Cold cathode
169146
223476379
2008-07-04T06:37:14Z
Neotesla
7343469
/* External links */
{{Unreferenced|date=May 2008}}
{{otheruses4|light sources and indicators|cold cathode ion sources|Ion source}}
[[Image:Cold Cathode Fluorescent Lamp.JPG|350px|thumb]]
A '''cold cathode''' is an element used within some [[Nixie tube]]s, [[gas discharge lamp]]s, [[gas filled tube]]s, and [[vacuum tube]]s. Cold cathodes do not employ a [[cathode heater]].
Cold cathode [[fluorescent lamp| fluorescent lamps]] ('''CCFLs''') are usually also called ''cold cathodes''. [[Neon lamp]]s are a very common example of a cold cathode lamp.
Cold Cathodes remain popular for LCD backlighting and enthusiast computer [[case modding|case modders]].
==Electron emission==
A [[cathode]] is any [[electrode]] that emits [[electron]]s. When used in electrical and electronic devices (most [[fluorescent lamp]]s, [[vacuum tube]]s, etc.), the cathode is explicitly heated, creating a [[hot cathode]]. By taking advantage of [[thermionic emission]], electrons can overcome the [[work 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 [[Room temperature|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 [[ion]]ized. A few cold cathode devices contain a [[vacuum]].
==Details==
Before discussing cathode ray tubes of any kind, a few basic terms must be defined:
The cathode is the negative electrode. Any gas discharge lamp has a pair of electrodes, acting as cathode and anode (the positive electrode). Both electrodes act alternating as anode and cathode because the devices run with [[alternating current]].
[[Image:Cold Cathodes.jpg|thumb|250px|right|A standard [[ATX]] computer case fitted with blue and green Cold Cathode tubes.]]
A ''cold cathode'' is distinguished from a [[hot cathode]] that is heated to induce [[thermionic emission]] of [[electron]]s. Discharge tubes with hot cathodes have an envelope filled with low pressure gas and containing a pair of cathodes. Examples are common [[fluorescent lamp]]s, high pressure discharge lamps and [completely-evacuated] [[electron tube]]s and [[vacuum fluorescent display]]s.
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 accelerating 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.
Another mechanism for generating free electrons from a cold metallic surface is [[field emission]]. It is used in some [[x-ray tube]]s and the [[field electron microscope]].
Cold cathodes sometimes have [[Rare earth element|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]] around the cathode is usually minimized, in favor of a so called positive column, filling the tube. The best example is the humble [[neon lamp]]. Another good example is [[Nixie tube]]s. [[Nixie tube]]s too are cold cathode, neon displays that also happen to be in-line, but not in-plane display devices.
[[Image:2007-09-02 CCFL lamp driver.jpg|thumb|right|Conventional type [[CCFL inverter]]]]
[[Image:CCFL Inverter.jpg|350px|thumb|Example of the advanced type [[CCFL inverter]] for notebook]]
A common cold cathode application is in [[neon sign]]age. Other examples include the [[thyratron]], [[krytron]], [[sprytron]], and [[ignitron]] tubes. Large-scale cold cathode [[fluorescent lamp]]s (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 [[backlight]]s for [[computer]] [[liquid crystal display]]s. Additionally, CCFLs are used by [[Case modding|computer modders]] to light the insides of their customized 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.)
Cold cathode devices typically use a complex high-voltage power supply with some mechanism for limiting current flow. 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 resistance to current flow begins to drop, requiring a gradually lower voltage to maintain operation. In the case of tubes with an ionizing gas, the gas can become a very hot [[plasma (physics)| plasma]] where electrical resistance is greatly reduced. If operated from a simple power supply without current limiting, this reduction in operational resistance would lead to damage to the power supply or the tube electrodes from overheating.
==See also==
* [[Ionization gauge]]
* [[Sumlock ANITA calculator|ANITA electronic calculator]]
==References==
<references/>
==Patents==
* {{US patent|2184910}} - [[Philo Farnsworth]] - ''Cold cathode electron discharge tube''
* {{US patent|2263032}} - Philo Farnsworth - ''Cold cathode electron discharge tube''
* {{US patent|2448527}} - [[Clarence Hansell|Clarence W. Hansell]] - ''Cold cathode electron discharge device and circuits therefore''
==External links==
*[http://www.ee.ualberta.ca/~schmaus/elect/pas1.html Pulse Power Switching Devices - An Overview- by John Pasley]
*[http://www.kingsway.de/index.php?option=com_content&task=view&id=20&Itemid=50 Replacing the CCFL of a notebook - a step by step procedure]
*[http://www.tlm.co.jp/web/gijyutu/ccfl.html Differences of CCFL and HCFL (Japan)]
*[http://www.islands.ne.jp/harison/index.html Harison Toshiba Lighting Corporation 液晶バックライト用冷陰放電灯 (Japan)]
*[http://www.nelt.co.jp/navi/oa/reiinn.htm NEC lighting (Japan)]
{{ArtificialLightSources}}
[[Category:Electrodes]]
[[Category:Vacuum]]
[[Category:Vacuum tubes]]
[[Category:Gas discharge lamps]]
[[Category:Lighting]]
[[de:Leuchtröhre]]
[[fr:Cathode froide]]
[[ja:冷陰極管]]
[[ko:냉음극관]]
[[nl:CCFL]]
[[sv:Kallkatodrör]]