Flexible self-regulating heating pad combination and associated method

A flexible, electric heating pad having a self-limiting heating element is provided. The heating element includes a pair of relatively spaced conductors surrounded by a positive temperature coefficient (PTC) material. A layer of electrically insulating material surrounds the PTC material. The PTC material includes a polyolefin resin having a relatively low flexural modulus. The heating element is disposed in a generally serpentine configuration within passages formed in the covering material of the heating pad. The passages are formed by joining together layers of covering material using ultrasonic welding or other suitable ways. The temperature of the heating element is controlled using controller circuit which utilizes a solid state timed interval control circuit. A safety circuit is provided for non-resettably disconnecting electrical power from the heating element if short or open circuit conditions occur therein. A temperature indicator and a variable automatic shutoff circuit are also provided with the controller circuit. A method of making the heating pad is also provided.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to an electric heating pad, and, more particularly, 
to a flexible electric heating pad having a self-limiting heating element 
and method of making such heating pad. 
2. Description of the Prior Art 
Electric heating pads are widely used for therapeutic and general comfort 
purposes. Heating pads currently available generally utilize fixed 
electrical resistance type heating elements. A problem that arises with 
fixed resistance type heating elements is that they are subject to 
overheat conditions if excess heat is not dissipated from the heating 
element. Overheat conditions often arise when the user of the heating pad 
places his or her body over the heating pad, as by laying or sitting on 
the pad. This often occurs when the heating pad is used in bed. When used 
in this manner, all of the surface area of the pad is covered and excess 
heat cannot dissipate from the heating element. Overheat conditions can 
also arise when clothing or other covering material is placed over the 
pad. Thermostats may be used to partially remedy the problem of overheat, 
but thermostats will not work if the area of overheat is located some 
distance from the thermostat. 
Because of the possibility of overheating occurring, fixed resistance 
heating pads must be placed over the area where the application of heat is 
desired with the opposed pad surface being exposed to the air. This may 
require that the user position himself or herself in an uncomfortable or 
inconvenient manner. 
When overheat conditions occur, the temperatures attained often result in 
breakdown or combustion of the insulation material on the heating element 
or combustion of the heating pad covering material. As a result, there is 
serious risk of fire and injury when overheat conditions occur in such 
heating pads. 
Another problem with fixed resistance heating pads is that the pads lack 
flexibility. That is, if the pad is flexed, or rolled up, it may damage 
the electrical integrity of the heating element. 
The use of positive temperature coefficient (PTC) material to provide 
self-limiting heating elements has been known. U.S. Pat. Nos. 4,271,350; 
4,309,596; 4,348,584; 4,818,439; and 4,277,673 disclose examples of 
heating elements which include various types of PTC materials. PTC 
material exhibits variable electrical resistance with temperature. As the 
temperature of the material increases, its electrical resistance also 
increases. When the material reaches a predetermined maximum temperature, 
its electrical resistance is effectively infinite and, the material will 
not heat past that predetermined maximum temperature. The predetermined 
maximum temperature is determined by the composition of the PTC material. 
PTC material typically includes a polymer in which electrically conductive 
particles are suspended. A typical example is disclosed in U.S. Pat. No. 
4,277,673. 
Because of the characteristics exhibited by PTC materials, heated articles 
using PTC containing heating elements are much less susceptible to 
overheat conditions. U.S. Pat. No. 5,081,339 discloses a waterbed heater 
which utilizes a PTC heating element. U.S. Pat. Nos. 4,309,596 and 
4,348,584 disclose examples of PTC heating elements which it said may be 
used in electric blankets. 
Some problems associated with PTC heating elements include decreased 
flexibility of the heating elements as the size of the heating element is 
increased and decreased stability of the PTC material with aging. In 
addition, small PTC heating elements which produce sufficiently high 
temperatures for use in the heating pads are often not sufficiently 
flexible for use in such articles. 
PTC heating elements may present fire hazards if short or open circuit 
conditions occur therein. It has been known to utilize safety circuits 
which non-resettably discontinue electrical power from the heating element 
should short or open circuit conditions occur. An example of such a 
circuit is disclosed in U.S. Pat. No. 4,436,986. 
Despite the prior art systems, there remains a need for a flexible electric 
heating pad which minimizes risk of overheating conditions occurring. 
There also remains a need for a PTC heating element that provides improved 
heating performance, which provides increased flexibility of the heating 
element and which provide improved stability of the PTC material. 
SUMMARY OF THE INVENTION 
The present invention has met the hereinbefore described needs. A flexible 
electric heating pad utilizing a self-limiting heating element is 
provided. The heating element includes two spaced conductors surrounded by 
PTC material. The PTC material may be surrounded by a layer of 
electrically insulating material. The PTC material preferably includes a 
polyolefin resin, such as medium density polyethylene for example, having 
a relatively low flexural modulus. The use of this type of material 
affords increased temperature resistance as compared with low density 
resins due to the increased density and accompanying crystallinity and 
also does not increase the stiffness of the heating element due to the low 
flexural modulus of the material. 
The PTC heating element is disposed in a generally continuous serpentine 
configuration within passages formed in the heating pad. The passages are 
formed by joining opposed layers of the material covering the pad into 
generally linear regions of joinder. Electrical plug means are provided on 
the heating pad for connecting the heating element to a power supply 
cable. The power supply cable is adapted to connect to a source of 
electrical power, thereby supplying electrical power to the heating 
element. 
