FULL SELECT-HALF SELECT PLASMA DISPLAY DRIVER CONTROL
United States Patent 3614769
A driver circuit in the form of a matrix formed by segment and character electrodes is utilized in a visual display for the selection of specific electroluminescent cells containing an encapsulated gas. Cells are driven into ignition by alternating energizing segment and character electrodes connected to the selected cells. The driver circuit utilizes a wall charge deposited on the individual cell walls during ignition of the cell, in the selection process for igniting selected cells. The wall charge is also utilized in preventing unselected cells connected to the energized segment or character electrode from igniting after the initial energization of the selected segment electrode and selected character electrodes.
US Patent References:
Electroluminescent apparatus
Rajchman et al. - June 1962 - 3041490
Electroluminescent-ferroelectric display with feedback control
Sack - May 1966 - 3252048
/3258644.html
Rajchman - June 1966 - 3258644
Circuit comprising writing and reproducing circuits using electroluminescent and ferrelectric cells
Duinker et al. - March 1967 - 3311781
Device for the digital display of data stored in electronic circuits
Rinaldi - September 1967 - 3340524
Electroluminescent apparatus
Rajchman et al. - June 1962 - 3041490
Electroluminescent-ferroelectric display with feedback control
Sack - May 1966 - 3252048
/3258644.html
Rajchman - June 1966 - 3258644
Circuit comprising writing and reproducing circuits using electroluminescent and ferrelectric cells
Duinker et al. - March 1967 - 3311781
Device for the digital display of data stored in electronic circuits
Rinaldi - September 1967 - 3340524
Inventors:
Coleman, William E. (Dayton, OH)
Skutt, Robert R. (Centerville, OH)
Skutt, Robert R. (Centerville, OH)
Application Number:
04/847141
Publication Date:
10/19/1971
Filing Date:
08/04/1969
View Patent Images:
Export Citation:
Assignee:
The National Cash Register Company (Dayton, OH)
Primary Class:
Other Classes:
345/208
International Classes:
G09G3/10; G09G3/28; H05B33/00; G09G3/04; G09F9/30
Field of Search:
340/324,343,344,166EL,324R 315/169,170,174
US Patent References:
| 3499167 | GAS DISCHARGE DISPLAY MEMORY DEVICE AND METHOD OF OPERATING | March 1970 | Baker et al. |
Primary Examiner:
Caldwell, John W.
Assistant Examiner:
Trafton, David L.
Claims:
What is claimed is
1. An electroluminescent driver control comprising, in combination,
2. THe driver control of claim 1 which said control means also includes means rendering it capable of repeatedly rendering only one of said normally nonconducting signal-translating devices conducting, in which event the electroluminescent cell is ignited only in response to the first rendering of said one of the signal-translating devices conductive by the control means, due to the subtractive effect of the wall charge upon subsequent voltages applied across the electroluminescent cell in the same direction as the initial application.
3. The driver control of claim 1 in which said electroluminescent cell comprises first and second transparent plates, said plates each having inner and outer surfaces, where said first plate has an image etched on its inner surface and said second plate has a mirror image of the image on said first plate etched on its inner surface, said plates assembled together so that said image and said mirror image are in full registration with each other, thereby forming a cavity in which an electroluminescent gas is encapsulated.
4. An electroluminescent driver control comprising, in combination,
5. The driver control of claim 4 in which each of said electroluminescent cells comprises first and second transparent plates, said plates each having inner and outer surfaces, where said first plate has an image etched on its inner surface and said second plate has a mirror image of the image on said first plate etched on its inner surface, said plates assembled together so that said image and said mirror image are in full registration, thereby forming a cavity in which a mixture of neon, argon, and nitrogen gas is encapsulated.
6. The driver control of claim 5 in which said first and second electrodes of the electroluminescent cells are attached to the outer surfaces of said first and second plates, at least one of said electrodes being transparent.
7. A matrix electroluminescent driver control comprising, in combination,
8. The driver control of claim 7 in which each segment electrode and each character electrode are serially connected by a corresponding diode and transistor to said potential source.
9. The driver of claim 7 in which each segment electrode and each character electrode are serially connected by a corresponding register to said potential source.
10. The driver control of claim 7 in which each of said electroluminescent cells comprises first and second substantially transparent plates, said plates each having inner and outer surfaces, where said first plate has an image etched on its inner surface and said second plate has a mirror image of said first plate etched on its inner surface, said plates assembled together so that said image and said mirror image are in full registration, thereby forming a cavity in which an electroluminescent gas is encapsulated.
11. The driver control of claim 10 in which the electroluminescent gas is a mixture of neon, argon, and nitrogen gas.
12. The driver control of claim 9 in which said segment and character electrodes are attached to the outer surfaces of said first and second plates.
13. The driver control of claim 12 in which the character electrodes are transparent.
14. The driver control of claim 7 in which said segment and character electrodes are connected to a common voltage source for the establishment of a voltage level commensurate with the ignition level of said electroluminescent cells.
1. An electroluminescent driver control comprising, in combination,
2. THe driver control of claim 1 which said control means also includes means rendering it capable of repeatedly rendering only one of said normally nonconducting signal-translating devices conducting, in which event the electroluminescent cell is ignited only in response to the first rendering of said one of the signal-translating devices conductive by the control means, due to the subtractive effect of the wall charge upon subsequent voltages applied across the electroluminescent cell in the same direction as the initial application.
3. The driver control of claim 1 in which said electroluminescent cell comprises first and second transparent plates, said plates each having inner and outer surfaces, where said first plate has an image etched on its inner surface and said second plate has a mirror image of the image on said first plate etched on its inner surface, said plates assembled together so that said image and said mirror image are in full registration with each other, thereby forming a cavity in which an electroluminescent gas is encapsulated.
