Cold cathode

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A set of cold cathode discharge tubes Gase-in-Entladungsroehren.jpg
A set of cold cathode discharge tubes

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.

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.

Thermionic emission thermally induced flow of charge carriers from a surface

Thermionic emission is the thermally induced flow of charge carriers from a surface or over a potential-energy barrier. This occurs because the thermal energy given to the carrier overcomes the work function of the material. The charge carriers can be electrons or ions, and in older literature are sometimes referred to as thermions. After emission, a charge that is equal in magnitude and opposite in sign to the total charge emitted is initially left behind in the emitting region. But if the emitter is connected to a battery, the charge left behind is neutralized by charge supplied by the battery as the emitted charge carriers move away from the emitter, and finally the emitter will be in the same state as it was before emission.

Neon lamp Light source based on gas discharge

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 are widely used as indicator lamps in the displays of electronic instruments and appliances.


Cold-cathode devices

The stacked digit arrangement in a Nixie tube is visible here ZM1210-operating edit2.jpg
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.

Geissler tube gas-discharge lamp

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 electrical 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.

Julius Plücker German mathematician and physicist

Julius Plücker was a German mathematician and physicist. He made fundamental contributions to the field of analytical geometry and was a pioneer in the investigations of cathode rays that led eventually to the discovery of the electron. He also vastly extended the study of Lamé curves.

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]

Negative resistance

In electronics, negative resistance (NR) is a property of some electrical circuits and devices in which an increase in voltage across the device's terminals results in a decrease in electric current through it.

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.

Thyratron type of gas filled tube

A thyratron is a type of gas-filled tube used as a high-power electrical switch and controlled rectifier. Thyratrons can handle much greater currents than similar hard-vacuum tubes. Electron multiplication occurs when the gas becomes ionized, producing a phenomenon known as Townsend discharge. Gases used include mercury vapor, xenon, neon, and hydrogen. Unlike a vacuum tube (valve), a thyratron cannot be used to amplify signals linearly.


The krytron is a cold-cathode gas-filled tube intended for use as a very high-speed switch, somewhat similar to the thyratron. It consists of a sealed glass tube with four electrodes. A small triggering pulse on the grid electrode switches the tube on, allowing a large current to flow between the cathode and anode electrodes. The vacuum version is called a vacuum krytron, or sprytron. The krytron was one of the earliest developments of the EG&G Corporation.

Nixie tube gas-filled digital indicator tube

A Nixie tube, or cold cathode display, is an electronic device for displaying numerals or other information using glow discharge.

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.

Xenon Chemical element with atomic number 54

Xenon is a chemical element with symbol Xe and atomic number 54. It is a colorless, dense, odorless noble gas found in the Earth's atmosphere in trace amounts. Although generally unreactive, xenon can undergo a few chemical reactions such as the formation of xenon hexafluoroplatinate, the first noble gas compound to be synthesized.

Stroboscope instrument used to make a cyclically moving object appear to be slow-moving, or stationary

A stroboscope also known as a strobe, is an instrument used to make a cyclically moving object appear to be slow-moving, or stationary. It consists of either a rotating disk with slots or holes or a lamp such as a flashtube which produces brief repetitive flashes of light. Usually the rate of the stroboscope is adjustable to different frequencies. When a rotating or vibrating object is observed with the stroboscope at its vibration frequency, it appears stationary. Thus stroboscopes are also used to measure frequency.


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.

Fluorescence emission of light by a substance that has absorbed light or other electromagnetic radiation

Fluorescence is the emission of light by a substance that has absorbed light or other electromagnetic radiation. It is a form of luminescence. In most cases, the emitted light has a longer wavelength, and therefore lower energy, than the absorbed radiation. The most striking example of fluorescence occurs when the absorbed radiation is in the ultraviolet region of the spectrum, and thus invisible to the human eye, while the emitted light is in the visible region, which gives the fluorescent substance a distinct color that can be seen only when exposed to UV light. Fluorescent materials cease to glow nearly immediately when the radiation source stops, unlike phosphorescent materials, which continue to emit light for some time after.

Cold-cathode fluorescent lamps are 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 20 mm 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 15 mm in diameter[ citation needed ] and typically operates at approximately 40 milliamperes. These lamps are commonly used for neon signs.


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 tubes Cold Cathodes.jpg
A standard computer case fitted with blue and green cold-cathode tubes
Cold-cathode fluorescent lamp Cold Cathode Fluorescent Lamp.JPG
Cold-cathode fluorescent lamp

A cold cathode is distinguished from a hot cathode that is heated to induce thermionic emission of electrons. Discharge tubes with hot cathodes have an envelope filled with low-pressure gas and containing two electrodes. Examples are most common fluorescent lamps, high-pressure discharge lamps and vacuum fluorescent displays.

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.

Another mechanism to generate free electrons from a cold metallic surface is field electron emission. It is used in some x-ray tubes, the field-electron microscope (FEM), and field-emission displays (FEDs).

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.


An illuminated cold-cathode CFL Cold-Cathode-CFL-illuminated.jpg
An illuminated cold-cathode CFL

Cold cathodes are used in cold-cathode rectifiers, such as the crossatron and mercury-arc valves, and cold-cathode amplifiers, such as in automatic message accounting and other pseudospark switching applications. Other examples include the thyratron, krytron, sprytron, and ignitron tubes.

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, 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: 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 backlightling 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[ by whom? ], but high-end manufacturers are now turning to high-efficiency white LEDs as a better solution.

See also

References and notes


  1. A negatively charged electrode emits electrons or is the positively charged terminal. For more, see field emission.
  2. Positive column is part of a glow discharge, such as in the Moore lamp.


  1. U.S. Patent 1,993,187 , Cold cathode discharge tube
  2. D. M. Neale, Cold Cathode Tube Circuit Design, Francis and Taylor, 1964. pp. 1–7.
  3. "Ifay guide info electric discharge lighting systems, cold cathode".
  4. "EGL lighting products". Archived from the original on October 26, 2010. Retrieved 9 February 2011.
  5. U.S. Patent 1,860,149 , Discharge tube.
  6. U.S. Patent 2,103,033 , Electron emissive electrode.
  7. U.S. Patent 1,316,967 , Gaseous-conduction lamp.
  8. Solé Lighting (commercial site advocating CCFLs).

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Fluorescent lamp Light source

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Neon sign electrified, luminous tube lights

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Electron gun

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Crookes tube

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Hot cathode Type of electrode.

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 lamp

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Townsend discharge gas ionisation process where free electrons are accelerated by an electric field, collide with gas molecules, and consequently free additional electrons

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