Cold-Cathode Devices
– Geissler tube and Plucker tube were early cold-cathode devices
– Early cathode-ray tubes led to the discovery of the electron
– Various types of thyratron and krytron were developed as cold-cathode switching tubes
– Dekatron is a cold-cathode tube used for counting
Cold-Cathode Lamps
– Cold-cathode fluorescent lamps (CCFLs) and neon lamps are examples of cold-cathode lamps
– Neon lamps rely on excitation of gas molecules to emit light
– CCFLs use a discharge in mercury vapor to develop ultraviolet light
– CCFLs were used for backlighting of LCDs
– Neon lamps are commonly used for neon signs
Cold Cathodes and Their Characteristics
– Cold cathodes are negative electrodes in gas-discharge lamps
– They are distinguished from hot cathodes that are heated to induce thermionic emission
– Cold cathodes can emit secondary electrons at a ratio greater than unity
– Cold-cathode discharge lamps use higher voltages than hot-cathode ones
– Field electron emission is another mechanism to generate free electrons from a cold metallic surface
Applications of Cold Cathodes
– Cold cathodes are used in rectifiers and amplifiers
– Examples include crossatron, mercury-arc valves, and pseudospark switching applications
– Neon signs often use cold cathodes
– Cold-cathode lighting is used in locations with low temperatures, such as The Clock Tower in the Palace of Westminster
– Large cold-cathode fluorescent lamps (CCFLs) are used for long-life linear light sources
Other Characteristics and Limitations
– Cold cathodes have a rare-earth coating to enhance electron emission
– Some types of cold cathodes contain a source of beta radiation to start ionization of the gas
– Glow discharge around the cathode is minimized in some tubes
– Cold-cathode devices require a high-voltage power supply with current limiting
– Miniature CCFLs have been produced and used for shaped, long-life linear light sources
– Effects of internal heating: Impinging electrons can cause substantial localized heating, often to red heat. Localized heating of tungsten-wire cathodes facilitates thermionic emission of electrons. Instant-start fluorescent lamps utilize this heating aspect. Backlights used for LCD TV displays face problems due to this aspect.
– Variable backlighting limitations: CCFLs have limited dimming capabilities due to lower plasma current affecting cathode temperature. Running the cathode at too low a temperature drastically shortens the life of the lamps. New energy-efficiency regulations propose variable backlighting for LCD TV sets. Variable backlighting improves perceived contrast range. High-end manufacturers are turning to high-efficiency white LEDs as a better solution.
References:
– U.S. Patent 1,993,187: Cold cathode discharge tube
– D. M. Neale, Cold Cathode Tube Circuit Design, Francis and Taylor, 1964, pp. 1–7
– Ifay guide info electric discharge lighting systems, cold cathode
– EGL lighting products. Archived from the original on October 26, 2010. Retrieved 9 February 2011
– U.S. Patent 1,860,149: Discharge tube
– U.S. Patent 2,103,033: Electron emissive electrode
– U.S. Patent 1,316,967: Gaseous-conduction lamp
– Solé Lighting (commercial site advocating CCFLs) Source: https://en.wikipedia.org/wiki/Cold_cathode
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A cold cathode is a cathode that is not electrically heated by a filament. A cathode may be considered "cold" if it emits more electrons than can be supplied by thermionic emission alone. It is used in gas-discharge lamps, such as neon lamps, discharge tubes, and some types of vacuum tube. The other type of cathode is a hot cathode, which is heated by electric current passing through a filament. A cold cathode does not necessarily operate at a low temperature: it is often heated to its operating temperature by other methods, such as the current passing from the cathode into the gas.
