Note: Descriptions are shown in the official language in which they were submitted.
1~:153i'7
BACK~ROUND OF THE INVENTION
A. Field of the Invention
This invention relates generally to temperature-
responsive protective circuits for electric bedcovers. It
relates particularly to an electric bedcover of a type wherein
a thermoresponsive device controls delivery of power to a heater
as an inverse function of temperature of the thermoresponsive
devlce .
In one type, a safety thermostat having a bimetallic
element is adapted to deenergize a heater when the bimetallic
element is heated sufficiently, as when an overheated condition
obtains. In another type, a thermistor having a positive tem-
perature coefficient of resistance is connected so as to control
the firing angle of a solid-state switch controlling delivery of
power to the heater, and the thermistor may be located so as
to respond to ambient temperature.
Adequate protective circuits are necessary in an electric
bedcover so as to prevent an overheated condition from starting
a fire in flammable bedclothes, scorching a fabric portion of
the electric bedcover, or injuring a person who may be either
asleep or bedridden. An overheated condition may be caused by
covering a substantial area of the electric bedcover as by a
conventional blanket, by tucking more than unheated marginal por-
tions of the electric bedcovèr under a mattress, by rumpling a
substantial area of the electric bedcover, and otherwise~ ;
B. Description of the Prior Art
It is conventional for an electric bedcover to have a
fabric shell, which has two or more plies and a network of ser-
pentine passages within the plies, and an elongated flexible
heater, which is deployed through the network of serpentine
passages, as exemplified by U. S. Patent No. 2,203,918 to
I. O. Moberg. It also is conventional for a thermostatic
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device, which is known in its conventional form as an ambient-
responsive control, to be used for adjustable control of an
on-off cycle of the heater as a function of ambient temperature.
Early electric bedcovers having such ambient-responsive
controls are disclosed in U. S. Patent No. 2,195,958 to
W. K. Kearsley and U. S. Patent No~ 2,344,820 to W~ K~ Kearsley.
As disclosed in U. S. Patent No. 3,708,649 to G. C. Crowley et al,
it is common to provide such a bedcover also with an array of
safety thermostats which are connected in series between series
segments of the heater, and which are located at strategic
places within the plies of the fabric shell, and it is known to
integrate an ambient-responsive control with the fabric shell.
It is known from U. S. Patent No. 2,565,478 to
G. C. Crowley, U. S. Patent No. 2,581,212 to D~ C. Spooner et al,
and U. S. Patent No. 2,846,560 to Jacoby et al, to employ an
elongated flexible component which is deployed as a cable through
the network of serpentine passages of the fabric shell of an
electric bedcover, which comprises a pair of elongated flexible
electrical conductors spaced from each other by a layer of
material having a negative temperature coefficient of resistance,
and in which one of the conductors constitutes the heater and - -
the other conductor constitutes a carrier for a signal indicative
of an overheated condition. An overheated condition causing
some part of the layer between the conductors to become a con-
ductor rather than an insulator causes closed contacts of an
electromagnetic relay to open so as to deenergize the heater~
It is known from U. S. Patent No. 2,846,559 to
J. Rosenberg, U. S. Patent No. 3,114,820 to R. G~ Hol~es, and
U. S. Patent No. 3,222,497 to W. H. Gordon, Jr., to employ an
elongated flexible component which acts as a sensor but not as
a heater, which is deployed as a cable alongside a separate
heater through the network of serpentine passages of the fabric
shell of an electric bedcover, and which also comprises a pair of
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elongated flexible electrical conductors spaced from each other
by a layer of material having a negative temperature coefficient
of resistance. Each conductor forms a part of a circuit
energizing a relay, whose contacts must be closed for the heater
to be energized, whereby an overheated condition causing some
part of the layer between the conductors to become a conductor
rather than an insulator opens the contacts. Cf~ U~ S~ Patent
No. 4,034,185 to G. C. Crowley.
It is known from U. S. Patent No. 3,418,454 to
W. D. Ryckman, Jr., to employ an elongated flexible component
which also acts as a sensor but not as a heater, which also
is deployed as a cable alongside a separate heater through the
network of serpentine passages of the fabric shell of an
electric bedcover having an ambient-responsive control as ~en-
tioned above, and which also comprises a pair of elongated flexi-
ble conductors spaced from each other by a layer of negative
temperature coefficient of resistance. Each conductor forms
a part of a circuit energizing resistive heaters associated
with respective ones of a pair of bimetallic arms of another
thermostatic device.
