Electric lamp and discharge devices: systems – With cathode or cathode heater supply circuit – Plural load device systems
Patent
1994-01-11
1995-07-25
Pascal, Robert J.
Electric lamp and discharge devices: systems
With cathode or cathode heater supply circuit
Plural load device systems
315 94, 315 96, 315 98, 315101, H05B 3700
Patent
active
054365279
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The present invention relates to circuits for operating fluorescent lamps.
BACKGROUND OF THE INVENTION
Graphic display devices using liquid-crystal technology are being used increasingly to display information in motor vehicles. Because of the high optical demands, so-called "active displays" with internal color filters are used with transparent lighting. Powerful back-lighting sources with adjustable luminance are required for this purpose.
Various requirements are imposed on these light sources. In addition to uniform illumination and high electro-optical efficiency, low heat emission and low temperature dependence of the luminance are required. Because of the considerable changes that occur in the lighting conditions in a motor vehicle, a luminance which can be controlled over a wide range is required. The minimum luminance must be less than 100 cd/m.sup.2 and the maximum luminance must be greater than 25,000 cd/m.sup.2. Finally, white light must be radiated over the entire dimming range.
Because of their efficiency and associated low heat emission, as well as their flat design, fluorescent lamps, especially compact fluorescent lamps, are suitable for this purpose. However, there are difficulties relating to the broad dimming range and temperature independence.
The goal of the present invention is to provide a circuit for operating a fluorescent lamp which permits fulfillment of the above requirements.
SUMMARY OF THE INVENTION
The present invention is directed to a circuit for operating a fluorescent lamp with a controllable luminance, including a discharge path and a plurality of heater coils, wherein the discharge path is connected through a first transformer to a controllable source of alternating current and wherein the current through the heater coils is controllable so that the total of the current through the discharge path and the current through the heater coils remains essentially constant.
The circuit according to the present invention allows control of luminance in the stated range with low temperature dependence of the luminance, good efficiency, and low heat emission. Another advantage of the circuit according to the present invention consists in the fact that the lamp current is limited in the negative area of the U-I curve.
The circuit according to the present invention is especially advantageously suited for back-lighting sources of liquid-crystal displays. Other applications for which similar requirements apply are not ruled out, however.
BRIEF DESCRIPTION OF THE FIGURES
FIG. 1 is a block diagram of an embodiment of the present invention, and
FIG. 2 is a detailed drawing of the essential features of another embodiment.
DETAILED DESCRIPTION
Equivalent parts have been given the same reference numerals in the figures.
In the embodiment shown in FIG. 1, a controllable DC power supply 1 is provided, connected at 2 to the vehicle electrical system of a motor vehicle. Controllable power supplies are known of themselves and are characterized by an at least dynamically large internal resistance, so that they deliver a current that is largely independent of the resistance of the connected load. A simple design for a controllable power supply consists of a transistor in an emitter-base circuit, in whose collector lead the load is connected and whose base is charged with a control voltage or control current.
Power supply 1 can be controlled by a selector switch 3 either through a manually adjustable voltage divider 4 or by a brightness meter 5, with automatic adjustment of the luminance to the ambient brightness. A first push-pull chopper 6 and a lamp transformer 7 produce a controllable alternating current whose amplitude is determined by the direct current supplied and which is fed to the discharge path of a fluorescent lamp 8.
Another chopper 9 likewise converts the output voltage from a power supply for heating coils 10 into an alternating voltage which is supplied through a heater transformer 11 to the heating coils 12, 13 of the fluorescent lamp 8.
The pow
REFERENCES:
patent: 3906301 (1975-09-01), Fisher
patent: 4358683 (1982-11-01), Seifert
patent: 5319281 (1994-06-01), Roth
Fiess Reinhold
Koenig Winfried
Scharlach Peter
Schwabe Florian
Pascal Robert J.
Ratliff Reginald A.
Robert & Bosch GmbH
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