CA1156300A - Electric blanket safety circuit - Google Patents

Abstract

Abstract of the Disclosure An electric blanket utilizing a positive coefficient material between two conductors as the heating means and having a safety circuit connected to the conductors to blow a fuse in the event of an open circuit condition in the conductors. The safety circuit is connected across each of the two conductors to sense an increase in voltage in response to which a low resistance circuit path is provided causing a current surge which disconnects the heater circuit by blowing the fuse.

Description

1 Background of the Invention The present invention relates to a safety circuit for use in connection with an electrically heated blanket or bed cover. More particularly, it relates to a circuit intended for use in a blanket of the type using a positive temperature coefficient material as the heating element.
Electric blankets are typically formed with fabric shells which include passageways throughout the area of the blanket in which a tortuous low wattage heating element is threaded. The blanket must be provided with some means for sensing overheat conditions along the heating element within the blanket so that the current to the blanket can be shut off or reduced before damage or injury is caused by the overheat condi-tion. The various means for sensing such overheat conditions have included discrete bimetallic thermostats positioned at spaced intervals along the blanket. In addition, continuous sensing wires have been used in conjunction with the heating element wire. The sensing wire responds to overheat conditions to operate a relay which opens the circuit to the main heating element.
More recently, there has been consideration of the use of positive temperature coefficient materials for the heating element so as to provide a blanket wire which would be self-limiting from a temperature standpoint in any areas in which an overheat condition occurred. The possible structure of such a wire and the manner in which the wire operates to supply heat to , the blanket and to respond to overheat conditions is described in the U. S. patent to Sandford et al ~Jo. 3,410,984. As des-cribed in the Sandford et al patent, the blanket wire consists of two spaced conductors which are enclosed by a positive temperature coeffici~ent material comprising polyethylene with 1 carbon black particles mixed therewith. The electrical current passes through the positive temperature coefficient material in passing from the one conductor to the other conductor and the PTC material acts as a heating element.
The formulation of the PTC material and the physical dimensions of its extrusion is selected so that the resistance, and, therfore, the heat dissipation per foot of length are reasonably constant at any given temperature. At low tempera-tures, the heat dissipation per foot will be greater than at normal room temperatures, When in an overheat or high temperature condition, the heat dissipation will be less than normal. The PTC material self limits to produce a given heat dissipation or wire temperature for every different ambient and insulation system. In this way, when a section of the heater is bunched up or abnormally restricted insofar as heat transfer is concerned (something on top of the blanket), the PTC wire reacts to the new environment and reduces its heat dissipation in that area, trying to keep its temperature reasonably constant.
Under normal circumstances, the type of PTC blanket wire described above operates well and eliminates the necessity for either the discrete bimetallic thermostats within the blanket or the various types of distributed sensing wires paralleling the heater wires in the blanket. However, it has been ascertained that significant problems ar~se when a broken ; or open cirucit occurs in connection with one of the two con-ductors in the PTC wire. In such an event, there occurs arcing or overheating at the specific areas in which the break occurred.
It would be desirable, therefore, to provide some me~ns in connection with a positive temperature coefficient heating wire blanket to interrupt the circuit to the blanket prior to there being a dangerous condition caused by the arcing of a broken conductor.