Solid state controller means are preferably provided for controlling the 
electrical power supply to the heating element to vary the temperature of 
the heating element. The controller means includes timed interval control 
means for varying the percentage of a predetermined cycle of time during 
which electrical power is supplied to the heating element. The timed 
interval control means may be adjusted using switch means to vary the 
temperature of the heating element. Indicator means may be provided to 
display the relative temperature of the heating element as related to the 
percentage of time during which electrical power is supplied thereto. 
A safety circuit may be provided to discontinue electrical power from the 
heating element if short or open circuit conditions occur therein. The 
safety circuit includes one-shot fuse means for non-resettably 
disconnecting electrical power if such conditions occur. In addition, the 
plug means includes means which ensures that the safety circuit is 
complete when the heating element is energized. 
The controller means and power cord may be releasably connected to the 
heating pad such that they may be disconnected to allow the heating pad to 
be washed. A textile covering may be used to enhance the washability and 
appearance of the pad and to provide a soft-feeling cover thereover. In 
addition, water may be applied to the textile covering such that the 
heating pad may be utilized to provide "moist heat". 
A method of making the heating pad is also provided. 
It is an object of this invention to provide a flexible electric heating 
pad having a self-limiting heating element and to provide a method of 
making such a pad. 
It is another object of this invention to provide a heating pad which 
utilizes a PTC heating element and to provide a method of making such a 
pad. 
It is a further object of this invention to provide an electric heating pad 
which provides increased resistance to overheat conditions. 
It is another object of this invention to provide an electric heating pad 
which utilizes a solid state controller. 
It is another object of this invention to provide a heating pad, and a 
method of making a heating pad, that does not require the use of a sealed, 
water tight envelope enclosing the heating element thereof. 
It is yet another object of this invention to provide a heating pad which 
is adapted to deliver constant heat, without significant temperature 
cycling. 
It is a further object of this invention to provide a heating pad which 
does not require the use of thermostats in the heating area thereof, and 
to provide a method of making such a heating pad. 
It is still another object of this invention to provide a PTC heating 
element that can be energized using a 12 volt, 120 volt or 240 volt power 
supply. 
It is yet another object of this invention to provide a heating pad, and a 
method of making a heating pad, that produces low electromagnetic field 
levels. 
It is an object of this invention to provide a heating pad, and a method of 
making a heating pad, that may be provided with a flexible, textile cover. 
It is further object of this invention to provide an electric heating pad 
which may be used when the pad is covered by the user's body or by other 
covering means without undue risk of overheating. 
It is still another object of this invention to provide a heating pad which 
may be used to apply "moist heat" or "dry heat" as desired. 
It is yet another object of this invention to provide a PTC heating element 
that utilizes medium density polyolefin material having a relatively low 
flexural modulus. 
It is another object of this invention to provide a PTC heating element 
that generates sufficient heat for use in heating pads, yet which is 
sufficiently flexible for such use. 
It is the object of this invention to provide a PTC heating element that 
provides improved stability of the PTC material. 
It is another object of this invention to provide a solid state controller 
for a PTC heating element which utilizes timed interval control means to 
adjust the temperature of the heating element. 
It is another object of this invention to provide controller for a PTC 
heating element that provides an indication of the relative temperature of 
the heating element. 
It is another object of this invention to provide controller for a PTC 
heating element that includes a safety circuit to non-resettably 
disconnect the electrical power from the heating element if short or open 
circuit conditions occur therein. 
These and other objects will be more fully understood from the following 
description on reference to the illustrations appended hereto.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
As used in herein, the term "flexible" as applied to a heating pad means 
the pad being able to bend such that opposite corners thereof may be 
brought into contact with one another without damaging the electrical 
integrity of the heating element within the heating pad. 
Referring to FIGS. 1 through 6 there is shown a preferred embodiment of the 
heating pad 2 of this invention. Heating pad 2 preferably includes a pad 
portion 4, a power cable 6, and a controller 8. Pad portion 4 includes a 
flexible outer covering material 10 forming an outer cover on pad portion 
4. As shown in FIGS. 4-6, disposed within outer cover 10 is self-limiting 
heating element 12 which contains positive temperature coefficient (PTC) 
material. As shown more particularly in FIG. 6, heating element 12 is 
preferably disposed in a generally serpentine configuration within cover 
10. Ends 14, 16 of heating element 12 are electrically connected to plug 
means 18. Power cable 6 and controller 8 are electrically connected to 
heating element 12 at plug means 18. 
Referring more particularly to FIGS. 1-3 and 5, opposed layers of outer 
cover 10 are joined together in at least one location to form regions of 
joinder 20. Regions of joinder 20 are preferably generally linear and 
defined generally linear passages 22 into which heating element 12 is 
received. Passages 22 are separated by generally linear barriers 24 formed 
by generally linear regions of joinder 20. Regions of joinder 20 are 
preferably discontinuous. Passages 22 are preferably about 1/2 to 11/2 
inches wide with the center to center distance between passages 22 being 
about 0.75 to 2 inches. 
