US3385956A - Protective device for electrically warming cloth - Google Patents

Description

May 28, 1968 EIICHI OH'ARA ETAL PROTECTIVE DEVICE FOR ELECTRICALLY WARMING CLOTH Filed July 15, 1965 FIG.

FIG.

mw imommz zk m0 .CEFDO TEMPERATURE OF ELECTRICALLY WARMING CLOTH United States Patent 3,385,956 PROTECTIVE DEVICE FOR ELECTRICALLY WARMING CLOTH Eiichi Ohara and Shinichi Nakarnura, Kamakura, and Tomiie Nakamura, Yoshihiro Nakano, and Yasushi Takanashi, Nitta-gun, Japan, assignors to Mitsubishi Denki Kabushilri-Kaisha, Tokyo, Japan, a corporation of Japan Filed July 15, 1965, Ser. No. 472,228 Claims priority, application Japan, July 30, 1964, 39/ 60,480 4 Claims. (Cl. 219-481) ABSTRACT OF THE DISCLOSURE A temperature control device for heating pads, blankets and the like having a differential transformer including a pair of primary windings one of which is connected to a length of electric resistance wire for controllably warming a warming blanket or pad and the other of which is connected to an operating relay winding through a rectifier. In normal heating operation the secondary winding of the transformer produces an output but overheating of the resistance wire produces no output. When energized the relay operating Winding closes the associated contacts connected in series tothe length of electric resistance wire. A transistor connected to the secondary transformer winding through a rectifier is non-conducting when normal heating is effected but in case of overheating it becomes conductive to short the operating relay winding thereby to open the contacts to deenergize the resistance wire.

This invention relates to a protective device for use with an electrically warming cloth to prevent excessive overheat thereof which may cause a fire and/or any damage to the user.

In the prior art type of protective devices :for use with electrically warming cloths such as blankets or beddings several thermostats have been usually connected in series circuit relationship to several sections of an electric resistance wire disposed within the cloth to electrically warm the latter. Such protective devices, however, can only decrease in dimension to a limited extent because each of the thermostats involved comprises a bimetal element, an operating member for opening and closing electric contacts and the like. This may cause the user to be given an impression of foreign matters. In addition the thermostats can not he arranged such that they can instantaneously sense any possible increase in temperature of every portion of the electric resistance wire.

Accordingly, an object of the invention is to provide a new and improved protective device for use with an electrically warming cloth capable of accurately sensing overheat of every portion of an electric resistance wire disposed in the associated electrically warming cloth whereupon a circuit with the resistance wire is immediately open thereby to terminate overheat of the wire.

Another object of the invention is to provide a new and improved protective device for use with an electrically energized warming cloth which is effective upon the local overheating of any portion of an electric resistance wire in the cloth and independently of the operation of thermostats involved.

Still another object of the invention is to provide a new and improved protective device for use with an electrically energized warming cloth substantially free from such malfunctions as premature interruption of an electrical warming circuit.

With the above cited objects in view, the invention resides in a protective device for use with an electrically energized warming cloth having a length of electric re- See sistance wire to warm the latter, comprising a differential transformer having an input connected to a circuit with the length of electric resistance wire to sense a change in resistance of the electric resistance wire. The differential transformer is normally placed in its unbalanced state and has an output decreased with an increase in resistance of the electric resistance wire due to an excessive rise of the temperature of the wire. A transistor normally in its non-conducting state is electrically connected to the out put of the differential transformer, and a relay is electrically coupled to the transistor and has contacts in circuit with the electric resistance wire. The arrangement is such that when an increase in resistance of the electric resistance wire due to overheat of the wire causes the output of the differential transformer to decrease it permits the transistor to conduct thereby to shortcircuit the relay. Then the relay is deenergized to open its closed contacts to interrupt the circuit with the electric resistance wire.