4. An electroluminescent driver control comprising, in combination,
5. The driver control of claim 4 in which each of said electroluminescent cells comprises first and second transparent plates, said plates each having inner and outer surfaces, where said first plate has an image etched on its inner surface and said second plate has a mirror image of the image on said first plate etched on its inner surface, said plates assembled together so that said image and said mirror image are in full registration, thereby forming a cavity in which a mixture of neon, argon, and nitrogen gas is encapsulated.
6. The driver control of claim 5 in which said first and second electrodes of the electroluminescent cells are attached to the outer surfaces of said first and second plates, at least one of said electrodes being transparent.
7. A matrix electroluminescent driver control comprising, in combination,
8. The driver control of claim 7 in which each segment electrode and each character electrode are serially connected by a corresponding diode and transistor to said potential source.
9. The driver of claim 7 in which each segment electrode and each character electrode are serially connected by a corresponding register to said potential source.
10. The driver control of claim 7 in which each of said electroluminescent cells comprises first and second substantially transparent plates, said plates each having inner and outer surfaces, where said first plate has an image etched on its inner surface and said second plate has a mirror image of said first plate etched on its inner surface, said plates assembled together so that said image and said mirror image are in full registration, thereby forming a cavity in which an electroluminescent gas is encapsulated.
11. The driver control of claim 10 in which the electroluminescent gas is a mixture of neon, argon, and nitrogen gas.
12. The driver control of claim 9 in which said segment and character electrodes are attached to the outer surfaces of said first and second plates.
13. The driver control of claim 12 in which the character electrodes are transparent.
14. The driver control of claim 7 in which said segment and character electrodes are connected to a common voltage source for the establishment of a voltage level commensurate with the ignition level of said electroluminescent cells.
Description:
FIELD OF THE INVENTION
This invention relates to a driver circuit for selectively energizing selected capacitively coupled electroluminescent cells of a visual display.
DESCRIPTION OF THE PRIOR ART
It is well known that an electroluminescent cell can be interposed between first and second electrodes and that, upon the application of a suitable electric potential between the first and second electrode connected to the cell, the cell will become luminescent because of the ionization which occurs within the cell. This characteristic lends itself quite readily for use in a display panel. A control circuit for driving such a display is exemplified in U.S. Pat. No. 3,343,128, which issued Sept. 19, 1967, on the application of Raymond J. Rogers.
A major problem associated with these displays is the prevention of spurious firings of unselected cells. The X-Y system described in the Rogers U.S. patent relates to circuitry for selectively exciting a crossed grid electroluminescent display. In order to prevent the firing of unselected cells, the above system includes means for applying suppression pulses to those drivers corresponding to unselected Y rows. This results in a potential difference at the crossover point of the unselected Y row and selected column electrodes, which is insufficient to ignite the cell interposed at that particular crossover point. Needless to say, this requires extensive and complex circuitry.
Other forms of control have been the half-select mode, in which one-half of the necessary voltage is applied to a first electrode and the other half applied to a second electrode, thus applying full voltage across the selected cell, which is connected between the first and second electrodes.
Still others have prevented the spurious ignition of unselected cells by applying exciting voltages having particular phase relationships at the selected point. The prior art also shows the application of variable impedances at the input of each row and column in order to prevent the luminescence of unselected cells.
SUMMARY OF THE INVENTION
The application of an electric field to an electroluminescent cell causes ionization to occur within the cell. The electric field imparts energy to electrons which collide with other atoms, thus releasing other electrons. This electron multiplication process continues until breakdown occurs, at which time ignition occurs; i.e., a gaseous discharge occurs within the cells, causing positive charges to be deposited on the cell walls connected to the cathode and electrons to be deposited on the cell walls connected to the anode. The charges deposited on the cell walls are trapped because of the capacitive coupling effect exerted by the cell walls. Since positive ions are attached to the cathode wall and electrons are attached to the anode wall, the wall charge will be of a polarity opposite to that of the electric field which instigated the gas discharge. In other words, the voltage contributed by the wall charge will be opposite in polarity to the applied electric field. Thus, it can be seen that, after discharge occurs, the total voltage impressed on the cell will be the algebraic sums of the voltage applied to the cell terminals plus the voltage contributed by the wall charge, which after ignition is negative with respect to the applied voltage, therefore resulting in a decreased cell voltage. The gas discharge which occurs in the cell continues until the wall voltage builds up to a certain value. This value is given by the relationship V a -V w <V e , where V a is the applied voltage, V w is the wall voltage, and V e is the voltage below which the cell is extinguished. In order to ignite the cell again using the same magnitude of applied voltage, it is necessary to reverse the polarity of the applied voltage to the cell, thereby impressing an applied voltage across the cell which is additive with the wall voltage left from the previous discharge, thus permitting a gas discharge to occur in the reverse direction. Since the wall charge is trapped within the cell, the wall voltage will always oppose the voltage which initiated the gas discharge.
Information is visually displayed in a display device in the form of characters, the characters being formed by a group of electroluminescent cells containing an encapsulated gas. The illumination is provided by a gaseous discharge within the cell which occurs upon the application of an electric field at the cell terminals, thereby igniting the cell. The invention comprises a control circuit for selectively energizing the electroluminescent cells, each of which is capacitively coupled between two electrodes, such as a segment electrode and a character electrode. The number of segment electrodes is determined by the number of cells per character, and the number of character electrodes is determined by the number of characters in the display device. Electrically, this easily takes on the form of a matrix in which the columns are called segment electrodes. Each individual cell connected in a column is called a segment cell, and the segment cells in each row are connected to a character electrode. One end of each segment electrode and each character electrode is connected to the same potential source. The other ends of the segment and character electrodes are each connected to ground through individual driver transistors. The energization of selected segment cells in a character line determines the information to be displayed. Means are provided for logically controlling the drive transistors.