1 It is well known in the electric blanket art to pro-vide overheat protection means which include means to blow a fuse in the event of such an overheat condition. One such circuit is shown in the U. S. Patent to Crowley No. 3,628,093 in which a short circuit is created in connection with an over-heat means and such short circuit is used to blow a protective fuse in the circuit. Another piece of prior art in which the safety circuit blows a fuse in connection with a malfunction in a blanket is the U. S. Patent to Crowley No. 4,034,185.
There are also many examples of protective circuits which include means for blowing the circuit fuse to protect the load in the event of an overvoltage condition. Examples of thé~e U. S. patents are Muench, Jr. No. 3,600,634; Wilson No.
3,968,407; Voorhoeve No. 3,878,434, Hurdle No. 3,493,815; and, Shatuck et al No. 3,215,896. Also of possible interest is the U. S. patent to McNulty No. 3,325,718 which senses a condition in a load and provides a circuit to overload and blow the cir-cuit fuse to disconnect the load from the power supply. Also of interest relative to the specfic circuitry used in such protective circuits is Lawson U. S. No. 3,845,355 which shows a photoresistor controlling an overload relay.
Summary of the Invention The instant invention provides an electric circuit for use with a positive temperature coefficient blanket which cuts off the power to the blanket whenever an open circuit has occurred in one of the conductors. The circuit has a character-istic which permits it to operate selectively on the overvoltage condition produced by the conductor breakage while not respond-ing to the types of momentary overvoltage conditions which are frequently found in household electrical power supply. The circuit is connected to respond to a break in either one of the 11~63~0 1 two conductors in the blanket wire to create an effective short circuit across the ends of the heating element thereby overload-ing the series fuse to open the circuit before any damage in the way of igniting the PTC material or the gas produced by arcing at the break is concerned.
The fuse provided is a slow blow type so that during the normal high inrush current encountered in the PTC material when first energized, the fuse will not blow out even though the peak currents are two or three times the fuse rating. This initial current surge is caused by the fact that the PTC wire when cool has a very low resistance which rises quickly upon energization of the circuit. If, however, there is a shorted condition in the blanket wire, the fuse will quickly blow out and de-energize the circuit. The fuse was selected to give the best protection during operation at all normal ambient tempera-tures and cold wire energization.
It is, therefore, an object of the present invention to provide an improved electric blanket having a positive temperature coefficient heating element with a sensing circuit connected to the element to disable the circuit in the event of a break in either of the heater conductor wires.
It is a further object of the present invention to provide an improved safety circuit for use in connection with à
positive temperature coefficient heating wire blanket in which an overvoltage sensing means is connected to the ends of the heating wire to respond to breaks in the individual conductors to short out the heating element wire for a sufficient period of time to blow the circuit fuse.
-; It is another object of the present invention to pro-vide a circuit protecting means for use with a positive tempera-ture coefficient electric blanket wherein the circuit means are insensitive to normal high voltage transients but disable the cir-cuit in the event of an interruption in either of the conductors in the heating element.
Further objects and advantages will become apparent as the following description proceeds and the features of novelty which characterize the invention will be pointed out in the claims annexed to and forming a part of the specification.
Brief Description of the Drawings Fig. 1 is a schematic diagram of an electric blanket in-cluding a safety control circuit embodying our invention.
Fig. 2 is an enlarged sectional view of the heating element wire which may be used in the electric blanket of Fig. 1.
Fig. 3 is a further schematic diagram of the blanket of Fig. 1 showing the heating element wire schematically to illustrate the connection to the safety control circuit.
Fig. 4 is a schematic diagram of an alternative circuit embodying our invention.
