Liquid crystal cells – elements and systems – Particular structure – Having significant detail of cell structure only
Reexamination Certificate
2000-03-09
2004-01-13
Chowdhury, Tarifur R (Department: 2871)
Liquid crystal cells, elements and systems
Particular structure
Having significant detail of cell structure only
C349S073000, C349S033000, C349S147000
Reexamination Certificate
active
06678033
ABSTRACT:
FIELD AND BACKGROUND OF THE INVENTION
The invention relates to a display device, in particular for a vehicle, having a screen which has a liquid crystal cell, with the liquid crystal cell having a front and a rear cell wall and a liquid crystal substance arranged in the cell space between these cell walls, and having a heating apparatus by means of which the display device can be heated.
In liquid crystal cells which, at least at times, are operated at low ambient temperatures, for example as is the case with displays in vehicles, the problem occurs that the switching times of the liquid crystal cells become undesirably long at low temperatures, so that displays equipped with them are subject to considerable inertia. In order to overcome this undesirable situation, it is known for a light box to be provided, which is used to illuminate a liquid crystal cell in a liquid crystal display and has a heating wire. The heating wire heats the liquid crystal cell and maintains it at a temperature level which, despite low ambient temperatures, allows the liquid crystal display to be operated with the necessary short switching times. Owing to the relatively long distance between the heating wire and the liquid crystal cell, this necessitates a large amount of power being supplied and the heating wire being strongly heated in order to allow the heating required for the liquid crystal cell to be produced. Furthermore, owing to its arrangement in the light box and its necessary high level of thermal emission, the heating wire heats the entire display unit, in an undesirable manner.
SUMMARY OF THE INVENTION
The invention is based on the object of designing a display device having a liquid crystal cell such that the display unit can be operated reliably, and with only short switching times, even at low ambient temperatures.
This object is achieved according to the invention in that the heating apparatus is arranged in the cell space of the liquid crystal cell directly or indirectly on the front and/or on the rear cell wall, and in that the heating apparatus is operated with a pure AC voltage.
By virtue of such a design, the liquid crystal substance is heated directly and very quickly, without any undesirable power losses occurring as the heat is passed on. The heating power produced by the heating apparatus may thus be particularly low. This prevents parts of the display device which must not be heated from being heated undesirably. Furthermore, the heating apparatus can easily be manufactured at the same time that the liquid crystal cell is produced, without any additional assembly measures being required. Operation of the heating apparatus with a pure AC voltage ensures that the display device has a long life and is operationally reliable. If a DC voltage is applied to a heater in a liquid crystal cell which can be heated, this results in a potential difference between the contact points; the liquid crystal layer that is in direct contact with the heating apparatus is then subject to a permanent DC voltage. Depending on the magnitude of the DC voltage or of the DC voltage component of an AC voltage, this leads to the liquid crystal decomposing irreversibly in the short term or medium term, and thus reduces the life and reliability of the display apparatus to an unacceptable extent. By using a pure AC voltage according to the invention to actuate the heating apparatus, with a low DC voltage component being allowed for the purposes of tolerances that cannot be avoided with reasonable complexity, the decomposition effects described above are effectively counteracted, so that the display apparatus according to the invention is distinguished by its long life and the clarity of the display.
As a result of the direct association of the heating apparatus with the liquid crystal substance and the lack of any thermally insulating layer, the display device heating-up and switching times are particularly short. Thus—and owing to the high operational reliability described above—the display device according to the invention is also particularly suitable for a tachometer display in a motor vehicle. In the case of a tachometer, short response times, immediate operating capability and unlimited reliability are particularly important, since it is of major importance to traffic safety to display the driving speed.
The display unit has a particularly simple and economic configuration if the liquid crystal cell is, preferably, a TN or STN cell.
The production costs of the display unit can advantageously be reduced if the heating apparatus has an electrically conductive layer. When current flows, this layer can then act as a resistance heater. It would be feasible for the electrically conductive layer to be arranged immediately adjacent to the liquid crystal substance.
However, for simple liquid crystal circuitry with a power consumption that is as low as possible, it is particularly advantageous for the electrically conductive layer to be arranged between a front electrode and/or a rear electrode of the liquid crystal cell and the front and/or rear cell wall.
The production of the liquid crystal cell is even further simplified, if, according to another advantageous development of the invention, the electrically conductive layer is arranged in the same plane as a front electrode and/or rear electrode of the liquid crystal cell. The electrically conductive layer can thus be applied, together with an electrode of the liquid crystal cell, in one operation.
It would be feasible for the electrically conductive layer to have a large area; however, the heating effect is particularly good if the electrically conductive layer has a meandering profile. It is also feasible for the electrically conductive layer to be arranged only in those areas in which there are switchable display elements. The use of electrical power required for heating can thus be further reduced, since this avoids heating areas of the liquid crystal cell which are not switched, and thus where there is no negative influence from low ambient temperatures.
Particularly in the case of dot-matrix displays using a liquid crystal cell, it is normal for the liquid crystal cell to have a front electrode and a rear electrode each having families of mutually parallel electrode strips, with the electrode strips of the front electrode and rear electrode being arranged such that they cross over one another. In order to achieve a good heating effect, covering an area, with such liquid crystal cells, it is particularly advantageous for the electrode in whose plane the electrically conductive layer is arranged to be in the form of strips, and for elongated sections of the electrically conductive layer to be arranged in the spaces between the strips.
In order to ensure that the elongated sections of the electrically conductive layer are connected in a manner which does not interfere with the contact with the electrodes that are in the form of strips, plated-through contacts, which connect the plane of the front electrode and the plane of the rear electrode, preferably electrically connect the elongated sections of the electrically conductive layer to one another by means of connecting sections.
The electrodes in liquid crystal cells are normally composed of indium tin oxide. The production of a display device according to the invention is thus considerably simplified if the electrically conductive layer is composed of indium tin oxide, so that no additional material need be handled and processed during the production of the liquid crystal cell.
According to another advantageous development of the invention, the functional reliability of the display device is increased if the electrically conductive layer is covered with an electrical insulation layer. The insulation layer is advantageously composed of glass. This may be applied, sintered and fused in as a glass powder.
According to an advantageous development of the invention, the AC voltage which operates the heating apparatus can be applied directly to the electrically conductive layer, in order to keep the power lo
Brandt Peter
Tippl Dietmar
Chowdhury Tarifur R
Mannesmann VDO AG
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