In a preferred embodiment of the invention, the protective device may comprise a differential transformer including a pair of primary windings disposed to ibe energized in opposite polarity relationship and a secondary winding. The differential transformer is normally in its unbalanced state. One of the primary winding being serially connected to a length of electric resistance wire embedded in the electrically warming cloth and having a relatively high temperature coefficient of resistance. The output voltage from the secondary winding of the differential transformer decreases with an increase in resistance of the electric resistance wire due to overheating of the wire. A rectifier is connected across the secondary winding of the differential transformer, a transistor including a base, an emitter and a collect-or electrode. The output of the rectifier is connected across the emitter and base electrodes of the transistor. A relay including an operating winding is connected across the emitter and collector electrodes of the transistor and has contacts disposed in circuit with the electric resistance wire and closed when the operating winding is energized. The transistor is arranged to be biased to be put in its non-conducting state when the electric resistance wire is at its normal operating temperature and to be brought into its conducting state when the output from the secondary winding of the differential transformer decreases to a predetermined value. The conduction of the transistor causes short cirouiting of the relay to deenergize the latter to open the closed contacts interrupting energization of the length of electric resistance wire.

In order to have the relay small-sized, a direct current relay may be advantageously used.

The invention will become more readily apparent from the following detailed description taken in conjunction with the accompanying drawings in which:

FIG. 1 is a schematic electric circuit of a protective device constructed in accordance with the invention; and

FIG. 2 is a graph illustrating the characteristics of the device shown in FIG. 1.

According to the teachings of the prior art, a length of electric resistance wire embedded in an electrically warming cloth could have a signalling wire wound around the same with a layer of any suitable heat sensitive material such as nylon interposed therebetween. If the electric resistance Wire is overheated, a current leaks through the heat sensitive layer and can be detected by means of the signalling wire.

According to the teachings of the invention, however, such an electric resistance wire may be composed, for example, of a foil of annealed copper, a thin nicke'l wire or any other suitable electrically conductive material having a relatively high positive temperature coefiicient of resistance. In operation, any change in resistance of the wire is utilized to detect overheating of the wire in a manner as will be hereinafter described in detail. This measure eliminates the variation and/ or deterioration of the operating characteristics of the device which could occur in the prior art devices using any organic material, for example, nylon as a material for the heat sensitive layer above described. Also the operating temperature ofthe device can be selected at will without any limitation as to materials used. For example, the operating temperature can be preset to range from 85 to 100 C. which ensures that not only a fear of fire but also any damage to the user is fully prevented.

Also, there has been previously proposed a protective device referred to comprising a differential transformer adapted to operate a relay to interrupt a power supply to the associated electrically warming cloth when the transformer is brought into its unbalanced state. Such a device is disclosed, for example, in U.S. Patent No. 2,592,525. In the proposed device just described it is not only difficult to predetermine the extent to Which the differential transformer can be unbalanced but also a range of operating temperatures increases dependent upon an operating range of the relay and the like. Therefore, such a protective device becomes unstable in operation.

On the contrary, the invention contemplates sensing overheating of the electric resistance wire by balancing the normally unbalanced differential transformers upon the occurrence of such overheat. In addition, the differential transformer has a transistor connected on the secondary side through a rectifier and preferably a full-wave rectifier for the purpose of control a relay to control a power supply to the associated cloth. This causes the device to be more sensitively operated with sharp characteristics in virtue of the fullwave rectification.

Referring now to FIG. 1, there is illustrated a protective device for an electrical warming cloth such as a blanket constructed in accordance with the teachings of the invention. An arrangement illustrated includes essentially a length of electric resistance wire embedded in the blanket to electrically warm the latter during energization, a differential transformer electrically connected on the primary side to the length of electric resistance wire, a transistor circuit connected to the differential transformer on the secondary side, and a direct current relay controlled by the transistor circuit to control power supply to the length of the electric resistance wire.

