HID questions

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Jun 9, 2005 | 06:00 AM

#11

Elistan

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Joined: Oct 2000

Posts: 15,323

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From: Longmont, CO

Quote:

Originally Posted by SgtKarj,Jun 8 2005, 10:21 PM

Chasmo is correct. The "arc tube" in an HID lamp is a small glass enclosure filled with a very finely measured combination of powdered metals and other compounds plus usually a lit-tle drop of mercury and xenon gas. When the ballast of the fixture hits these metals with high voltage, the electricity starts to vaporize them into their gaseous form, which they generally do quite slowly, the larger the lamp (that's why when you go to a ballfield or other large sports field, the lighting seems to take 10 or 15 minutes to reach full song. It DID take a long time. I don't know how they make them come up so quick in the automotive field, but it must be magic.

I'm not sure how the baseball field versions work, but my understanding is that the ballast mentioned above is responsible for the quick turn-on of HIDs by giving the tube a 20,000 Volt jolt of electricity when you flip the switch. (That's why the units have "This can kill you" stickers on them.) That's the voltage required to force electricity across the spark gap. Once that is done the contents of the HID tube not only vaporize, they actually ionize - that is, the electrons disassociate from the atoms and tend to float around. That reduces the electrical impedance of the spark gap and allows the HIDs to operate at more sane voltages.