Claims
- 1. In a display system including a vacuum fluorescent tube in which electrons generated at a filament element and attracted by a grid element bombard a phosphored anode element to emit light for display purposes, and control apparatus for supplying operating voltages to the filament, grid and anode elements from a variable voltage source, the improvement wherein:
- the voltage supplied to the anode element follows the voltage of said source; and
- the voltage supplied to the grid element is varied in inverse relation to that of the voltage supplied to the anode element, thereby to reduce fluctuations in the brightness of the emitted light despite substantial variation of the source voltage.
- 2. In a display system including a vacuum fluorescent tube in which electrons generated at a filament element and attracted by a grid element bombard a phosphored anode element to emit light for display purposes, and control apparatus for supplying operating voltages to the filament, grid and anode elements from a source having a variable output voltage, the improvement wherein:
- the anode element is operated substantially at the output voltage of said source; and
- the grid is operated at a voltage lower than the output voltage of said source and which varies in inverse relation thereto, at least when said output voltage exceeds a reference voltage.
- 3. The improvement of claim 2, wherein:
- the source includes an automotive storage battery having a nominal open-circuit output voltage, the battery being adapted to be charged at voltages in excess of such nominal open-circuit voltage; and
- the reference voltage substantially corresponds to said nominal open-circuit voltage.
- 4. In a display system including a vacuum fluorescent tube in which electrons generated at a filament element and attracted by a grid element bombard a phosphored anode element to emit light for display purposes, control apparatus for supplying operating voltages to the filament, grid and anode elements from a source having a variable output voltage so as to minimize source voltage related variations in the brightness of the emitted light, comprising:
- anode supply means connected between the source and the anode element for supplying an operating voltage to the anode element which follows the output voltage of said source; and
- grid supply means connected between the source and the grid element for supplying the grid element with an operating voltage which varies in inverse relation to the voltage supplied to the anode element.
- 5. In a display system including a vacuum fluorescent tube in which electrons generated at a filament element and attracted by a grid element bombard a phosphored anode element to emit light for display purposes, control apparatus for supplying operating voltages to the filament, grid and anode elements from a source having a variable output voltage so as to minimize source voltage related variations in the brightness of the emitted light, comprising:
- anode supply means connected between the source and the anode element for supplying an operating voltage to the anode element which is substantially equal to the output voltage of said source; and
- grid supply means connected between the source and the grid element for supplying the grid element with an operating voltage which is lower than the voltage supplied to said anode element and which varies in inverse relation thereto, at least when said output voltage exceeds a reference voltage.
- 6. The apparatus set forth in claim 5, wherein:
- the source includes an automotive storage battery having a nominal open-circuit output voltage, the battery being adapted to be charged at voltages in excess of such nominal open-circuit voltage; and
- the reference voltage substantially corresponds to said nominal open-circuit voltage.
- 7. In a display system including a vacuum fluorescent tube in which electrons generated at a filament element and attracted by a grid element bombard a phosphored anode element to emit light for display purposes, control apparatus for supplying operating voltages to the filament, grid and anode elements from a source having a variable output voltage so as to minimize source voltage related variations in the brightness of the emitted light, comprising:
- anode supply means connected between the source and the anode element for supplying the anode element with an operating voltage substantially equal to the output voltage of said source;
- filament supply means connected between the source and the filament element for supplying the filament element with a relatively low operating voltage ratiometrically related to the output voltage of said source;
- grid supply means connected between the source and the grid element for supplying the grid element with an operating voltage intermediate the operating voltages supplied to said anode and filament elements, at least when said output voltage is less than a reference voltage; and
- brightness control means for reducing the operating voltage supplied to the grid element by said grid supply means in relation to the amount by which the operating voltage supplied to the anode element exceeds said reference voltage.
- 8. The apparatus set forth in claim 7, wherein:
- the source includes an automotive storage battery having a nominal open-circuit output voltage, the battery being adapted to be charged at voltages in excess of such nominal open-circuit voltage; and
- the reference voltage substantially corresponds to said nominal open-circuit voltage.