Fig. 5 is a schematic diagram of an alternative circuit embodying our invention.
Fig. 6 is a schematic diagram of an alternative circuit embodying our invention.
Fig. 7 is an enlarged showing of one of the gas tubes shown in Fig. 6.
Fig. 8 is a schematic diagram of an alternative circuit embodying our invention.
Summary of the Invention Referring to Fig. 1 of the drawings, there is shown a schematic circuit diagram of a preferred embodiment of our invention wherein the electric blanket wire and the associated safety circuit are generally referred to by reference numeral10. Included therein is an elongated blanket wire 12 which is typically on the order of 119 feet long for twin bed blankets and 162 feet long for double bed blankets and is looped back and forth through channels formed in an electric blanket shell 1 ~56300 1 13 to provide heat evenly across the surface of the blanket in a well-known manner. The blanket wire 12 is of the type utiliz-ing a positive temperature coefficient material 14 which is extruded between and around a pair of spaced conductors 16 and 18 as is shown best in the sectional view of Fig. 2.
The general type of wire involved is disclosed in the U. S. Patent to Sandford et al No. 3,410,984. The PTC material is typically a polyethylene, silicone rubber or the like having carbon black particles mixed therein in such a manner as to give the desired temperature/resistance characteristics. As is indicated in the aforementioned Sandford et al patent, the conductors are spaced apart and enveloped by the PTC material which is extended into intimate engagement with the conductors.
A suitable electrically insulating layer 19 is extruded over the PTC material.
The conductors 16 and 18 are shown schematically in Fig. 2 as closely spaced with parallel resistances positioned between the conductors, There are actually no discrete resist-ances between the conductors 16 and 18 since the PTC material 14 which is positioned between the conductors 16 and 18 throughout the entire length acts as a single, continuous resistance heater as the current flows ~hrough the PTC material from the conductor 16 to the conductor 18. The conductors 16 and 18 have their opposite ends connected to a suitable source of electrical power by means of leads 20 and 22 respectively.
In order to have a uniform voltage drop between the conductors 16 and 18 at any point throughout the length of the blanket wire 12, the power leads 20 and 22 are connected to opposite ends of the blanket wire 12 as is best shown schematically in Fig. 3.
With such an arrangement, the voltage drop between the adjacent conductors 16 and 18 at any point is essentially equal to the 1 line voltage less the voltage drop resulting from the current passing through one length of either the conductor 16 or 18.
As an example, at the end of conductor 16 adjacent the power lead 20, the voltage would equal the line voltage less the drop caused by the current passing through the length of conductor 18. Similarly, at the end of conductor 18 adjacent the power lead 22, the voltage drop between the conductors 16 and 18 would equal the line voltage less the voltage drop caused from the current passing through the length of the conductor 16. Simi-larly, at the middle of the blanket wire 12, the drop acrossthe conductors 16 and 18 would equal the line voltage less the voltage drop caused by the current passing through half of the conductor 16 and half of the conductor 18. This arrangement results in a uniform heating effect being obtained throughout the length of the blanket wire 12.
Electric blankets are conventionally used in much the same manner as a nonelectric blanket being spread acr~ss a bed and overlying the user. During use and between uses, the blanket may be flexed or folded repeatedly. In addition, when stored or when washed, the electric blanket is again subjected to repeated folding and flexing. In view of the demands made on an electric blanket in normal use, it is necessary that the blanket wire 12 including the associated conductors 16 and 18, the PTC material 14 and the coating 19 be made of suitable dimensions and materials so that they can be repea~edly flexed without breaking or causing any other problems. In spite of careful design and ~anufacture of the blanket wire, there will be occasions in which a break or fault will develop in the ~ conductors 16 and 18. When such a break or fault occurs and the blanket is connected to a source of electrical power, an elec-trical arc will often occur at the break. This arc will often cause burning of the PTC material.

