A
vacuum fluorescent display (VFD) is a
display deviceA display device is an output device for presentation of information in visual or tactile form...
used commonly on consumer-electronics equipment such as video cassette recorders,
car radioCar audio/video , auto radio, mobile audio, 12-volt and other terms are used to describe the sound or video system fitted in an automobile. While 12-volt audio and video systems are also used, marketed, or manufactured for marine, aviation, and buses, this article focuses on cars as the most common...
s, and
microwave ovenA microwave oven is a kitchen appliance that heats food by dielectric heating, using microwave radiation to heat polarized molecules within the food...
s. Invented in Japan in 1967, the displays became common on calculators and other consumer electronics devices. Unlike
liquid crystal displayA liquid crystal display is a flat panel display, electronic visual display, or video display that uses the light modulating properties of liquid crystals . LCs do not emit light directly....
s, a VFD emits a very bright light with high contrast and can support display elements of various colours. VFDs can display
seven-segmentA seven-segment display , or seven-segment indicator, is a form of electronic display device for displaying decimal numerals that is an alternative to the more complex dot-matrix displays...
numerals, multi-segment alpha-numeric characters or can be made in a dot-matrix to display different alphanumeric characters and symbols. In practice, there is little limit to the shape of the image that can be displayed: it depends solely on the shape of phosphor on the anode(s). Hundreds of millions of units are made yearly.
Design
The device consists of a
hot cathodeIn vacuum tubes, a hot cathode is a cathode electrode which emits electrons due to thermionic emission. In the accelerator community, these are referred to as thermionic cathodes. The heating element is usually an electrical filament...
(filaments),
anodeAn anode is an electrode through which electric current flows into a polarized electrical device. Mnemonic: ACID ....
s (
phosphorA phosphor, most generally, is a substance that exhibits the phenomenon of luminescence. Somewhat confusingly, this includes both phosphorescent materials, which show a slow decay in brightness , and fluorescent materials, where the emission decay takes place over tens of nanoseconds...
) and grids encased in a
glassGlass is an amorphous solid material. Glasses are typically brittle and optically transparent.The most familiar type of glass, used for centuries in windows and drinking vessels, is soda-lime glass, composed of about 75% silica plus Na2O, CaO, and several minor additives...
envelope under a high
vacuumIn everyday usage, vacuum is a volume of space that is essentially empty of matter, such that its gaseous pressure is much less than atmospheric pressure. The word comes from the Latin term for "empty". A perfect vacuum would be one with no particles in it at all, which is impossible to achieve in...
condition. The cathode is made up of fine
tungstenTungsten , also known as wolfram , is a chemical element with the chemical symbol W and atomic number 74.A hard, rare metal under standard conditions when uncombined, tungsten is found naturally on Earth only in chemical compounds. It was identified as a new element in 1781, and first isolated as...
wireA wire is a single, usually cylindrical, flexible strand or rod of metal. Wires are used to bear mechanical loads and to carry electricity and telecommunications signals. Wire is commonly formed by drawing the metal through a hole in a die or draw plate. Standard sizes are determined by various...
s, coated by alkaline earth metal oxides, which emit
electronThe electron is a subatomic particle with a negative elementary electric charge. It has no known components or substructure; in other words, it is generally thought to be an elementary particle. An electron has a mass that is approximately 1/1836 that of the proton...
s when heated by an electric current. These electrons are controlled and
diffusedMolecular diffusion, often called simply diffusion, is the thermal motion of all particles at temperatures above absolute zero. The rate of this movement is a function of temperature, viscosity of the fluid and the size of the particles...
by the grids, which are made up of thin metal. If electrons impinge on the phosphor-coated plates, they
fluoresceFluorescence is the emission of light by a substance that has absorbed light or other electromagnetic radiation of a different wavelength. It is a form of luminescence. In most cases, emitted light has a longer wavelength, and therefore lower energy, than the absorbed radiation...
, emitting light. Unlike the orange-glowing cathodes of traditional vacuum tubes, VFD cathodes are efficient emitters at much lower temperatures, and are therefore essentially invisible.
The principle of operation is identical to that of a vacuum tube
triodeA triode is an electronic amplification device having three active electrodes. The term most commonly applies to a vacuum tube with three elements: the filament or cathode, the grid, and the plate or anode. The triode vacuum tube was the first electronic amplification device...
. Electrons can only reach (and "illuminate") a given plate element if both the grid and the plate are at a positive potential with respect to the cathode. This allows the displays to be organized as
multiplexed displayMultiplexed displays are electronic displays where the entire display is not driven at one time. Instead, sub-units of the display are multiplexed, that is, driven one at a time, but the electronics and the persistence of vision...
s where the multiple grids and plates form a matrix, minimizing the number of signal pins required. In the example of the VCR display shown to the right, the grids are arranged so that only one digit is illuminated at a time. All of the similar plates in all of the digits (for example, all of the lower-left plates in all of the digits) are connected in parallel. One by one, the
microprocessorA microprocessor incorporates the functions of a computer's central processing unit on a single integrated circuit, or at most a few integrated circuits. It is a multipurpose, programmable device that accepts digital data as input, processes it according to instructions stored in its memory, and...
driving the display enables a digit by placing a positive voltage on that digit's grid and then placing a positive voltage on the appropriate plates. Electrons flow through that digit's grid and strike those plates that are at a positive potential. The microprocessor cycles through illuminating the digits in this way at a rate high enough to create the illusion of all digits glowing at once via
persistence of visionPersistence of vision is the phenomenon of the eye by which an afterimage is thought to persist for approximately one twenty-fifth of a second on the retina....
