Patent GB-1215857-A (GB)
Title: Improvements relating to light sensitive electric switch arrangements
IPC: H03K17/78
Filed: 1968-04-18
Abstract: 1,215,857. Photo-electric cells. GENERAL ELECTRIC CO. 18 Aoril, 1968 [26 April, 1967]. No. 18404/68. Heading H1D. [Also in Division G1] A photo-electric switching device for connecting and disconnecting a supply voltage to a load (7), Fig. 1 (not shown), such as a street lamp, comprises at least one gaseous discharge device (1) which is connected to an A.C. or D.C. supply (5) and which is rendered conductive at a predetermined level of illumination such that when conducting, its cathode, which is thermally coupled to a thermal switch (6), is heated up to cause actuation of the thermal switch. The gaseous discharge device which, in the embodiment described, is a photoemissive tube filled with a neon-argon mixture (in proportions of 99: 1), is arranged to have a large area cathode compared with its anode so that the device can carry a large current i.e. about 0À5 amperes at an operating voltage of 120 V. A small current limiting resistor 2 (80-100 Q) is connected in series with the tube 1. The pressure of the gas filling is 50-100 torr to allow for clean-up. The difference in the sizes of the electrodes provides for deionisation during each half cycle of an A.C. supply and so a rectifier to protect against reverse breakdown is not required although it may be provided for safety. The photocathode of tube is made of a metal such that the " dark effect " of the device is enhanced i.e. breakdown is delayed or absent even though the operating voltage is applied to the tube, in the absence of illumination. Suitable metals are nickel and molybdenum, and higher work function materials such as tungsten, rhenium, rhodium. Low work function materials and oxide films are avoided. Various constructions are described. In that of Fig. 2, a glass envelope 16 passing U.V. and visible in the range 3000-6000 Š contains a small anode 18 and comprises one side constituted by a large disc-shaped photocathode 17. The thermal switch is in direct contact with the cathode. In the arrangement of Fig. 3 the envelope 23 is of metal and has two tubular extensions one 28 closed at its end by a glass globule carrying the anode wire 27 and the other by a glass window 29 through which the light passes to activate the inside of the envelope which acts as the photocathode. Again the thermal switch contacts the envelope to provide thermal coupling. The arrangement of Fig. 4 has a hollow reentrant glass cylinder 31 with a cylindrical photo-cathode 34 carried around the inside of the re-entrant portion, which surrounds a thermally conductive probe 37 connected to the thermal switch. The anode 35 is cylindrical and surrounds and is spaced from the cathode. Operation.-The switching device is said to exhibit an averaging effect on the incidence of light. A spurious event e.g. a cosmic ray will not activate the tube but steadily increasing illumination, e.g. at dawn, causes a cumulative ionisation to take place on successive half cycles of the A.C. supply, finally causing heating of the cathode and operating of the switch 6. The switch operates in reverse with decreasing illumination intensities. The apparatus is adapted for use over a range of supply voltages 5 by the addition of a discharge lamp (40) and discharge tube (42), Fig. 5 (not shown), mounted in a light-tight box and connected in parallel with tube 1. The tube is always illuminated by lamp (42) which is continuously conductive such that tube (40) has a voltage limiting effect on the voltage across tube 1.