Vacuum Fluorescent Display. Basically, a small vacuum tube with the anode coated with phosphor so that it will glow. Used as a display device. Those ancient calculators with red numbers prior to LCDs were VFDs.

vacuum fluorescent display(VFD); A VFD is built up of several vacuum fluorecent cells. Each cell is built in a similar fashion to triode vacuum tubes; It consists of an anode, a grid and several cathode filaments. The anode is given a layer of phosphors.

phosphor coated
 anode

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    || : .
    || :
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    || :   cathode
    || : . filaments
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    |  :

      grid

This assembly is put inside a glass container, the air is pumped out, and the container is sealed.

To operate the cell, the anode is given a potential difference to the cathode of between 150V and 200V(and the filament is given a glow current). When the grid is energized, it creates an electron flow between the cathode and the anode. This flow exites the phosphors and they emit light. By adjusting the voltage on the grid, the intencity of the light can be changed.

By arranging several cells in a 7-segment or a matrix pattern a display can be created. The VFD's usualy have a green or blueish glow. They have a medium to good contrast, but has been replaced by LCDs in most applications, since the VFD has a higher power consumption.

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