Referring now to FIGS. 4 and 5, in a preferred embodiment, pad portion 2 
includes two adjacent layers of scrim material 26. The two layers of 
batting material 28 are disposed in surface to surface contact with scrim 
material 26. Outer cover material 10 forms outer cover of scrim material 
26 and batting material 28. Passages 22 into which heating element 12 is 
received are defined between the layers of scrim material 26. Opposing 
layers of outer cover material 10 are preferably joined together through 
intervening layers of scrim material 26 and batting material 28 in regions 
and joinder 20. 
In a preferred embodiment, scrim material 26 is preferably a spun bonded 
polyester material about 2 to 10 mils thick and batting material 28 is 
preferably a high loft polyester needle punch material about 1/8 to 1/4 
inches thick. However, it will be appreciated that any suitable materials 
may be used for scrim material 26 and batting material 28. Outer cover 
material 10 is preferably a textile material containing at least one of 
cotton and polyester. In a preferred embodiment, outer cover 10 is a 
velour material, the exposed surface thereof being soft to the touch. 
Outer cover 10 is preferably made of a washable textile material. In 
addition, water may be applied to outer cover 10 in any desired manner, 
such as sprinkling, while the pad is in use such that "moist heat" or "dry 
heat" may be applied using the pad. 
In a preferred embodiment, opposed layers of outer cover material 10 are 
joined together to form barriers 24 using ultrasonic welding. It will be 
appreciated, however, that radio frequency welding, heat welding, sewing 
or any other suitable means of joining the layers of material together may 
be used. The peripheral edges of outer cover material 10 of pad portion 4 
are preferably sewn together. However, it will be appreciated that any 
suitable manner of joining the material may be used. 
In a preferred embodiment, pad portion 4 is preferably generally 
rectangular in shape and about 10 to 24 inches long, about 6 to 18 inches 
wide and about 0.375 to 1.25 inches thick. However, it will be appreciated 
that pad portion 4 may be of virtually any desired shape and of any 
desired size. In addition, in a preferred embodiment, about 35 to 80 
percent of the surface of pad portion 4 is heated. It will also be 
appreciated that any desired percentage of the surface of pad portion 4 
may be heated. 
Referring more particularly to FIGS. 4-6, heating element 12 is preferably 
disposed in a generally continuous serpentine configuration in passages 
22. In a preferred embodiment the total length of heating element 12 in 
pad portion 4 is about 10 to 40 feet. However, it will be appreciated that 
the length of heating element used may vary depending upon the size of pad 
portion 4 and the size of the area of pad portion 4 which is to be heated. 
Each passage 22 preferably contains at least one generally U-shaped loop 
40 of heating element 12. Each loop 40 contains a total of about 1 to 3 
feet of heating element 12 which preferably has been doubled back on 
itself such that two parallel lengths 42, 44 of heating element 12 are 
positioned directly adjacent and parallel to one and another. The 
electrical current in each length 42 will be flowing in generally the 
opposite direction of that flowing in each length 44. As a result, by 
positioning parallel lengths 42, 44 of heating element 12 directly 
adjacent to one another, the electromagnetic field generated around each 
length 42, 44 when electrical circuit is flowing therethrough will tend to 
cancel one another. This electromagnetic field cancellation effect will 
reduce the overall electromagnetic field generated by the heating pad 
during operation. 
In a preferred embodiment, about 3 to 30 loops 40 of heating elements 12 
are disposed within pad portion 4. However, it will be appreciated that 
any desired number of loops 40 may be used in order to ensure that heating 
element 12 is provided over the desired surface area of the pad portion 4 
which is to heated. In addition, the total length of heating element 12 
utilized in each loop 40 may be varied to accommodate pad portions 4 of 
various dimensions. 
Referring now to FIG. 7, there is shown a cross-sectional view of heating 
element 12 of this invention. Heating element 12 includes a pair of 
relatively spaced conductors 46, 48 surrounded by PTC material 50. A layer 
of insulating material 52 preferably surrounds PTC material 50. In a 
preferred embodiment, heating element 12 is preferably generally dog bone 
shaped in cross-section, having two generally circular portions 54, 56 and 
a connecting portion 58. One conductor 46, 48 is preferably generally 
centrally disposed in each circular portion 54, 56. In a preferred 
embodiment, the thickness of connecting portion 58 will be generally 
uniform and less than the diameter of generally circular portions 54, 56. 
However, it will be appreciated that the thickness of connecting portion 
58 may be equal to the diameter of generally circular portions 54, 56, 
with the cross-sectional shape of heating element 12 being generally oval. 
The thickness of the layer of PTC material 50 surrounding conductors 46, 48 
in generally circular portions 54, 56 is preferably about 0.003 to 0.015 
inches. The thickness of PTC material 50 in connecting portion 58 is 
preferably about 0.020 to 0.060 inches. The center to center distance 
between conductors 46, 48 is preferably about 0.055 to 0.090 inches. 
Electrical insulating layer 52 is preferably about 0.010 to 0.020 inches 
thick. Overall, heating element 12, is preferably about 0.10 to 0.20 
inches wide by 0.060 to 0.090 inches high. 
