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Fluorescent lamps require special control gear to be dimmed.
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Last month (page 26), I compared the special demands fluorescent lamps’ electronic control gear to well-known magnetic ballasts’ requirements. Large sign projects that include projected signs often require adjustable light levels to avoid “light pollution” or other neighborhood issues. Architectural-lighting applications often mandate lamp dimming.
Dimming cold-cathode or neon tubing is rather simple if installers observe a few facts (see ST, June 2005, page 20). Let’s first examine how fluorescent dimming works.
Electrode construction and operation differentiates cold-cathode and fluorescent lamps. A fluorescent-lamp electrode must reach a minimum temperature to release electrons into the gas. In normal lamp operation, ions bombard the electrode to heat it. Ion impingement maximizes at the hottest part of the electrode, the electron-emission point, thus forming a “hot spot.” Only a small part of the electrode surface carries the lamp's operating current.
If an electrode hasn’t reached operating temperature, and isn’t ready to emit electrons thermally, and if high voltage is applied to force a current through the lamp (the scenario for “instant start”), the operates similarly to a cold-cathode electrode.
As Georges Claude’s first neon patent claimed in 1910, a cold-cathode electrode can carry only a limited amount of current per surface, or it will be destroyed by sputtering very quickly, and the maximum load was 6.5mA per square centimeter of electrode surface.
A fluorescent lamp electrode has a small surface compared to the current, and if it operates like a cold-cathode electrode, sputtering will destroy it quickly, even when the lamp operates at reduced current.
Dimming a neon lamp is achieved by reducing the effective (average) operating current flowing through the lamp. This also applies to dimming a fluorescent lamp, but there’s a problem: When the ion current that’s heating the electrode to the proper operating temperature drops, the electrode won’t be kept at the temperature needed to emit electrons and will act as a cold-cathode electrode. Below a certain current level, sputtering will destroy it. To overcome this problem, the electrode must always be maintained at operating temperature.
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