US2510040A - Electric blanket control - Google Patents

Description

May 30, 1950 A. E. RUDAHL ELECTRIC BLANKET CONTROL Filed Jan. 17, 1947 arwa/nio b ARA/OLD E. PUD/4 HL Patented May 30, 1950 UNITED STATES PATENT OFFICE ELECTRIC BLANKET CONTROL Application J afruary 17, 1947, Serial No. 722,57 6

18 Claims. 1

The present invention relates to electric blankets, and more particularly to control circuits therefor whereby the heat generated in the blanket may be closely regulated so as to avoid undesirable temperature variations within the blanket and to control heat losses therethrough within relatively narrow limits.

- It is an object of the invention to provide a control circuit for electric blankets which is more highly sensitive than controls heretofore utilized so as to maintain the temperature of the blanket within a close range under all conditions of operation.

It is a further object of the invention to provide a control circuit for electric blankets which is sensitive to temperatures within the blanket itself and also to ambient temperature, if desired, thereby assuring the close control of the temperature, as referred to above, and also providing a safety factor in that any abnormal heating of the blanket is immediately detected and corrected thereby eliminating the hazards of fire.

A further object of the invention is to provide a control circuit which is reliable and efficient during use, and which can be used with safety particularly insofar as heat will not be generated within the blanket unless the control circuit is functioning properly.

Other objects will be in part obvious and in part pointed out more in detail hereinafter.

The invention accordingl consists in the features of construction, combination of elements and arrangement of parts which will be exemplifled in the construction hereafter set forth and the scope of the application of which will be indicated in the appended claims.

In the drawing the single figure is a wiring diagram of a specific embodiment of the heating and control circuits of an electric blanket arranged in accordance with the invention.

Referring to the drawing, the reference numeral I denotes a blanket in which is arranged a heating element The blanket It may be formed of any conventional blanketing material and the heating element may be in the form of a thin, flexible resistance wire suitably coated with a thin layer of insulating material which is disposed throughout the blanket in a plurality of convolutions whereby heat is supplied substantially uniformly throughout the blanket. This construction of an electric blanket is conventional and well known in the art and therefore need not be described in more detail here.

The resistance element l2, which also is disposed within the blanket III, is an element whose resistance varies substantially in relationship to its temperature. Preferably, the resistance element I2 is one whose resistance decreases in substantially equal increments for each degree of temperature decrease of the element, such resistances being well known in the art. The resistance element l2 may be formed of an insulated thin pliable resistance wire or may be in the form of a series of resistance buttons, or both. Whatever form of resistance element is utilized, the same is preferably distributed throughout the blanket so as to be affected by the temperature conditions throughout the blanket.

The heating element II is connected directly to the power lines l3 and M which, for example, may be of an ordinary household lighting circuit, through a thermal relay IE to be described more full hereinafter, and does not operate through the transformer l6 which is provided merely to furnish the electrical energy required to operate the control circuit and therefore may be quite small. This transformer l6 has its primary connected to the power lines I 3 and It while the secondary of the transformer is tapped at a suitable number of points denoted by the reference numerals I1, [8, I9, 2|, 22 and 23 to provide the various voltages required in the control circuit.

Taps I9, 2| and 22 are selected so that the voltages between taps I9 and 2| and taps 2| and 22, respectively, are substantially in the same proportion as the resistance ratio to one another of resistance elements 24 and I2. These voltages are applied to a bridge circuit which comprises the said portions of the secondary between the taps l9 and 2| and taps 2| and 22, respectively, a fixed resistance 24 having a zero or negligible temperature coeiiicient of resistance, hereinafter referred to as the reference resistance, and the blanket resistance element I2. In series with the blanket resistance element l2 there is shown a second resistance element 25 which is optional and may be included if it is desired to have the control circuit responsive to ambient temperature. The resistance element 25 is adapted to function in a simi lar manner to that of resistance l2, i. e., it is so constructed that its resistance changes in relationship to changes in its temperature. The resistance element 25 is external to the blanket I0 and may be located in the control box, for example whereby it is affected only by room temperatures as distinct from temperatures within the blanket thereby adding a component into the control circuit which is sensitive to ambient temperatures. If desired, however, the resistance element 25 may be omitted and the blanket resistance element I2 may be directly connected to the tap 22.

