Patent GB-1445839-A (GB)
Title: Liquid crystal display devices
IPC: G02F1/1365
Filed: 1972-11-25
Abstract: 1445839 Matrix displays FERRANTI Ltd 20 Nov 1973 [25 Nov 1972] 54592/72 Heading G5C A liquid crystal display device, Fig. 1, comprises liquid crystal material 11 between parallel glass plates 12, 13. The device is viewed through plate 12, which has a conducting, transparent coating 16 with a terminal 17. Electrodes 18 are mounted on an insulating layer 26 carried by the plate 13 in a rectangular array, and are in contact with the liquid crystal 11. The surface of plate 13 carries a first set of conductors 19 running parallel to and between the columns of electrodes 18, Fig. 2, and each conductor 19 is connected by resistors 20 to respective electrodes 18 of the corresponding column. A second set of conductors 21 run parallel to and between the rows of electrodes 18, and each conductor 29 is connected by switching resistors 22 to respective electrodes 18 of the corresponding row. The switching resistors 22 are resistors of which the resistance falls abruptly from <SP>1</SP>/ 10  to <SP>1</SP>/ 100  of its initial value when the temperature is raised above a given value, e.g. 70‹ C. A third set of conductors 23 run parallel to the rows of electrodes 18, and each conductor 23 is connected to the mid-points 24 of all the switching resistors 22 connected to the corresponding conductor 21. The conductors 19, 21, 22, the resistors 20, and the switching resistors 22 are all deposited on the face of plate 13 and insulated from the liquid crystal 11 by the insulating layer 26. To produce a display, the tin oxide layer 16 is maintained at OV, while a. positive voltage is applied to selected electrodes 18. If the voltage is between 10 and 15 v., the neighbouring area of the liquid crystal will become opaque. To select electrodes 18, a negative voltage pulse is applied to a conductor 19. The current thus caused to flow in the corresponding switching resistors 22 is not sufficient to change their resistances, but when simultaneous positive pulses are applied to selected conductors 23, enough current flows from a selected conductor 23, to the junction at 24, through part of switching resistor 22, electrode 18, resistor 20 and conductor 19 to heat part of switching resistor 22. This causes a fall in resistance of the heated' part of switching resistor 22, with consequent. increase in current through the whole resistor until it is all heated and its resistance falls to the minimum. On removal of the pulses, the reduced resistance enables sufficient current to flow through the switching resistance to keep its resistance low, and since it forms a potentiometer with resistor 20, the potential of electrode 18 is changed, thus causing an area of the liquid crystal to opacify. This procedure is repeated for each column in turn. The display may be wholly erased by raising the potential of the column conductors 19, or reducing that of the row conductors 21. For selective erasure, a positive pulse is applied to a selected column conductor 19. The duration of the pulse is 6 Ásecs., and the voltage required to maintain sufficient heating current is removed thereby; however, no change in the resistance of resistor 22 takes place in such a short time. A 6 Ásec. pulse which is applied immediately after the first pulse to a selected row conductor 21, extends the period during which maintaining current is removed from a selected resistor 22 to 12 Ásecs., so that the resistance of this resistor is raised to its initial value, and the potential of the associated electrode 18 returns to its initial value. It is stated that the device is operable by an A.C. signal.