In order to illuminate a selected cell for display purposes, it is necessary to alternately energize the segment and character electrodes connected to the selected cells. However, this causes all the cells connected to the energized segment and character electrodes to ignite. These cells are referred to as unselected cells.
The wall charge produced in the unselected cells by the initial energization of the selected segment electrode always opposes the electric field created by all succeeding segment energizations and will no longer ignite. This is also true for the unselected cells connected to the energized character electrode, in which the wall voltage always opposes the electric field produced by the initial character electrode energization. However, such is not the case for the selected cell connected at the crossover point of the segment and character electrodes being energized. The wall charge in the selected cell will reverse in polarity with each energization of the cell. This occurs because the alternate energization of the segment and character electrodes reverses the polarity of the electric field applied to the selected cell. Therefore the electric field will be of the same polarity as that of the wall charge.
It can therefore be seen that the instant invention provides a simple driver circuit which is capable of exciting selected cells in a visual display and in which the power dissipation is small. The invention further provides novel means for suppressing the spurious ignition of unselected cells by the proper utilization of the wall charge in the unselected cells and also permits the ignition of the selected cells with a wide range of firing voltages because of the wall charge associated with the selected cells.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1A is a sectional view of a plasma cell that can be utilized with the instant invention.
FIG. 1B is a plan view of a representative visual display.
FIG. 2 is a schematic diagram of the basic driver scheme comprising the invention.
FIGS. 3 and 4 show a plurality of waveforms illustrating the operation of the circuits shown in FIG. 2.
FIG. 5 is a schematic diagram of a first embodiment of the invention.
FIG. 6 shows a plurality of waveforms illustrating the operation of the circuits shown in FIG. 5.
FIG. 7 is a schematic diagram of a second embodiment of the invention.
DESCRIPTION OF THE PREFERRED EMBODIMENT
FIG. 1A is representative of an electroluminescent display cell which can be used with the present invention. The cell 1 usually comprises a glass sandwich encapsulating a gas at a particular pressure. A discharge which occurs in the encapsulated gas and provides sufficient illumination for use in visual displays will occur within the cell 1 upon the application of a particular potential V a between electrodes 2 and 3, the electrodes being located externally of the cell in order to utilize its capacitive properties. The electrons and ions created by the discharge will attach to the anode and cathode sides of the glass cell, respectively, to produce what is commonly referred to as the wall charge. The voltage V w attributed to the wall charge has a polarity opposite to that of the applied voltage V a which initiated the discharge. Upon reversal of the applied voltage V a , it can be seen that V a and V w are additive, thereby causing another discharge to occur and permitting the use of a voltage V a which can be at a lower level than that which originally initiated the discharge.
FIG. 1B shows a plurality of cells of the type illustrated in FIG. 1A combined to form a conventional seven-bar code matrix 62, comprising seven individual segments 21. Individual ones of these segments can be selectively energized to form desired numbers and symbols.
The electroluminescent cell 1 of FIG. 1A is shown in FIG. 2 as being capacitively coupled to the cell electrodes 2 and 3, in which at least one of the electrodes is transparent for the passage of light. The two coupling capacitances 17 and 18 exist because of the glass dielectric between each exterior electrode and the adjacent interior glass wall surface. Although two coupling capacitors 17 and 18 are shown in FIG. 2, one coupling capacitor could be eliminated, and the combination would still be referred to as a capacitively coupled cell. The electroluminescent cell electrodes 2 and 3 are connected to the collectors 4 and 5, respectively, of two signal-translating devices, represented here as NPN transistors 6 and 7. The cell electrodes 2 and 3 are also connected, respectively, by resistors 8 and 9 to a common voltage source 10. The emitters 11 and 12 are both connected to ground. The bases 13 and 14 of the transistors 6 and 7, respectively, are each connected to pulse generators 15 and 16.
The following voltages, gas compositions, and pressures utilized in the operation of the circuit shown in FIG. 2 are given by way of example only and are by no means exclusive. The voltage V a applied to the cell electrodes 2 and 3 is 250 volts. The voltage necessary to ignite the cell 1 is, for all practical purposes, equal to V a , the applied voltage to the cell. The cell contains a gas mixture of 99.7 percent neon, 0.2 percent nitrogen, and 0.1 percent argon at a pressure of 200 millimeters of mercury.
Referring to FIGS. 2 and 3, a pulse is applied to the base 14 at time T 1 , causing the base to become positive with respect to the emitter 12, thereby switching the transistor 7 into a conducting state. The impedance of the transistor 7, when conducting, is very low; therefore the cell electrode 3 is effectively driven to ground. The cell electrode 2 remains at 250 volts, therefore impressing a positive voltage V 2 -3 across the cell 1 by means of a path which extends from the voltage source 10, through the resistor 8, across the cell 1 and the coupling capacitors 17 and 18 and through the collector-emitter path of the conducting transistor 7 down to ground. The cell ignites, and discharge occurs at T 1 + , causing a wall charge to be deposited on the inside glass surface walls of the cell 1. The wall charge produces a wall voltage opposite in polarity to that of the applied voltage which initially drove the cell into ignition at time T 1 + . It will be assumed for purposes of illustration that the wall charge in the illustrated embodiment contributes a voltage of 125 volts. Using this voltage, it can be seen from waveform C, in which waveform C illustrates the voltage across the cell 1, that the cell voltage V c drops to 125 volts after ignition, since the wall voltage V w is negative with respect to the applied voltage V a ; i.e., V c =V a +(-V w ). The pulse to the base 14 is turned off at time T 2 , thus switching the transistor 7 into a nonconducting state and also switching the cell electrode 3 back to 250 volts. The cell voltage is now -125 volts, since only the wall voltage V w , contributed by the wall charge, is across the cell 1. At time T 3 , a pulse is applied to the base 13 of the transistor 6, thus switching the transistor 6 into a conducting state. The cell electrode 3 remains at a potential at 250 volts, and the electrode 2 is driven to ground, thereby impressing a negative voltage -V a = V 3 -2 across the cell electrodes 3 and 2. The applied negative voltage V a will add to the negative voltage V w contributed by the wall charge from the previous discharge, thereby increasing V c to -375 volts. cell will ignite at T 3 + , causing a wall charge to be deposited on the cell walls of opposite polarity to that of the applied negative voltage initiating the discharge. At time T 4 , the pulse to the base 13 is turned off, thereby switching the applied voltage across the cell electrodes 3 and 2 to zero and leaving the cell voltage V c at +125 volts due to the wall charge. The above operations occur so long as the transistors 6 and 7 are alternately pulsed into conduction.
