NZ197765A

NZ197765A - Electric blanket safety circuit

Info

Publication number: NZ197765A
Application number: NZ19776581A
Authority: NZ
Inventors: G C Crowley
Original assignee: Sunbeam Corp
Priority date: 1980-11-03
Filing date: 1981-07-20
Publication date: 1985-07-12
Also published as: AU7701481A; FR2493625A1; MX149865A; JPS57105989A; CA1162598A; GB2086676B; DE3137754A1; AU541235B2; FR2493625B1; GB2086676A

Description

197765 . Friarfh/ Oa.'f-e. 3-1) -§?o Ccnip^eftf S^sciSksrifiirU'sd:7 Ciass: k<PA„ PUtf!f3Et'0'"5 n;~',-?v ^ JUL 1985 i "-1JW. ...............

F.O, journal, N.<x ..... J.^rT7. /,. -s>/ NEW ZEALAND "Lmi PATENTS ACT, 1953 Jr;r' No.: Date: COMPLETE SPECIFICATION "PROTECTIVE CIRCUIT FOR ELECTRIC BEDCOVER OR THE LIKE" jttfWe, SUNBEAM CORPORATION, a corporation organized and existing under the laws of the State of Delaware, of 5400 West Roosevelt Road, Chicago, Illinois 60650, United States of America hereby declare the invention for whicbcibs' we pray that a patent may be granted to ?»8/us, and the method by which it is to be performed, to be particularly described in and by the following statement:- I (Followed by paqe la) M.

Title of tho Invention ■6-0250" PROTECTIVE CIRCUIT FOR ELECTRIC BEDCOVER OR THE LUCE Background of the Invention This invention pertains to a protective circuit for an electric bedcover, electric heating pad, or other electrical appliance comprising a supporting substrate, which may be a fabric shell of an electric bedcover or similar electric appliance, and a heating wire, which traverses the supporting substrate. Such an appliance comprising a fabric shell may be a heated muff, boot, garment, pillow, fi etc* it* Prior protective circuits of pertinent interest^^,^^ isclosed in U.K. Patent Application No. 2,028,608In each of such protective circuits, a heating wire is arranged to be supplied with an alternating current, and a sensing wire is insulated electrically from the heating wire by temperature-sensitive material, which in some disclosed embodiments melts to effect electrical contact between the sensing wire and the heating wire in an overheated condition. Similar uses of temperature-sensitive materials were exemplified in U.S. Patent No. 3,493,727, U.S. Patent No. 3,628,093, British Patent Specification No. 1,155,118, and British Patent Specification No. 1,456,684.

In each of the protective circuits disclosed in U.K. Patent Application No. 2,028,608A, a first diode and a first resistor in series are connected between a first end of the heating wire and the sensing wire, a second diode and a second resistor in series are connected between a second end of the heating wire and the sensing wire, and polarities of the two diodes are such as not to allow current to bypass the heating wire through the two diodes. The resistors are coupled thermally to a thermal fuse, which is connected so as to disable the heating wire when heated sufficiently.

In the protective circuits disclosed in U.K. Patent Application No. 2,028,608A, an overheated condition at either extreme end of the heating wire causes the insula-tive layer between the sensing wire and the heating wire to melt so as to allow current to flow at alternate half-cycles of the alternating current supplied to the heating wire through the sensing wire, and through the particular resistor and the associated diode connected to the opposite end of the heating wire, whereupon the conducting resistor heats the thermal fuse. The other resistor does not conduct as there is no drop in voltage across it. An overheated condition at an intermediate part of the heating wire thus allows current to flow alternatingly through the respective resistors. U.K. Patent^Application No. 2,028,608A also discloses that the diodes are protected against reverse transient damage by the resistors.

Other protective circuits of pertinent interest -disclosed in U.K. Patent Application No. 2,028,607A.T In ^each of such protective circuits, a sensing wire is insulated electrically from a first heating wire by temperature-sensitive material, which may melt in like manner, and a second heating wire is interlocated with the first heating wire and connected at a given end to the first heating wire so as to divide an applied voltage between the heating wires, which are arranged to be supplied with an alternating current. Also, both ends of the sensing wire are connected to the opposite end of the second heating wire through a resistor, which is coupled thermally to a thermal fuse. In one such circuit, as shown in Figure 3 of U.K. Patent Application No. 2,028,607A, a diode is connected between the respective heating wires. 197765 As discussed In U.K. Patent Application No. 2,028,607A and U.K.