--`" 1156300 1 In order to prevent any such condition, the electric blanket of the instant invention is provided with a safety cir-cuit designated generally by reference numeral 25 and shown in Fig. l enclosed in dotted lines. The safety circuit includes a fuse 26 which is connected in series with the power lead 20 to interrupt the circuit when the current to the blanket exceeds a predetermined minimum. In a preferred embodiment, the fuse 26 was a slow blow fuse which would blow after the current exceeded 2 amperes for a period of more than eight miliseconds.
As will be explained in greater detail below, it is important that the fuse be capable of withstanding brief pulses of current in excess of two amp rating due to the inrush current caused by the low cold resistance of the PTC material, and it would be undesirable to have the fuse blown on such occasions.
For the purpose of responding to overvoltage condi-tions in either of the conductors 16 or 18, there is provided a pair of neon lights 28 and 30 which are connected across the conductors 16 and 18 respectively as best shown in Fig, 3, The neon lights 28 and 30 require a voltage of 65 volts minimum to break down and, as a consequence during normal operating condi-tions for the blanket, the lights 28 and 30 are nonconducting.
Associated with the neon lights 28 and 30 is a photoresistor 32.
~he two neon lights and the photoresistor 32 are enclosed in a light-tight enclosure 34 so that the photoresistor 32 will respond only to the light from the neon lights 28 and 30. A
suitable conductor 35 connects one terminal of the photo-resistor 32 to the power lead 22 while the other terminal of the photoresistor is connected by a wire 36 to the gate 38 of a triac 40. The triac 40 is connected by wires 42 and 44 across the power leads 20 and 22 and is essentially in parallel with the blanket wire 12. Also connected in series with the triac 40 l is a two or four ohm resistor 46 which is intended to protect the triac 40 and control the blowing time of the fuse.
To permit washing the blanket 13, there are separable connections in the conductors extending between the circuit 25 and the heater 12 contained in the blanket. This arrangement presents the possibility that the circuit 25 may be energized while the heater 12 is not connected as shown in Fig. 1 and 3.
In such circumstances, leakage currents present in the circuit may result in the breakdown of the neon lights 28 and 30 and conduction by the triac 40 causing the fuse 26 to blow in an undesired manner. To overcome this problem, a pair of one megohm leakage shunting resistances 47 have been connected across the neon lights 28 and 30. These resistances 47 prevent the above-described blowing of the fuse 26 from occurring.
In normal operation of the safety circuit 25, either of the neon lights 28 or 30 may sense an overvoltage created by a break in either of the conductors 16 or 18. When any such break occurs, the voltage across the conductor in which the break occurs rises over the 65 volt breakdown level and the neon light associated with that conductor conducts and is illuminated.
Illumination of either the lights 28 or 30 causes the photo-resistor 32 to decrease in resistance thereby causing the triac 40 to become conductive. Since the triac 40 and the limiting resistor 46 are connected across the power leads 20 and 22, conduction of the triac 40 causes a high current which results in blowing the fuse 20. Even though a slow blow fuse is employ-ed, the safety circuit 25 reacts quickly enough to prevent arc-ing which would otherwise be associated with the open in the conductor 16 or the conductor 18.
The fuse 26 must be a slow blow fuse to prevent blow-ing the fuse when there are simply high voltage transients in 1 the line or high inrush currents and no overvoltage created by a break in either of the conductors 16 or 18. The safety cir-cuit 25, therefore, provides a simple and effective means for interrupting the power to the blanket 10 in the event of a break or arcing condition in the conductors 16 and 18.
As an alternative embodiment to the circuit of Fig. 1, it would be possible and practical to substitute a high power photocell or a light activated SCR or triac for the photo-resistor 32 and thereby eliminate the triac 40 as shown in Fig.
4. In the embodiment of Fig. 4, a high power photocell 80 is connected to power leads 20 and 22 in parallel with the blanket wire 12 providing a short circuit route in series with the fuse 26 between the leads 20 and 22 to be operative in the event that there were a break in either the conductor 16 or conductor 18.