The triode 2! is utilized for simplicity of presentation to illustrate a type of vacuum tube amplifier, but this. has been done merely for convenience. If desired, other types of electronic tubes, such as amplifier or relay tubes, or a plurality of tubes, could be utilized as will be apparent to one skilled in the art. The heater voltage for the filament 28 is obtained from across the taps IT and I8. The cathode 29 is connected to. the tap 2i while the tap 23 is connected to the plate 3% through a heater wire 38 of the relay I5. The grid 3| is connected through a limiting resistance 32 to the terminal 39 of the bridge whereby it is biased to control current flow through the tube 27 responsive to the degree of balance or unbalance of the bridge.

The thermal relay I comprises a pair of bimetallic switch arms 33 and 34 having contact points 35 and 33 respectively. Under normal conditions, that is, at ordinary room temperature, the contact points 35 and 36 are separated. However, the bi-metallic switch arms 33 and 34 are so arranged that, when switch arm 34 is heated, as, for example, by means of the heater wire 38, the heated bi-metallic switch arm will bend inwardly so as to close the contacts 35 and 36. On the contrary, when switch arm 33 is heated as, for example, by means of the heater wire 31, that arm will bend outwardly thus tending to separate the contact points 35 and 36. The heater wire 38 is connected in series between the plate 30 and the transformer tap 23 as previously mentioned. The heater wire 3? is connected through the thermal relay I5 with the power lines I3 and I l.

The operation of the above described embodiment of the invention will be readily understood from the above description taken in connection with the following explanation. Beginning with the circuits in the condition shown in the drawing and assuming that the blanket IE is in a relatively cold condition, the total resistance of resistance I2 and resistance 25, if the latter resistance is included, is chosen to be such that the voltage, if any, impressed on the grid 3-I of the tube 21 by the bridge circuit will permit the fiow of current through the tube 27 and thus through the heater wire 38. The flow of current through the heater wire 38 causes the bi-metallic switch arm 3d to become heated and thus close the contact points 35 and 36. Immediately upon this happening, the heating circuit to the resistance element II is closed and heat is then supplied to the blanket. The closing of this heating, circuit also closes the circuit through the heater wire 31. Accordingly, the bi-metallic switch arm 33 will become heated and will eventually open the thermal relay I5 thereby opening the circuit through the heater element I I and the heater wire 31. The bi-metallic switch arm 33 is then permitted to cool, thereby returning the contact point 35 toward the contact point 33. If the blanket is still in a cold condition, a current will be passing through the heater wire 38, as previously described, thereby flexing the bi-metallic switch arm 34 toward the bi-metallic switch arm 33 so that, upon cooling of the bi-metallic switch arm 33, the thermal relay I5. will again close and energize the heater element I I. If, on the other hand, the blanket is now at a desired higher temperature, the resulting change in resistance of resistance element I2 causes a change in the degree of balance or unbalance of the bridge circuit which is sufficient by reason of the change in grid voltage to cause a reduced current or no current to fiow through the tube 27 and hence through the heater wire 38, thereby permitting contact point 36 to move further away from contact point 35 so that the thermal relay I5 will not close when the bi-metallic switch arm 33 cools. The use of the bridge circuit not only provides greater sensitivity in detecting changes in the resistance of the control resistances I2 and 25, but also assures that fluctuations in line voltage will be of no consequence.

In efiect, the thermal relay I5, by reason of the heater wire 31, functions as a simple cycling relay for opening and closing the contacts 35 and 36 periodically. However, by reason of the second bi-metallic arm 34 under control of the control circuit, the relative duration of the off and on periods is controlled by the temperature within the blanket. The greater the difference between the resistance I2 and reference resistance 26, and therefore the colder the blanket, the nearer will be contact 36 to contact 35, thus causing the heating periods tobe of greater duration. This arrangement is highly sensitive because of the infinite variations in the setting of: the thermal relay I5 in response to temperature conditions with the result that the temperature within the blanket I3 is maintained at substantially a constant or at least without being subject to any wide fluctuations in temperature.

By inclusion of the resistance 25., the control circuit may be made responsive to both the temperature within the blanket and to ambient temperature. If the resistance isomitted, the control circuit, of course, is sensitive only to temperatures within the blanket itself.

The arrangement is effective for overcoming the hazards of fire because of the sensitivity of the control circuit to temperatures within the blanket. A further safety factor lies in the fact that, if there is any failure of the control circuit, such as by failure of the tube 21 or the like, the thermal relay IE will not close and therefore the system fails safe.

There is thus provided, in accordance with the invention, a control circuit for electric blankets which is highly sensitive in operation so as to provide close control of the heat generated within the blanket, which circuit is safe and reliable in operation and is not likely to malfunction or re quire frequent replacement or repair.

As many changes could be made in the above construction and many apparently widely different embodiments of this invention could be made without departing from the scope thereof, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

It is also to be understood that the language used in the following claims is intended to cover all of the generic and specific features of the invention herein described and all statements of the scope of the invention which, as a matter of language, might be said to fall therebetween.

