Fluorescent tube
Fluorescent tubes are gas discharge tubes , between the electrodes of which a glow discharge ignites when a high voltage is applied . The extended positive column lights up in color depending on the filling gas. The cathode is not heated and therefore hardly emits thermal electrons, - the emission takes place by secondary electron emission from the cathode fall accelerated positive ions strike the cathode.
The red neon tubes were the first practically usable fluorescent tubes, developed around 1909 by the French Georges Claude . They actually contain neon as filling gas and gave the fluorescent tubes their trivial name. Other colors are by other gases, but also by a fluorescent -Innenbeschichtung as fluorescent lamps reach (cold cathode fluorescent lamps, Engl .: cold cathode fluorescent lamps , shortly CCFL ). In this case, the gas mixture consists of argon and mercury and emits primarily ultraviolet, which is converted into the desired light color by the fluorescent material. White-emitting fluorescent tubes are used for lighting when a long service life is required, for example for the backlighting of the first ( LCD ) flat-screen monitors or, earlier, for design elements and light fields in city centers.
Fluorescent tubes are also widely used today for neon advertising , but are increasingly being replaced by light-emitting diodes .
For fluorescent tubes for demonstration purposes in physics lessons, see Geissler tube .
Structure, mode of operation, operation
Electrodes are fused into the ends of a thin, gas-filled glass tube, both of which are referred to as the cathode, although in the almost exclusively used operation with alternating current, one is always the anode. The reason lies in their function of releasing electrons.
The term cold cathode tube does not mean that the electrodes remain cold during operation, but that the mechanism of electron emission is not thermal emission . This would be a coating of the electrodes with a material of low work function required for the electrons, as in fluorescent lamps whose life is limited by the slow evaporation of this material. In contrast, fluorescent tubes work with secondary electrons . The applied voltage creates an electric field between the cathode and anode, which quickly moves gas ions present in the gas towards the respective cathode. The energy released on impact releases electrons from the cathode of some ions, but in most of them it is completely converted into heat, which means that "cold cathodes" often become hotter than hot cathodes at high currents. Another undesirable side effect is the removal of material . A ring or cup shape of the electrodes leads to redeposition of the material.
For the secondary emission there is a steep voltage drop, i. H. a strong electric field is necessary immediately in front of the cathode, as the ions lose speed and thus energy over short distances due to collisions with gas atoms. At the same time, this field accelerates the released electrons away from the cathode so strongly that gas atoms lose electrons when they collide (impact ionization), which multiplies the number of electrons. However, until the electrons in the positive column carry the majority of the current, the field strength remains high, so that a considerable part of the operating voltage, 50 to 100 volts , has already been lost by then. The area in front of the cathode is therefore called the cathode fall . Its expansion depends 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 lower. It is approx. 400 volts per meter for tubes with a diameter of 30 mm and up to 1000 V / m for a diameter of approx. 80 mm. Collisions between electrons and gas atoms make them glow, the rarer impact ionization only replaces the loss of charge carriers through recombination.
In order for the losses in the cathode case to be less significant, operating voltages of several hundred volts are usually chosen; the ignition voltage is considerably higher. VDE regulations limit the permissible voltage (and thus the length of the tubes) to 7.5 kV.
While fluorescent lamps can be operated with a simple choke on mains voltage (with incandescent emission, the cathode drop is in the order of magnitude of the ionization energy of the gas and the operating voltage is much lower at 100 to 200 volts), this is not possible with fluorescent tubes.
A stray field transformer was previously used as the ballast for fluorescent tubes . These often have the option of setting the operating current or adapting it to different numbers of tubes connected in series, as is typical for neon advertising. The current was set with a mechanically adjustable magnetic shunt. Usual voltages are 2 × 2.5… 4 kV. Operation with currents below the nominal current is - as with other cold cathode tubes - not critical, so neon tubes can be dimmed with phase-cut dimmers . Today mostly electronic ballasts are used based on the principle of a switched-mode power supply ; these usually have a setting option for the current themselves. In the case of battery or DC voltage supplies, they are called resonance converters or (from English) inverters .
When idling, the ballast supplies a high ignition voltage, which drops to approx. 30% during operation. The power consumption of fluorescent tubes is approx. 30 W / m, the luminous efficiency is 30–100 lm / W. Uncoated cold cathode lamps have a service life of up to 20 years, depending on the filling gas. It is independent of switching on and off; a property that is beneficial for flashing neon signs.
The resonance converters used for cold cathode lamps for backlighting LC and TFT displays , a special form of inverter with a resonance transformer , allow the current to be regulated by means of a control signal . In terms of circuitry, they are usually designed as a resonance push-pull converter that uses two transistors as switching elements. Ballasts for mains operation are also often controllable. The advantages (flicker-free instant start, dimming from 0 to 100%, different colors) are also advantageous for art installations and RGB color changers.
Since gas discharge lamps have a negative differential internal resistance (the more current flows through the tube, the less voltage it drops), ballasts have to limit the output current. In inverters, idling can sometimes lead to their destruction, electronic ballasts have an automatic switch-off in the event of no ignition.
