A cold cathode[1] is a cathode that is not electrically heated by a filament.[note 1] 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.
The stacked digit arrangement in a Nixie tube is visible here
A cold-cathode vacuum tube does not rely on external heating of an electrode to provide thermionic emission of electrons. Early cold-cathode devices included the Geissler tube and Plucker tube, and early cathode-ray tubes. Study of the phenomena in these devices led to the discovery of the electron.
Neon lamps are used both to produce light as indicators and for special-purpose illumination, and also as circuit elements displaying negative resistance. Addition of a trigger electrode to a device allowed the glow discharge to be initiated by an external control circuit; Bell Laboratories developed a "trigger tube" cold-cathode device in 1936.[2]
Many types of cold-cathode switching tube were developed, including various types of thyratron, the krytron, cold-cathode displays (Nixie tube) and others. Voltage regulator tubes rely on the relatively constant voltage of a glow discharge over a range of current and were used to stabilize power-supply voltages in tube-based instruments. A Dekatron is a cold-cathode tube with multiple electrodes that is used for counting. Each time a pulse is applied to a control electrode, a glow discharge moves to a step electrode; by providing ten electrodes in each tube and cascading the tubes, a counter system can be developed and the count observed by the position of the glow discharges. Counter tubes were used widely before development of integrated circuit counter devices.
The flash tube is a cold-cathode device filled with xenon gas, used to produce an intense short pulse of light for photography or to act as a stroboscope to examine the motion of moving parts.
Lamps
Cold-cathode lamps include cold-cathode fluorescent lamps (CCFLs) and neon lamps. Neon lamps primarily rely on excitation of gas molecules to emit light; CCFLs use a discharge in mercury vapor to develop ultraviolet light, which in turn causes a fluorescent coating on the inside of the lamp to emit visible light.
Cold-cathode fluorescent lamps were used for backlighting of LCDs, for example computer monitors and television screens.
In the lighting industry, “cold cathode” historically refers to luminous tubing larger than 20mm in diameter and operating on a current of 120 to 240 milliamperes. This larger-diameter tubing is often used for interior alcove and general lighting.[3][4] The term "neon lamp" refers to tubing that is smaller than 15mm in diameter[citation needed] and typically operates at approximately 40 milliamperes. These lamps are commonly used for neon signs.
Details
The cathode is the negative electrode. Any gas-discharge lamp has a positive (anode) and a negative electrode. Both electrodes alternate between acting as an anode and a cathode when these devices run with alternating current.
A standard computer case fitted with blue and green cold-cathode tubesCold-cathode fluorescent lamp backlight
The surface of cold cathodes can emit secondary electrons at a ratio greater than unity (breakdown). An electron that leaves the cathode will collide with neutral gas molecules. The collision may just excite the molecule, but sometimes it will knock an electron free to create a positive ion. The original electron and the freed electron continue toward the anode and may create more positive ions (see Townsend avalanche). The result is for each electron that leaves the cathode, several positive ions are generated that eventually crash onto the cathode. Some crashing positive ions may generate a secondary electron. The discharge is self-sustaining when for each electron that leaves the cathode, enough positive ions hit the cathode to free, on average, another electron. External circuitry limits the discharge current. Cold-cathode discharge lamps use higher voltages than hot-cathode ones. The resulting strong electric field near the cathode accelerates ions to a sufficient velocity to create free electrons from the cathode material.
Cold cathodes sometimes have a rare-earth coating to enhance electron emission. Some types contain a source of beta radiation to start ionization of the gas that fills the tube.[5] In some tubes, glow discharge around the cathode is usually minimized; instead there is a so-called positive column, filling the tube.[6][7][note 2] Examples are the neon lamp and nixie tubes. Nixie tubes too are cold-cathode neon displays that are in-line, but not in-plane, display devices.
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 electric current through the lamp. To offset this effect and maintain normal operation, the supply voltage is gradually lowered. In the case of tubes with an ionizing gas, the gas can become a very hot plasma, and electrical resistance is greatly reduced. If operated from a simple power supply without current limiting, this reduction in resistance would lead to damage to the power supply and overheating of the tube electrodes.
Applications
A photo of an illuminated compact fluorescent lamp (CFL) of the cold-cathode variety
A common cold-cathode application is in neon signs 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 undesirable. Large 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. As of 2011[update], miniature CCFLs were extensively used as backlights for computer and television liquid-crystal displays. CCFL lifespans vary in LCD televisions depending on transient voltage surges and temperature levels in usage environments.
Due to its efficiency, CCFL technology has expanded into room lighting. Costs are similar to those of traditional fluorescent lighting,[clarification needed] but with several advantages: it has a long life, the light emitted is easier on the eyes[clarify], bulbs turn on instantly to full output and are also dimmable.[8]
Effects of internal heating
In systems using alternating current but without separate anode structures, the electrodes alternate as anodes and cathodes, and the impinging electrons can cause substantial localized heating, often to red heat. The electrode may take advantage of this heating to facilitate the thermionic emission of electrons when it is acting as a cathode. (Instant-start fluorescent lamps employ this aspect; they start as cold-cathode devices, but soon localized heating of the fine tungsten-wire cathodes causes them to operate in the same mode as hot-cathode lamps.)