Waveform A of FIG. 4 illustrates the condition when only the transistor 6 is pulsed into conduction. It can be seen that the cell will fire only at time T 1 + , because on the next succeeding pulse, at time T 5 , the wall charge deposited by the initial discharge at T 1 +opposes the applied voltage V a , therefore lowering the cell voltage V c to a level insufficient for ignition. This is true so long as the level of the applied voltage V a does not exceed the algebraic sum of the wall voltage and the firing voltage. In the example cited, applied voltage V a could increase to a maximum of just less than 375 volts without causing the cell to fire. A similar analysis holds for the situation where only the transistor 7 is pulsed into conduction.
FIG. 5 illustrates a first embodiment of the invention in matrix form operating in the time shared full select-half select mode. The illustrated circuit arrangement is designed to drive a seven-segment, six-character display, with each segment representing an electroluminescent cell. The characters may represent alphabetic, numeric, or alpha-numeric characters, depending upon the configuration of the cells, such as that shown in FIG. 1B, and also depending upon the order in which the cells are selected for ignition. The matrix is formed by segment electrodes 19 in the Y direction crossing character electrodes 20 in the X direction. For purposes of simplicity only, the electroluminescent cells are shown as one capacitor in FIG. 5. One side of each capacitively coupled electroluminescent cell constituting a segment 21 is connected to one of the common segment electrodes 19. One end of each segment electrode 19 extends to the collector 22 of a segment driver transistor 23. The base 24 of the driver transistor 23 is connected to an AND gate 25 for biasing, and the emitter 26 is grounded. The AND gate 25 is connected to a pulse generator 27 and to a line 43, which extends to a serial input-parallel output buffer 28, which can be composed of four parallel flip-flops feeding a diode matrix. The buffer 28 is connected to a recirculating shift register 29 through an AND gate 45. The other end of the segment electrode 19 is connected to a resistor 30, which in turn is connected to a voltage source 31.
The other side of each capacitively coupled electroluminescent cell forming a character symbol is connected to one of the common character electrodes 20, which extends to the collector 32 of a character driver transistor 33. The base 35 of the driver transistor 33 is connected to an AND gate 36, and the emitter 34 of said transistor 33 is grounded. The AND gate 36 is connected to a delay line 37, which is connected to a pulse generator 27. The AND gate 36 is also connected to a line 41, which extends to a character counter 38. The counter 38 is connected to a clock 44, and the clock 44 is further connected to the AND gate 45. The other end of each of the character electrodes 20 extends to a resistor 40, which terminates in the voltage source 31. No specific configuration is given for the character counter 38, which can consist of conventional flip-flop circuits, nor for the clocks or pulse generators, which can be crystal operated oscillators.
In operation, the circuit of FIG. 5 will scan (i.e., turn on for display purposes) each character for a specific length of time. Upon displaying all six characters, the scanning process will revert back to character one, thereby repeating the operation.
FIG. 6 is a detailed representation of the pulse sequences which occur for a typical operation of the circuit shown in FIG. 5. The pulse sequences are shown only for the first three characters; however, the operation to be described below applies to all six characters. The cell 21aa is selected during the time that the line 41a is energized, the cell 21bb is selected during the time that the line 41b is energized, and the cells 21ac and 21cc are selected during the time that the line 41c is energized. The six characters to be displayed are serially entered at the input 46 into the recirculating shift register 29 in the form of bits, four bits per character. The clock 44 and the shift register form the two inputs necessary to turn the AND gate 45 on, at which time the first of six characters will be entered serially by bit into the serial input-parallel output buffer 28. The four bit signals are applied to four flip-flops in the buffer 28, the outputs of which flip-flops are applied to a diode encoding matrix of conventional design, which converts the four-bit input code to a seven-line output code, which is applied to the segment lines for energizing selected ones of the segments 21, shown in FIG. 1B. The selected segment line (in this example, line 43a ) is energized for a period of 160 microseconds. The clock 44, at the time the AND gate 45 is turned on, also turns on the selected character line (in this instance, the line 41a) for a period of 160 microseconds. The signal of the pulse generator 27, which has an 8-microsecond period and a 2-microsecond pulse width, together with the signal on the line 43a, turns on the AND gate 25, thereby switching the transistor 23a into conduction for 2 microseconds at time T 1 . The conduction of the transistor 23a drives the segment electrode 19a to ground, which results in a voltage of 250 volts impressed across all the cells connected to the segment electrode 19a. The charging path for every cell connected to the segment electrode 19a includes the resistor 40a, the segments 21, and the transistor 23a. This results in a gaseous discharge and an accompanying wall charge at time T 1 + , occurring in all the cells connected to the segment electrode 19a. It should be mentioned that the words "gaseous discharge" and "ignition" are used interchangeably. At time T 2 , the pulse is terminated, thus switching the transistor 23a off. At time T 3 , the delayed pulse at the delay line 37 and the signal on the line 41a form the two inputs necessary to turn on the AND gate 36 for 2 microseconds, which renders the transistor 33a conducting. Therefore, all segments 21 connected to the character electrode 20a will experience a gaseous discharge and an associated wall charge at time T 3 + . The selected segment 21aa has a negative wall voltage V w from the previous discharge at time T 1 +which is additive with the negative applied voltage V a impressed across the cell 21aa at time T 3 , thereby permitting the gaseous discharge at time T 3 + . The polarity of voltage V w at time T 5 will be additive to the applied voltage, driving the segment 21aa into ignition. The unselected cells connected to the segment electrode 19a will not fire at time T 5 + , since the cells still retain a wall charge from the gaseous discharge which occurred at time T 1 +and will therefore be of a polarity which opposes the applied voltage at time T 5 . For the same reasons as above, only the selected cell 21aa will ignite at time T 7 + , since the unselected cells connected to the character electrode still retain a voltage V w from the gaseous discharge at time T 3 . Therefore the voltage V w associated with those cells is opposed to the voltage V a at time T 7 , and the cell will not ignite. Thus only the cell 21aa will continue to experience gaseous discharges for the remainder of the 160-microsecond period. The remaining character segments are all displayed in the same manner. While the actual energization of the characters takes place sequentially, the characters appear to a viewer to be displayed continuously, due to retention of vision.