. Several amateurs have experimented with the possibilities of using VFDs as triodes.
The extra indicators (in our example, "VCR", "Hi-Fi", "STEREO", "SAP", etc.) are arranged as if they were segments of an additional digit or two or extra segments of existing digits and are scanned using the same multiplexed strategy as the real digits. Some of these extra indicators may use a phosphor that emits a different colour of light, for example, orange.
The light emitted by most VFDs contains many colours and can often be filtered to enhance the colour saturation providing a deep green or deep blue, depending on the whims of the product's designers. Phosphors used in VFDs are different from those in cathode-ray displays since they must emit acceptable brightness with only around 50 volts of electron energy, compared to several thousand volts in a CRT.
Use
Besides brightness, VFDs have the advantages of being rugged, inexpensive, and easily configured to display a wide variety of customized messages, and unlike LCDs, VFDs are not limited by the response time of rearranging liquid crystals and are thus able to function normally in cold, even sub-zero, temperatures, making them ideal for outdoor devices in cold climates. Early on, the main disadvantage of such displays was their use of significantly more power (0.2
wattThe watt is a derived unit of power in the International System of Units , named after the Scottish engineer James Watt . The unit, defined as one joule per second, measures the rate of energy conversion.-Definition:...
s) than a simple LCD. This was considered a significant drawback for battery-operated equipment like calculators, so VFDs ended up being used mainly in equipment powered by an
ACIn alternating current the movement of electric charge periodically reverses direction. In direct current , the flow of electric charge is only in one direction....
supply or heavy-duty rechargeable batteries.
During the 1980s, this display began to be used in automobiles, especially where car makers were experimenting with digital displays for vehicle instruments such as speedometers and odometers. A good example of these were the high-end
Subaru; is the automobile manufacturing division of Japanese transportation conglomerate Fuji Heavy Industries .Subaru is internationally known for their use of the boxer engine layout popularized in cars by the Volkswagen Beetle and Porsche 911, in most of their vehicles above 1500 cc as well as...
cars made in the early 1980s (referred to by Subaru enthusiasts as a
digi-dash, or digital
dashboardA dashboard is a control panel placed in front of the driver of an automobile, housing instrumentation and controls for operation of the vehicle....
). The brightness of VFDs makes them well suited for use in cars. Current models of Renault MPV, Scenic and Espace both use VFD panels to show all functions on the dashboard including the radio and multi message panel. They are bright enough to read in full sunlight as well as dimmable for use at night. This panel uses four colours; the usual blue/green as well as deep blue, red and yellow/orange.
This technology was also used from 1979 to the mid-1980s in portable
electronic gameAn electronic game is a game that employs electronics to create an interactive system with which a player can play. The most common form of electronic game today is the video game, and for this reason the terms are often mistakenly used synonymously. Other common forms of electronic game include...
units. These games featured bright, clear displays but the size of the largest vacuum tubes that could be manufactured inexpensively kept the size of the displays quite small, often requiring the use of magnifying
Fresnel lensA Fresnel lens is a type of lens originally developed by French physicist Augustin-Jean Fresnel for lighthouses.The design allows the construction of lenses of large aperture and short focal length without the mass and volume of material that would be required by a lens of conventional design...
es. While later games had sophisticated multi-colour displays, early games achieved colour effects using transparent filters to change the colour of the (usually light blue) light emitted by the phosphors. High power consumption and high manufacturing cost contributed to the demise of the VFD as a videogame display. LCD games could be manufactured for a fraction of the price, did not require frequent changes of expensive batteries (or AC adapters) and were much more portable. Since the late 1990s, backlit colour active-matrix LCD displays have been able to cheaply reproduce arbitrary images in any colour, a marked advantage over fixed-colour, fixed-character VFDs. This is one of the main reasons for the decline in popularity of VFDs, although they continue to be made. Many low-cost DVD players still feature VFDs.
From the mid-1980s onwards, VFDs were used for applications requiring smaller displays with high brightness specifications, though now the adoption of high-brightness
organic light-emitting diodeAn OLED is a light-emitting diode in which the emissive electroluminescent layer is a film of organic compounds which emit light in response to an electric current. This layer of organic semiconductor material is situated between two electrodes...
s (OLEDs) is pushing VFDs out of these markets.
In addition to the widely used fixed character VFD, a graphic type made of an array of individually addressable pixels is also available. These more sophisticated displays offer the flexibility of displaying arbitrary images, and may still be a useful choice for some types of consumer equipment. One such example is the Logitech music player with its 320 x 32 pixel
Noritakeis a porcelain maker headquartered in Nagoya, Aichi Prefecture, Japan.- History :Noritake Co., Limited, commonly known as "Noritake," grew out of a trading company established in Tokyo and in New York City by the Morimura Brothers in 1876. In 1904, key members of this trading company created the...
gray scale VFD display.
Fade
Fading is sometimes a problem with VFD displays. Light output drops over time due to falling emission and reduction of phosphor efficiency. How quickly and how far this falls depends on the construction and operation of the VFD. In some equipment, loss of VFD output can render the equipment inoperable.
Emission may usually be restored by raising filament voltage. Thirty-three percent voltage boost can rectify moderate fade, and 66% boost severe fade. This can make the filaments visible in use, though the usual green-blue VFD filter helps reduce any such red or orange light from the filament.
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