Patent Application No. 2,028,608A, some types of temperature-sensitive material have negative temperature coefficients of resistance. Pertinent Information of related Interest may be found In U.S. Patent No. 2,581,212, U.S. Patent No. 2,846,559, and U.S. Patent No. 2,846,560.

Additional background may be obtained from U.S. Patent No. 2,195,958, U.S. Patent No. 4,205,223, British Patent Specification No. 1,566,005, French Patent Pub I I cat Ion No. 2,406,330, French Patent Pub I Icat I on No. 2,408,932, and French Patent Publication No. 2,416,611.

Summary Of The Invention The Invention consists In an electric appliance Including a supporting substrate, a heating wire having a first and a second end which transverses the supporting substrate and which Is adapted to be supplied with an alternating current, and a protective circuit comprising: wire, which Is Insulated electrically from the heating wire by an insulatlve layer being deployed along the heating wire and having a fusing temperature correspond Ing to an overheated condition of the electric applicance, and which Is Interwound with the heating wire so as to effect electrical contact between the sensing wire and the heating wire if at least part of the insulatlve layer fuses between the sensing wire and the heating wire, (a) a sensing wire, which is deployed with the heating 197765 (b) a disabling means comprising a resistance element with first and second ends, the first end of which Is connected to the sensing wire, and thermal means responsive to a current flowing In said resistance element for disconnecting the heating wTre from the alternating current supply, (c) a first diode connected between the first end of the heating wire and the second end of the resistance element, (d) a second diode connected between the second end of the heating wire and the second end of the curront conduction element so as not to allow current to bypass the heating wire through the first and second diodes, whereby an overheated condition causing at least part of the Insulatlve layer to fuse closes the circuit through said resistance element to activate the disabling means.

Brief Description Of The Drawings Figure 1 Is a fragmentary, partly broken-away, elevatlonal view of a coaxial cable comprising a heating wire, a sensing wtre, and associated Insulative layers, as used in an electric bedcover, electric heating pad, or other electric appliance embodying this invention. Figure 2 Is a cross-sectional view taken along I I ne 2- - 2 of _ Oal: " 3* Figure 1, in the direction of the arrows, and on an enlarged scale. Figure 2 also shows a fabric shell, within which the coaxial cable is deployed.

Figure 3 is a circuit diagram of a protective circuit utilizing the coaxial cable of Figures 1 and 2 and according to a preferred embodiment of this invention. Figure 4 is a circuit diagram of a protective circuit utilizing the coaxial cable of Figures 1 and 2 and according to an alternative embodiment of this invention.

Figure 5 is a pictorial view of a thermal switch, which comprises a fusible junction, and which is utilized in the protective circuit of Figures 3 and 4.

Figures 6 and 7 are respective circuit diagrams of other protective circuits utilizing the coaxial cable of Figures 1 and 2 and according to other alternative embodiments of this invention.

Detailed Description of the Preferred Embodiment A protective circuit according to this invention utilizes a coaxial cable 10, which is shown in Figures 1 and 2, and which is similar to prior coaxial cables, as exemplified in U.S. Patent No. 3,62§",093 and U.S.

Patent No. 3,493,727. As shown in Figure 2, the coaxial cable 10 is deployed in a network of serpentine channels 12, one of which is shown in cross-section, within a fabric shell 14, which is shown fragmentarily, of an electric bedcover, which may be an electric overblanket or an electric underblanket.

As shown in Figures 1 and 2, the coaxial cable 10 comprises a textile core 16, which comprises numerous twisted strands, a first conductor 18, which is wound tautly around the textile core 16, an insulative layer 20, which surrounds the first conductor 18 and insulates the first conductor 18 electrically, and which fuses at a predetermined temperature corresponding to.an overheated condition of the electric bedcover, a second conductor 22, 197765 y % the bronze strip 74 are good conductors of heat and current. The bronze strip 74, which acts as a spring leaf, has suitable spring characteristics.

The contacting flange 78 is soldered to the flat top 66 by a eutectic alloy providing a fusible junction 84, which opens when heated sufficiently, and which cannot be reset under normal operating conditions. The eutectic alloy may be composed of 63 parts of tin and 37 parts of lead, so as to fuse at approximately 361°F.