Each conductor 46, 48 preferably includes at least one conductive wire 60 
spirally wrapped around a flexible core 62. In a preferred embodiment, two 
wires 60 are wrapped around each core 62 to form conductors 46, 48. Wires 
60 are preferably two 30-36 AWG hard tinned cadmium copper alloy wires. It 
will be appreciated that other suitable types of conductive wire which are 
sufficiently flexible may be used for wire 60. In addition, it will 
appreciated that other materials may be used to coat the copper alloy 
wire, such as nickel or silver, for example. Core 62 preferably includes 
at least one polyester filament. In a preferred embodiment, core 62 
includes two strands of 1100 denier polyester yarn, with each yarn having 
a diameter of about 0.010 to 0.025 inches. A suitable yarn is sold by E. 
I. DuPont DeNemours & Company, Inc. under the designation Type 68 Dacron 
Polyester Filament with K6418 Finish. However, it will be appreciated that 
any suitable material which is sufficiently strong and flexible, such as 
fiberglass for example, may be utilized as core 62. Wires 60 are wrapped 
around core 62 at a rate of about 10 to 40 turns per inch of core. In a 
preferred embodiment, conductors 46, 48 may be coated with an electrically 
conductive colloidal graphite containing material 64. Such a coating 64 
improves the stability of the interface between conductor 46, 48 and PTC 
material 50. Coating 64 may also improve electrical conductivity across 
the interface between conductors 46, 48 and PTC material 50 and within 
conductors 46, 48. A suitable material for coating 64 is sold by Acheson 
Colloids Company under the designation ELECTRODAG. However, any suitable 
colloidal graphite material may be used. 
PTC material 50 preferably includes a mixture of one or more types of 
polyolefin resin, polyolefin copolymers and carbon black particles. In a 
preferred embodiment, the polyolefin used is polyethylene resin. However 
any suitable polyolefin resin, such as polypropylene or polyethylene, may 
be used. It has been found that the use of medium density polyethylene 
resin having a relatively low flexural modulus will result in a heating 
element having increased temperature resistance resulting from increased 
density and accompanying crystallinity of the polyethylene, yet which does 
not suffer from greatly increased stiffness due to the low flexural 
modulus of the polyethylene material. PTC material 50 preferably includes 
about 30 to 70 percent by weight polyolefin resin having a density of 
about 0.90 to 0.96 grams/cc and a flexural modulus of about 20,000 to 
150,000 psi. In a preferred, embodiment, PTC material includes about 0 to 
70 percent by weight polyethylene resin having a density of about 0.928 to 
0.958 grams/cc and a flexural modulus of about 50,000 to 150,000 psi. It 
has been found that a material that is particularly well suited for this 
application is sold by Phillips 66 Corporation under the designation 
MARLEX HHM TR-400. However, it will be appreciated that any suitable 
material having the desired characteristics may be used. 
In a preferred embodiment, PTC material 50 also includes about 30 to 70 
percent by weight of a blend of polyolefin copolymer resin, such as 
ethylene ethyl-acrylate, ethylene methyl-acrylate, or ethylene 
vinyl-acetate, for example, and carbon black. This blend includes about 45 
to 55 percent by weight polyolefin copolymer resin having a density of 
about 0.920 to 0.940 grams/cc and flexural modulus of about 5,000 to 
20,000 psi and about 45 to 55 percent by weight carbon black having a 
particle size of about 10 to 40 nanometers. This blend will yield a PTC 
material having a total of about 14 to 32 percent carbon black. A suitable 
polyolefin copolymer resin is sold by Union Carbide Corporation under the 
designation DPDA 9169. Suitable carbon black is sold by Cabot Corporation 
under the designation BLACK PEARLS L. It will be appreciated, however, 
that any suitable polyolefin copolymer and carbon black may be used. 
The percentages of polyethylene, polyolefin copolymer and carbon black in 
the PTC material may be altered by the addition of various types of 
inorganic filler material, such as alumina trihydrate, calcium carbonate, 
or magnesium hydroxide, for example. Such inorganic fillers may be 
incorporated to improve economy, flame retardance, and/or arc suppression. 
The total percentage of inorganic filler material added to the PTC 
material may be as high as 10-30% by weight. 
In a preferred embodiment, a layer of insulating material 52 includes a 
thermoplastic polyolefin material. A suitable material is sold by Furon, 
Inc. under the designation FMO-201. This material provides sufficient 
flexibility for this application and provides desired flame retardant 
characteristics. It will be appreciated, however, that any suitable 
electrically insulating material that is sufficiently flexible may be 
used. 
Heating element 12 preferably delivers about 1.0 to 4.0 watts per linear 
foot of heating element 12 and generates a maximum surface temperature of 
about 50 to 80 degrees Centigrade in free air. 
It has been found that the described combination of polyolefin resins, 
polyolefin copolymers, and carbon black particles produces a PTC material 
that is particularly well suited for use in heating pad applications. A 
heating pad element using this PTC material is capable of producing the 
higher temperatures necessary for heating pad applications, yet is 
sufficiently flexible to be formed into the tight loops required for such 
applications. The use of a flexible heating element enhances the 
flexibility of the heating pad, or other heated article, in which it is 
used. 