Referring to the waveform for the cell 21ac during the time that the line 41c is energized, it is seen that the cell did not ignite, even though it was a selected cell. This occurred because the cell 21ac experienced ignition at time T 1 +during the time that the line 41a was energized. Since the cell 21ac was never selected after that, it still retained the original wall charge, which is opposite in polarity to the applied voltage at time T 1 during the time that the line 41c is energized. Thus, the voltage is insufficient to ignite the cell 21ac. However, on the succeeding pulse at time T 3 , at which time the voltage V a is reversed, the cell will ignite because the voltages V a and V w are now additive. These occurrences, both the spurious ignition of unselected cells, and the nonignition of selected cells, are not objectionable, because they are not noticeable when compared to the total illumination provided by the selected cell.
The circuit of FIG. 7 differs from that of FIG. 5 in that the resistors 30 and 40 have all been replaced by diodes 47 and 48, respectively, and in the addition of transistors 50 and 51. Since the circuits are substantially the same, identical elements of FIGS. 5 and 7 have been given the same reference numeral designations. The segment electrodes are connected to the voltage source 31 through the transistor 50, and the character electrodes are connected to the voltage source 31 through the transistor 51. The mode of operation of this embodiment is similar to that of the circuit illustrated in FIG. 5. Assuming that the line 43a is energized, a pulse from the pulse generator 27 will turn on the transistor 51 and will also turn on the segment driver transistor 23a through the AND gate 25a. The conducting driver transistor 23a drives the segment electrode 19a to ground, thereby impressing a voltage across all the cells connected to the segment electrode 19a. The charging path for the cells connected to the segment electrode 19a extends from the voltage source 31, through the conducting transistor 51, the diodes 48, the segments 21, and the segment driver transistor 23a down to ground, thus resulting in a gaseous discharge in all of the segments 21 connected to the segment electrode 19a.
Similarly, the delayed pulse at the delay line 37 will turn on the transistor 50 and will also turn on the segment driver transistor 33 through an AND gate 60a. The charging path for the cells connected to the character electrode 20a extends from a potential source 49, through the conducting transistor 50, the diodes 47, the segments 21, and the character driver transistor 33 down to ground, thus resulting in the occurrence of a gaseous discharge in all of the segments 21 connected to the character electrode 20a. The diodes 47 and 48 prevent the formation of any sneak paths. The transistors 50 and 51 provide a fast switching time, thus permitting a higher discharge current with no resultant damage to the driver transistors. Power loss is reduced in the embodiment of FIG. 7 because of the elimination of the resistors employed in the embodiment of FIG. 5.
This invention relates to a driver circuit for selectively energizing selected capacitively coupled electroluminescent cells of a visual display.
DESCRIPTION OF THE PRIOR ART
It is well known that an electroluminescent cell can be interposed between first and second electrodes and that, upon the application of a suitable electric potential between the first and second electrode connected to the cell, the cell will become luminescent because of the ionization which occurs within the cell. This characteristic lends itself quite readily for use in a display panel. A control circuit for driving such a display is exemplified in U.S. Pat. No. 3,343,128, which issued Sept. 19, 1967, on the application of Raymond J. Rogers.
A major problem associated with these displays is the prevention of spurious firings of unselected cells. The X-Y system described in the Rogers U.S. patent relates to circuitry for selectively exciting a crossed grid electroluminescent display. In order to prevent the firing of unselected cells, the above system includes means for applying suppression pulses to those drivers corresponding to unselected Y rows. This results in a potential difference at the crossover point of the unselected Y row and selected column electrodes, which is insufficient to ignite the cell interposed at that particular crossover point. Needless to say, this requires extensive and complex circuitry.
Other forms of control have been the half-select mode, in which one-half of the necessary voltage is applied to a first electrode and the other half applied to a second electrode, thus applying full voltage across the selected cell, which is connected between the first and second electrodes.
Still others have prevented the spurious ignition of unselected cells by applying exciting voltages having particular phase relationships at the selected point. The prior art also shows the application of variable impedances at the input of each row and column in order to prevent the luminescence of unselected cells.