Such a solder is available commercially from Newark Electronics Div., Premier Industrial Corp., 500 North Pulaski Road, Chicago, Illinois 60625. The bronze strip 74 is biased so as to separate from the bronze base 64 if the fusible junction 84 fuses.

Suitable thermal switches, which comprise suitable fusible junctions having specified fusing temperatures as low as 140°F, are available commercially from Emerson Electric Co. , Micro Devices Division, 1881 Southtown Boulevard, Dayton, Ohio 45439, under its trademark "Microtemp".

A resistance element - 88, which may^be a conventional carbon or wire-wound resistor having opposite leads 88a, 88b, is mounted to the circuit board 62, by conventional grommets 96 receiving the leads 94, so as to be disposed beneath the. flat top 66 for good transfer of heat from the resistance element 88, through the bronze base 64, to the fus-ible junction 84. The thermal switch 60 and the , resistance element 88 may be encased in a protective capsule (not shown) outside the fabric shell 14, of a type used conventionally to encase controls for an electric bedcover, electric heating pad, or other electric appliance.

Ab shown in Figure 3, the heating wire 18 preferably is energized by unrectified alternating current from a conventional source 100 through a switch 122, which may be an on-off switch, an ambient-responsive control, or any other conventional switch, as may be appropriate in an electric bedcover, electric heating pad, or other electric 197765 1 appliance. If the switch 122 is to be an ambient-responsive control in an electric bedcover, the switch 122 may be similar to the ambient-responsive control described in U.S. Patent No. 2,195,958.

In the preferred embodiment of Figure 3, the first conductor 18 serves as a heating wire, and the second conductor 22 serves as a sensing wire. Both ends of the second conductor 22 are connected to each other at a shunting connection 102 and through the shunting con-10 nection 102 to a given lead 88a of the . resistance element 88. The opposite lead 88b of the resistance element- 88 is connected to a common connection 104. The shunting connection 102 provides two parallel paths for current through the resistance element 88. If the second conductor 15 22 breaks so as to open one such path, the shunting connection 102 provides an alternate path for current through the auxiliary heater.

A first diode 110 is connected between a given end 18a of the first conductor 18 and the resistance element 88. 20 A second diode 120 is connected between the.opposite end. - t 18b of the first conductor 18 and the resistance elejnent g8. The first dipde 110 and the second diode 120 are connected to the resistance element 88 through the common connection 104. The respective polarities of the first diode 110 25 and the second diode 120 are such as not to allow current to bypass the first conductor 18 through the first diode 110 and the second diode 120. If the illustrated polarity of the first diode 110 is reversed, the illustrated polarity of the second diode 120 must be reversed, and vice-versa. 30 An overheated condition causing at least part of the insulative layer 20 to fuse clpses an electrical circuit through the resistance element 88, which thus is enabled to heat the thermal switch 60 sufficiently for the thermal switch 60 to open so as to disable the first conductor 18 35 serving as a heating wire.

In an overheated condition at the first end 18a of the first conductor 18, the resistance element- 88 and the second diode 120 conduct alternate half-cycles (of a given 197765 -io- polarity) of the alternating current supplied to the first conductor 18 by the source 100. In an overheated condition at tjie second end 18b of the first conductor 18, the resistance element 88 and the first diode 110 conduct alternate half-cycles (of the opposite polarity) of such alternating current. In an overheated condition at £i? intermediate part of the first conductor 18, the resistance element 88 and the second diode 120 conduct alternate half-cycles (of the given polarity} of such alternating current, and the resistance element ^8 and the first diode 110 conduct alternate "half-cycles (of the opposite polarity) of such alternating current.

A capacitor 130 is connected in parallel with the first conductor 18. The capacitor 130 protects the first 15 diode 110 and the second diode 120 against damage from reverse spikes of voltage.

For any capacitor, capacitive reactance in ohms is equal to X = 1 = 0.159 c 27TfC fC where f equals frequency in hertz and ^ equals capacitance in farads. Hence, as reverse spikes of voltage exhibit higher frequencies than line frequency, and as capacitive reactance of the capacitor 130 varies inversely with the frequency, the capacitor 130 conducts reverse spikes of 25 current preferentially so as to protect the first diode 110 and the second diode 120 from damage.