This aspect is problematic in the case of backlights used for LCD TV displays. New energy-efficiency regulations being proposed in many countries will require variable backlighting; variable backlighting also improves the perceived contrast range, which is desirable for LCD TV sets. However, CCFLs are strictly limited in the degree to which they can be dimmed, both because a lower plasma current will lower the temperature of the cathode, causing erratic operation, and because running the cathode at too low a temperature drastically shortens the life of the lamps.[citation needed] Much research is being directed to this problem, but high-end manufacturers are now turning to high-efficiency white LEDs as a better solution.[citation needed]
Cathode rays or electron beams (e-beam) are streams of electrons observed in discharge tubes. If an evacuated glass tube is equipped with two electrodes and a voltage is applied, glass behind the positive electrode is observed to glow, due to electrons emitted from the cathode. They were first observed in 1859 by German physicist Julius Plücker and Johann Wilhelm Hittorf, and were named in 1876 by Eugen Goldstein Kathodenstrahlen, or cathode rays. In 1897, British physicist J. J. Thomson showed that cathode rays were composed of a previously unknown negatively charged particle, which was later named the electron. Cathode-ray tubes (CRTs) use a focused beam of electrons deflected by electric or magnetic fields to render an image on a screen.
A cathode is the electrode from which a conventional current leaves a polarized electrical device. This definition can be recalled by using the mnemonic CCD for Cathode Current Departs. A conventional current describes the direction in which positive charges move. Electrons have a negative electrical charge, so the movement of electrons is opposite to that of the conventional current flow. Consequently, the mnemonic cathode current departs also means that electrons flow into the device's cathode from the external circuit. For example, the end of a household battery marked with a + (plus) is the cathode.
A vacuum tube, electron tube, valve, or tube, is a device that controls electric current flow in a high vacuum between electrodes to which an electric potential difference has been applied.
A Nixie tube, or cold cathode display, is an electronic device used for displaying numerals or other information using glow discharge.
A fluorescent lamp, or fluorescent tube, is a low-pressure mercury-vapor gas-discharge lamp that uses fluorescence to produce visible light. An electric current in the gas excites mercury vapor, which produces short-wave ultraviolet light that then causes a phosphor coating on the inside of the lamp to glow. A fluorescent lamp converts electrical energy into useful light much more efficiently than an incandescent lamp. The typical luminous efficacy of fluorescent lighting systems is 50–100 lumens per watt, several times the efficacy of incandescent bulbs with comparable light output. For comparison, the luminous efficacy of an incandescent bulb may only be 16 lumens per watt.
A neon lamp is a miniature gas-discharge lamp. The lamp typically consists of a small glass capsule that contains a mixture of neon and other gases at a low pressure and two electrodes. When sufficient voltage is applied and sufficient current is supplied between the electrodes, the lamp produces an orange glow discharge. The glowing portion in the lamp is a thin region near the cathode; the larger and much longer neon signs are also glow discharges, but they use the positive column which is not present in the ordinary neon lamp. Neon glow lamps were widely used as indicator lamps in the displays of electronic instruments and appliances. They are still sometimes used for their electrical simplicity in high-voltage circuits.
In the signage industry, neon signs are electric signs lighted by long luminous gas-discharge tubes that contain rarefied neon or other gases. They are the most common use for neon lighting, which was first demonstrated in a modern form in December 1910 by Georges Claude at the Paris Motor Show. While they are used worldwide, neon signs were popular in the United States from about the 1920s to 1950s. The installations in Times Square, many originally designed by Douglas Leigh, were famed, and there were nearly 2,000 small shops producing neon signs by 1940. In addition to signage, neon lighting is used frequently by artists and architects, and in plasma display panels and televisions. The signage industry has declined in the past several decades, and cities are now concerned with preserving and restoring their antique neon signs.
A flashtube (flashlamp) is an electric arc lamp designed to produce extremely intense, incoherent, full-spectrum white light for a very short time. A flashtube is a glass tube with an electrode at each end and is filled with a gas that, when triggered, ionizes and conducts a high-voltage pulse to make light. Flashtubes are used most in photography; they also are used in science, medicine, industry, and entertainment.
An electron gun is an electrical component in some vacuum tubes that produces a narrow, collimated electron beam that has a precise kinetic energy.
A gas-filled tube, also commonly known as a discharge tube or formerly as a Plücker tube, is an arrangement of electrodes in a gas within an insulating, temperature-resistant envelope. Gas-filled tubes exploit phenomena related to electric discharge in gases, and operate by ionizing the gas with an applied voltage sufficient to cause electrical conduction by the underlying phenomena of the Townsend discharge. A gas-discharge lamp is an electric light using a gas-filled tube; these include fluorescent lamps, metal-halide lamps, sodium-vapor lamps, and neon lights. Specialized gas-filled tubes such as krytrons, thyratrons, and ignitrons are used as switching devices in electric devices.
