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Cold Cathode Lighting

Cold cathode is an element used within some gas discharge lamps and vacuum tubes The term cold cathode refers to the fact that the cathode is not independently heated. In spite of this, the cathode itself may still operate at temperatures as high as if the cathode were heated. Cold cathode fluorescent lamps (CCFLs) are usually also called cold cathodes.

Electron emission A cathode is any electrode that emits electrons. When used in electrical and electronic devices , the cathode is explicitly heated, creating a hot cathode. By taking advantage of thermionic emission, electrons can overcome the function of the cathode without an electric field to pull the electrons out. But if sufficient voltage is present, electrons can still be stripped even out of a cathode operating at ambient temperature. Because it is not deliberately heated, such a cathode is referred to as a cold cathode, although several mechanisms may eventually cause the cathode to become quite hot once it is operating. Most cold cathode devices are filled with a gas which can be ionized. A few cold cathode devices contain a vacuum.

The interior surface of cold cathodes are capable of producing secondary electrons at a ratio greater than unity (amplification) upon electron and ion impact. For acceleration of the ions to a sufficient velocity for creating free electrons from the cathode material, cold cathode discharge lamps need higher voltages than hot cathode ones, causing a strong electric field near the cathodes.

A common cold cathode application is in neon signage and other locations where the ambient temperature is likely to drop well below freezing, The Clock Tower, Palace of Westminster (Big Ben) uses cold cathode lighting behind the clock faces where continual striking and failure to strike in cold weather would be less than ideal. Cold cathode devices typically use a complex high-voltage power supply with some mechanism for limiting current. Although creating the initial space charge and the first arc of current through the tube may require a very high voltage, once the tube begins to heat up, the electrical resistance drops, thus increasing the electrical current through the lamp. To offset this effect and maintain normal operation, the supply voltage is gradually lowered.

Cold Cathode
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