The alternating voltage generated by the inverter has a high frequency of 30 ... 100 kilohertz , often a value of <50 kHz, as the interference radiation of the third harmonic is below 150 kHz (lower limit of EMC measurements). The electrical supply lines between the inverter and the cold cathode must therefore not be too long or run over conductive surfaces (e.g. the computer case), otherwise part of the inverter power is lost due to the high capacity and the tube becomes darker, goes out or no longer lights up to its full length .
The cables and the lamp ends have high-voltage-proof insulation (usually silicone rubber ) that must not be damaged.
Stray field transformers and inverters usually provide an alternating voltage that is symmetrical to earth potential and potential-free in terms of direct current; Ballasts of neon signs can detect shorts to ground in this way. This also reduces the radiation of interference.
Colours
The color of the fluorescent tube depends on the type of gas filled in (non-noble gases susceptible to aging in brackets):
- Neon : orange red
- Neon with mercury in a yellow tube: green
- Helium : white pink
- Helium in yellow tube: yellow
- ( Nitrogen : yellow pink)
- ( Carbon dioxide : bluish white)
- Krypton : white
- Argon : pink
- Argon with mercury: blue
Colored fluorescent tubes often work like 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 by different phosphors that convert the ultraviolet emission of the gas discharge of a mercury - argon filling into visible light.
Colorless tubes, d. H. those without a colored glass filter and / or fluorescent dye are rarely used because they are difficult to see when switched off and are therefore less suitable for illuminated advertising. However, by flashing switching they produce a clear color change effect on decorations.
Neon tubes
Neon tubes are filled with the noble gas neon and glow red-orange according to its emission spectrum. Colloquially, fluorescent lamps are also incorrectly called neon tubes - however, fluorescent lamps contain mercury vapor and have a fluorescent substance on the inside of the glass. Neon tubes have an uncoated clear or red tinted glass envelope. Neon tubes have long been used for illuminated advertising (neon advertising) and for lighting tall structures, while smaller designs are known as glow lamps .
Along with the Moore light, neon tubes were among the first fluorescent tubes. They were invented in 1909 by the French Georges Claude , who received US patent number 1,125,476 on January 19, 1915.
Neon tubes are still used today as neon signs and for decorative purposes. For this purpose, they are often bent into lettering. It is cheaper to make lettering from a long tube and cover the transitions between the letters instead of connecting a separate tube for each letter. The ballasts required for operation (stray field transformers or electronic devices) are often called neon transformers or NST (from Neon Sign Transformer ) for short .
In the emission spectrum of neon (see picture) one can find not only the intense lines in the red area but also those in orange and yellow, while the green lines only shine weakly. Therefore, a neon tube usually appears bright red. In order to generate dark red light (stronger signal red), the glass tubes are additionally colored red in order to absorb the yellow (and green) part.
application
The main area of application for fluorescent tubes is traditionally in neon advertising. Lettering is formed by appropriately curved tubes that are painted black between the letters. Several tubes can be operated one behind the other (in series) on one ballast.
Cold cathode tubes illuminate the originals in scanners and fax machines .
They became the backlighting for flat-panel televisions , LC - or TFT - flat panel displays used, but are increasingly here of LED replaced lamps. As a rule, two CCFLs were installed in a notebook , which have an expected service life of around 15,000 hours. After this time, they do not fail, but only shine with lower brightness - the end of service life is defined by half the brightness. However, newer control devices work with a constant current source so that the loss of brightness due to the automatic readjustment is not visible. When the tube has aged so far that the control range of the control unit is no longer sufficient to achieve the previous brightness, it usually switches off completely. This leads to a sudden failure; by bridging the current monitoring, further operation with reduced brightness is usually possible. Further failure probabilities are often based on inadequate isolation of the very high voltages.
Cold cathode tubes are also used for effect lighting in computer cases ( case modding ). They are used to illuminate the transparent housing in different colors. Such lights are provided with an inverter suitable for operation on 12 volts and have wire connections or a plug connection suitable for the computer power supply. An approximately 30 cm long cold cathode tube with an inverter consumes an electrical power of approximately 4.5 watts.
Fluorescent tubes are also used in modern art. The first works were created in the 1960s by Bruce Nauman and Dan Flavin, among others .
Cold cathode tubes with 12 or 24 volt inverters for vehicle tuning illuminate the interior, the engine compartment or are installed under the vehicle. Such changes mostly violate the approval regulations in Germany and other countries .
literature
- Christoph Ribbat : Flickering modernity. The story of neon light. Steiner, Stuttgart 2011. ISBN 978-3-515-09890-8 .
- Günter Springer: Expertise in electrical engineering. 18th edition, Europa-Lehrmittel, Wuppertal, 1989, ISBN 3-8085-3018-9 ; 29th edition 2014, ISBN 978-3-8085-3190-7 .
- Adolf Senner: Electrical engineering. 4th edition. Europe-Lehrmittel, Haan-Gruiten 1965 DNB 451091205
