US3400252A - Electrical heating device - Google Patents

Description

P 1968 SHIGERU HAYAKAWA ETAL 3,400,252

ELECTRICAL HEATING DEVICE Filed Oct. 20, 1965 4 Sheets-Sheet 1 FlG.l

APPLIED VOUAGE CURRENT FLOW 3mm INVENTORS I SHIGERU HAYAKAWA TAKASHI l 6U CHI YUKI O KASAHA RA WWWVM P 1968 SHIGERU HAYAKAWA ETAL 3,400,252

ELECTRICAL HEATING DEVICE 4 Sheets-Sheet 2 Filed Oct. 20, 1965 FIG.3

304m .hzmmmzu I? T EMPERATURE 3mm INVENTORS SHIGERU HAYAKAWA TAKAS'II YUKIO IGUCHI KASAHMA A I M M P 1968 SHIGERU HAVYAKAWA ETAL 3,400,252

ELECTRICAL HEATING DEVICE Filed Oct. 20, 1965 4 SheetsSheet 5 F l G. 5

3mm INVENTQRS SH] GERU HAYAKNIA TAKASHI l6 UCHI YUKIO KASAHARA United States Patent Oifice 3,400,252 Patented Sept. 3, 1968 3,400,252 ELECTRICAL HEATING DEVICE Sliigeru Hayakawa, Hirakata-shi, Osaka-fu, Takashi Iguchi, Kyoto-shi, Kyoto-fu, and Yukio Kasahara, Kadoma-shi, Osaka-fa, Japan, assignors t Matsushita Electric Industrial (30., Ltd., Osaka, Japan Filed Oct. 20, 1965, Ser. No. 498,548 1 Claim. (Cl. 219-504) ABSTRACT OF THE DISCLOSURE A temperature responsive resistance device which is responsive to change the flow of current therethrough at the ends of the range of temperatures, so that it can be used as a self controlled heating device, as part of a temperature control system, or as a temperaturecurrent switch. The device has a thermistor which has a positive temperature coeflicient of electrical resistance connected in series of an external impedance, the series connected elements being adapted to be coupled to a source of current and having a combined temperatureresistance characteristic which causes the current therethrough to drop abruptly when the temperature of the thermistor rises above the upper limit of the temperature range and to increase abruptly when the temperature of the thermistor drops below the lower limit of the temperature range.

This invention relates to electric heating devices and the like, such as a Warming device, and particularly to an improved self-controllable device including a heating element which is temperature sensitive.

Prior art heating elements by themselves are difficult to control as to the ambient temperature thereof and it is necessary to have additional equipment for such control, such as a switch having a contact. However, such a contact, which turns electric current on or off depending upon the ambient temperature, is apt to become oxidized, contaminated and welded together over a long period of operating time and finally ceases working.

Recently, a thermistor with a positive temperature ooeflicient of electrical resistance has been discovered. This thermistor is a ceramic body consisting essentially of barium titanate. A heating element made from this ceramic has a characteristic such that an electric current flowing through the element decreases with increasing ambient temperature and vice versa. Therefore, said heating element is responsive to overheating. However, theam bient temperature controlled by the heating device varies with the heat capacity of the object or material to be heated when the heating device comprises only heating elements of the aforesaid ceramics. Furthermore, the range of controllable temperature also varies with the heat capacity of the heated object or material.

The object of the present invention is to provide a heating device which eliminates these drawbacks and which controls the ambient temperature of the object or material being heated within a given range of appropriate temperatures while using no additional switch.

It is another object of the present invention to provide a temperature responsive control system which no longer requires an additional circuit opening and closing device.

It is a further object of the present invention to provide a novel temperature current switch responsive to a temperature change to a temperature outside a range of temperatures.

For a better understanding of our invention reference may be had to the following description taken in connection with the accompanying drawings, in which:

FIG. 1 is a circuit diagram of the temperature sensitive control system of the present invention;

FIG. 2 is a graph showing the characteristic current vs. voltage curves of the heating element operating at several ambient temperatures and the voltage-current load line of the external impedance consisting of reactance and resistance;

FIG. 3 is a graph showing variation in the current with the ambient temperature for the heating element;

FIG. 4 is a circuit diagram of a temperature responsive control system incorporating the temperature sensitive control of FIG. 1.

FIG. 5 is a cross sectional view of a milk warming device incorporating the temperature sensitive control system of the present invention;

FIG. 6 is a graph showing the temperature dependence of the electrical resistance of the thermistor; and

FIG. 7 is a graph showing the relation of current and temperature for the device of FIG. 5.

