US3432728A - Photoelectric control of bimetal-type switch with plural heaters - Google Patents

Description

March 11, 1969 YASUTAKA KAWASE 3,432,728

PHOTOELECTRIG CONTROL OF BIMETAL-TYPE SWITCH WITH PLURAL HEATERS Filed Feb. 27, 1967 FIG.

FIG.

FIG.

no 5 (Km DAY RESISTANCE OF PHOTOCONDUCTIVE ELEMENT I NVENTOR YASUTAKA KAWASE y pwla gi M United States Patent 2 Claims ABSTRACT OF THE DISCLOSURE A light sensitive control apparatus having a thermal switch consisting of a photocell, a pair of electric heating resistors which are thermally connected to a pair of bimetal strips, one of which being main and the other temperature compensating. The apparatus can be operated to turn on and otf a lamp located outdoors in response to ambient light.

The present invention relates to a light sensitive control means, and more particularly, to an improved photosensitive electric switch which is in response to ambient light for automatically controlling a circuit connected to a lamp positioned outdoors.

Prior to the description of the present invention, the conventional light sensitive switch of the prior art is summarized hereinbelow. As shown in FIG. 1 of the accompanying drawing, there are shown a photoconductive or photosensitive cell or photocell 1 for detecting ambient light, a heating resistor or heater 2 which is connected with a source of commercial, say 100 v. A.C. supply across the photocell 1, a main bimetal strip 4 which is thermally connected with the heater 2, and a temperature compensating bimetal strip 5 positioned in proximity thereto. A thermal switch 6 consists of both bimetal strips and the heater 2.

In the light sensitive switch of the above construction, the photocell 1 exhibits a very low value of resistance during the daylight time, hence the heater 2 attains a maximum heat value so that the main bimetal strip 4 curves outwardly or leftwardly in FIG. 1 as shown by a broken line. In this state, the thermal switch 6 maintains an open circuit wherein there is no light. However, the photocell 1 shows a very high value of resistance during the night time where an ambient intensity of illumination is low, hence the heat value of heater 2 becomes minimum. As a result, the main bimetal strip 4 stands erect so that the thermal switch 6 maintains a closed circuit wherein there is light.

In the conventional photosensitive switch described above, where a street lighting post is employed as a load to be controlled, it happens that the street lamp is on while it is still light in the evening twilight, and the street lamp is olf while it is already in the daylight after sunrise. As a result, it follows that a futile consumption of electric energy together with a shortened life of an electric bulb would be inevitable. To overcome the above disadvantage has been one of the keen desires in industry.

The cause of the disadvantage is described as follows: During the dark night, a high voltage equal to a peak value, say, 130-150 v. of the commercial 100 v. A.C. source applies to the photocell continuously. To meet the safety requirement, the photocell is required to have a 3,432,728 Patented Mar. 11, 1969 characteristic of withstanding a voltage of 250-300 v. In view of this requirement, the photocell having such a particular characteristic has been in general use.

In reference to the photoconductive or photosensitive element, it is known that sensitivity varies inversely to the square of peak-voltage rating between peak-voltage rating and sensitivity. The sensitivity of the photosensitive element is shown by the relation between illumination intensity and resistance, and a high sensitivity refers to a low resistance.

For example, to increase two times as high as the peakvoltage rating of the photoconductive element is to increases two times as distant as the space between comblike electrodes. Hence, the number of comb tooth per unit area of light received decreases one half, so the value of series resistance of the photoconductive element increases two times while the value of parallel resistance decreases one half. In consequence, its sensitivity will be reduced to one-fourth.

In view of the above, in general, the sensitivity of the photoconductive element having the characteristic of resistance to a high voltage, say, 250-300 v. is considerably low, and the street lamp turns on in response to a higher ambient intensity of illumination than desired, and otf to a relatively lower ambient light.

In accordance with a preferred embodiment of the invention, the voltage applied to the photoconductive element at night is reduced to about one half of that of the conventional one, a photoconductive element of a relatively low peak-voltage rating, in other words, of a relatively high sensitivity can be used so that the above disadvantage can be obviated.

Accordingly, it is an object of the invention to provide an improved light sensitive control means in which a photoconductive element of a lower peak-voltage rating resistance to voltage, in other words, of a higher sensitivity than before can be used.

