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HID headlamp bulbs do not run on low-voltage DC, such as the 12 volt system commonly found in passenger vehicles, so they require a ballast with either an internal or external igniter. The igniter is integrated into the bulb in D1 and D3 systems, and is either a separate unit or integral with the electronic ballast in D2 and D4 systems. The ballast controls the current to the bulb. The ignition and ballast operation proceeds in three stages:
1. Ignition: a high voltage pulse is used to produce a spark, in a manner similar to a spark plug, which ionizes the Xenon gas, creating a conducting tunnel between the tungsten electrodes. In this tunnel, the electrical resistance is reduced and current flows between the electrodes.
2. Initial phase: the bulb is driven with controlled overload. Because the arc is operated at high power, the temperature in the capsule rises quickly. The metallic salts vaporize, and the arc is intensified and made spectrally more complete. The resistance between the electrodes also falls; the electronic ballast control gear registers this and automatically switches to continuous operation.
3. Continuous operation: all metal salts are in the vapor phase, the arc has attained its stable shape, and the luminous efficacy has attained its nominal value. The ballast now supplies stable electrical power so the arc will not flicker.
Stable operating voltage is 85 volts AC in D1 and D2 systems, 42 volts AC in D3 and D4 systems. The frequency of the square-wave alternating current is typically 400HZ or higher. |