SUMMARY OF THE INVENTION
The application of an electric field to an electroluminescent cell causes ionization to occur within the cell. The electric field imparts energy to electrons which collide with other atoms, thus releasing other electrons. This electron multiplication process continues until breakdown occurs, at which time ignition occurs; i.e., a gaseous discharge occurs within the cells, causing positive charges to be deposited on the cell walls connected to the cathode and electrons to be deposited on the cell walls connected to the anode. The charges deposited on the cell walls are trapped because of the capacitive coupling effect exerted by the cell walls. Since positive ions are attached to the cathode wall and electrons are attached to the anode wall, the wall charge will be of a polarity opposite to that of the electric field which instigated the gas discharge. In other words, the voltage contributed by the wall charge will be opposite in polarity to the applied electric field. Thus, it can be seen that, after discharge occurs, the total voltage impressed on the cell will be the algebraic sums of the voltage applied to the cell terminals plus the voltage contributed by the wall charge, which after ignition is negative with respect to the applied voltage, therefore resulting in a decreased cell voltage. The gas discharge which occurs in the cell continues until the wall voltage builds up to a certain value. This value is given by the relationship V a -V w <V e , where V a is the applied voltage, V w is the wall voltage, and V e is the voltage below which the cell is extinguished. In order to ignite the cell again using the same magnitude of applied voltage, it is necessary to reverse the polarity of the applied voltage to the cell, thereby impressing an applied voltage across the cell which is additive with the wall voltage left from the previous discharge, thus permitting a gas discharge to occur in the reverse direction. Since the wall charge is trapped within the cell, the wall voltage will always oppose the voltage which initiated the gas discharge.
Information is visually displayed in a display device in the form of characters, the characters being formed by a group of electroluminescent cells containing an encapsulated gas. The illumination is provided by a gaseous discharge within the cell which occurs upon the application of an electric field at the cell terminals, thereby igniting the cell. The invention comprises a control circuit for selectively energizing the electroluminescent cells, each of which is capacitively coupled between two electrodes, such as a segment electrode and a character electrode. The number of segment electrodes is determined by the number of cells per character, and the number of character electrodes is determined by the number of characters in the display device. Electrically, this easily takes on the form of a matrix in which the columns are called segment electrodes. Each individual cell connected in a column is called a segment cell, and the segment cells in each row are connected to a character electrode. One end of each segment electrode and each character electrode is connected to the same potential source. The other ends of the segment and character electrodes are each connected to ground through individual driver transistors. The energization of selected segment cells in a character line determines the information to be displayed. Means are provided for logically controlling the drive transistors.
In order to illuminate a selected cell for display purposes, it is necessary to alternately energize the segment and character electrodes connected to the selected cells. However, this causes all the cells connected to the energized segment and character electrodes to ignite. These cells are referred to as unselected cells.
The wall charge produced in the unselected cells by the initial energization of the selected segment electrode always opposes the electric field created by all succeeding segment energizations and will no longer ignite. This is also true for the unselected cells connected to the energized character electrode, in which the wall voltage always opposes the electric field produced by the initial character electrode energization. However, such is not the case for the selected cell connected at the crossover point of the segment and character electrodes being energized. The wall charge in the selected cell will reverse in polarity with each energization of the cell. This occurs because the alternate energization of the segment and character electrodes reverses the polarity of the electric field applied to the selected cell. Therefore the electric field will be of the same polarity as that of the wall charge.
It can therefore be seen that the instant invention provides a simple driver circuit which is capable of exciting selected cells in a visual display and in which the power dissipation is small. The invention further provides novel means for suppressing the spurious ignition of unselected cells by the proper utilization of the wall charge in the unselected cells and also permits the ignition of the selected cells with a wide range of firing voltages because of the wall charge associated with the selected cells.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1A is a sectional view of a plasma cell that can be utilized with the instant invention.
FIG. 1B is a plan view of a representative visual display.
FIG. 2 is a schematic diagram of the basic driver scheme comprising the invention.
FIGS. 3 and 4 show a plurality of waveforms illustrating the operation of the circuits shown in FIG. 2.
FIG. 5 is a schematic diagram of a first embodiment of the invention.
FIG. 6 shows a plurality of waveforms illustrating the operation of the circuits shown in FIG. 5.
FIG. 7 is a schematic diagram of a second embodiment of the invention.
DESCRIPTION OF THE PREFERRED EMBODIMENT
FIG. 1A is representative of an electroluminescent display cell which can be used with the present invention. The cell 1 usually comprises a glass sandwich encapsulating a gas at a particular pressure. A discharge which occurs in the encapsulated gas and provides sufficient illumination for use in visual displays will occur within the cell 1 upon the application of a particular potential V a between electrodes 2 and 3, the electrodes being located externally of the cell in order to utilize its capacitive properties. The electrons and ions created by the discharge will attach to the anode and cathode sides of the glass cell, respectively, to produce what is commonly referred to as the wall charge. The voltage V w attributed to the wall charge has a polarity opposite to that of the applied voltage V a which initiated the discharge. Upon reversal of the applied voltage V a , it can be seen that V a and V w are additive, thereby causing another discharge to occur and permitting the use of a voltage V a which can be at a lower level than that which originally initiated the discharge.
FIG. 1B shows a plurality of cells of the type illustrated in FIG. 1A combined to form a conventional seven-bar code matrix 62, comprising seven individual segments 21. Individual ones of these segments can be selectively energized to form desired numbers and symbols.
The electroluminescent cell 1 of FIG. 1A is shown in FIG. 2 as being capacitively coupled to the cell electrodes 2 and 3, in which at least one of the electrodes is transparent for the passage of light. The two coupling capacitances 17 and 18 exist because of the glass dielectric between each exterior electrode and the adjacent interior glass wall surface. Although two coupling capacitors 17 and 18 are shown in FIG. 2, one coupling capacitor could be eliminated, and the combination would still be referred to as a capacitively coupled cell. The electroluminescent cell electrodes 2 and 3 are connected to the collectors 4 and 5, respectively, of two signal-translating devices, represented here as NPN transistors 6 and 7. The cell electrodes 2 and 3 are also connected, respectively, by resistors 8 and 9 to a common voltage source 10. The emitters 11 and 12 are both connected to ground. The bases 13 and 14 of the transistors 6 and 7, respectively, are each connected to pulse generators 15 and 16.