As shown in FIG. 1, any suitable source 1 of alternating current power such as a commercial power line is connected to a length of electric resistance wire 2 embedded in the associated cloth such as a pad or blanket (not shown). The electric resistance wire 2 is preferably composed of any suitable electrically conducting material,

such as annealed copper, nickel, or any suitable electrically conductive material having a relatively high temperature coefficient of resistance. A thermostat 3 is serially connected to the length of electric resistance wire 2 and is operatively associated with a heater element 4 for indirectly heating the thermostat 3 with a flow current through the element and the length of resistance wire 2. The blanket is normally maintained at a temperature predetermined by the thermostat 3. While one thermostat is illustrated in FIG. 1 it will be understood that more than one of the thermostats may be used if desired.

As well known, use of only a thermostat or thermostats can not always maintain the temperature of the blanket at a predetermined temperature. To ensure that the temperature of the blanket is maintained at a substantially constant value determined by the thermostat 3, a differential transformer generally designated by the reference numeral 5 is provided. The differential transformer 5 has a primary winding 6 including a pair of primary winding portions 6a and b adapted to excite the associated magnetic core in opposite polarities. One of the primary winding portions, for example, the winding portion 6a has one end connected to the length of resistance wire 2 and the other end 7 connected directly to one terminal of the source '1 and is also connected to the adjacent end of the other primary winding portion 6b. Thus the other end 7 constitutes an intermediate tap on the primary winding 6 of the differential transformer 5. The other end of the other winding portion 61) is connected to the other ter minal of the source 1 through a full wave rectifier bridge 8, a balancing resistor 9 and contacts as will be described hereinafter.

The differential transformer 5 includes a secondary winding 6c having a full-wave rectifier bridge 11 connected across it. The output of the rectifier bridge 11 is connected to a pnp transistor 12 having a base, an emitter and a collector electrode. Thus one of the output terminals, for example, the upper terminal as viewed in FIG. 1, of the rectifier bridge 11 is connected to the emitter electrode of the transistor 12 and the other output terminal of the bridge 11 is connected to the base electrode thereof. A filtering capacitor 13 is connected across the output of the rectifier bridge 11 and hence across the emitter and base electrodes of the transistor 12 and serves to smooth the output voltage from the rectifier. A control resistor 14 is connected between the base and collector electrodes of the transistor 12 to control the firing point of the transistor.

Connected across the emitter and collector electrodes of the transistor 12 is an operating winding 15a of a direct current relay generally designated at dotted line 15 and a current limiting resistor 16 for the relay in a series circuit relationship. A filtering capacitor 17 may be preferably connected across the emitter and collector electrodes of the transistor 12. The relay includes contacts 15b serially connected in a circuit with the electric re s-istance wire 2. In FIG. 1 the contacts 15b are shown as having a stationary contact connected to one terminal of the source 1 and through a pair of serially connected pushbutton switches 18 and 19 to the junction of one input terminal of the rectifier bridge 8 and the lower terminal of the other winding portion 6b of the differential transformer 5 and a movable contact connected to both the heater element 4 and the balancing resistor 9.

The output of the rectifier bridge 8 is connected across the transistor 12 and accordingly across the series arrangement of the operating winding 15a of the relay and the current limiting resistor 16.

In the protective device of the invention it is to be noted that the differential transformer 5 should be in its unbalanced state in the normal operation of the blanket or at the normal operating temperature of the resistance wire 2. In other words, primary winding portions 6a and 6c should be designated and constructed such that they have different ampere-turns within a range of normal operating temperatures of the resistance wires. However, upon excessively increasing the temperature of the resistance wire 2, the pair of primary winding portions 6a and 6b should be in its balanced state where the winding portions have substantially equal numbers of turns. To this end, both winding portions can be chosen to be different in both diameter of wire used and the number of turns. 1

Furthermore, the rectifier bridge 11 must be poled such that, within the range of normal operating temperatures of the resistance wire 2 it can apply a positive potential to the base electrode of the transistor 12 to reversely bias the transistor 13 and hence put the latter in its nonconducting state.

