A cold cathode is an element used within some Nixie tubes, gas discharge lamps, gas filled tubes, and vacuum tubes. A nixie tube is an electronic device for displaying numerals or other information. Gas discharge lamps are a family of artificial light sources that generate light by sending an Electrical discharge through an ionized gas i A gas-filled tube, also known as a discharge tube, is an arrangement of Electrodes in a Gas within an insulating, temperature-resistant Envelope This article is about the electronic device not an evacuated pipe used for experiments in Free-fall. Cold cathodes do not employ a cathode heater. A cathode heater is a filament used to heat the Cathode in a Vacuum tube or Cathode ray tube.
Cold cathode fluorescent lamps (CCFLs) are usually also called cold cathodes. A fluorescent lamp or fluorescent tube is a Gas-discharge lamp that uses Electricity to excite mercury Vapor. Neon lamps are a very common example of a cold cathode lamp. A neon lamp is a Gas discharge Lamp containing primarily Neon gas at low Pressure.
Cold Cathodes remain popular for LCD backlighting and enthusiast computer case modders.
Contents |
Electron emission
A cathode is any electrode that emits electrons. A cathode is an Electrode through which (positive Electric current flows out of a polarized electrical device An electrode is an Electrical conductor used to make contact with a nonmetallic part of a circuit (e The electron is a fundamental Subatomic particle that was identified and assigned the negative charge in 1897 by J When used in electrical and electronic devices (most fluorescent lamps, vacuum tubes, etc. A fluorescent lamp or fluorescent tube is a Gas-discharge lamp that uses Electricity to excite mercury Vapor. This article is about the electronic device not an evacuated pipe used for experiments in Free-fall. ), the cathode is explicitly heated, creating a hot cathode. Hot cathode is also a name for a Hot filament ionization gauge, a vacuum measuring device By taking advantage of thermionic emission, electrons can overcome the work function of the cathode without an electric field to pull the electrons out. Thermionic emission is the flow of Charge carriers from a surface or over some other kind of Electrical potential barrier caused by thermal vibrational energy In Solid state physics, the work function is the minimum Energy (usually measured in Electron volts needed to remove an Electron from a solid In Physics, the space surrounding an Electric charge or in the presence of a time-varying Magnetic field has a property called an electric field (that can But if sufficient voltage is present, electrons can still be stripped even out of a cathode operating at ambient temperature. Room temperature (also referred to as ambient temperature) is a common term to denote a certain Temperature within enclosed space at which humans are accustomed 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. This page is about the physical properties of gas as a state of matter An ion is an Atom or Molecule which has lost or gained one or more Valence electrons giving it a positive or negative electrical charge A few cold cathode devices contain a vacuum. This vacuum means "absence of matter" or "an empty area or space" for the cleaning appliance see Vacuum cleaner.
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. An alternating current ( AC) is an Electric current whose direction reverses cyclically as opposed to Direct current, whose direction remains constant
A cold cathode is distinguished from a hot cathode that is heated to induce thermionic emission of electrons. Hot cathode is also a name for a Hot filament ionization gauge, a vacuum measuring device Thermionic emission is the flow of Charge carriers from a surface or over some other kind of Electrical potential barrier caused by thermal vibrational energy The electron is a fundamental Subatomic particle that was identified and assigned the negative charge in 1897 by J Discharge tubes with hot cathodes have an envelope filled with low pressure gas and containing a pair of cathodes. Examples are common fluorescent lamps, high pressure discharge lamps and [completely-evacuated] electron tubes and vacuum fluorescent displays. A fluorescent lamp or fluorescent tube is a Gas-discharge lamp that uses Electricity to excite mercury Vapor. A vacuum fluorescent display (VFD is a Display device used commonly on consumer-electronics equipment such as Video cassette recorders Car radios
The interior surface of cold cathodes are capable of producing secondary electrons at a ratio greater than unity (amplification) upon electron and ion impact. Secondary electrons are electrons generated as ionization products 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. Field emission (FE is the emission of electrons from the surface of a condensed phase into another phase due to the presence of high electric fields It is used in some x-ray tubes and the field electron microscope. An X-ray tube is a Vacuum tube that produces X-rays They are part of X-ray machines X-rays are part of the Electromagnetic spectrum, an
Cold cathodes sometimes have rare earth coating on them for enhancing electron emission. Rare earth elements and rare earth metals are according to IUPAC, the collection of seventeen Chemical elements in the Periodic table, namely Some types contain a source of beta radiation to start ionization of the gas that fills the tube. Beta particles are high-energy high-speed Electrons or Positrons emitted by certain types of Radioactive nuclei such as Potassium -40 Ionization is the physical process of converting an Atom or Molecule into an Ion by adding or removing charged particles such as Electrons In such a tube, glow discharge around the cathode is usually minimized, in favor of a so called positive column, filling the tube. Electric glow discharge is a type of plasma formed by passing a current at 100 V to several kV through a gas usually Argon or another Noble The best example is the humble neon lamp. A neon lamp is a Gas discharge Lamp containing primarily Neon gas at low Pressure. Another good example is Nixie tubes. A nixie tube is an electronic device for displaying numerals or other information. Nixie tubes too are cold cathode, neon displays that also happen to be in-line, but not in-plane display devices. A nixie tube is an electronic device for displaying numerals or other information.
A common cold cathode application is in neon signage. Neon signs are luminous-tube signs that contain neon or other inert gases at a low pressure Other examples include the thyratron, krytron, sprytron, and ignitron tubes. A thyratron is a type of Gas filled tube used as a high Energy electrical Switch and controlled Rectifier. The Krytron is a cold-cathode Gas filled tube intended for use as a very high-speed switch and was one of the earliest developments of the EG&G Corporation. The Krytron is a cold-cathode Gas filled tube intended for use as a very high-speed switch and was one of the earliest developments of the EG&G Corporation. An ignitron is a type of controlled Rectifier dating from the 1930s 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. A fluorescent lamp or fluorescent tube is a Gas-discharge lamp that uses Electricity to excite mercury Vapor. Nowadays, miniature CCFLs are extensively used as backlights for computer liquid crystal displays. A backlight is the form of illumination used in a Liquid crystal display (LCD A computer is a Machine that manipulates data according to a list of instructions. Additionally, CCFLs are used by computer modders to light the insides of their customized computer cases. Case modification (commonly referred to as case modding) is the modification of a computer chassis (often just referred to as the case) or a
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. An alternating current ( AC) is an Electric current whose direction reverses cyclically as opposed to Direct current, whose direction remains constant An anode is an Electrode through which Electric current flows into a polarized electrical device 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. Tungsten (ˈtʌŋstən also known as wolfram (/ˈwʊlfrəm/ is a Chemical element that has the symbol W and Atomic number 74 Hot cathode is also a name for a Hot filament ionization gauge, a vacuum measuring device )
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 where electrical resistance greatly reduced. In Physics and Chemistry, plasma is an Ionized Gas, in which a certain proportion of Electrons are free rather than being bound 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
References
Patents
- U.S. Patent 2,184,910 - Philo Farnsworth - Cold cathode electron discharge tube
- U.S. Patent 2,263,032 - Philo Farnsworth - Cold cathode electron discharge tube
- U.S. Patent 2,448,527 - Clarence W. Hansell - Cold cathode electron discharge device and circuits therefore
External links
- Pulse Power Switching Devices - An Overview- by John Pasley
- Replacing the CCFL of a notebook - a step by step procedure
Dapyx Software network: MP3 Explorer | Ebook Manager | Zenithic