In the alternative embodiment of Figure 4 , a multi-pole double-throw switch 140 enables alternative modes of operation to be selected. In a first mode of operation, 30 the first conductor 18 serves as a heating wire, which dissipates power at a given rate, and the second conductor 22 serves as a sensing wire, as in the preferred embodiment. In a second mode of operation, the second conductor 22 serves as a heating wire, which dissipates power at a 35 different rate, and the first conductor 18 serves as a sensing wire. 19 The switch 140 has a pole 140a, which can be switched between a contact 140b for the first mode of operation and a contact 140c for the second mode of operation, and which is connected to the first diode 110, to a given side 130a of the capacitor 130, and to a given side 100a of the source 100. The contact 140b is connected to the given side 18a of the first conductor 18. The contact 140c is connected to a given side 22a of the second conductor 22.

The switch 140 has a pole 140d, which can be switched between a contact 140e for the first mode of operation and a contact 140f for the second mode of operation, and which is connected to the lead 88a of the resistance element "88 at the shunting connection 102. The contact 140e is connected to the given end 22a of the second conductor 22. The contact 140f is connected to the given end 18a of the first conductor 18.

The switch 140 has a pole 140&, which can be switched between a contact 140h for the first mode of operation and a contact 140k for the second mode of operation^ an4 which is connected to the lead 88a of the resistance element 88 at the shunting connection 102. The contact 140h is connected to the opposite end 22b of the second conductor 22. The contact 140k is connected to the opposite end 18b of the first conductor 18.

The switch 140 has a pole 140|>, which can be switched between a contact 140^ for the first mode of operation and a contact 140r for the second mode of operation, and which is connected to the second diode 120, to the opposite side 130b of the capacitor 130, and through the thermal switch 60 and the switch 102 to the opposite side 100b of the source 100. The contact 140£ is connected to the opposite side 18b of the first conductor 18. The contact 140r is connected to the opposite side 22b of the conductor 22.

Thus, the switch 140 enables the first conductor 18 and the second conductor 22 to be interchanged selectively, whereupon different rates of dissipation of power can be .selected. However, in other respects, the 197765 embodiment of Figure 4 is similar to the preferred embodiment of Figure 3.

In either described embodiment, the source 100 may deliver unrectified alternating current at 110-120 VAC, 60 Hz, whereupon the carbon nr wire-wound resistor constituting the resistance element 88 must be of such size and value to raise the temperature of the fuse alloy to its melting point (361°F) in a reasonable time after an overheated condition effects electrical contact between a sensing wire and a heating wire. A resistor of 1 or 2 watt size with a resistance value between 500 ohms and 3,500 ohms suffices. The capacitor 130 may have a capacitance of about .01 microfarad.

Similarly, at 220-240 VAC, 50 Hz, a resistor of 1 or 2 watt size with a resistance value between 1,800 ohms and 8,200 ohms suffices. The exact values of resistance used depend on the design of the thermal switch and more specifically on its adequacy with respect to thermal conductivity between the resistors and the fusing alloy. The capacitor 130 may have a capacitance of about 0.005 microfarad.

As shown in Figure 6, a thermistor 160 having a positive temperature coefficient of resistance may be used as a thermal switch and may be substituted for the thermal switch 60, in the embodiment of Figure 3, so long as the thermistor 160 has suitable characteristics. The thermistor 160 is connected so as to disable the heating wire 18 when heated sufficiently, and so as to enable the heating wire 18 otherwise, as when the thermistor 160 is cooled sufficiently after the thermistor 160 has disabled the heating wire 18. Similarly, the thermistor 160 may be substituted for the thermal switch 60, in the embodiment of Figure 4.

Thermistors having suitable characteristics are available commercially from Raychem Corporation, 300 Constitution Drive, Menlo Park, California 94025, as PolySwitch™ Thermal Limit Devices, and are described in Specification TLD-01, which is published by Raychem Corporation, am ^es 197765 such devices as temperature-triggered switching devices used for thermal protection of electrical equipment.

Herein, all references to thermal means contemplate such devices and equivalent devices, as well as thermal switches 5 having fusible links.

As shown in Figure 7, an electromagnetic relay, which comprises normally closed contacts 170 and a coil 180 coupled magnetically to the contacts 170, and which is connected so as to disable the heating wire 18 when the coil 10 180 conducts sufficient current for the contacts 170 to be opened, may be substituted for the thermal switch 60 and the auxiliary heater 88, in the embodiment of Figure 3, whereby an overheated condition causing at least part of the insulative layer 20 to fuse closes an electrical cir-15 cuit through the coil 180, which conducts sufficient current for the contacts 170 to be opened so as to disable the heating wire 18. Similarly, such a relay may be substituted for the thermal switch 60 and the auxiliary heater 88, in the embodiment of Figure A.