Referring to FIG. 1, terminals adapted to be coupled to an alternating current source are connected to a heating element 1, which is a thermistor with a positive temperature coefiicient of electrical resistance, and to an external impedance such as a reactance 2 and a resistance 3, the thermistor 1, reactance 2 and resistance 3 being in series relation to each other. The controlled temperature and its range are determined by the voltage-current characteristics of the thermistor and the load line of the external impedance comprised of the reactance and the resistance.

Referring to FIG. 2, reference characters 4, 5, 6, and 7 designate the characteristic curves of said thermistor with regard to various ambient temperatures there-of, respectively, and 8 represents the load line of said external impedance. A current flowing through the thermistor at a certain temperature increases with an increase in the applied voltage and at the same time the temperature of the thermistor itself goes up. Since the thermistor has a positive temperature coefficient of electrical resistance, the current flowing through the thermistor decreases with increasing applied voltage where the temperature of thermistor exceeds a specified temperature which depends upon the ambient temperature and the temperature-resistance characteristics of the thermistor. The characteristic voltage-current curves for the temperatures of the thermistor are curved upwardly and then downwardly as shown in FIG. 2. When the ambient temperature is higher, the specified temperature is achieved by a lower current flowing through the thermistor. Therefore, the characteristic curves for the temperature of thermistor shift down with an increase in the ambient temperature as shown in FIG. 2 where the curve 4 is for a lower limit of the temperature and the curve 7 is for an upper limit of the temperature. An upper limit of a controllable temperature range can be a temperature at which the characteristic curve of the thermistor is tangent to the load line of the external im pedance at the peak of the characteristic curve as shown in FIG. 2. A lower limit of a controllable temperature range can be a temperature at which the characteristic curve is tangent to the load line at a downward curvature of the characteristic curve. The load line intersects the characteristic curve for the temperature between the upper limit and lower limit temperature at three points. Of course, the external impedance has little temperature sensitivity and the load line thereof is nearly independent of the temperature. Operating points represented by intersecting points of the characteristic curves of said thermistor and the load line of the external impedance vary successively in the order 9, 10, 11, 12 with increasing temperature, and at the same time the current flow decreases as shown in the FIG. 2. Since the characteristic curve 7 for the upper limit temperature is tangent to the load line at the peak 12, the operating point immediately moves to a point 13 from a point 12 as soon as the temperature exceeds the upper limit temperature. Consequently, the current flow drops abruptly. On the other hand, as the temperature goes down from the upper limit of the temperature range, the operating points move successively in the order 13, 14, 15, 16 along the load line 8, and at the same time the current flow increases. Since the characteristic curve 4 is tangent to the load line at the point 16 as shown in FIG. 2, the operating point immediately moves from a point 16 to another operating point 9 with a further decrease in the temperature. Thus, the current flow increases abruptly.

This combined thermistor and impedance can be used to make a novel self controllable heating device wherein both the thermistor and external impedance are used as heating elements.

FIG. 3 shows the temperature-current characteristics of a combined thermistor and external impedance. In order to clarify the operation, the operating points on the curve in FIG. 3 correspond to the operating points in FIG. 2 and they are designated by the same numbers. As the temperature rises from the lower limit 19 of the temperature range, the flowing current decreases gradually in the order 9, 10, 11 and 12. When the temperature of the thermistor rises above the upper limit 18 of the temperature range, the current flow drops abruptly from point 12 to point 13. This abrupt decrease in the current flow lowers the temperature. As the temperature goes down the current flow gradually increases in the order 13, 14, 15 and 16 with the up-shift of characteristic voltage-current curves for the temperature of the thermistor. When the temperature of the thermistor drops below the lower limit 19 of the temperature range, the current flow increases abruptly. Thus, the heating device according to the present invention can control an ambient temperature.

Referring to FIG. 3, the temperature range 17 between the upper limit 18 and the lower limit 19 can be predetermined by a selection of charatceristics of said thermistor and the load line of the external impedance, such as a reactance and a resistance, because, the upper limit and the lower limit of the temperature range can be determined by a combination of the charatceristic voltagecurrent curve for the temperature of the thermistor and the load line of the external impedance as explained in the preceding description.