I claim as my invention:

1. A heat control arrangement for an electrically heated blanket comprising, in combination, a blanket, an electrical heating element in said blanket, a resistance element whose resistance increases in relationship to increases in its temperature, circuit means for-connecting the heating element to a source of electrical energy, a pair of cooperating contact points in said circuit adapted to open and close the circuit, electrically operated means for moving one of said points in a closing direction, means for supplying said electrically operated means with an electric current of a magnitude proportionate to decreases in the resistance of said resistance element, electrically operated means for actuating the other contact point in an opening direction, and means for supplying an electric current to said means responsive to closing of the contact points.

2. A heat control arrangement for an electrically heated blanket comprising, in combination, a blanket, an electrical heating element in said blanket, a resistance element in said blanket whose resistance increases in relationship to increases in its temperature, a circuit for connecting the heating element to a source of electrical energy, a switch arm normally opening said circuit and movable in one direction for closing said circuit, electrically operated means for causing said arm to move in said one direction, means responsive to changes in the resistance of said resistance element for supplying an electric current to said electrically operated means when the resistance of the resistance element falls below a predetermined minimum, a second arm cooperating with the first arm and movable in the opposite direction for opening said circuit to terminate the heating period, electrically operated means for actuating the second arm, nd means for supplying an electric current to the last named means responsive to closing of the switch arms.

3. A heat control arrangement for an electrica1ly heated blanket comprising, in combination, a blanket, an electrical heating element in said blanket, a resistance element in said blanket and subject to temperatures within the blanket, a second resistance element exterior to said blanket and subject to ambient temperatures, said resistance elements exhibiting resistance changes in relationship to changes in their temperatures, a circuit for connecting the heating element to a source of electrical energy, means movable in one direction for closing said circuit to initiate a heating period, electrically energized means for actuating said movable means in said one direction, means responsive to changes in the resistance of said resistance elements for energizing the actuating means with an electric current which varies in magnitude in relationship to changes in the resistance of said resistance elements, means cooperating with the first named movable means and movable in the opposite direction for opening said circuit to terminate the heating period, electrically energized means for actuating the last named movable means in said opposite direction, and means for energizing the last named means responsive to closing of the circuit.

4. A heat control arrangement for an electrically heated blanket comprising, in combination, a blanket, an electrical heating element in said blanket, a resistance element in said blanket whose resistance increases in relationship to increases in its temperature, circuit means for connecting the heating element to a source of electrical energy a switch having a bimetallic switch arm constructed and arranged to open said circuit when heated, means energized by closing of the switch for applying heat to said arm, and a second bimetallic switch arm constructed and arranged to close said circuit when heated, and means for applying heat to said arm responsive to decreases in the resistance of said resistance element below a predetermined minimum.

5. A heat control arrangement for an electrically heated blanket comprising, in combination, a blanket, an electrical heating element in said blanket, a circuit for connecting the heating element to a source of electrical energy comprising a normally open thermal relay having a pair of cooperating bimetallic switch arms one of which when heated is operable to close the relay and circuit and the second of which arms when heated is operable to open the relay and circuit, a resistance element in said blanket whose resistance changes in relationship to changes in its temperature, means for heating the first switch arm, means for energizing the last named means at varying rates in proportion to the resistance of said resistance element, means for heating the second switch arm, and means for energizing the last named means conditioned for operation by closing of the relay.

6. A heat control arrangement for an electrically heated blanket comprising, in combination, a blanket, an electrical heating element in said blanket, resistance element whose resistance increases in relationship to increases in its temperature, a heating circuit comprising said heating element and a normally open thermal relay having cooperating bimetallic switch arms one of which when heated is operable to close the circuit and the second of which arms when heated is operable to open the circuit, electrical heating means for the second of said switch arms, means for energizing the said heating means connected thereto through the relay only when said relay is closed, electrical heating means for the first of said switch arms, and means for applying electrical energy to said last named heating means responsive to changes in resistance of said resistance element.

7. A heat control arrangement for an electrically heated blanket comprising, in combination, a blanket, an electrical heating element in said blanket, resistance element in said blanket whose resistance increases in relationship to increases in its temperature, a normally open thermal relay having cooperating bimetallic switch arms one of which when heated is operable to close the switch and the second of which arms when heated is operable to open the switch, electrical heating means for each of said switch arms, circuit means for connecting the heating element to a source of electrical energy through said switch arms, means for energizing the heating means of the second switch arm actuated in response to the closing of said circuit means by the switch arms, and means for energizing the heating means of the first switch arm inversely in relationship to the resistance of said resistance element.

