page_title: Metal-halide lamp
section_title: Ballasts
hierarchical_section_title: Metal-halide lamp / Components / Ballasts
caption_attribution_description: English: electronic power supply for metal halide lamps
context_page_description: A metal-halide lamp is an electrical lamp that produces light by an electric arc through a gaseous mixture of vaporized mercury and metal halides. It is a type of high-intensity discharge gas discharge lamp. Developed in the 1960s, they are similar to mercury vapor lamps, but contain additional metal halide compounds in the quartz arc tube, which improve the efficiency and color rendition of the light.
  The most common metal halide compound used is sodium iodide. Once the arc tube reaches its running temperature, the sodium dissociates from the iodine, adding orange and reds to the lamp's spectrum from the sodium D line as the metal ionizes.
  As a result, metal-halide lamps have high luminous efficacy of around 75–100 lumens per watt, which is about twice that of mercury vapor lights and 3 to 5 times that of incandescent lights and produce an intense white light. Lamp life is 6,000 to 15,000 hours. As one of the most efficient sources of high CRI white light, metal halides as of 2005 were the fastest growing segment of the lighting industry.
context_section_description: The electric arc in metal-halide lamps, as in all gas discharge lamps has a negative resistance property; meaning that as the current through the bulb increases, the voltage across it decreases. If the bulb is powered from a constant voltage source such as directly from the AC wiring, the current will increase until the bulb destroys itself; therefore, halide bulbs require electrical ballasts to limit the arc's current.  There are two types:  
  Inductive ballast - Many fixtures use an inductive ballast, also known as a magnetic ballast, similar to those used with fluorescent lamps. This consists of an iron-core inductor.  The inductor presents an impedance to AC current. If the current through the lamp increases, the inductor reduces the voltage to keep the current limited.
  Electronic ballast - These are lighter and more compact. They consist of an electronic oscillator which generates a high frequency current to drive the lamp. Because they have lower resistive losses than an inductive ballast, they are more energy efficient. However, high-frequency operation does not increase lamp efficiency as for fluorescent lamps.
  Pulse-start metal-halide bulbs don't contain a starting electrode which strikes the arc, and require an ignitor to generate a high-voltage (1–5 kV on cold strike, over 30 kV on hot restrike) pulse to start the arc. Electronic ballasts include the igniter circuit in one package. American National Standards Institute (ANSI) lamp-ballast system standards establish parameters for all metal-halide components (with the exception of some newer products).
caption: Electronic ballast for 35 W metal halide light bulbs
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