As an example, the one primary winding portion 6a of the differential transformer 5 is composed of an enamel copper wire having a diameter of 0.45 mm. and has eighteen turns while the other primary winding portion 6b is composed of an enamel copper wire having a diameter of 0.08 mm. and has turns numbering 1,000. Such a differential transformer normally provides an output voltage of from 3 to 4 volts across its secondary winding having the number of turns of 750 with an enamel copper wire whose diameter was 0.05 mm. When the warming circuit including the length of electric resistance wire 2, the indirect heater element 3 and the like has an operating resistance reaching a magnitude of approximately 110 ohms the differential transformer is brought into its balanced state providing null output voltage across the secondary winding.

The full-wave rectifier bridge 8 electrically coupled to the primary winding portion 6b of the differential transformer 5 provides at its output a rectified cur-rent of 30 rnilliamperes at its input voltage of 30 volts and cooperates with the smoothing capacitor 18 to sensitively operate the relay 16. The purpose of the rectifier bridge 8 is to supply an alternating current to the other primary winding portion 6b thereby to bring the differential transformer into its balanced state when necessary.

The transistor 12 used is of a low frequency amplifier type and has a rated collector current 1 of 50 rnilliamperes, a collector-to-base voltage V of 25 volts, and an emitter-to-base voltage V of 12 volts. Since the transistor is reversely biased as previously described within the range of normal operating temperatures of the resistance wire 2 it has applied thereto a base-to-emitter voltage of from 3 to 4 volts, a baseto-collector voltage of from 17 to 18 volts and a emitterto-collector voltage of from 13 to 14 volts with the collector current substantially null.

Upon the occurrence of an extraordinary or emergency state of the blanket, the differential transformer 5 is balanced to provide null output. This causes removal of the reverse voltage from the transistor 12 to permit the transistor 13 to be instantaneously brought into its conducting state where the base current on the order of 200 microamperes and the collector current on the order of 17 milliamperes flowing therethrough. The conduction of the transistor 12 causes shortcircuiting of the operatting winding 15a of the DC relay. Thus the relay 15 is deener-gized to open its contacts 15b thereby to interrupt the warming circuit.

Further the device includes a pair of starting or on and off pushbuttons 18 and 19 connected serially between the one terminal of the source 1 and the junction of the input terminal of the rectifier bridge 8 and the lower terminal of the other primary winding portion 6b of the differential transformer 5 with the junction of both pushbuttons connected to an intermediate tap on the balancing resistor 9.

If one desires to use an electrical warming blanket associated with the protective device thus far described then the pushbutton 18 can be operated to energize the rectifier bridge 8 from the source 1 through that portion of the balancing resistor 9 not parallelling the pushbutton 18. Then the rectifier bridge 8 applies a rectified voltage to the DC relay 15 to energize it. When energized, the relay 15 closes its contacts 15b to complete the warming circuit for the blanket.

With certain circuit parameters having the figures as previously specified, the DC relay 15 may have such a rating that an operating voltage is 15 volts and a reset voltage is 5 volts with a DC resistance of approximately 550 ohms. Under these circumstances, in order to ensure that the rectifier bridge 8 supplies to the DC relay 15 its operating voltage after the starting pushbutton 17 has been released that pushbutton is arranged to be capable of shortcircuiting one portion of the balancing resistor 9. With the circuit elements having the figures as previous specified, that portion of the resistor 9 having a resistance of about 2.3 kilohms may be shortcircuited whereby the relay 15 will be maintained at approximately 9 volts.

While the certain circuit parameters have been described in terms of specific figures it is to be noted that, if desired, they may have any desired figures different from those specified above.

After the relay contacts 15b have been closed in the manner as above described, a current from the source 1 flows through the heating element 4, the thermostat 3, the electric resistance wire 2 and one of the primary winding portions 6a of the differential transformer 5. When the resistance wire 2 is within the range of normal operating temperatures, the same has a relatively low resistance whereby a flow of current through the primary winding portion 6a is allowed to be relatively high. This causes the differential transformer 6 to be brought into its unbalanced state because a magnetomotive force caused by the one primary winding portion 6a is different in magnitude from that caused by the other primary winding portion 6b. Therefore, the differential transformer 6 applies an alternating current output voltage to the rectifier bridge 11 where the voltage is rectified. After having been smoothed, the rectified voltage is applied across the base and emitter electrodes of the transistor 12 with the voltage at the base electrode positive with respect to the emitter electrode. As previously described, this maintains the transistor in its non-conducting state. Under these circumstances, the rectifier bridge 8 continues to apply to the DC relay 15 a direct current sufficient to hold the relay in its operating state whereby the relay contacts 15b remain closed.