8. A heat control arrangement for an electrically heated blanket comprising, in combination, a blanket, an electrical heating element in said blanket, a resistance element in said blanket whose resistance increases in relationship to increases in its temperature, a heating circuit for connecting the heating element to a source of electrical energy, means for closing and opening said circuit including a pair of normally spaced apart contact means, electrically energized means for moving one of said contact points in a circuit closing direction and electrically energized means for moving the other of said contact points in a circuit opening direction, a bridge circuit including said resistance element having a balance af- 7 fectedby the change of resistance of said element, means for energizing the first named electrically energizedmeans responsive to the balance of said bridge circuit, and means for energizing the last named electrically energized means responsive to closing of the contact points.

9 A. heat control arrangement for an electricall'y heated blanket comprising, in combination, a blanket, an electrical heating element in said blanket, a resistance element in said blanket subject to temperatures within the blanket, a secondresistance element exterior to the blanket subj'ect to ambient temperatures, said resistance elementsexhibiting resistance changes in relationship to changes: in their temperatures, a normally open thermal relay having a. pair of bimetallic switch arms, one of said arms when heated being. adapted to move in a relay closing direction and the second of said arms when heated being adapted to move in a relay opening direction, a heater wire for each of said arms, a circuit including said relay for connecting the heating element to a sourceof electrical energy, means for energizing the heater wire of said second switch arm of the relay responsive to closing of the relay, and means for energizing the heater wire of the first switch arm of the relay proportionately to decreases in resistance of said resistance elements.

10; A heat. control arrangement for an electrically heated blanket comprising, in combination, a blanket, an electrical heating element in said blanket, a resistance element in said blanket whose resistance changes in relationship to changes in its temperature, a normally open heating circuit for connecting the heating element.

to a source of electrical energy, a bridge circuit including said resistance element for producing a voltage drop substantially in proportion to the change in resistance of said resistance element, electrically operated means for closing said heating circuit, amplifying means responsiveto said voltage drop for energizing the last namedmeans, electrically operated means operative for opening said heating-circuit, and means connected to the heating circuit for energizing the last named electrically operated: means responsive to: closing of the heating circuit.

11. A heat control. arrangement for an electrically heated blanket comprising, in combination, a blanket, an electrical heating element in said blanket, a resistance element in said blanket whose resistance changes in relationship to changes in itstemperature, a normally open thermal relay having a pair of cooperatingbimetallic switch: arms, one of said arms when heated being adapted to move in a relay closing direction and the second'oi said arms when heated being adapted to move in a relay opening direction, a heater wire for each of said switch arms, a circuit. including: said relay for connecting the heating element and the heater Wire of the second switch arm to asource. of electrical energy, said circuitbeing closed byclosing of the relay, and amplifying means for energizing the heater Wire of the first switch, arm of the relay responsive to changes in resistance of said resistance element.

12:. A heat control arrangement for an electrically heated blanket comprising, in. combination, a blanket, a resistance element in said blanket whose resistance changes in relationship to, changes in its temperature, a normally open thermalrelay having; a; pair of cooperating bimetallic switch arms, one. of said: arms when heated being adapted to move in a; relay closing direction (ill and the second of said arms when heatedbeing adapted to move in a relay opening direction, a heater Wire for each of said switch arms, a circuit including said relay for connecting the heating element and the heater Wire of the second switch arm to a source of electrical energy, said circuit being closed by closing of the relay, a bridge circuit including said resistance element adapted to be balanced at a predetermined temperature of said element, and means for energizing the heater wire of the first switch arm of the relay in relationship to the degree of unbalance of said bridge circuit.

13. A heat control. arrangement for an electrically heated blanket comprising, in combination, a blanket, an electrical heating element in said blanket, a resistance element in said blanket where it is subject to temperatures within the blanket, a, second resistance element external to the blanket where it is subject to ambient temperatures, said resistance elements exhibiting resistance changes in relationship to changes in their temperature, a normally open thermal relay having a pair of cooperating bimetallic switch,

-- arms, one of said arms when heated being adapted to move in a relay closing direction and. the secondof said arms when heated being adapted to more in a relay opening direction, a heater wire for each of said switch arms, a circuit including said relay forconnecting the heating element and the heater wire of the second switch arm to a source of electrical energy, said circuit being closed by closing of the relay, a bridge circuit for producing a voltage drop in relationship to the change in resistance of said resistance elements, and amplifying means for energizing the heater wire of the first switch arm in relationship to changes in said voltage drop.