- 9. In a display system including a vacuum fluorescent tube in which electrons generated at a filament element and attracted by a grid element bombard a phosphored anode element to emit light for display purposes, control apparatus for supplying operating voltages to the filament, grid and anode elements from a source having a variable output voltage so as to minimize source voltage related variations in the brightness of the emitted light, comprising:
- ratiometric supply means connecting the source to the anode and filament elements for supplying the anode and filament elements with relatively high and low operating voltages, respectively, which ratiometrically follow the output voltage of said source; and
- grid supply means connected between the source and the grid element for supplying the grid element with an independently variable operating voltage intermediate the operating voltages of said anode and filament elements such that increases in the anode voltage are accompanied by decreases in the grid voltage to effect a lower voltage difference between the filament and grid elements, whereby the anode is bombarded by fewer electrons and the brightness of the emitted light tends to remain relatively constant.
BACKGROUND OF THE INVENTION
This invention relates to the control of an automotive vacuum fluorescent (VF) display, and more particularly, to a method and apparatus for minimizing the display brightness variations which occur due to variations in the supply voltage.
Vacuum fluorescent (VF) displays are generally defined by an evacuated envelope enclosing one or more phosphored anodes arranged in a pattern of desired light emission, a filament and a grid disposed between the anodes and filament. The filament is electrically heated at a relatively low voltage to generate a cloud of electrons, and the grid is maintained at a relatively high voltage to accelerate electrons onto any of the anodes which are also maintained at a relatively high voltage. The anodes bombarded by electrons emit light due to the phosphor coating.
In automotive applications, the anodes, filament and grid are generally referenced to the storage battery, as shown in the PRIOR ART drawing of FIG. 1. Referring to FIG. 1, the storage battery 10 is connected by ignition switch 12 to a supply terminal T which, when referenced to the vehicle frame, is generally referred to as the ignition voltage or IGN. The VF display is generally designated by the reference numeral 14 and comprises a filament 16, a grid 18 and a plurality of anode segments 20. The anode segments 20 are individually and selectively connected to the ignition voltage IGN through an anode driver array 22 and a dimming circuit 24. The anode driver array 22 and a dimming circuit 24. The anode driver array comprises a plurality of solid state switches 26 which are individually controlled to define the pattern of desired light emission, and the dimming circuit 24 comprises a solid state switch 28 which is pulse-width-modulated to control the average anode voltage and therefore the overall brightness of the display 14. A control of this sort is generally required for operator adjustment of the display brightness in night driving conditions. The grid 18 is maintained substantially at the ignition voltage IGN through the resistor R.sub.g and the filament is energized at a relatively low potential via a dropping resistor R.sub.f or a separate low voltage power supply (not shown). When multiplexing is employed, a grid supply switch 30 may be provided for open-circuiting the grid 18 to turn off the entire portion of the display 14 situated under the grid.
A drawback of the above-described drive circuit is that the display brightness tends to vary with the terminal voltage of the battery 10. In certain displays, brightness variations of 60% or more have been observed when the battery voltage is allowed to fluctuate over a 12-16 volt range. The usual solution is to insert a regulated power supply between the battery and the display. This, of course, is quite expensive, especially if a switching regulator is required.
The present invention is directed to an improved VF display control apparatus operated directly from an automotive storage battery, wherein the display brightness variation is minimized by controlling the relationship between the anode and grid voltages in relation to the fluctuation of the battery voltage. In essence, we have discovered that the brightness fluctuations of a VF display can be reduced or substantially eliminated over a range of supply voltages by driving the anode and grid such that the grid voltage varies in inverse relation to that of the anode voltage.
In operation, the anode and filament voltages are ratiometrically related to the battery voltage, and the grid is supplied with an independently variable voltage intermediate that of the anode and filament. In the illustrated embodiment, the anodes of the display are operated substantially at the battery voltage, and the voltage supplied to the grid is reduced in relation to the amount by which the anode (supply) voltage exceeds the nominal open-circuit terminal voltage of the battery. The voltage increase at the filament is relatively slight compared to the voltage increase at the anode, and the reduced grid voltage compensates for the higher anode-to-filament potential difference by reducing the grid-to-filament potential difference. As a result, the anode is bombarded by fewer but more energetic electrons and the display brightness tends to remain relatively constant. In a mechanization of the illustrated embodiment, the overall display brightness variation over a supply voltage range of 12-16 volts was reduced to less than 10%.
US Referenced Citations (3)
Non-Patent Literature Citations (1)
| Entry |
| Brochure Entitled "Fluorescent Indicator Panel Application Notes" NEC Electronics Inc., Issue 2. |
Patent Grant
4970441