If under these circumstances, the resistance wire 2 increases excessively in temperature, its resistance will increase as shown at upper curve a in FIG. 2 wherein the abssissa represents a temperature of the electrical warming blanket, curves a, b and 0 represent the resistance in ohms of circuit for warming the blanket, an output voltage in volts from the differential transformer and a collector current flowing through the transistor 12 respectively, assuming that the circuit parameters have the values as previously described.

An increase in resistance of the warming circuit causes a decrease in current flowing through the circuit and hence through the primary winding portion 6a of the differential transformer 5 until the magnetomotive force caused by that current will balance that caused by current flowing through the primary winding portion 6b. Therefore, the output voltage from the differential transformer 5 will gradually decrease as shown at curve b in FIG. 2 until the transformer is balanced and provides substantially null output.

After the differential transformer 5 has reached its balanced state, the collector current through the transistor rapidly increases as shown at curve 0 in FIG. 2 whereupon the emitter-to-colle-ctor circuit of the transistor conducts and is effectively connected in parallel to the operating winding 15a of the DC relay 15. In other words, the now conducting transistor 12 shortcircuits the operating winding leading to deenergization of the relay 15.

The deenergization of the relay opens its contacts 15b to interrupt the warming circuit for the blanket to permit the latter to be cooled.

If the temperature of the blanket decreases to a predetermined point to decrease the resistance of the heating circuit to the corresponding predetermined magnitude then the differential transformer provides again the output voltage thereby to bring the transistor into its nonconducting state to permit the relay to be effective for connecting the warming circuit to the source 1 as will be readily understood from the foregoing description.

As previously described, the bias resistor 14 can be controlled in resistance to adjust the firing point of the transistor 12.

If power supply to the resistance wire is desired to be interrupted then the pushbutton 18 can be closed to shortcircuit the rectifier bridge 8 to prevent the latter from energizing the relay 15. Therefore the relay contacts 15b are open to interrupt the warming circuit for the blanket.

From the foregoing it will be appreciated that the present invention provides a protective device for an electrical warming cloth by which the differential transformer 5 senses a change in resistance of the electric resistance wire due to an increase in temperature thereof,

to render conductive the normally non-conducting transistor to shortcircuit the DC relay, leading to opening of the warming circuit for the cloth whereby excessive generation of heat in the cloth rapidly terminates with the result that the various faults are effectively prevented from occurring due to excessive heat of the resistance wire.

The invention has several advantages. For example, the use of the normally unbalanced differential transformer to detect a change in resistance of the electric resistance wire can sense excessive heat on any portion of the wire, without any delay, upon generation of such heat, ensuring that any fault due to such excessive heat is prevented from occurring. As the normally blocked transistor is used to control the operation of the relay any variation in source voltage and/or the operation of the thermostat or thermostats involved does not affect a flow of current through the relay winding thereby to completely prevent the premature deenergization of the operating relay. On the other hand, if the blanket has its extraordinarily high temperature the relay can be immediately deenergized to open its contact to disconnect the warming circuit from the source. Since the relay is arranged to be deenergized by having connected thereto a shortcircuit circuit involving the transistor brought into its conducting state, the differential transformer is required only to provide a low voltage to ensure controlling of the relay. The use of the DC relay decreases the effect of variation in frequency and prevents the relay contact from vibrating during resetting movement. This vibration can be further suppressed by using a full-wave rectifier and a capacitor. Also the use of the DC relay permits the entire device to be small-sized as compared with the use of an AC relay.

In addition the use of an electric resistance wire having a relatively high temperature coeflicient of resistance results in an electrical warming blanket less in power capacity thereof and compatible in rapid heating with the conventional blanket. For example, the blankets according to the invention can be designed to have a power capacity of 120 watts a the beginning of power supply thereto and still have a power capacity of 100 watts in an operating state.