The following voltages, gas compositions, and pressures utilized in the operation of the circuit shown in FIG. 2 are given by way of example only and are by no means exclusive. The voltage V a applied to the cell electrodes 2 and 3 is 250 volts. The voltage necessary to ignite the cell 1 is, for all practical purposes, equal to V a , the applied voltage to the cell. The cell contains a gas mixture of 99.7 percent neon, 0.2 percent nitrogen, and 0.1 percent argon at a pressure of 200 millimeters of mercury.
Referring to FIGS. 2 and 3, a pulse is applied to the base 14 at time T 1 , causing the base to become positive with respect to the emitter 12, thereby switching the transistor 7 into a conducting state. The impedance of the transistor 7, when conducting, is very low; therefore the cell electrode 3 is effectively driven to ground. The cell electrode 2 remains at 250 volts, therefore impressing a positive voltage V 2 -3 across the cell 1 by means of a path which extends from the voltage source 10, through the resistor 8, across the cell 1 and the coupling capacitors 17 and 18 and through the collector-emitter path of the conducting transistor 7 down to ground. The cell ignites, and discharge occurs at T 1 + , causing a wall charge to be deposited on the inside glass surface walls of the cell 1. The wall charge produces a wall voltage opposite in polarity to that of the applied voltage which initially drove the cell into ignition at time T 1 + . It will be assumed for purposes of illustration that the wall charge in the illustrated embodiment contributes a voltage of 125 volts. Using this voltage, it can be seen from waveform C, in which waveform C illustrates the voltage across the cell 1, that the cell voltage V c drops to 125 volts after ignition, since the wall voltage V w is negative with respect to the applied voltage V a ; i.e., V c =V a +(-V w ). The pulse to the base 14 is turned off at time T 2 , thus switching the transistor 7 into a nonconducting state and also switching the cell electrode 3 back to 250 volts. The cell voltage is now -125 volts, since only the wall voltage V w , contributed by the wall charge, is across the cell 1. At time T 3 , a pulse is applied to the base 13 of the transistor 6, thus switching the transistor 6 into a conducting state. The cell electrode 3 remains at a potential at 250 volts, and the electrode 2 is driven to ground, thereby impressing a negative voltage -V a = V 3 -2 across the cell electrodes 3 and 2. The applied negative voltage V a will add to the negative voltage V w contributed by the wall charge from the previous discharge, thereby increasing V c to -375 volts. cell will ignite at T 3 + , causing a wall charge to be deposited on the cell walls of opposite polarity to that of the applied negative voltage initiating the discharge. At time T 4 , the pulse to the base 13 is turned off, thereby switching the applied voltage across the cell electrodes 3 and 2 to zero and leaving the cell voltage V c at +125 volts due to the wall charge. The above operations occur so long as the transistors 6 and 7 are alternately pulsed into conduction.
Waveform A of FIG. 4 illustrates the condition when only the transistor 6 is pulsed into conduction. It can be seen that the cell will fire only at time T 1 + , because on the next succeeding pulse, at time T 5 , the wall charge deposited by the initial discharge at T 1 +opposes the applied voltage V a , therefore lowering the cell voltage V c to a level insufficient for ignition. This is true so long as the level of the applied voltage V a does not exceed the algebraic sum of the wall voltage and the firing voltage. In the example cited, applied voltage V a could increase to a maximum of just less than 375 volts without causing the cell to fire. A similar analysis holds for the situation where only the transistor 7 is pulsed into conduction.
FIG. 5 illustrates a first embodiment of the invention in matrix form operating in the time shared full select-half select mode. The illustrated circuit arrangement is designed to drive a seven-segment, six-character display, with each segment representing an electroluminescent cell. The characters may represent alphabetic, numeric, or alpha-numeric characters, depending upon the configuration of the cells, such as that shown in FIG. 1B, and also depending upon the order in which the cells are selected for ignition. The matrix is formed by segment electrodes 19 in the Y direction crossing character electrodes 20 in the X direction. For purposes of simplicity only, the electroluminescent cells are shown as one capacitor in FIG. 5. One side of each capacitively coupled electroluminescent cell constituting a segment 21 is connected to one of the common segment electrodes 19. One end of each segment electrode 19 extends to the collector 22 of a segment driver transistor 23. The base 24 of the driver transistor 23 is connected to an AND gate 25 for biasing, and the emitter 26 is grounded. The AND gate 25 is connected to a pulse generator 27 and to a line 43, which extends to a serial input-parallel output buffer 28, which can be composed of four parallel flip-flops feeding a diode matrix. The buffer 28 is connected to a recirculating shift register 29 through an AND gate 45. The other end of the segment electrode 19 is connected to a resistor 30, which in turn is connected to a voltage source 31.
The other side of each capacitively coupled electroluminescent cell forming a character symbol is connected to one of the common character electrodes 20, which extends to the collector 32 of a character driver transistor 33. The base 35 of the driver transistor 33 is connected to an AND gate 36, and the emitter 34 of said transistor 33 is grounded. The AND gate 36 is connected to a delay line 37, which is connected to a pulse generator 27. The AND gate 36 is also connected to a line 41, which extends to a character counter 38. The counter 38 is connected to a clock 44, and the clock 44 is further connected to the AND gate 45. The other end of each of the character electrodes 20 extends to a resistor 40, which terminates in the voltage source 31. No specific configuration is given for the character counter 38, which can consist of conventional flip-flop circuits, nor for the clocks or pulse generators, which can be crystal operated oscillators.
In operation, the circuit of FIG. 5 will scan (i.e., turn on for display purposes) each character for a specific length of time. Upon displaying all six characters, the scanning process will revert back to character one, thereby repeating the operation.