In order to provide a more detailed disclosure of the invention, an example 
will be considered. In this example, each conductor includes two 35 AWG 
hard tinned cadmium copper alloy (99% percent copper, 1% cadmium) wires 
wrapped around two strands of 1100 denier polyester core yarns at 14 turns 
per inch. The conductors are coated with a layer of colloidal graphite 
consisting of ELECTRODAG 154 sold by Achenson Colloid Company. The PTC 
material includes 49.5 percent polyethylene resin having a density of 
0.939 grams/cc and a flexural modulus of 90,000 psi, 27.0 percent ethylene 
ethyl-acrylate having a density of 0.931 grams/cc and flexural modulus of 
9,000 PSI, and 23.5 percent carbon black having a particle size of 24 
nanometers. The electrical insulating material is Furon FMO-201 flame 
retardant thermoplastic polyolefin. 
The heating element described in this example delivers about 2.5 watts per 
linear foot of heating element at 120 volts and produces a maximum surface 
temperature of about 80 degrees Centigrade in free free. 
It will be appreciated that this heating element may be effectively 
utilized with power supply voltages of about 12 to 240 volts. 
However, when a 12 volt power source is utilized, the configuration of the 
heating element may preferably be somewhat different than that used with 
120 volt and 240 volt power sources. Referring to FIG. 8, there is shown a 
cross-sectional view of 12 volt heating element 66. Heating element 66 
includes a pair of relative space conductors 68, 70 surrounded by a layer 
of PTC material 72. In a preferred embodiment, each conductor 68, 70 is 
stranded and has a diameter of about 22 to 30 AWG. No flexible core is 
provided. Each stranded conductor 68, 70 may include about 7 to 65 strands 
71 of about 27 to 44 AWG hard drawn tinned cadmium-copper alloy wire. 
Conductors 68, 70 are preferably rope lay or bunch stranded. The use of 
stranded conductors results in greater flexibility of the conductor, while 
providing greater cross-sectional area to carry electrical current. The 
use of lower voltage requires significantly higher electrical current than 
is required with higher voltage systems in order to produce the same 
wattage. Accordingly, a 12 volt heating element may be required to carry 
ten or more times the current than is required to be carried by 120 volt 
heating elements in order to deliver similar wattage. 
PTC material 72 of 12 volt heating element 66 is generally similar to the 
PTC material utilized in the 120 volt heating element. PTC material 72 
preferably includes about 25 to 65 percent polyolefin resin, such as 
polyethylene, having a density of about 0.90 to 0.96 g/cubic centimeter 
and a flexural modulus of about 20,000 to 150,000 psi. In a preferred 
embodiment, the polyolefin is a polyethylene resin having a density of 
about 0.928 to 0.958 grams/cubic centimeter and a flexural modulus of 
about 50,000 to 150,000 psi. PTC material also preferably includes about 
75 to 40 percent of a blend of polyolefin copolymer resin and carbon 
black. This blend preferably includes about 55 to 45 percent polyolefin 
copolymer, such as ethylene ethylacrylate, having a density of about 0.920 
to 0.940 grams/cubic centimeter and a flexural modulus of about 5,000 to 
20,000 psi, and about 45 to 55 percent carbon black having a particle size 
of about 10 to 40 nanometers. This yields a PTC material containing about 
19 to 35 percent carbon black. Because of the lower voltage, it is 
preferred to have a slightly higher percentage of carbon in the PTC 
material than is required with higher voltages. Suitable materials for the 
ingredients of the PTC material of heating element 66 have been discussed 
hereinbefore with respect to heating element 12 shown in FIG. 7. 
Because of the low voltage, no layer of electrical material is required 
over the PTC material, and no insulating layer is used in the preferred 
embodiment. However, if a layer of insulating material is used, it will be 
generally identical to that discussed hereinbefore with respect to the 
other embodiment of the heating element. A colloidal graphite coating on 
conductors 68, 70 may also be used, as discussed hereinbefore. 
The dimensions of the heating element of this embodiment are substantially 
identical to those of the other embodiment discussed hereinbefore. 
Referring now to FIGS. 1 and 9, there is shown the controller 8 of this 
invention. Controller 8 is for controlling the electrical power supplied 
to the heating element. Controller 8 includes housing 76. Housing 76 
includes on/off switch 78 for turning the electric power on and off. In a 
preferred embodiment, on/off switch 78 is a single button. However, it 
will be appreciated that separate buttons or other types of switch means 
may be provided for selectively turning power on and turning power off. 
Housing 76 also includes switches 80, 82 for manually varying the 
temperature of she heating element. Manipulation of switches 80, 82 
activates timed interval control means, the operation of which will be 
more fully discussed hereinafter, which controls the temperature of the 
heating element of heating pad 2. In a preferred embodiment, separate 
buttons 80, 82 are provided for increasing and decreasing the temperature 
of the heating element. In addition, in a preferred embodiment, raised 
arrows may be provided on buttons 80, 82 to indicate which button raises 
temperature and which button lowers temperature. 
Controller 8 also includes indicator means 84 for displaying the relative 
temperature of the heating element. In a preferred embodiment, indicator 
means 84 includes a plurality of lighted segments 86 which form a bar 
graph display 88. In a preferred embodiment, bar graph display 88 is a 
backlit LCD type element. It will be appreciated, however, that any 
suitable display may be used, such as LEDs for example. Bar graph display 
88 preferably includes about nine segments 86 illuminated in three colors. 