It is another object of the invention to provide an improved light sensitive control or photosensitive switch means for automatically controlling a circuit connected to a lamp in response to ambient light wherein a less futile consumption of electric energy and a less shortened life of a bulb than before can be attained.

Further objects and advantages of the present invention will be apparent from the following description, reference being had to the accompanying drawing, wherein a preferred embodiment of the present invention is shown.

In the drawing:

FIGURE 1 is a diagrammatic view of an electric circuit of a light sensitive control means of the prior art.

FIG. 2 is a diagrammatic view of an electric circuit of a light sensitive control means in accordance with the principle of the present invention.

FIG. 3 is a diagrammatic side view showing the relation between a thermal switch and a heating resistor or heater of the control means of this invention.

FIG. 4 is a characteristic diagram showing the current consump;tion of two heaters in response to the ambient illumination versus the resistance value of photoconductive element.

In the drawing, it is to be noted that the numerals indicting the parts shown in FIG. 1 are different from those of parts in FIGS 23, though the parts are the same.

A photosensitive switch embodying the principle of the invention will be described hereinbelow in connection with a preferred embodiment thereof shown in FIGS. 2-3.

In FIG. 2, there is shown a circuit comprising a photoconductive element 7 consisting of CdS (Cadmium sulfide) or CdSe (Cadmium selenide) for detecting an ambient light connected in series with a first heating resistor or heater 8 with a terminal 9 and a terminal 10, a second heating resistor or heater 11 connected in parallel with the photoconductive element 7, a thermal switch 12 connected with terminals and 13, a load or lamp to be controlled connected with terminals 9 and 13, and a commercial 100 v. A.C. source connected with terminals 9 and 10.

In FIG. 3, the thermal switch 12 comprises a main bimetal strip 16 which bends leftwardly as shown in a broken line or opens the circuit in response to the increase of temperature, and a temperature compensating bimetal strip 17 positioned in proximity thereto. The temperature compensating bimetal strip 17 bends leftwardly or inwardly in response to the increase of temperature, too. The main bimetal strip 16 is thermally connected with the first heater 8 while the temperature compensating bimetal strip 17 is also thermally connected with the second heater 11. The term, thermally connected means that two parts are connected by heat, in other words, when a predetermined heat value is reached, two parts are connected, and when it is lowered, they part from each other. The load connected with the terminals 9' and 13 refers to a street lamp located outdoors, other lamp or equip ment. The source 15 connected with the terminals 9 and 10 is the commercial 100 V. AC.

The first heater 8 and the second heater 1.1 are preferably of the same configuration, and it is to be noted that the resistance value of both heaters are specified to be of 15 kiloohms, respectively. In addition, the main bimetal strip and the temperature compensating bimetal strip are preferred to be of substantially the same material and configuration. Both bimetal strips can be formed integral with two legs in the form of H. However, the invention is not limited as above.

In many kinds of the conventional photosensitive switch of the similar type, the heating resistor or heater has been thermally connected with the main bimetal strip by winding the former around the latter or bonding the former to the latter. Besides, in the conventional device of the same type including the main bimetal strip and the temperature compensating bimetal strip, it is found that the temperature compensation effect obtained by the two bimetal strips is not satisfactory due to a mechanical resistant action of the main bimetal strip by the heater when subjected to bending. In consequence, it has often led to an erroneous action.

However, in the light sensitive control means of this invention, the heating resistor or heater is thermally connected with both temperature compensating bimetal strip and main bimetal strip, hence the above disadvantage has been overcome simultaneously.

In the photosensitive switch of this invention during the daylight time, the photoconductive element 7 reduces its resistance value to the order of about 100 ohms to make the heat value of the first heater 8 to be maximum while that of the second heater 11 to minimum, almost zero. Accordingly, during the daylight time the main bimetal strip 16 bends outwardly or leftwardly in a broken line as shown in FIG. 3 to maintain the thermal switch 12 to be open.

On the contrary, however, during the dark night period, the photoconductive element 7 shows a higher resistance value than 1 megaohm to apply about 50 v. on the heaters8 and 11, respectively, hence the heat values of both heaters 8 and 11 are substantially equal so that both main bimetal strip 16 and temperature compensating bimetal strip 17 bend in the same direction in the same amount to maintain the thermal switch 12 to be closed.