The pair of bimetallic arms of such thermostatic device
carry respective contacts, through which the heater is energized,
and which are closed when sufficient heat is supplied to each
arm by the heater associated with such arm. An overheated con-
dition causing the layer between the conductors of the sensor
to become a conductor rather than an insulator divides a current,
so as to cause less heat to be supplied by the heater associated
with one arm, whereupon such arm tends to open the contacts.
In each circuit wherein an element comprising a pair
of conductors separated by a layer of material having a negative
temperature coefficient of resistance acts as a sensor but not
as a heater as mentioned above, the conductors are intended to
carry current during normal operation of the electric blanket.
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~ 3~7
The conductors and associated components of the circuits com-
prising the conductors have inherent resistance and thus con-
tribute to overall power dissipation of the electric bedcover
during its normal operation.
U. S. Patent No. 2,782,290 to P. E. Lannan et al
discloses, for an electric bedcover, several protective circuits
employing an elongated flexible sensor, which comprises a pair
of conductors separated by a layer of material having a negative
temperature coefficient of rectification. Other protective
devices for such electric bedcovers are disclosed in U~ S. Patent
l~o. 3,628,093 to G. C. Crowley, U. S~ Patent Re~ 28,656
(originally U. S. Patent No. 3,673,381) to G~ C. Crowley et al
and U. S. Patent No. 3,683,151 to E. R. Mills et al.
It is known from U. S. Patent No. 3,588,446 to E~ R.
Mills et al, and from U. S. Patent No. 3,588,447 to E~ R. Mills
et al, to employ triacs and similar solid-state switches for
control of power to electric bedcovers, in circuits including
circuits wherein thermistors having positive temperature
coefficients of resistance are connected so as to control the
firing angles of the solid-state switches. As in certain embo-
diments disclosed in U. S. Patent ~o. 3,588,446, the thermistors
may be located so as to respond to ambient temperatures~
It is known from British Patent Specification No.
l,372,627 to Dimplex Limited to employ, in an electrical room-
heating appliance, a thermistor, by which is understood a compact,
rigid, semiconductive component to be mounted on a bracket, but
which has a negative temperature coefficient of resistance, in
circuits controlling current through a resistor, which is mounted
for heat dissipation influencing a thermostatic control for a
main heater of the appliance. Similar circuits are understood
to have been used in hair dryers.
A need remains for a protective circuit which may be used
advantageously either independently or to supplement conventional
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safety thermostats, which dissipates little power during normal
operation of the electric bedcover, and which does not re~uire
any separate relays or separate thermoresponsive devices.
Summary of the Invention
Accordingly, this invention provides an improvement
in an electric bedcover of a type comprising a fabric shell,
which has a network of serpentine passages within its plies, an
elongated flexible heater, which is deployed through at least
some of the passages, a thermoresponsive de~ice which is loca-
ted outside said fabric shell so as to respond to ambient tem-
perature, and which controls delivery of power to the heater
as an inverse function of temperature of said thermoresponsive
device, a resistor, which is mounted for heat dissipation from
the resistor to the thermostatic device and an elongated flex-
ible sensor, which comprises a pair of elongated flexible con-
ductors spaced from each other by an elongated flexible layer
of material having a negative temperature coefficient of re-
sistance, which is deployed through at least some of the pas-
sages, and which is adapted to influence the thermoresponsive
device through the resistor.
By the improvement of this invention, a circuit branch
comprising the resistor, the conductors, and the layer between
the conductors, in series with each other, is connected so as to
maintain the conductors at relatively different electrical po-
tentials at least when the heater operates, so as to conductlittle current except when some part of the layer between the
conductors is heated so as to act as an electrical conductor
having a low impedance rather than as an electrical insulator
having a high impedance, as in an overheated condition of at
least part of said electric bedcover, and so as to conduct suffi-
cient current to cause the resistor to dissipate sufficient heat
to cause the thermoresponsive device essentially to terminate
delivery of power to the heater when the layer between the
conductors thus acts as a shunt between the conductors of the
sensor. The thermoresponsive device receives little heat from
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the resistor except when some part of the layer thus acts as a
shunt between the conductors of the sensor.
