Vacuum fluorescent display

A fluorescent display, and vacuum - fluorescent display VFD or referred to, english Vacuum Fluorescent Display, VFD abbreviated as, is an electron tube and is based on the lighting of certain materials (ie fluorescence ) when impinging on these electrons.

Principle of operation

Vacuum fluorescent displays are constructed from a transparent glass plate and a rear base plate, which is usually also made ​​of glass. The plates are connected at the edge with glass solder or glued together, the display system is intervening in a vacuum. Prior to the anode forming luminous segments is a thin, coated with oxides of tungsten filament stretched ( directly heated cathode) of this thermally emit electrons ( thermionic emission ) and fly when voltage is applied to the anode segments. The phosphor layer ( " phosphorus " ), with which the anodes are covered begins to shine like the " magic eye " or in picture tubes when the electrons strike. Directly in front of the anode segments, a grid is mounted, with the control of the electron current and the electrons can be ejected (see triode). One segment of the indicator lights when both the grid and the anode are electrically positive relative to the cathode. The voltage between the anode and cathode is between 10 and 50 volts.

Electrical actuation

The voltage between the anode and cathode determines the uniform illumination and the life. As the filament current is removed, its two ends are at different potentials. In DC operation, this results in an uneven brightness of the display, so the heating wire is usually operated with an AC voltage.

While previously extremely effective voltage transducers were used with transformers for generating relatively high VFD operating voltages, these voltages are today usually created with this ICs specially manufactured or simpler voltage transformers, or it will VFDs operate with lower anode voltages are available anyway, (z. example 12 V).

To avoid that you need for each segment a separate line with its own drive voltage ( at 10 digits in seven-segment display with decimal points would be the 80 lines ), VFDs are usually driven in the time-division multiplexing. For the anodes of the same segments of all digits are connected ( for example, all the anodes for the upper horizontal line). Well first the grid for the first digit is briefly placed on positive voltage and those anodes to be lit at the first digit; all other digits remain dark. The same then takes place in rapid succession for the second and all subsequent figures. For the mentioned example of a display with 10 digits so are only 8 10 = 18 lines for the anode and grid needed. Since the digits light up very quickly in succession, the display will appear almost flicker-free.

The temporal sequence of activation is controlled by Integrated circuits ( control circuits and drive circuits ), often with the VFD form a unit and have a serial or parallel interface.

Recently there is also "Chip In Glass " displays with integrated in the display tube driver ICs to reduce the number of connecting wires led out on. It also helps to dot matrix displays with lots of light spots realize. The driver block in the tube is thereby directly controlled by serial data transmission.

Historical

The VFD technology was developed in the 1960s in Japan to industrial maturity. Fluorescent displays replaced the glow lamps functioning as Nixie tubes. They existed first as a cylindrical tube for displaying a single, displayable segments of digit. Later fluorescent displays were also produced as a flat complex display element. Especially in applications for office computer, the term " Digitron " indicator remained preserved.

Applications

The modern fluorescent displays provide a wide viewing angle and crisp, clear signs and graphic symbols due to the flat design. Also bargraph displays for meter can be realized. In front of the displays is often a filter plate mounted so as to increase the contrast between dark and driven segments.

Currently, there are color display with up to nine different colors; the blue-green color most commonly used has the advantage of high light yield.

Advantages of the fluorescent displays are liquid crystal displays as compared with a high luminance up to more than 1000 cd / m², no backlight, and a wide range of operating temperature. A disadvantage is the rapid aging of the phosphor, especially quickly at high initial brightness.

Together with the high reliability and relatively long lifetime have these advantages, for example, widespread use in consumer electronics, such as video recorders, out. Even in automobiles are used, although their structure is impact-sensitive than, for example that of liquid crystal or LED displays. Rarely they are mostly used on models that are on the table in computers.

For battery-powered portable devices fluorescent displays, however, are less suitable because of the unilluminated compared to liquid crystal displays more power is required and the required higher voltages. A disadvantage here is the large thickness which is not negligible due to the one necessary for the vacuum to thick glass plates.