US3290554A - Electroluminescent display screen and circuit therefor - Google Patents

Description

E- A. SACK Dec. 6, 1966 ELECTROLUMINESCENT DISPLAY SCREEN AND CIRCUIT THEREFOR Filed Dec. 17, 1962 Voltage Across C Fig.3 RB

INVENTOR.

EDGAR A. SACK ATTORNEY United States Patent 3,290,554 ELECTRULUMINESQENT DISPLAY SCREEN AND CIRCUIT THEREFOR Edgar A. Sack, Pittsburgh, Pa, assignor, by mesne assignments, to the United States of America as represented by the Secretary of the Navy Filed Dec. 17, 1%2, Ser. No. 245,330 1 Claim. (Cl. 315169) This invention relates to display devices and, more particularly, to sol-id state display screens formed of an array of electroluminescent ferroelectric elements.

In the operation of such display screens, a contro charge or signal is applied to each element, which functions as a non-linear capacitor, and the charge is later removed by another signal or charge. The elements are provided with an electroluminescent material and the period during which the element is charged by the control signal sets the brightness of the element.

Ferroelectric capacitor elements have been used as the storage means for controlling the light from the electroluminescent material since a screen formed of these elements provides a display having high brightness and a minimum of flicker. The electroluminescent material or element may be applied to or formed on the ferroelec-tric element in various ways and emits light in accordance with an alternating or time varying potential appearing across it, while the ferroelectric capacitor causes this potential to be altered in accordance With the applied video control potential.

This invention concerns an electroluminescent ferroelectric display screen formed of barium titanate crystals or square loop barium titanate ceramic elements in which the control charge is supplied to the elements in a manner to provide the elements with inherent controlled persistence characteristics in order that the electroluminescent material or element may be operated to on condition by a control signal and remain on for a pre-selected period and return to the off state without the necessity of using an oil signal.

One object of the invention resides in the provision of a display screen formed of electroluminescent ferroelectric elements having circuitry for controlling the supply of control charge to the elements to provide excitation of the electroluminescent material for a selected time period.

Another object of the invention resides in the provision of a display screen formed of electroluminescent ferroelectric elements having circuitry for controlling the supply of the control charge to the elements and also for excitation of the electroluminescent material for predetermined time periods.

Other objects, advantages and features of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings, wherein:

FIG. 1 shows diagrammatically an outline of a display screen with a representative group of barium titanate ferroelectric elements incorporated therein and greatly enlarged;

FIG. 2 shows the circuitry for one of the cells of FIG. 1;

FIG. 3 is a graph showing hysteresis loops for the barium titanate elements of FIG. 1;

FIG. 4 shows a wave form when the elements are of the voltage responsive type;

FIG. 5 shows a wave form when the elements are of the current responsive type;

FIG. 6 is a schematic showing the circuitry of FIG. 2 arranged for supplying the charge to the representative group elements of FIG. 1.

Referring to FIG. 1, the display screen 10 shows a small group of elements 11 which are greatly enlarged in the figure but are actually of miniature size and a practical display screen may contain upwardly of 250 elements per square inch. Regardless of the actual number of elements used to form a screen, the functions of the elements 11 are those of emitting light in accordance with the A.-C. potential appearing across an element and of causing this potential to be altered in accordance with the applied video control potential.

In FIG. 2, the circuitry for a single element 11 of the display screen is shown. In this figure, an element 11 is separated into two of its component parts which are indicated at 12 and 13. The component 12 represents the barium titanate ferroelectric material while the component 13 represents the electroluminescent material. The circuit for the signal charge is from the voltage source E by line 14, to line 15 having resistor R therein, components 12 and 13 and return line 16 to source E. Resistance R is in parallel with the electroluminescent material 13 and represents the inherent screen resistance which determines the leakage of voltage from the element 13.