The photocell 80 is enclosed in a light-tight envelope 82 along with neon lights 84 and 86 which break down in the same manner described above in connection with neon lights 28 and 30.
However, the photocell becomes sufficiently conductive to blow the fuse 26 eliminating the need for the separate triac 40 included in the embodiment of Figs. 1 and 3.
A further alternative embodiment is shown in the cir-cuit of Fig. 5. In lieu of the neon lights 28 and 30, there are provided coils 50 and 52 which are portions of a two coil relay 54 having one of the coils connected across each of the con-ductors 16 and 18. The relay 54 includes a switch 56 connected across the leads 20 and 22 in order to short out the power supply through the fuse 26 in the event of an interruption in either of the conductors 16 and 18. The coils are associated with a core and armature for closing switch 56 at a voltage which would be produced in the case of an open circuit in either of the conductors 16 and 18. Under the normal blanket operating 1 conditions, there would not be sufficient flux developed to close the switch 56. However, in the event of a break in either of the conductors 16 or 18, the relay 54 closes switch 56 to provide the short circuit through the fuse 26 which, in turn, blows the fuse.
In Fig. 6, there is shown an embodiment of the inven-tion which makes use of a three element gas switch in lieu of the neon lights and photoresistor of the preferred embodiment.
As shown in Fig. 6, a three element gas switch 60 is connected across each of the conductors 16 and 18. As is shown in Fig, 7, each of the tubes 60 has three terminals 60a, 60b and 60c which are connected as shown to three elements 60d, 60e and 60f within the tube. When there is sufficient voltage applied to terminals 60a and 60b, in excess of 65 volts, the gas ionizes and there is conduction through the tube 60. The element 60d includes a U-shaped bimetallic portion 64 which deflects when the tube 60 begins to conduct, This deflection of bimetallic portion 64 causes a switch 66 connecting elements 60d and 60f to close.
As is evident from Fig. 6, the terminals 60a and 60b of each of the gas tubes 60 are connected to opposite ends of the conduc-tors 16 and 18 while the terminal 60c is connected to the opposite side of the line from its respective terminal 60a.
When the potential between elements 60d and 60e reaches 65 volts, the tube 60 begins to conduct. This results in a bimetallic portion 64 associated with the numbered terminal 60a to deflect closing the switch 66 and resulting in a connec-tion between terminals 60a and 60c of the tube 60. Thus, when either of the tubes 60 are subjected to an overvoltage condition in either of the conductors 16 or 18, the ~ubes 60 begin conduct-ing which results in the closing of switch 66 thereby providinga short circuit across the line through the fuse 26. This -- i 1 56300 1 results in the fuse 26 being blown thereby preventing any arcing or fire in connection with the break which has occurred in the conductors 16 or 18.
In Fig. 8, there is shown a further alternative embodiment for deactivating the circuit by blowing the fuse in the event of a break in either of the conductors 16 or 18. In Fig. 8, a thermal fuse 70 is employed rather than the more conventional current responsive fuse element. The thermal fuse 70 includes a fuseable element 72 and heaters 74 and 76 arranged in close proximity to the fuse element 72. When there is a break in either of the conductors 16 or 18, there will be a higher voltage applied to the heaters 74 or 76 which will generate sufficient heat to melt the fuse element 72 thereby opening the circuit.
Although there have been disclosed a number of differ-ent embodiments of the invention for deactivating a blanket heater circuit in the event of a break occurring in either of the conductors included in the blanket wire, all of the various applications serve to eliminate the arcing and fire hazard as~ociated with such PTC wire, While several embodiments of the present invention have been shown, it will be understood that various changes and modifications will occur to those skilled in the art, and it is contemplated in the appended claims to cover all such changes and modifications as fal]. within the true spirit and scope of the present invention.
j

