Note: Descriptions are shown in the official language in which they were submitted.
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ELECTRIC BLANKET OR PAD HAVING IMPROVED POSITIVE
TEMPERATURE COEFFICIENT ~EATER CIRCUIT
Back~round of the Invention
This invention relates generally to electric
blankets, heating pads and mattress pads and, more
specifically, to a novel heating element circuit for use
in such blankets and pads.
Electric blankets, electric mattress pads and
heating pads are silimar in that all three include some
type of elongated flexible heating element which is
disposed in a tortuous configuration within a fabric or
plastic sandwich designed to enclose and obscure the
heating element. The fabric or plastic sandwich is
sometimes termed a shell and it includes a plurality o~
passageways through which the heating element is threaded
so that the heat therefrom is deli~ered uniformly across
the face of the blanket or pad. There are normally a
number of spaced lengths of the heating element which are
disposed across the surface of the blanket or pad with
these portions of the heating element being spaced close
enough so that when the element is energized the user
gets the illusion of more or less uniform warmth across
the entire surface of the blanket or pad.
In the prior art blankets and pads there have
typically been some means to control the application of
electrical energy to the pad and also means to sense any
overheat conditions at local areas in the pad. The local
heat sensing is nec~ssary since the control means for the
blanket or pad might sense that power is required at the
same time an overheat oondition exists at some point in
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the blanket or pad. Such a local overheat condition may
be caused by the heating element being folded upon itself
or covered by the mattress or a blanket so that
temperature tended to increase in that particular area.
Where such folding or covering of the heating element
occurs, the heat is not dissipated from the element at
the same speed as in other areas of the blanket or pad
and dangerous overheat conditions begin to develop.
The temperature sensing means to respond to
such overheat conditions have typically taken the form oi
bimetallic thermostats positioned throughout the pad or
continuous sensing wires which more or less parallel the
heating element to sense any overheat conditions through-
out the length of the element. A third alternative
involves the use of a heating element which includes
positive temperature coefficient material as the heating
element. This type of material is self-regulating in
that increases in local temperature along the length of
the heating element cause that portion of the element to
receive less current and thereby reduces the power input
to the overheated area. There are a number of prior art
patents disclosing heating element configurations for use
in such PTC blankets.
One of the problems encountered in the prior
art PTC heating element circuits is the fact that a break
in one of the conductors included in the heating element
causes serious arcing at the location of the break and
has required that certain safety modules be included in
the circuits for such blankets. These safety modules
include means for disabling or shutting off the blanket
when an open circuit is sensed in either one of the
conductors. Since the inclusion of such circuit module
to shut off the blanket in the event of an open circuit
is expensive, it would be desirable to devise a circuit
which would eliminate the necessity for the safety module
or safety circuit.
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Summary of the Invention
The present invention involves an electric
blanket, heating pad or mattress pad having a positive
temperature coefficient heating cable including two
conductors which are separated by a layer of positive
temperature coefficient material. The cable is disposed
in a tortuous manner in the blanket, heating pad or
mattress pad with the ends of the cable being connected
so that the ends of each of the two conductors are
connected together. A cord connector is provided to
supply power to each of the two conductors. In the event
of a break or open circuit in either of the two
conductors in the heating cable, there is a very small
voltage drop across the break since the ends of the
brokèn conductor are connected together. Thus, the only
difference in voltage at the break would be a consequence
of the location of the break with respect to the ends of
the conductor. A break at the middle would produce no
drop across the break whereas a break at one end of the
conductor would produce a drop equal to the total voltage
drop through the length of the conductor which would be
on the order of less than 10 volts.
Accordingly, it is an object of the present
invention to provide an improved electric blanket circuit
utilizing a positive temperature coefficient heating
cable which has the ends of the heating cable connected
together to minimize the voltage drop across any brea~
which may occur in either one of the conductors in the
cable.
It is a fuxther object of the present invention
to provide an improved PTC heating element circuit which
avoids the need for a safet~ module to interrupt the
circuit in the event of an open circuit in one of the
conductors.
It is another object of the present invention
to provide an improved heating circuit for an electric
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blanket, mattress pad or heating pad having an elongated
PTC heatin~ element which is connected to be supplied
with power from both ends to eliminate any arcing problem
which might occur in the event of a break in one of the
conductors.
Further ob]ects 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 showing an
electric blanket embodying my invention;
Fig. 2 is a greatly enlarged cross-sectional
view of the heating cable of the type used in the blanket
of Fig. 1, and
Fig. 3 is a schematic wiring diagram of the
blanket of Fig. 1 showing the positive temperature
coefficient heating material as a series of distributed
parallel resistances.
Description of the Preferred Embodiment
Referring to the drawings, there is shown in
Fig. 1 an electric blanket, heating pad or mattress pad
10. The blanket or pad 10 includes a shell 13 which
conventionally consists of several layers of fabric
secured together to form passageways through which a
heating element 12 is threaded. The element 12 as shown
in Fig. 1 is disposed in a tortuous configuration having
a plurality of parallel legs interconnected together to
dispose the heating element 12 across the entire surface
of the shell 13 so that heat is distributed relatively
uniformly to the entire surface. The heating element 12
terminates in ends 12a and 12b which extend into a
connector module 14 which is shown schematically in
greatly enlarged form. The connector module 14 would be
no more than a few inches square and would be attached to
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the blanket 13. Also extending into the connector module
14 are conductors 16 and 18 which form parts of a power
cord 20 which terminates in a plug 22.
