HID headlamp bulbs do not run on low-voltage DC current, so they require a ballast with either an internal or external ignitor. An electrical ballast is a device intended to limit the amount of current in an electric circuit. The ignitor is integrated into the bulb in D1 and D3 systems, and is either a separate unit or part of the 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 Phase. A high voltage pulse is used to produce a spark, in a manner similar to a spark plug, which ionises the Xenon gas, creating a conducting tunnel between the tungsten electrodes. Electrical resistance is reduced within the tunnel, and current flows between the electrodes. 2. Turn-On phase. The bulb is driven with controlled load. Because the arc is operated at high power, the temperature in the bulb rises quickly. The metallic salts vaporise, and the arc is intensified and made visually 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 .............. That’s just an excerpt to this short, but concise video. Get the scoop here. Presented using advanced software CG animation technology to help you understand. Part of our Automotive Technology series shown here on this channel. Enjoy...