The bottom three segments are preferably illuminated in yellow, indicating 
that lowest temperatures; the middle three segments are preferably 
illuminated in amber; indicating moderate temperatures; and the top three 
segments are preferably illuminated in red, indicating the highest 
temperatures. When the power is turned on controller 8, at least one 
segment 86 become visible. As the temperature of the heating element is 
adjusted upward, the segment immediately above the last visible segment of 
the bar graph display 88 becomes visible. As temperature is adjusted 
downward, the topmost segment disappears from view. Bar graph display 88 
thereby indicates the relative temperature setting of the heating element 
in a number of ways. First, the color of the topmost segment that is 
illuminated indicates relative temperature setting, as discussed 
hereinbefore. Secondly, the total number of segments of the bar graph are 
simultaneously visible or illuminated, or the height of the bar graph, 
indicates relative temperature level of the heating element. In addition, 
visual and tactile indication means may be provided on housing 76 to show 
the general temperature setting indicated by a predetermined height of bar 
graph display 88. Visual indication means may be words, such as Hi, Lo, 
and Med. Tactile indication means may be one or more raised dots molded 
into housing 76. Indicator means 84 may alternatively be a digital display 
of the temperature of the heating element. In addition, indicator means 84 
may include an audible signal which indicates that an adjustment has been 
made to the temperature of the heating element. Such audible signal may 
include a beep or other tone which may be provided in a manner well known 
to those skilled in the art. 
Controller 8 is preferably made from LEXAN or ABS plastic material, or some 
other suitable material. Controller 8 is preferably of a size that is 
conveniently hand held. In a preferred embodiment, controller 8 is about 4 
to 6 inches long, about 1 1/2 to 2 1/2 inches wide, and about 1/2 to 1 
inches thick. It will be appreciated, however, that controller 8 may be of 
any desired size that will accommodate the necessary circuitry. 
Controller 8 is electrically connected to power cable 6 and to pad portion 
4. In a preferred embodiment power cable 6 includes connector means 96 on 
one end thereof for connection to a source of electrical power. Connector 
means 96 preferably includes adaptor 98 to connect power cable 6 to a 
conventional 120 volt household outlet. Because the heating element of 
this invention may be energized from 12 volt or 240 volt power supplies, 
in addition to 120 volt power supplies, the adaptor for connecting power 
cable 6 to a power supply may be of a type for connection to 240 volt or 
12 volt power supply. The other end of power cable 6 is electrically 
connected to the heating element of heating pad 2 through controller 8, as 
will be more fully discussed hereafter. 
Referring now to FIG. 10, there is shown a circuit diagram of controller 8. 
Electrical power input leads 100, 102 are connected to a 120 volt A.C. 
electrical power supply, whereby A.C. electrical power is provided to 
power control section 104 (indicated by the dashed rectangle), heating 
element 12, and safety circuit means 106 (indicated by the dashed 
rectangle). One shot fuse means 108 is preferably provided in lead 100. 
Fuse means 108 will non-resettably discontinue electrical power to heating 
element 12 and power control section 104 if short or open circuit 
conditions occur in heating element 12, as discussed hereinafter, or if 
fault conditions occur in controller 8. In a preferred embodiment, fuse 
means 108 is a fuse wire rated at about 5 amps continuous. However, it 
will be appreciated that any suitable fuse means may be utilized. 
Input leads 100, 102 provide electrical power to power control section 104. 
In a preferred embodiment, power control section 104 converts the 120 volt 
or 240 volt A.C. power to about +5 volts D.C., which is then provided to 
solid state microprocessor means 110. Microprocessor means 110 provides 
timed interval control means for controlling the relative temperature of 
heating element 12. Timed interval control means operate by controlling 
the percentage of a predetermined cycle of time during which electrical 
power is supplied to heating element 12. In a preferred embodiment, the 
cycle of time is approximately 1 minute. A higher percentage of the cycle 
of time that electrical power is supplied to heating element 12 results in 
heating element 12 reaching a higher temperature. In a preferred 
embodiment, the percentage of time during which electrical power is 
supplied to heating element 12 may be varied, thereby permitting the 
adjustment of the temperature of heating element 12. The percentage of 
time during which electrical power is supplied to heating element 12 may 
preferably be varied between about 5 to 100 percent. 
In a preferred embodiment, switches 80, 82 are used to adjust the relative 
temperature of heating element 12. Activation of switch 80 increases the 
temperature of heating element 12 by increasing the percentage of the 
predetermined cycle of time during which electrical power is supplied to 
heating element 12. Activation of switch 82 decreases the temperature of 
heating element 12 by decreasing the percentage of time during which 
electrical power is supplied thereto. 
In a preferred embodiment, the percentage of the cycle time during which 
power is supplied to heating element 12 is controlled by microprocessor 
110 sending a D.C. control signal to triac 112, preferably through a 
resistor 114. In response to that control signal, triac 112 switches the 
A.C. power provided to heating element 12 on and off such that A.C. power 
is provided to heating element 12 for the desired predetermined percentage 
of time. 
On/off switch 78 is used to turn electrical power to heating element 12, 
microprocessor 110, and indicator means 84 on and off. Electrical power is 
supplied to indicator means 84 through microprocessor processor 110. 