Preferably, the thermostatic device comprises a bimetallic
element and electrical contacts arranged to be opened and closed
by the bimetallic element as the bimetallic element is heated
and cooled sufficiently, to which the resistor is added, whereupon
the resistor is separate from another resistor which may con-
ventionally be connected in series with one of the contacts so
as to heat when the contacts are closed, and which thus may be
used to heat the bimetallic element in a conventional way~ When
the bimetallic element is heated sufficiently, the contacts are
opened, and delivery of power to the heater thus is terminated.
Alternatively, the thermoresponsive device may be a
thermistox, which has a positive temperature coefficient of
resistance, and which is connected so as to control the firing
angle of a solid-state switch controlling delivery of power to
the heater as a function of temperature of the thermistor.
When heated ~ufficiently, the thermistor essentially terminates
delivery of power to the heater.
In either case, the circuit branch comprising the
resistor, the conductors, and the layer beyween the conductors,
in series with each other, conducts little current and thus
dissipates little power during normal operation of the electric
bedcover. No separate relays or separate thermostati~ devices
are required.
A protective circuit according to this invention may
be used either independently, as sole protective circuit for
an electric bedcover, or to supplement conventional safety ther-
mostats so as to sense an overheated condition that may obtain
in a location away from any safety thermostat. Supplemental
protective circuits are desirable for enhanced protective
capability in an electric bedcover, which is used by a person
who is either asleep or bedridden, and who may not be able
either to sense an overheated condition or to correct it
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promptly.
Because an elongated flexible sensor of the type used
in this invention does not rely upon mechanical motion, it is
less susceptible to accidental damage than conventional safety
thermostats, which are exposed to possible damage whenever a
person sits, jumps, or lies on an electric bedcover having such
safety thermostats. If the ambient-responsive control is
located outside the fabric shell, as it is conventionally, it
is also less susceptible to such damage~
Brief Descript~on of the Drawin~s
Figure 1 is a schematic diagram of a preferred embodi-
ment of this invention, in an electric bedcover having an ambient-
responsive control, which comprises a bimetallic el~ment~
Figure la, in which similar reference characters are applied to
similar components, is a schematic diagram of an alternative
embodiment of some components of Figure 1~
Figure 2 is a partly fragmentary layout of a heater,
a sensor, and an array of safety thermostats, for the preferred
embodiment of Figure 1, and also for the alternative embodiment
of Figure la.
Figure 3 is a schematic diagram of an alternative
embodiment of this in~ention, in an electric bedcover having a
solid-state control, which comprises a thermistor controlling
the firing angle of a solid-state switch.
Detailed Description of Preferred Embodiment
As shown in Figure l and 2, an electric bedcover 10,
in which this invention is embodied, has several conventional
features including a fabric shell 12, which has an upper ply 14
and a lower ply 16 and a network of serpentine passages 18
within the plies, and an elongated flexible heater 20, which
is deployed through substantially all of the ne~work of serpentine
passa~es 18. Nine safety thermostats 22 are confined within the
ne~work of serpentine passages 18 and connected respectively
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between series segments of the heater 20 in conventional manner.
Conventionally, the heater 20 may be connected to a
source of electrical power, which may be 120 VAC, 60 Hz, through
a connector 24, which is indicated schematically in Figure l.
A male component 24a of the connector 24 is shown in Figure 2.
The connector 24 receives electrical leads 26 and 2~, which ~-
are connected respectively to opposite ends of the heater 20,
and which thus are connected through associated pins 26a and ~
28a of the male component 24a, to associated sockets (not shown) ~-
of a female component (not shown) for further connection as
described below. The connector 24 is similar to conventional
connectors having two pins and two sockets except ~or its
provision for another lead 30 and an associated pin 30a to be
described below.
The connector 24 connects the heater 20 to the source
of electrical power through an ambient-responsive control 32
which is located outside the fabric shell 12, which comprises
a resistor 34, a bimetallic arm 36,and a pair of electrical
contacts 38, one of which is carried by the bimetallic arm
36, in series with each other and with the heater 20 through
the lead 26, and which also comprises adjustable means 40 to
bias the bimetallic arm 36 in conventional manner~ The
ambient-responsive control 32 cycles in conventional manner,
over a period that is a function of ambient temperature, except
wnen an overheated condition is sensed as discussed below.