To better understand the nature of the heating
element 12 reference should be had to Fig. 2 which is a
greatly enlarged cross-sectional view of the heating
element 12. The element 12 includes a pair of spaced
conductors 24 and 26 which are separated by a layer of
PTC material 28 which also surrounds the conductors 24
and 26 to form a somewhat barbell type configuration.
The PTC material 28 is in turn enclosed in an insulating
coating 30. The PTC material 28 may be any suitable
conductive polymer which exhibits positive temperature
coefficient resistance characteristics at suitable
temperatures as is well-known in the art. Possible
compositions for the PTC material 28 are disclosed in the
Kelly U. S. Patent No. 4,277,673. The conductors 24 and
26 must be of sufficiently high conductivity so that the
primary heating effect created in the heating cable 12 is
in the PTC material 28 and not in the conductors 24 and
26. Details of the flexible conductors suitable for use
in such application are disclosed in Crowley U. S. Patent
No. 4,309,596.
In a typical electric blanket application, the
length of the heating element 12 would be on the order of
150 feet and the voltage drop over the length of one of
the conductors 24 or 26 would be on the order of 10 volts
when connected to a conventional 110 volt AC power
supply. It has been conventional in the past to connect
one of the conductors 24 at one end to one side of the
power line and the other end of the other conductor to
the other side of the power line. The purpose of
connecting opposite ends to the opposite sides of the
power line is to obtain a uniform potential between the
two conductors 24 and 26 throughout the length of the
heating element 12. Thus, the heat dissipated at any
place along the leng~h of the heating element 12 would be
the same. Although less desirable, it is also possible
to connect the conductors at one end of the cable 12 to
each of the power supply terminals to thereby energize
the element 12
One of the problems which was found to exist
with electric blankets using this type of heating element
involved the results of having a break in one of the
conductors. In such a situation, there would be a
substantial voltage drop across the broken ends of the
wire, the drop being on the order of 100 volts. This
type of voltage drop would cause an arc which had some
tendency to dissociate the polymer material into a
flammable gas. It was therefore necessary to devise some
type of circuit module which would disable the circuit or
open the circuit whenever there was a break in one of the
conductors which might cause the arcing situation
discussed above. An example of one such safety circuit
module is disclosed in the U. S. Patent to Carlson No.
4,436,986. However, such circuit modules tend to be
costly and are difficult to justify 'o the consumer who
knows nothing of the need for such a safety module. In
our present invention, we have devised a method of
connecting the conductors so that the circuit module is
rendered unnecessary.
Referring to Fig. 1, the end of the heating
element 12a includes conductor ends 24a and 26a and the
end 12b includes conductor ends 24b and 26b. Within the
module 14, the ends of conductor 24 which comprise 24a
and 24b are connected together at 32. The ends of
conductor 2& which are designated as 26a and 26b are
connected at 34. These common connections 32 and 34 are
connected to the power cord conductors 16 and 18
respectively. There is provided a fuse 36 in series with
the cord conductor 16 and the heating element 12.
Fig. 3 provides a schematic diagram ~f the
circuit described above in connection with Fig. 1. In
Fig. 3, the heating element 12 is shown as including the
conductors 24 and 26 with a plurality of parallel
resistances 28a connected therebetween. The resistances
28a are merely employed to illustrate how the PTC layer
28 would function in the circuit. The I2R heating
produced in the heating element 12 is primarily a result
of the current flowing through the PTC material 28 which
may be considered an infinite n~mber of parallel
resistances connected between the conductors 24 and 26.
The diagram of Fig. 3 shows clearly the loop
connection of each of the conductors 24 and 26 wherein
the ends of each conductor are connected together. In
this arrangem~nt, power cord conductor 16 supplies power
to both ends of conductor 24 of the heating element 12
and power cord conductor 18 supplies power to both ends
of the conductor 26. As a consequence, if either of the
conductors 24 or 26 breaks and opens the circuit at some
particular point over its length, there will be a minimal
voltage drop at the break because both segments of the
broken conductor will still be connected to the power
supply. If the break is in the middle of the conductox,
there would be no voltage drop while if it were at one
end there would exist whatever drop occurred over the
length of the longer conductor which would only be about
10 volts.
In contrast with only one end of each conductor
connected to the power supply, a break in one of the
conductors will result in a voltage drop across the break
of about 100 volts since the unpowered end of the
conductor away from the break tends to assume the voltage
of the other side of the line giving almost full line
voltage across the break.
As is evident from Fig. 1, the ends of each
conductor 24 or 26 are close together making it a simple
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matter to complete the loop connection. The resulting
circuit provides a simple and effective means of over-
coming the very serious arcing problem which existed
with respect to the prior art PTC blankets.
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