Electrodeless lamps

Posted on 6:54 PM by Admin

In contrast with all other electrical lamps that use electrical connections through the lamp envelope to transfer power to the lamp, in electrode less lamps the power needed to generate light is transferred from the outside of the lamp envelope by means of (electro)magnetic fields. There are two advantages of eliminating electrodes. The first is extended bulb life, because the electrodes are usually the limiting factor in bulb life. The second benefit is the ability to use light-generating substances that would react with metal electrodes in normal lamps.

Aside from the method of coupling energy into the mercury vapor, these lamps are very similar to conventional fluorescent lamps. Mercury vapor in the discharge vessel is electrically excited to produce short-wave ultraviolet light, which then excites the phosphors to produce visible light. While still relatively unknown to the public, these lamps have been available since 1990. The most common form has the shape of an incandescent light bulb. Unlike an incandescent lamp or conventional fluorescent lamps, there is no electrical connection going inside the glass bulb; the energy is transferred through the glass envelope solely by electromagnetic induction.

In the most common form, a glass tube (B) protrudes bulb-wards from the bottom of the discharge vessel (A). This tube contains an antenna called a power coupler, which consists of a coil wound over tubular ferrite core.

In lower-frequency versions of induction systems, the lamp consists of two long parallel glass tubes, connected by two short tubes that have coils mounted around them.

The antenna coils receive electric power from the electronic ballast (C) that generates a high frequency. The exact frequency varies with lamp design, but popular examples include 13.6 MHz, 2.65 MHz and 250 kHz (in physically large lamps). A special resonant circuit in the ballast produces an initial high voltage on the coil to start a gas discharge; thereafter the voltage is reduced to normal running level.

The system can be seen as a type of transformer, with the power coupler forming the primary coil and the gas discharge arc in the bulb forming the one-turn secondary coil and the load of the transformer. The ballast is connected to mains electricity, and is generally designed to operate on voltages between 100 and 277 VAC at a frequency of 50 or 60 Hz. Most ballasts can also be connected to DC voltage sources like batteries for emergency lighting purposes.

In other conventional gas discharge lamps, the electrodes are the part with the shortest life, limiting the lamp lifespan severely. Since an induction lamp has no electrodes, it can have a very long service life. For induction lamp systems with a separate ballast, the service life can be as long as 100, 000 hours, which is 11.4 years continuous operation, or 22.8 years used at night or day only. For induction lamps with integrated ballast, the life is 15, 000 to 30, 000 hours. Extremely high-quality electronic circuits are needed for the ballast to attain such a long service life. Such expensive lamps have special application areas in situations where replacement costs are high.

Research on electrodeless lamps continues, with variations in operating frequency, lamp shape, the induction coils and other design parameters. Low public awareness and relatively high prices have so far kept the use of such lamps highly specialized.

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