Indicator means 84 displays the relatives temperature setting of heating 
element 12, as discussed hereinbefore, as related to the percentage of 
time during which electrical power is supplied to heating element 12. 
In a preferred embodiment, microprocessor 110 includes a preheat mode which 
supplies power to heating element 12 continuously (100% of the time) for a 
predetermined period of time immediately after activation of on/off switch 
78. The preheat mode permits rapid initial heating of heating element 12. 
The time during which the preheat mode is activated may preferably be 
varied depending upon the temperature setting of the heating element. A 
higher temperature setting will result in the preheat mode being activated 
for a longer period. In a preferred embodiment, the preheat mode may be 
reset by activating on/off switch 78. 
In a preferred embodiment, microprocessor means 110 includes automatic 
shutoff means for automatically discontinuing the electrical power to 
heating element 12 after a predetermined period of time. In a preferred 
embodiment, the period time after which power will be discontinued to 
heating element is variable. This period of time is preferably variably 
between about 1/2 to 2 hours. The period is preferably reset by activation 
of on/off switch 78. 
In a preferred embodiment, safety circuit means 106 is electrically 
connected to heating element 12. Safety circuit means 106 preferably 
includes a pair of two element SIDACtors 115, 116 electrically between the 
ends 117, 118 of heating element 12 which are connected directly to the 
electrical power supply. One element of each SIDACtor 115, 116 is 
electrically connected to either end 117 or 118 of heating element 12. The 
other element of each SIDACtor 115 is electrically connected to the other 
element of the other SIDACtor 116 such that SIDACtors 115, 116 are 
electrically connected in series between ends 117, 118 of heating element 
12. The junction of SIDACtors 115, 116 is electrically connected to a pair 
of capacitance means 119, 120 which are electrically connected between the 
ends 121, 122 of heating element 12 which are not directly connected to 
the power supply. At least one impedance element 123 is electrically 
connected between at least one SIDACtor 115, 116 and ends 117, 118 of 
heating element 12. If short or open conditions occur in heating element 
12, safety circuit means 106 will cause sufficient current to flow through 
fuse means 108 to cause it to open, thereby non-resettably discontinuing 
electrical power to heating element 12. 
Referring now to FIG. 11, there is an alternative embodiment of safety 
circuit means 106 (indicated by the dashed rectangle). In this embodiment, 
the outside elements of a three element gas to tube 124 are electrically 
connected between the ends 125, 126 of heating element 127 which are 
electrically connected to the power supply. A pair of resistors 128, 129 
are electrically connected between the ends 144, 145 of heating element 
127 which are not directly connected to the power supply. The center 
element 146 of gas tube 124 is electrically connected between resistors 
128, 129. At least one impedance means 147 is electrically connected 
between at least one of the outside elements of gas tube 124 and at least 
one of ends 125, 126 of heating element 127. If short or open circuit 
conditions occur in heating element 127, circuit means 106 causes 
sufficient current to flow through the fuse means 108 to non-resettably 
discontinue power to heating element 127. The safety circuit of the 
embodiment is more fully discussed in U.S. Pat. No. 4,436,986, the 
disclosure of which is expressly incorporated herein by reference. 
Referring to FIG. 12, there is shown an alternative embodiment for 
connecting heating element 180 to a power supply. In this embodiment, the 
ends 182, 184 and 186, 188 of each conductor 190, 192 are electrically 
connected together and then connected to a power supply. That is, each 
conductor 190, 192 is connected so as to form a continuous loop. Each loop 
is then connected to the power supply. This manner of connection tends to 
result in a relatively uniform drop voltage between conductors 190, 192 
thereby resulting in greater uniformity in the wattage generated 
throughout heating element 180. This manner of connecting heating element 
180 to a power supply tends to reduce the likelihood that arcing will 
occur across a break in either conductor 190, 192. 
Referring now to FIGS. 1 and 13, there is shown plug means 18 or 
electrically connecting power cable 6 and controller 8 to heating element 
12. Plug means 18 includes male portion 130 and female portion 132. Ends 
14 and 16 of heating element 12 are preferably connected to male portion 
130. Female portion 132 is preferably connected to power cable 6. However, 
it will be appreciated that either portion of plug means 18 may be 
connected to either power cable 6 or heating element 12 as desired. 
Female portion 132 includes two receptacles 134 preferably electrically 
connected to power cable 6. The connection may be made using any means 
known to those skilled in the art, such as soldering or crimping. The 
capacitance or resistance means of the safety circuit are also preferably 
electrically connected to female portion 132 of plug means 18. The safety 
circuit is preferably electrically connected to female portion 132 at 
receptacles 136. The connections of power cable 6 and the safety circuit 
to receptacles 134 and 136 are preferably enclosed within the body of 137 
of female portion 132. Body 137 is preferably made of an electrically 
insulating material and insulates the connections from one another as well 
as protects accidental contact with those electrical connections. 
Ends 117, 118 of conductors 46, 48 of heating element 12 are preferably 
electrically connected to prongs 138 of male portion 130 plug means 18. 