FIG. 6 is a detailed representation of the pulse sequences which occur for a typical operation of the circuit shown in FIG. 5. The pulse sequences are shown only for the first three characters; however, the operation to be described below applies to all six characters. The cell 21aa is selected during the time that the line 41a is energized, the cell 21bb is selected during the time that the line 41b is energized, and the cells 21ac and 21cc are selected during the time that the line 41c is energized. The six characters to be displayed are serially entered at the input 46 into the recirculating shift register 29 in the form of bits, four bits per character. The clock 44 and the shift register form the two inputs necessary to turn the AND gate 45 on, at which time the first of six characters will be entered serially by bit into the serial input-parallel output buffer 28. The four bit signals are applied to four flip-flops in the buffer 28, the outputs of which flip-flops are applied to a diode encoding matrix of conventional design, which converts the four-bit input code to a seven-line output code, which is applied to the segment lines for energizing selected ones of the segments 21, shown in FIG. 1B. The selected segment line (in this example, line 43a ) is energized for a period of 160 microseconds. The clock 44, at the time the AND gate 45 is turned on, also turns on the selected character line (in this instance, the line 41a) for a period of 160 microseconds. The signal of the pulse generator 27, which has an 8-microsecond period and a 2-microsecond pulse width, together with the signal on the line 43a, turns on the AND gate 25, thereby switching the transistor 23a into conduction for 2 microseconds at time T 1 . The conduction of the transistor 23a drives the segment electrode 19a to ground, which results in a voltage of 250 volts impressed across all the cells connected to the segment electrode 19a. The charging path for every cell connected to the segment electrode 19a includes the resistor 40a, the segments 21, and the transistor 23a. This results in a gaseous discharge and an accompanying wall charge at time T 1 + , occurring in all the cells connected to the segment electrode 19a. It should be mentioned that the words "gaseous discharge" and "ignition" are used interchangeably. At time T 2 , the pulse is terminated, thus switching the transistor 23a off. At time T 3 , the delayed pulse at the delay line 37 and the signal on the line 41a form the two inputs necessary to turn on the AND gate 36 for 2 microseconds, which renders the transistor 33a conducting. Therefore, all segments 21 connected to the character electrode 20a will experience a gaseous discharge and an associated wall charge at time T 3 + . The selected segment 21aa has a negative wall voltage V w from the previous discharge at time T 1 +which is additive with the negative applied voltage V a impressed across the cell 21aa at time T 3 , thereby permitting the gaseous discharge at time T 3 + . The polarity of voltage V w at time T 5 will be additive to the applied voltage, driving the segment 21aa into ignition. The unselected cells connected to the segment electrode 19a will not fire at time T 5 + , since the cells still retain a wall charge from the gaseous discharge which occurred at time T 1 +and will therefore be of a polarity which opposes the applied voltage at time T 5 . For the same reasons as above, only the selected cell 21aa will ignite at time T 7 + , since the unselected cells connected to the character electrode still retain a voltage V w from the gaseous discharge at time T 3 . Therefore the voltage V w associated with those cells is opposed to the voltage V a at time T 7 , and the cell will not ignite. Thus only the cell 21aa will continue to experience gaseous discharges for the remainder of the 160-microsecond period. The remaining character segments are all displayed in the same manner. While the actual energization of the characters takes place sequentially, the characters appear to a viewer to be displayed continuously, due to retention of vision.
Referring to the waveform for the cell 21ac during the time that the line 41c is energized, it is seen that the cell did not ignite, even though it was a selected cell. This occurred because the cell 21ac experienced ignition at time T 1 +during the time that the line 41a was energized. Since the cell 21ac was never selected after that, it still retained the original wall charge, which is opposite in polarity to the applied voltage at time T 1 during the time that the line 41c is energized. Thus, the voltage is insufficient to ignite the cell 21ac. However, on the succeeding pulse at time T 3 , at which time the voltage V a is reversed, the cell will ignite because the voltages V a and V w are now additive. These occurrences, both the spurious ignition of unselected cells, and the nonignition of selected cells, are not objectionable, because they are not noticeable when compared to the total illumination provided by the selected cell.
The circuit of FIG. 7 differs from that of FIG. 5 in that the resistors 30 and 40 have all been replaced by diodes 47 and 48, respectively, and in the addition of transistors 50 and 51. Since the circuits are substantially the same, identical elements of FIGS. 5 and 7 have been given the same reference numeral designations. The segment electrodes are connected to the voltage source 31 through the transistor 50, and the character electrodes are connected to the voltage source 31 through the transistor 51. The mode of operation of this embodiment is similar to that of the circuit illustrated in FIG. 5. Assuming that the line 43a is energized, a pulse from the pulse generator 27 will turn on the transistor 51 and will also turn on the segment driver transistor 23a through the AND gate 25a. The conducting driver transistor 23a drives the segment electrode 19a to ground, thereby impressing a voltage across all the cells connected to the segment electrode 19a. The charging path for the cells connected to the segment electrode 19a extends from the voltage source 31, through the conducting transistor 51, the diodes 48, the segments 21, and the segment driver transistor 23a down to ground, thus resulting in a gaseous discharge in all of the segments 21 connected to the segment electrode 19a.
Similarly, the delayed pulse at the delay line 37 will turn on the transistor 50 and will also turn on the segment driver transistor 33 through an AND gate 60a. The charging path for the cells connected to the character electrode 20a extends from a potential source 49, through the conducting transistor 50, the diodes 47, the segments 21, and the character driver transistor 33 down to ground, thus resulting in the occurrence of a gaseous discharge in all of the segments 21 connected to the character electrode 20a. The diodes 47 and 48 prevent the formation of any sneak paths. The transistors 50 and 51 provide a fast switching time, thus permitting a higher discharge current with no resultant damage to the driver transistors. Power loss is reduced in the embodiment of FIG. 7 because of the elimination of the resistors employed in the embodiment of FIG. 5.