FIG. 4 shows a circuit diagram of a temperature responsive control system wherein reference characters 22, 23 and 24 represent a thermistor having a positive temperature coeflicient of electrical resistance and consisting essentially of barium titanate, an external impedance and a current actuated means, such as a solenoid coil and a solenoid switch, respectively. The control action of the control system is brought about by an abrupt change in current flow responsive to change of temperatures to temperatures outside a range of temperatures. The said range of temperature can be determined by a combination of the characteristic voltage-current curves for the temperature of the thermistor 22 and the load line of both the external impedance 23 and the current actuated means 24 in a similar way to that explained above in connection with FIGS. 2 and 3.

Furthermore, a temperature sensitive electric switch can be easily prepared by employing the aforesaid temperature responsive control system.

The temperature sensitive electric switch comprises a thermistor having a positive temperature coefficient of electrical resistance and consisting essentially of barium titanate and an external impedance connected in series with said thermistor. The said series connected thermistor and impedance are adapted to be coupled to a source of current. Referring to FIG. 3, the current flow indicated by point 12 is a critical point at the upper limit 18 of the temperature range and the current flow indicated by point 16 is another critical point according to the lower limit 19 of the temperature range. When the temperature slight- 1y exceeds the upper limit 18 of the temperature range, the current flow decreases immediately to a point indicated by 20 in accordance with the novel control action of the present invention, and then the temperature decreases to the lower limit 19 of the temperature range. When the temperature drops slightly down below the lower limit 19 of the temperature range, the current flow suddenly increases up to a point 21 and then the temperature increases. Repeated cycles of this type actuate the current actuated means which can in turn be used to control current flow to and from an apparatus. The control system thus acts as a stable and sensitive thermal switch. This thermal switch is operable even when there exists a great variation in the applied voltage, because a variation in the applied voltage results in a variation in the temperature of thermistor.

The following examples of specific new devices are given by way of illustration and should not be construed as limitative.

FIG. 5 is a cross sectional view of a milk warming device which maintains a substantially constant temperature by using the temperature sensitive control system of the present invention. It is preferred to have the temperature of the milk about 50 C. for a baby. In FIG. 5, reference character 25 is a nursing bottle, water 26 is used as a heating medium in container 31 having cover 32, and a coating 27 of electrically insulating material such as polytetrafluoroethylene is positioned over the heating ele ment 28, which element is mounted in the bottom of the container 31 and is prepared by the following procedure.

An equimolar mixture of barium carbonate and titanium oxide is Wet-milled with the addition of 2 percent by weight of silver oxide and 3 percent by weight of an excess of titanium oxide. The resultant mixture is pressed into a disk 15 cm. in diameter and 0.5 cm. in thickness and fired in flowing nitrogen gas at 1350 C. for 2 hours. A semiconductive sintered body black in color results, and is heated in air at 1100 C. for 1 hour to increase the positive temperature coefficient of electrical resistance. The sin- J tered ceramic disk 8 cm. in diameter and 0.3 cm. in thick- 0 ance 30 is connected in series to the termistor 28 by an electric lead. The external impedance comprises a coil 30 having a reactance of IS/Lh. and a resistor having a resistance of 35 ohm. FIG. 6 shows the temperature-current curve for the warming device illustrated in FIG. 5, and indicates that the center temperature and range of controlled temperature are 50 C. and :5 C., respectively in accordance with the present invention. These characteristics are, of course, independent of the amount of heated milk.

It will be understood by those skilled in the art that systems according to the invention can be modified in various respects without departing from the essence of the invention and within the essential features of the invention as set forth in the claim annexed hereto.

What is claimed is:

1. A temperature responsive resistance device responsive to change flow of current therethrough at the ends of a range of temperatures, said device comprising a thermistor having a positive temperature coefficient of electrical resistance and consisting essentially of barium titanate, the characteristic voltage-current curves for the temperatures of the thermistor at the ends of the range of temperatures being curved upwardly and then downwardly, and an external impedance connected in series with said thermistor, the voltage-current load line of said impedance intersecting the characteristic curve for the lower temperature of the range adjacent the origin end of the curve and being tangent to the other end of said last-mentioned characteristic curve, and said load line being tangent to the peak of the characteristic curve for the higher temperature of the range adjacent the origin end of the curve and intersecting the other end of said last-mentioned curve, said series connected thermistor and impedance being adapted to be coupled to a source of current, whereby the current fiow drops abruptly when the temperature of the thermistor rises above the upper limit of the temperature range and the current flow to increase abruptly when the 9/1962 Swanson 2l9505 6/1965 Andrich 219--504 RICHARD M. WOOD, Primary Examiner.

L. H. BENDER, Assistant Examiner.

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