Cold cathode
From Free net encyclopedia
Note: Principles are mostly the same for cold cathode ion sources as in particle accelerators to create electrons. A cold cathode gas filled tube or vacuum tube is one in which there is no filament or cathode heater.
Specifics include: Types of Cold Cathodes, color temperature, color rendering index, manufacturing processes.
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Description
Before looking at cathode ray tubes of any kind, a few basic terms must be defined.
1) Cathode- The negative electrode in a cathode ray tube
2) Cathode rays- the positive ions created when some electrons are removed form the molecules of a gas at low pressure
A cold cathode is distinguished from a hot cathode that is heated to induce thermionic emission of electrons. These electron discharge tubes have an envelope evacuated or filled with low pressure gas and containing a pair of cathodes, usually parallel to one another. The interior surface of the cathodes are capable of producing secondary electrons at a ratio greater than unity (amplification) upon electron impact. A third node is present in the device, acting as an accelerator between the two cathodes. These devices use longitudinal magnetic fields and electrostatic focusing.
Cold cathodes sometimes have rare earth coating on them for enhancing electron emission. Some types contain a source of beta radiation to start ionization of the gas that fills the tube. In such a tube, glow discharge is usually minimized, in favor of arc discharge. The best example is the humble neon lamp. Another good example is Nixie tubes. Nixie tubes too are cold cathode, neon displays that also happen to be in-line, but not in-plane display devices.
A common cold cathode application is in neon signage. Other examples include the thyratron, krytron, sprytron, and ignitron tubes. Large-scale cold cathode fluorescent lamps (CCFLs) have been produced in the past, and are still used today when shaped, long life linear light sources are required. Nowadays, miniature CCFLs are extensively used as backlights for computer liquid crystal displays. Additionally, CCFLs are directly used by computer modders to light the insides of their highly customized transparent computer cases.
Despite their name, cold cathodes don't necessarily remain cold as they operate; they can get painfully hot. In systems using alternating current but without separate anode structures, the cathodes alternate as anodes and the impinging electrons can cause substantial localized heating, often to red heat. The cathode may or may not take advantage of this heating to facilitate the thermionic emission of electrons when it is acting as a cathode. (Instant start fluorescent lamps definitely do employ this aspect; they start as cold-cathode devices but soon localized heating of the fine tungsten wire cathodes causes them to operate as ordinary hot cathode lamps.)
See also
Patents
- Template:US patent - Philo Farnsworth - Cold cathode electron discharge tube
- Template:US patent - Philo Farnsworth - Cold cathode electron discharge tube
- Template:US patent - Clarence W. Hansell - Cold cathode electron discharge device and circuits therefore