A glow discharge is a plasma formed by the passage of electric current through a gas. It is often created by applying a voltage between two electrodes in a glass tube containing a low-pressure gas. When the voltage exceeds a value called the striking voltage, the gas ionization becomes self-sustaining, and the tube glows with a colored light. The color depends on the gas used.
A Geissler tube is an early gas discharge tube used to demonstrate the principles of electrical glow discharge, similar to modern neon lighting. The tube was invented by the German physicist and glassblower Heinrich Geissler in 1857. It consists of a sealed, partially evacuated glass cylinder of various shapes with a metal electrode at each end, containing rarefied gasses such as neon, argon, or air; mercury vapor or other conductive fluids; or ionizable minerals or metals, such as sodium. When a high voltage is applied between the electrodes, an electric current flows through the tube. The current dissociates electrons from the gas molecules, creating ions, and when the electrons recombine with the ions, the gas emits light by fluorescence. The color of light emitted is characteristic of the material within the tube, and many different colors and lighting effects can be achieved. The first gas-discharge lamps, Geissler tubes were novelty items, made in many artistic shapes and colors to demonstrate the new science of electricity. In the early 20th century, the technology was commercialized and evolved into neon lighting.
An electric arc is an electrical breakdown of a gas that produces a prolonged electrical discharge. The current through a normally nonconductive medium such as air produces a plasma, which may produce visible light. An arc discharge is initiated either by thermionic emission or by field emission. After initiation, the arc relies on thermionic emission of electrons from the electrodes supporting the arc. An arc discharge is characterized by a lower voltage than a glow discharge. An archaic term is voltaic arc, as used in the phrase "voltaic arc lamp".
A vacuum arc can arise when the surfaces of metal electrodes in contact with a good vacuum begin to emit electrons either through heating or in an electric field that is sufficient to cause field electron emission. Once initiated, a vacuum arc can persist, since the freed particles gain kinetic energy from the electric field, heating the metal surfaces through high-speed particle collisions. This process can create an incandescent cathode spot, which frees more particles, thereby sustaining the arc. At sufficiently high currents an incandescent anode spot may also be formed.
Neon lighting consists of brightly glowing, electrified glass tubes or bulbs that contain rarefied neon or other gases. Neon lights are a type of cold cathode gas-discharge light. A neon tube is a sealed glass tube with a metal electrode at each end, filled with one of a number of gases at low pressure. A high potential of several thousand volts applied to the electrodes ionizes the gas in the tube, causing it to emit colored light. The color of the light depends on the gas in the tube. Neon lights were named for neon, a noble gas which gives off a popular orange light, but other gases and chemicals are used to produce other colors, such as hydrogen (purple-red), helium, carbon dioxide (white), and mercury (blue). Neon tubes can be fabricated in curving artistic shapes, to form letters or pictures. They are mainly used to make dramatic, multicolored glowing signage for advertising, called neon signs, which were popular from the 1920s to 1960s and again in the 1980s.
A Crookes tube is an early experimental electrical discharge tube, with partial vacuum, invented by English physicist William Crookes and others around 1869-1875, in which cathode rays, streams of electrons, were discovered.
In vacuum tubes and gas-filled tubes, a hot cathode or thermionic cathode is a cathode electrode which is heated to make it emit electrons due to thermionic emission. This is in contrast to a cold cathode, which does not have a heating element. The heating element is usually an electrical filament heated by a separate electric current passing through it. Hot cathodes typically achieve much higher power density than cold cathodes, emitting significantly more electrons from the same surface area. Cold cathodes rely on field electron emission or secondary electron emission from positive ion bombardment, and do not require heating. There are two types of hot cathode. In a directly heated cathode, the filament is the cathode and emits the electrons. In an indirectly heated cathode, the filament or heater heats a separate metal cathode electrode which emits the electrons.
Gas-discharge lamps are a family of artificial light sources that generate light by sending an electric discharge through an ionized gas, a plasma.
In electromagnetism, the Townsend discharge or Townsend avalanche is a ionisation process for gases where free electrons are accelerated by an electric field, collide with gas molecules, and consequently free additional electrons. Those electrons are in turn accelerated and free additional electrons. The result is an avalanche multiplication that permits electrical conduction through the gas. The discharge requires a source of free electrons and a significant electric field; without both, the phenomenon does not occur.
A glow switch starter or glowbottle starter is a type of preheat starter used with a fluorescent lamp. It is commonly filled with neon gas or argon gas and contains a bimetallic strip and a stationary electrode. The operating principle is simple, when current is applied, the gas inside ionizes and heats a bimetallic strip which in turn bends toward the stationary electrode thus shorting the starter between the electrodes of the fluorescent lamp. After about a second the starter's bimetallic strip cools and opens the circuit between the electrodes, and the process repeats until the lamp has lit. One disadvantage of glow switch starters is that when the lamp is at the end of its life it will continuously blink on and off until the glow switch starter wears out or an electrode on the fluorescent lamp burns out. Glow starters have a relatively short life, and light fittings enable the starter to be changed easily. Electronic starters, being interchangeable and using the same casing as a glow starter, last for many years.
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