In an embodiment described above, the insulative layer 20 may be made of a selected material not only having a fusing temperature corresponding to an overheated condition but also having a negative temperature coefficient of resistance, whereby an overheated condition of less 25 severity causes the insulative layer 20 to conduct leakage current before the insulative layer 20 fuses.

In such an. embodiment comprising the thermal switch 60 and the resistance element 88, such current may be sufficient for the thermal switch 60 to open before the insula-30 tive layer 20 fuses, so as to disable the heating wire(s) without permanent damage to the insulative layer 20. Once opened, the thermal switch 60 cannot be reset automatically.

In such an. embodiment comprising the thermistor 160 and the resistance element ^8, such current may be sufficient 35 for the thermistor 160 to disable the heating wirc(s) before the insulative layer 20 fuses, so as to provide modulated control of the heating wire(s) without permanent^M|age to dtw~v # /f -y£- >0: / £ u the insulative layer 20. As compared to the thermal switch 60, the thermistor 160 becomes reset automatically if, as, and when the overheated condition of less severity passes.

In such an embodiment comprising the electromagnetic relay, which comprises the contacts 170 and the coil 180, such current may be sufficient for the contacts 170 to be opened, so as to provide modulated control of the heating wire(s) without permanent damage to the insulative layer 20. As well, such a relay becomes reset automatically if, as, and when the overheated condition passes.

In an electric bedcover embodying a protective circuit according to any embodiment described above, a suitable material having a negative temperature coefficient of resistance is doped polyvinyl chloride, which is available commercially from The B.F. Goodrich Co. , B.F. Goodrich Chemical Div., Independence, Ohio, as "GEON" No. 82726-natural-024, 0.8% doped with "Triton X-400" dopant.

Claims

197765 15 WHAT-^WE CLAIM IS.:

1. An electric appliance Including a supporting substrate, a heating wire having a first and a second end which transverses the supporting substrate and which Is adapted to be supplied with an alternating current, and a protective circuit comprising: (a) a sensing wire, which Is deployed with the heating wire, which Is Insulated electrically from the heating wire by an Insulatlve layer being deployed along the heating wire and having a fusing temperature corresponding to an overheated condition of the electric appllcance, and which Is Interwound with the heating wire so as to effect electrical contact between the sensing wire and the heating wire If at least part of the Insulative layer fuses between the sensing wire and the heating wire, (b) a disabling means comprising a resistance element with first and second ends, the first end of which Is connected to the sensing wire, and thermal means responsive to a current flowing In said resistance element for disconnecting the heating wire from the alternating current supply, (c) a first diode connected between the first end of the heating wire and the second end of the resistance element, (d) a second diode connected between the second end of the heating wire and the second end of the current conduction clement so as not to allow current to bypass the heating wire through the first and second diodes, whereby an overheated condition causing at least part of the Insulatj layer to fuse closes the circuit through said resistance elemenj activate the disabling means.

2. An electric appliance according to claim 1 Including a capacitor which Is connected In paral lei with the heating wire to protect the first and second diodes against damage from, reverse spikes of voltage. 197765 16

3. An electric aDDliance^to either of claims 1 or 2 wherein the thermal means comprises a junction which opens when heated sufficiently.

4. An electric appliance according to either of claims 1 or 2 wherein the thermal means comprises a temperature-triggered switching device having a positive temperature characteristic of resistance.

5. An electric appliance according to any one of the preceding claims wherein the Insulatlve layer has a negative temperature co-efficient of resistance whereby an overheated condition of less severity causes the' Insulatlve layer to conduct leakage current through the resistance element before the Insulatlve layer fuses.

6. An electric appliance according to any one of the preceding claims wherein the heating wire dissipates power at a given rate, wherein the sensing wire comprises another heating wire, which dissipates power at a different rate, and wherein the electric appliance comprises selective switching means, which is connected so as to enable the heating wires to be interchanged selectively.

7. An electric appliance Including a protective circuit substantially as herein before described with reference to Figures 1 to 6 of the accompanying drawings.