Note: Descriptions are shown in the official language in which they were submitted.
- 1156300
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 156300
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.
--2--
1 156300
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
--3--
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
1 156300
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
-5-
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 156300
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
1 156300
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 156300
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
- 10 -
1 156300
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
-12-