Refer to FIG. 3, wherein the vertical axis represents electrical displacement and the horizontal axis represents voltage applied across the ferroelectric component 12. Assume that the ferroelectric material is barium titanate and point A is the on condition and point B is the off condition, that is, point A represents the charged condition and point B the discharged condition. By selection of the values of pulse voltage E and resistor R, a desired net charge may be supplied to component 12 during a cycle to set the component to the point A or on condition in order to charge this component for exciting the component 13 during a discharge period.

If component 13 is voltage responsive and the charge is a voltage sufficient to excite electroluminescence, the wave form across C will be as shown in FIG. 4. The charge period 214 is made a small fraction of the period of electroluminescence t t and the values of E and R will be adjusted so that substantially all of the charge leaks off component 13 by R during the period 1 -h.

If component 13 is current responsive, the wave form is then as shown in FIG. 5. Assume N pulses of charge to be supplied to set the material 12 to on or point A of FIG. 3, then, after the material is filled, the first pulse thereafter finds the component 12 incapable of accepting the charge and component 13 is excited during a frame period of succeeding pulses. The net result of this operation is that with component 12 starting at point A of FIG. 3, 13 will emit N light pulses [over a time N (t t and then goes out and stays out. Only by resetting 12 to point A, or by reversing the polarity of B, will the element again emit light.

In FIG. 6, the circuitry of FIG. 2 is shown applied schematically to the group of elements 11 of the screen It and accordingly a circuit is shown for each of the 9 elements of FIG. 1. In FIG. 6', the circuits for each horizontal row are supplied in parallel from voltage source E which, because of the rather large resistances R, will overcome problems in scanning which may be involved if the circuits were connected in series.

The following paragraph is for the purpose of clarifying the operation of the screen '10 or elements 11.

Assume the capacitance of component 12 to he 10 micromicrofarads and that volts must appear across 13 in order to provide the desired brightness. Then the charge B would be 10- farads 100 volts resulting in 10- coulom'bs or 1 millimicrocoulomb of current. As sume further that a repetition rate of 1000 pulses per second is necessary for adequate light, and if a frame time is second, .a change of 1000 p.p.s. 30 sec.

X 1 millimicroeoulomb 10 volts 10- amps or have a resistance value of ohms.

A display screen having the characteristic of controlled inherent persistence would have particular application with conventional television or C.R.T. equipment where the frame time of the screen scanning is of fixed duration and consequently would eliminate the necessity of equipment for turning the elements to off condition. Various modes of scanning the above described screen may be used. It may be placed in a CRT. envelope and the electron beam, for example, used as the current source or current diode distribution systems of the prior art may be used requiring only one half this number of diodes necessary since only the write or on signals would need to be distributed.

Obviously many modifications and variations of the present invention are possible in the light of the above teachings. It it therefore to be understood that within the scope of the appended claim the invention may be practiced otherwise than as specifically described.

I claim:

In a display screen formed of a plurality of inde-pend' ently controlled display elements, the improvement in which,

(a) said display elements are formed of barium titanate material having a surface of electroluminescent material,

(b) an electrical circuit including a voltage source and a first resistor connected in series with each of said display elements to apply a charge to the barium titanate material,

(c) a second discharging resistor in said circuit and connected in parallel with the electroluminescent material,

(d) said volt-age source and first resistor being operative to supply an electrical charge of predetermined value to the barium titanate material and (e) said discharging resistor bypassing the barium titanate material and having a value selected to control the rate of discharge of the electrical energy from the barium titanate material and provide a predetermined period of excitation of the electroluminescent material.

References Cited by the Examiner UNITED STATES PATENTS 2,860,322 11/1958 Stadler 340-1732 2,917,667 12/1959 Sack 315169 3,011,157 11/1961 Anderson 340173.2 3,042,823 7/ 1962 Williard 3136 3,154,720 10/1964 Cooperman 31569 JOHN W. HUCKERT, Primary Examiner.

A. I. JAMES, Assistant Examiner.

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