14. A. heat control arrangement for an electrically heated blanket comprising, in combination, a. blanket, an electrical heating element in said blanket, a. resistance element in said blanket whose resistance changes in relationship to changes irritstemperature, a normally open thermal: relay having a pair of cooperating bimetallic:

heater wire for each of said switch arms, a cirsupplying electrical energy to the heater Wire of the first switch, arm from the secondary of said transformer controlled by said bridge circuit.

15. A heat control arrangement for an electrisally heated blanket comprising, in combination, a blanket, an electrical heating element in said blanket, a resistance element. in said blanket. Where it is; subject to temperatures within the blanket, a second resistance element exterior to said blanket where it is subject. to ambient temperatures, said resistance elements exhibiting re-* sistance changes in relationship to changes. in their temperature, av normally open thermal. relay having a pair of cooperating bimetallic switch ar-ms ,.one of said arms when heated being: adapted to move in a relay closing direction and the second of said arms when heated being adapted to move in a relay opening direction, a heater wire for each of said switch arms, a circuit including said relay for connecting the heating element and the heater wire of the second switch arm to a source of electrical energy, said circuit being closed by closing of the relay, a transformer adapted to be connected to the source of electrical energy, a bridge circuit comprising, in sequence, two sections of the secondary of said transformer, said resistance elements and a reference resistance having a negligible temperature coefficient of resistance, and amplifying means for energizing the heater wire of the first switch arm in relationship to the voltage drop across said bridge circuit.

16. A heat control arrangement for an electrically heated blanket comprising, in combination, a blanket, an electrical heating element in said blanket, a resistance element in said blanket whose resistance changes in relationship to changes in its temperature, a normally open thermal relay having a pair of cooperating bimetallic switch arms, one of said arms when heated being adapted to move in a relay closing direction and the second of said arms when heated being adapted to move in a relay opening direction, a heater wire for each of said switch arms, a circuit including said relay for connecting the heating element and the heater Wire of the second switch arm to a source of electrical energy, said circuit being closed by closing of the relay, a transformer adapted to be connected to the source of electrical energy, a bridge circuit comprising two sections of the secondary of said transformer, said resistance element and a reference resistance having a negligible temperature coefficient of resistance, and amplifying means for supplying electrical energy to the heater wire of the first switch arm in relationship to the unbalance of said bridge circuit.

17 A heat control arrangement for an electrically heated blanket comprising, in combination, a blanket, an electrical heating element in said blanket, a resistance element in said blanket whose resistance changes in relationship to changes in its temperature, a normally open thermal relay having a pair of cooperating bimetallic switch arms, one of said arms when heated being adapted to move in a relay closing direction and the second of said arms when heated being adapted to move in a relay opening direction, a heater wire for each of said switch arms, a circuit including said relay for connecting the heating element and the heater wire of the second switch arm to a source of electrical energy, said circuit being closed by closing of the relay, a transformer adapted to be connected to the source of electrical energy, a bridge circuit comprising two sections of the secondary of said transformer, said resistance element and a reference resistance having a negligible temperature coefiicient of resistance, and a circuit for energizing the heater wire of the first switch arm comprising the secondary of said transformer and a vacuum tube having a grid voltage determined by the degree of balance of said bridge circuit.

18. A heat control arrangement for an electrically heated blanket comprising, in combination, a blanket, an electrical heating element in said blanket, a resistance element in said blanket whose resistance changes in relationship to changes in its temperature, a normally open thermal relay having a pair of cooperating bimetallic switch arms, one of said arms when heated being adapted to move in a relay closing direction and the second of said arms when heated being adapted to move in a relay opening direction, a heater wire for each of said switch arms, a circuit including said relay for connecting the heating element and the heater wire of the second switch arm to a source of electrical energy, said circuit being closed by closing of the relay, transformer adapted to be connected to the source of electrical energy, a bridge circuit comprising two sections of the secondary of said transformer, said resistance element and a reference resistance having a negligible temperature coefficient of resistance, a circuit for energizing the heater wire of the first switch arm comprising the secondary of said transformer and a vacuum tube having a grid voltage determined by the balance of said bridge circuit, said bridge circuit being designed to provide a grid bias on said tube sufficient to prevent flow of current while said resistance element is above a predetermined temperature.

ARNOLD E. RUDAHL.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,276,589 Steinmetz Aug. 20, 1918 1,694,264 Hull Dec. 4, 1928 2,302,603 Davis et a1 Nov. 17, 1942 FOREIGN PATENTS Number Country Date 144,669 Great Britain Sept. 9, 1921 338,880 Great Britain Nov. 18, 1930

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