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This allows the displays to be organized as multiplexed displays where the multiple grids and plates form a matrix, minimizing the number of signal pins required. 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.
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The principle of operation is identical to that of a vacuum tube triode. The oxides in the cathodes are not stable in air, so they are applied to the cathode as carbonates, the cathodes are assembled into the VFD, and the cathodes are heated by passing a current through them while inside the vacuum of the VFD to convert the carbonates into oxides. The cathode wire to which the oxides are applied is made of tungsten or ruthenium-tungsten alloy. The most widely used phosphor is Zinc-doped copper-activated Zinc oxide, which generates light at a peak wavelength of 505 nm. The shape of the phosphor will determine the shape of the VFD's segments. This transfers energy from the trace to the segment. The anode consists of a glass plate with electrically conductive traces (each trace is connected to a single indicator segment), which is coated with an insulator, which is then partially etched to create holes which are then filled with a conductor like graphite, which in turn is coated with phosphor. Unlike the orange-glowing cathodes of traditional vacuum tubes, VFD cathodes are efficient emitters at much lower temperatures, and are therefore essentially invisible.
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If electrons impinge on the phosphor-coated anode plates, they fluoresce, emitting light. These electrons are controlled and diffused by the grids (made using Photochemical machining), which are made up of thin (50 micron thick) stainless steel. The cathode is made up of fine tungsten wires, coated by alkaline earth metal oxides (barium, strontium and calcium oxides ), which emit electrons when heated to 650☌ by an electric current. The device consists of a hot cathode ( filaments), grids and anodes ( phosphor) encased in a glass envelope under a high vacuum condition. Macro image of a VFD digit with 3 horizontal tungsten wires and control grid. In the late 1980s hundreds of millions of units were made yearly. The displays became common on calculators and other consumer electronics devices. The first multi-segment VFD was a 1967 Japanese single-digit, seven-segment device. The first VFD was the single indication DM160 by Philips in 1959. 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). VFDs can display seven-segment numerals, multi-segment alpha-numeric characters or can be made in a dot-matrix to display different alphanumeric characters and symbols. Cadmium was commonly used in the phosphors of VFDs in the past, but the current RoHS-compliant VFDs have eliminated this metal from their construction, using instead phosphors consisting of a matrix of alkaline earth and very small amounts of group III metals, doped with very small amounts of rare earth metals. The choice of color (which determines the nature of the phosphor) and display brightness significantly affect the lifetime of the tubes, which can range from as low as 1,500 hours for a vivid red VFD to 30,000 hours for the more common green ones. Standard illumination figures for VFDs are around 640 cd/m 2 with high-brightness VFDs operating at 4,000 cd/m 2, and experimental units as high as 35,000 cd/m 2 depending on the drive voltage and its timing. Unlike liquid crystal displays, a VFD emits a very bright light with high contrast and can support display elements of various colors. In fact, each tube in a VFD is a triode vacuum tube because it also has a mesh control grid. Each tube in a VFD has a phosphor-coated carbon anode that is bombarded by electrons emitted from the cathode filament. different VFD tubesĪ vacuum fluorescent display ( VFD) is a display device once commonly used on consumer electronics equipment such as video cassette recorders, car radios, and microwave ovens.Ī VFD operates on the principle of cathodoluminescence, roughly similar to a cathode ray tube, but operating at much lower voltages. All segments are visible due to external ultraviolet illumination. Vacuum fluorescent display from a CD and dual cassette Hi-Fi.