Claims (14)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. An electric blanket comprising an elongated flex-ible wire heating element having a pair of spaced conductors extending the length of said element, a positive temperature coefficient resistance material positioned between said con-ductors to function as a self-limiting heater when said conductors are connected to a source of electrical power, a sheath of electrically insulating material enclosing said conductors and said resistance material, circuit means connected to the ends of said conductors to sense an open circuit in said conductors, said circuit means including a voltage sensitive element connected across each of said conductors, a fuse connected in series with one of said conductors, said voltage sensitive element sensing an increase in voltage caused by a break in the respective conductor it is connected across, switch means connected in series with said fuse and in parallel with said heating element, said switch means being responsive to said voltage sensitive element to increase current flow through said fuse to interrupt the circuit at said fuse.

2. The electric blanket of claim 1 wherein said voltage sensing element comprises neon bulbs which conduct at a voltage in excess of the drop across the length of each conduc-tor, and a photoresistor associated with said neon bulbs to actuate said switch means in the event of an open circuit in one of said conductors.

3. The electric blanket of claim 2 wherein said switch means comprises a triac connected in parallel with said heating element.

4. The electric blanket of claim 1 wherein said posi-tive temperature coefficient material comprises a material which will ignite if heated by an arc formed between the ends of a broken conductor, said fuse having slow response characteristics so as to be unaffected by surges caused by normal voltage transients which will be sensed by said voltage sensing element and results in momentary actuation of said switch means, said fuse interrupting said circuit in the event of an open circuit in said conductors prior to ignition of said PTC material.

5. The electric blanket of claim 1 wherein said voltage sensitive elements comprise coils of relay, said relay including a mechanical switch connected in parallel with said heating element and in series with said fuse,

6. The electric blanket of claim 1 wherein said voltage sensing elements comprise neon lights which conduct and become lighted at a voltage in excess of the drop across the length of each conductor, and said switch means includes a light responsive cell which conducts when either of said lights becomes conductive to cause the fuse to interrupt the circuit at the fuse.

7. The electric blanket of claim 1 wherein said voltage sensing elements comprise gas tubes which conduct when the voltage exceeds the normal drop across the length of each conductor, said switch means including a bimetallic switch with-in each of said gas tubes responding to conduction in each of said tubes to close the respective bimetallic switch in the conducting tube to cause the fuse to interrupt the circuit at the fuse.

8. An electric blanket comprising an elongated flex-ible wire heating element having a pair of spaced conductors extending the length of said element, a positive temperature coefficient resistance material positioned between said con-ductors are connected to a source of electrical power, a sheath of electrically insulating material enclosing said conductors and said resistance material, circuit means connected to the ends of said conductors to sense an open circuit in said con-ductors, said circuit means including a voltage sensitive ele-ment connected across each of said conductors, a fuse connected in series with one of said conductors, each said voltage sensitive element sensing an increase in voltage caused by a break in the respective conductor across which it is connected, means being responsive to said voltage sensitive element to interrupt the circuit at said fuse.

9. The electric blanket of claim 1 wherein said voltage sensing elements comprise heating elements in close heat transfer association with said fuse, said fuse being a thermal fuse which opens the circuit in response to overvoltage conditions caused by an open circuit in either of said conductors.

10. An electric blanket comprising an elongated wire including a pair of closely spaced conductors having a positive temperature coefficient material between said conductors, means for connecting opposite ends of said wire to a source of elec-trical power with one end of one conductor and the distal end of the other conductor being joined by said connecting means to said power source, a current sensitive means in series with said connecting means and said wire, a voltage sensitive means con-nected across each of said conductors, means operated by said voltage sensitive means to short circuit said wire to cause said current sensitive means to open said circuit externally of said wire when there is an open circuit in either of said con-ductors.

11. An electric blanket comprising an elongated wire including a pair of closely spaced conductors having a positive temperature coefficient material between said conductors, means for connecting opposite ends of said wire to a source of electrical power with one end of one conductor and the distal end of the other conductor being joined by said connecting means to said power source, current interrupting means in series with said connecting means and said wire, sensing means con-nected across each of said conductors to cause said current interrupting means to open said circuit externally of said wire when there is an open circuit in either of said conductors.

12. The electric blanket of claim 11 wherein said spaced conductors are of low resistance, a heating effect which is produced by energizing said wire resulting primarily from the current flow through said positive temperature coefficient material.

13. The electric blanket of claim 10 including a fabric shell having spaced layers between which said elongated wire is disposed in a tortuous configuration to deliver heat to the entire blanket shell.

14. The electric blanket of claim 12 wherein said current interrupting means comprises a slow blow fuse and said sensing means comprises a circuit for short circuiting said wire in response to a voltage increase over the length of one of said conductors.