While the invention has been illustrated and described in conjunction with the preferred embodiments thereof it is to be understood that various changes and modifications may be made in the embodiments illustrated and that the invention is not limited thereby or thereto. For example, instead of the pnp transistor 12 an npn transistor may be used with the rectifiers poled reversely from those illustrated. Further, from the foregoing it will be readily appreciated that the thermostat 3 and the associated heating element 4 may be omitted because the differential transformer 5 is effective for presetting the range of normal operating temperature of the electrically warming wire 2 and maintaining such range.

What we claim is:

1. For an electrical warming cloth having embedded therein a length of electric resistance Wire (2) to warm the same, a protective device comprising, a thermostat (3) connected for controlling the temperature of the length of electric resistance wire and a differential transformer (5) connected to said resistance wire and including a pair of primary windings (6a and 6b) and a secondary winding, (60) means comprising one of the primary windings (6a) defining a heat generating circuit, means in said protective device responsive to a change in output voltage across the secondary transformer winding to open said heat generating circuit when said length of electric resistance wire is overheated, means including the differential transformer (5) connected such that the pair of primary windings (6a and 6b) are energized in the opposite directions and have ampere turns maintained thereof in an unbalanced state to permit the sec ondary winding (6c) to induce an output thereacross when the length of electric resistance wire is being normally heated and in a balanced state of said primary windings to cause the secondary winding to produce no output thereacross when the length of electric resistance wire is overheated, a relay (15) including an operating Winding (15a), rectifier means (8) connecting the operating winding to one of the primary windings, relay contacts (151)) connected in the heat generating circuit closed upon energization of the operating winding (15a) and self-held thereby to close the heat generating circuit, a transistor (12) including emitter and base electrodes, rectifier means (11) connecting said electrodes across the secondary transformer winding (6c), means coupling the transistor to the secondary winding so that it is nonconducting under control of a reverse voltage induced across the secondary winding when the length of electric resistance wire is normally heated, and when the length of electric resistance wire is overheated the transistor is conductive in response to a decrease in output from the secondary transformer winding (6c), and means connected to the transistor to short out the Operating winding (15a) to open the relay contacts (15b) and the heat generating circuit when said transistor is conductive.

2. For an electrical warming cloth, having embedded therein a length of electric resistance wire having a relatively high temperature coefficient of resistance, a protective device comprising, in combination, a differential transformer having a pair of primary winding portions energized with opposite polarities and a secondary winding, one of said primary Winding portions being connected in series circuit relationship to said length of electric resistance wire and the other of said primary winding portions being energized so as to cause said differential transformer to be put in its unbalanced state so long as said length of electric resistance wire has a normal operating temperature, rectifier means connected across said secondary winding of said differential transformer, a transistor having a base, a collector and an emitter electrode, said rectifier means being connected across said emitter and base electrodes of said transistor such that so long as said length of electric resistance wire has a normal operating temperature said rectifier means applies a reverse voltage across said emitter and base electrodes of said transistor to put it in its nonconducting state, a relay connected across said emitter and collector electrodes of said transistor and having contacts closed when energized, means connecting said contacts in circuit with said length of electric resistance wire, said differential transformer being responsive to overheating of said electric resistance wire to decrease its output voltage to permit said transistor to conduct thereby to shortcircuit said relay to open said contacts to interrupt energization of said length of electric resistance wire.

3. A protective device as claimed in claim 2, wherein said relay comprises a direct current relay.

4. A protective device as claimed in claim 2, wherein said relay comprises a direct current relay, and full-wave rectifier means connected for energizing said direct current relay.

References Cited UNITED STATES PATENTS 2,592,525 4/1952 Huck 219-481 3,259,832 7/1966 Summerer 210-501 X 3,270,184 8/1966 Negromanti 2l9505 X RICHARD M. WOOD, Primary Examiner.

L. H. BENDER, Assistant Examiner.

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