In FIG. 4 the characteristic diagram showing the current consumption of two heaters is shown in which A refers to an operating diagram of heater 8 while B to that of heater 11.

It is understood that a known chattering prevention means made of magnet or spring is preferably provided between the main bimetal strip 16 and the opposite bimetal strip 17, though not shown in the drawing.

In the light sensitive control means of this invention, the voltage applied to the photoconductive element 7 in the commercial V. AC. source never exceeds about 70 v. even at its peak day and night. Therefore, the photoconductive element 7 having the characteristic of resistance to voltage as small as half of that of the prior art device, say, in the order of v. can be employed, hence the sensitivity thereof can be developed to four times as high as that of prior art in comparison with the conventional device.

In the conventional device of FIG. 1, in order to effect an assured design for operation of thermal switch 6 which is in response to the variation of illumination of photocell 1, it is known that the central point of operation of thermal switch should be determined as a point where the resistance value of photocell 1 is equal to that of heater 2. Accordingly, when the resistance value of photocell 1 is 15 kiloohms, in other words, when the heater 2 evolves the heat value of 166 mw. (suppose the source voltage 100 v.), it is specified as the central point of operation of thermal switch 6.

However, in'the device of this invention, when the heat difference of 166 mw. takes place between two heaters 8 and 11, the resistance value of photocell 7 is 22.5 kiloohms as clearly shown in FIG. 4. Therefore, in the device of this invention, the central point of operation of thermal switch can be shifted to the side of low intensity of illumination compared with the conventional one, in other words, 1.5 times (22.5 kiloohms divided by 15 kiloohms) as high as before. Thus, the sensitivity of the device of this invention can be made 1.5 times as high as ever. As described above, the device of this invention can develop its sensitivity about four times as high as prior art, and finally improve its sensitivity to such a considerable degree as about six times as high as ever when put together.

To put it concretely, assuming that the conventional device can operate to turn on lamp at the illumination of 60 luxes, and off at luxes, it has been found that the device of the present invention can operate to turn on lamp at 10 luxes and off at 30 luxes.

Furthermore, as the heater 11 is thermally connected with the main bimetal strip 16 as well as the temperature compensating bimetal strip 17, the mechanical resistance of heater against the bending action of bimetal acts on the main bimetal strip 16 as well as the temperature compensating bimetal strip 17 in a uniform manner. As a result, a satisfactory effect of temperature compensation can 'be achieved.

As fully described in the foregoing, the light sensitive control means of this invention can improve its sensitivity considerably by a simple reconstruction comprising adding a single heating resistor or heater to the conventional device of the same type. In addition, an improved advantage is obtained from the device of this invention in order to utilize the temperature compensation effect efficiently even when the heater winds around the bimetal strip or attaches to it.

While the embodiment of the invention as herein disclosed constitutes a preferred form, it is to be understood that other forms might be adopted.

What is claimed is as follows:

1. In a light sensitive control comprising a photoconductive element adapted to detect an ambient illumination, a first heater connected in series with said element, a thermal switch comprising a main bimetal strip tending to bend outwardly in response to the increase of its temperat-ure, said main bimetal strip having a first contact, a temperature compensating bimetal strip tending to bend inwardly in response to the increase of its temperature, said temperature compensating bimetal strip having a second contact opposing said first contact, and said heater being thermally connected with said two bimetal strips, the improvement comprising a second heater connected in parallel with said element, and said second heater being also thermally connected with said temperature compensating bimetal strip whereby said contacts 10 open and close in response to light and the absence of light on said photoconductive element.

2. In a light sensitive control means as claimed in claim 1 wherein said photoconductive element is provided with a pair of comb-like electrodes positioned face-to-face.

References Cited UNITED STATES PATENTS 2,654,865 10/1953 Klug 324-106 2,796,553 6/1957 Ronning 250206 X 2,967,981 1/1961 Wise 317-124 JAMES W. LAWRENCE, Primary Examiner. E. R. LA ROCHE, Assistant Examiner.

US. Cl. X.R. 250206; 307-117

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