Ends 121, 122 of conductors 46, 48 are preferably connected to prongs 140 
of male portion 130. Ends 117, 118, 121, 122 may be connected to prongs 
138, 140 in any manner known to those skilled in the art, such as 
soldering or crimping for example. Body 142 of male portion 130 is 
preferably made of electrically insulating material. The connections of 
ends 117, 118, 121, 122 to prongs 138, 140 are preferably enclosed within 
body 142 of male portion 130 of plug means 18. This protects the 
connections from moisture when heating pad 2 is washed and also 
electrically insulates the conductors from one another and the surrounding 
environment. This also eliminates the need for a sealed water-tight cover 
over the heating element which is typically required with fixed resistance 
heating pads. 
Body 142 is preferably secured to pad portion 4 in such a manner as to 
permit prongs 138, 140 to be exposed. Body 142 may be secured to at least 
one of the scrim material, batting material and outer cover material using 
any means known to those skilled in the art, such as sewing or ultrasonic 
welding, for example. 
Power cord 6 and controller 8, along with the associated safety circuit may 
be releasably connected to heating element 12, and to pad portion 4, at 
plug means 18. To make the connections, prongs 138, 140 of male portion 
130 are inserted into receptacles 134, 136 of female portion 132, thereby 
completing the electrical circuit. This permits pad portion 4 to be washed 
without exposing the electrical components to water. In a preferred 
embodiment, prongs 140 are slightly longer than prongs 138. This permits 
the electrical connection of ends 121,122 of conductors 46, 48 to the 
safety circuit to be made prior to the electrical condition of heating 
element 12 to the power supply. As a result, the safety circuit will be 
complete at the time heating element 12 is energized, even if it becomes 
energized immediately upon connection of plug means 18. By insuring that 
the safety circuit is completed at the time heating element 12 is 
energized, the risk of damage or injury is lessened should short or open 
conditions be present in the heating element at the time it is energized. 
It will be appreciated that plug means 18 may be of any suitable 
configuration which facilitates the desired electrical connection of power 
cord 6 to heating element 12. 
Referring now to FIGS. 14A through F, there is shown the steps of the 
preferred embodiment of the method of making the flexible heating pad 150 
of this invention. Referring more particularly to FIG. 14A, at least two 
adjacent layers of flexible scrim material 152 are provided. Two layers of 
flexible batting material 154 disposed in surface to surface contact with 
the layers of scrim material 152. Two layers flexible textile coring 
material 156 is preferably placed in surface to surface contact with 
buffing material 154 thereby forming an outer cover 158 over heating pad 
150. All of the layers of scrim material 152, batting material 154, and 
covering material 156 are generally coextensive. 
Referring now to FIGS. 14A and 14B, the layers of textile material 156 are 
preferably separated by the scrim material 152 and the batting material 
154. At a peripheral portion 160 the layers of scrim material 152, batting 
material 154, and covering material 156 are joined such that at least one 
opening 162 remains to permit access to an area between the layers of 
scrim material 152. In a preferred embodiment, the assembly of the layers 
of scrim material 152, batting material 154 and covering material 156 is 
reversed and the peripheral portion of the layers is joined while there 
are in this reversed orientation. The joined layers are then turned inside 
out, such that covering material 156 is disposed on the outside and scrim 
material 152 is disposed on the inside, with batting material 154 
therebetween. This produces a smooth seam at peripheral portion 160. 
Referring to FIGS. 14A, 14C, and 14D the layers of covering material 156 
are joined together through intervening layers of scrim material 152 and 
batting material 154 to form at least one passage 164 between the layers 
of scrim material 152. In a preferred embodiment, the layers of covering 
material 156 are joined in a plurality of generally linear regions of 
joinder 166 to form a plurality of passages 164. In a preferred 
embodiment, regions of joinder 166 are preferably discontinuous. Regions 
of joinder 166 are preferably formed using ultrasonic welding. However, it 
will be appreciated that any suitable manner of joining the layers of 
material may be used, such as radio frequency welding, heat welding and 
sewing for example. 
Self-limiting heating element 168 is inserted into heating pad 150. A 
length of self-limiting heating element 168 is preferably formed using 
extrusion. Heating element 168 is generally identical to the heating 
element discussed hereinbefore. A die, of a type known to those skilled in 
the art, is preferably used to extrude the PTC material onto the 
conductors and to extrude the layer of electrically insulating material 
over the PTC material. 
At least one loop 170 of heating element 168 is inserted into each passage 
164 through opening 162 to form a generally continuous serpentine 
configuration. Plug means 172 is connected to the ends of heating element 
168. Plug means 172 is for electrically connecting heating 168 to a source 
of electrical power. As shown in FIG. 14E, plug means 172 is preferably 
secured to at least one of the scrim material 152, body material 154 and 
covering material 156. Referring to FIG. 14F, opening 162 is then closed 
along seam 174 using any suitable means known to those skilled in the art, 
such as sewing or ultrasonic welding. 
It will be appreciated that this invention provides a flexible heating pad, 
and a method for making the same, which utilizes a PTC-containing heating 
element to minimize the risk of overheat conditions occurring during the 
operation of the heating pad. It will also be appreciated that this 
invention provides a PTC heating element which delivers increased heating 
performance and which is sufficiently flexible for use in flexible heated 
articles. 
Whereas, particular embodiments of this invention have been described 
herein for purposes of illustration, it will be evident to those skilled 
in the art that numerous variations in the details may be made without 
departing from the invention as described in the claims appended hereto.