Cold cathode

Fluorescent tubes are cold-cathode tubes, between the electrodes by applying a high voltage lights a glow discharge whose extensive positive column colored lights depending on the filling gas.

The red neon tubes were the first practically usable light tubes, developed around 1909 by the Frenchman Georges Claude. They find wide application in the neon sign. For fluorescent tubes for demonstration purposes in physics classes see Geissler tube.

Fluorescent tubes, if it does not fluorescent lamps are meant with heated electrodes, as these are coated with a phosphor, the UV light into visible radiation converts ( eng.: Cold Cathode Fluorescent Lamp, CCFL short ). They are used for lighting purposes, where it depends on a long life, such as for the backlighting of monitors.

Design, operation, operation

In the ends of thin, gas-filled glass tube electrodes are sealed, which are both referred to as a cathode, although in the operation with an alternating current almost exclusively used, the anode is always one. The reason lies in its capacity to release electrons.

The term cold cathode tube does not mean that the electrodes remain cool during operation, but that the mechanism of electron emission is not thermal emission. This would require coating of the electrodes with a material of low work function for the electrons, such as fluorescent lamps whose life is limited by the slow evaporation of this material. Contrast, fluorescent tubes work with secondary electrons. The applied voltage is generated between the cathode and the anode, an electric field, which gas ions present moves quickly in the direction of the respective cathode in the gas. The energy released during the impact energy leaves a few ions out electrons from the cathode, in most but it is implemented fully in heat, thus " cold cathode " at high currents are often hotter than thermionic cathodes. Another undesirable side effect is the removal of material. A ring or cup shape of the electrodes leads to the re-deposition of the material.

For the secondary emission is a steep voltage drop, ie a strong electric field directly in front of the cathode necessary because the ions have to lose over short distances by collisions with gas atoms at speed and thus energy. At the same time this box the released electrons accelerated so strongly away from the cathode that gas atoms lose electrons in collisions ( impact ionization ), bringing the number of electrons multiplied. Until the electrons in the positive column carry the major part of the current, the field strength remains large, so by then, have lost a significant portion of the operating voltage, 50 to 100 volts. Therefore, the area in front of the cathode is called the cathode fall. Its expansion is dependent on the gas pressure.

In the positive column, which fills the remaining length of the tube, the field strength and thus the energy of the electrons is less. It is about 400 volts per meter for tubes of 30 mm in diameter and up to 1000 V / m for about 80 mm in diameter. Collisions of electrons with gas atoms stimulate them to glow, the rarer impact ionization only replaces the loss of carriers by recombination.

The losses in the cathode fall less important, arc voltages are usually chosen from several hundred volts; the ignition voltage is considerably higher. However, VDE regulations limit the allowable voltage (and hence the length of the tubes ) to 7.5 kV.

While fluorescent lamps with a simple throttle can be operated on line voltage ( with thermionic emission is the cathode fall in the order of the ionization energy of the gas and the operating voltage is 100 to 200 volts is much smaller ), this is not possible with fluorescent tubes.

As ballast for fluorescent tubes were used earlier, a leakage transformer. They often have the capability to adjust or to adapt to different numbers of series-connected tubes, as is typical for the operating current neon sign. The power setting was made with a mechanically adjustable magnetic shunt. Typical voltages are 2 × 2.5 ... 4 kV. The operation at currents below the rated current is - as with other cold-cathode tubes also - not critical, therefore, can be neon phase cut dimmers dim. Today, most electronic ballasts according to the principle of switching power supply will be used; these are usually themselves have a setting for the current. When the battery or DC power supply they are called resonant converter or (from the English ) inverter.

Idle, the ballast provides a high sparking voltage, which drops to about 30% during operation. The power consumption of tubes is about 30 W / m, the light output of from 30 to 100 lm / W. Have Uncoated cold cathode lamps, depending on the filling gas, a lifespan of up to 20 years. It is independent of switching on and off; a property that is advantageous for flashing neon sign.

The resonant converter used for cold cathode fluorescent lamps for backlighting of LCD and TFT displays, a special type of inverter with resonant transformer that allow regulation of the current by the control signal. They are mostly designed as a resonant circuit technology push-pull converter, which utilizes two transistors as switching elements. Also ballasts for mains operation are often controllable. The advantages ( flicker-free instant start, dimming from 0 to 100 %, different colors ) are also good for art installations and RGB color changer.

Since gas discharge lamps have a negative differential internal resistance (the more current flows through the tube, the less voltage drops her off ), ballasts must limit the output current. The idle can sometimes lead to their destruction inverters, electronic ballasts have an automatic shut off when not firing.

