tactical soon thereafter, released the same light in an AC out version which greatly reduced and/or eliminated the flickering. I don't know of any 35 Watt and above HID's that are DC output except my 300 Watt Locators which are circa 1970's and have a whopping bulb life of 25 hours. My Locator bulb is very small by today's short arc HID bulb standards at about 3 to 3 1/2" long with an arc chamber around 3/8" to 7/16" whereas 150 Watt short arc bulbs I've been looking at recently are in the neighborhood of 5" to 7" long with arc chambers around 3/4". To me, lamp structure looks the same - AC and DC. Specifications-wise, and this may be an old "wives-tale", the "receiving" electrode (not sure if it's the Anode or Cathode) on the DC bulb might be larger in proportion to its' AC counter-part because all the electrode wear is from one electrode due to one-way current flow whereas in AC, electrode wear is more even due to two-way flow. Someone jump in if this is inaccurate.

Thanks for your answer, i thougt AC lamps should flicker, even with invisible to the eye few hundred hz, but turns out DC types have more flicker problems. I guess DC flicker caued by incorrect burning position or just quick change of position by holding flashlight in hand? Or it is natural for DC lamps?
According to wiki, the short-arc HIDs are pure xenon, and always operates DC, and small ones, like automotive are metal halides, with small xenon addition for faster start, and DC is not good for metal halides. I guess your 300 Watt Locators are the first type "xenon short arc", right?

Again, THANKS A LOT, i'm "read-only" to HIDs, trying to figure out wich type is without flicker problems...

Actually, I believe the Locator bulb is Mercury Xenon because of its' very long warm-up. All the pure Xenon short arcs I know of, provide about 80% of their brightness within a second or 3 of turn on. I don't know for sure, though. I should look it up.

For DC lamps it is typical that the anode is much bigger than the cathode. Bulbs like the small brightstar (10W/21W/24W/35W) are most driven with DC but these are AC lamps from construction. In the past I built some AC ballast for these lamps and got much more light output and a much better light colour. Even it was possible to drive the lamps with much more power (the 24W Brightstar with 35-40W at AC is no problem).

What BVH said about Xenon is right, real Xenon bulbs only have gas (Xenon) inside, so no warm-up time is required. The warm up time is only required when the additional materials inside have to heat up and evaporate. In automotive lights Xenon is only a starting gas. You get -with enough power, at last more than 100W at 35W lamps- the for standards required light flux directly after ignition and when the materials inside are evaporated the power can be reduced to the nominal wattage.

regards
Thomas

I found numerous WEB pages that indicate it's a mercury fill but most pages, including ones that called themselves reprints from old GE catalogs indicate a Metal Halied fill. Many also say it's a Short Arc bulb which I don't understand with a gap of about 4-5mm.

Interesting, so Brightstar "DC" units are actually AC... But are "real DC" lamps are alse flickering and less efficient than AC ones? Or AC is always better for metal halides? Thanks.

You can't say every bulb runs better with AC. DC bulbs like Xenon-Lamps for cinema are designed for DC and your must not run it with AC. But Arc-Lamps with symmetrical Electrodes are not for DC operation, because the anode will get much hotter than the cathode. DC-Lamps have assymetrical electrodes with a bigger anode.
Theoretical a arc is more stable at DC, so you should not have flickering at DC-Lamps. At AC you could have problems with acoustic resonances in the bulb at some frquencies, but when you have a well working ballast and you take care to the lamp datasheet you won't have problems with flickering at any lamp. At last the most efficient lamps are AC types (my Osram HTI 1500 is one of the most efficient types), but I don't think this is a problem of AC or DC...

regards
Thomas

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