The AC voltage generated by the inverter has a high frequency of 30 ... 100 kilohertz, often a value of < 50 kHz, since the noise radiation of the third harmonic at 150 kHz (lower limit of EMC measurements ). The electrical leads between the inverter and cold cathode may therefore not (for example, the computer case ) are too long or via conductive surfaces out, otherwise go through the high capacity part of the inverter power is lost and the tube is dark, turns off or does not light at full length.

The lines and the light ends have a high voltage-resistant insulation ( usually silicone rubber), which must not be violated.

Reactance transformers and inverters usually provide a symmetrical to ground, dc- potential- free change-over voltage is available; Ballasts of neon signs can determine in this way ground faults. In addition, by radiated interference is reduced.

Colors

The color of the fluorescent tube depends on the kind of the charged gas ( aging vulnerable non- inert gases in brackets):

• Neon: Orange-red
• Neon yellow tube with mercury in green: green
• Helium White Pink
• Helium in a yellow tube: Yellow
• (Nitrogen: Yellow Pink )
• ( Carbon dioxide: Bluish White )
• Krypton: White
• Argon: blue ( intense by enclosing of mercury).

Colored neon tubes often work as fluorescent lamps with fluorescent dyes. There are both cold cathode and hot-cathode lamps for different colors. The colors are then not achieved by the gas filling, but with different phosphors, which convert the ultraviolet emission of the gas discharge a mercury -argon fill into visible light.

Colorless tubes, that is, those without colored glass filters and / or fluorescent dye are rarely used because they are in the off state poorly visible and therefore less suitable for illuminated advertising.

Fluorescent tube filled with neon.

Argon mercury

Krypton

Xenon

Neon lights

Neon tubes are filled with the inert gas neon and light up according to its emission spectrum red-orange. Colloquially also called neon lamps are incorrect - but fluorescent lamps contain mercury and have a phosphor on the glass inside. Neon lights have an uncoated clear or red tinted glass flask. Neon lights are very long for light advertising ( neon sign ) and for firing high buildings used smaller designs are referred to as a glow lamp.

Neon lights were next to the Moore - light to the first tubes. They were invented in 1909 by Frenchman Georges Claude, who on January 19, 1915 was awarded U.S. patent number 1,125,476.

Neon lights are also still used today as a neon sign and for decorative purposes. They are often bent into this lettering. It is cheaper to produce lettering from a long tube and cover the transitions between the letters, rather than to connect a separate tube for each letter. The necessary for the operation of ballasts ( reactance transformers or electronic devices ) are often short Neontrafo or NST called (of English. Neon Sign Transformer).

In the emission spectrum of neon (see picture) can be found next to the intense lines in the red area also which in orange and yellow, while the green lines radiate only weakly. Therefore, a neon tube normally appears bright red. In order to produce far-red light (signal stronger red ), stained glass tubes in addition to the red.

Application

The main field of application of fluorescent tubes is traditionally the neon sign. Strokes are formed by appropriately curved tubes that are painted black between the letters. On a ballast multiple tubes can be connected in series operated ( in series).

CCFL light scanners and faxes in the templates.

They became the backlight for flat screen TV, used LCD or TFT screens, but here are increasingly being replaced by LED lamps. In a notebook CCFL two were installed in the rule, which have a life expectancy of around 15,000 hours. After this time, they do not fall out, but emit only with lower brightness - the end of life is defined by the half brightness. However, recent ECUs operate at a constant current source such that the loss of light through the automatic readjustment is not visible. When the tube is so far decayed that the control range of the controller is no longer sufficient to achieve the original brightness, it usually turns off completely. This leads to a sudden failure, due to bridging of the power control further operation with reduced brightness is usually possible. More PDs are often based on inadequate isolation of very high voltages.

Cold cathode tubes are also used for decorative lighting of computer cases ( case modding ). They are used here for the different colored light inside the transparent casing. Such lamps are provided with a suitable for operation on 12 volt inverter and have wire connections or a suitable computer for the power supply connector. An approximately 30 cm long cold-cathode tube inverter receives an electric power of approximately 4.5 watts.

Light tubes are also used in modern art. First works were written in the 1960s, among others, Bruce Nauman and Dan Flavin.

Cold -cathode tubes, 12 - or 24 -volt inverter for car tuning illuminate the interior, the engine compartment or mounted under the vehicle. Such changes hurt in Germany and other countries, mostly the licensing requirements.