In normal operation, when no overheated condition is
sensed as discussed below, the resistor 34 heats the bimetallic
arm 36, so long as the contacts 38 are closed. As ambient
temperature increases, it takes less time to heat the bimetallic
arm 36 sufficiently, so that the contacts 38 are opened. Con-
versely, as ambient temperature decreases, it takes more time
to do so. Hence, the period over which the ambient-responsive
control 32 cycles is an inverse function of ambient temperature,
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as is average heat dissipated by the heater 20.
Conventionally, a neon lamp 42 and a series resistor 44,
which limits current through the neon lamp 42, are connected in
parallel with the heater 20 and the ambient-responsive control
32, and an on-off switch 46, which may be a snap-action toggle
switch, is connected between the neon lamp 42 and one side of
the source of electrical power. A conventional plug 48 is used
to connect both sides of~the heater 20, through the leads 26
and 28 and the connector 24 as mentioned above, to the source
of electrical power.
As discussed above, the electric bedcover 10 resembles
conventional electric bedcovers and operates as conventional -
electric bedcovers operate except when on overheated condition
is sensed as described below. As mentioned in an earlier para-
graph, an overheated condition in an electric bedcover may be
caused by covering a substantial area of the electric bedcover
as by a conventional blanket, by tucking more than unheated
marginal portions of the electric bedcover under a mattress, by
rumpling a substantial area of the electric bedcover, and
otherwise.
In the electric bedcover 10, as in conventional electric
bedcovers, if any one of the safety thermostats 22 is heated
sufficiently as when an overheated condition obtains, it opens
so as to deenergize the heater 20, until it is cooled sufficiently
to close so as to reenergize the heater 20. The safety thermo-
stat 22 that is affected may cycle between its opened and closed
conditions if the overheated condition persists~
Additionally, as improved by this invention, the electric
bedcover 10 has a supplemental, st~ndby, protective circuit,
which dissipates little power except when an overheated condition
is sensed as discussed below. Tw~ principal components are
required, namely, a resistor 50 which is added to the ambient-
responsive control 32, and which is mounted for heat dissipation
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11~53i7
to the bimetallic arm 36 of the ambient-responsive control 32,
and an elongated flexible sensor 52 which is deployed through at
least some of the passages 18 alongside the heater 20> as noted ~ -
in Figure 2.
It is preferable for the sensor 52 to be deployed, in
every two or.three passages 18, so that not more than two
adjacent parallel passages 18 fail to contain an elongated run
of the sensor 52, and so that all heated areas of the electric
bedcover 10 are traversed by the sensor 52~ .Some parallel
passages 1~ thus contain not only an elongated run of the heater
20 but also an elongated run of the sensor 52,
The sensor 52 may be constructed as described in U~ S.
Patent No. 2,581,212 to D. C. Spooner et al, U~ S~ Patent No.
2,846,559 to J. ~osenberg, and U. S. Patent No. 2,846,560 to
J. F. Jacoby et al, so as to form.a cable of small diameter and
so as to comprise a pair of elongated flexible electrical con-
ductors spaced from each other by a layer of material having a
negative temperature coefficient of resistance~ As sho~m
schematically in Figure 1, one such conductor 54 is spaced from
the other conductor 56 by a layer 58 of such material, which may
be chosen from a variety of suitable materials described in these
patents.
Such material is chosen so that at normal temperatures,
which obtain during normal operation of an electric bedcover, such
material acts as an electrical insulator having a high impedance,
whereas at elevated temperatures, which obtain during an overheated
condition of an electric bedcover, such material acts as an
electrical conductor having a low impedance. Suitable materials
having suitable negative temperature coefficients of resistance
are available commercially from The B. F. Goodrich Co., B~ F.
Goodrich Chemical Div.~ Independence, Ohio, as "GEON" No~ 82726-
natural-024, 0.8% doped with "Trito~ X-400" dopant.
Pr~ferably, as shown in Figure 1, a circuit branch com-
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prising the resistor 50, the conductor 54, the layer 58, and
the conductor 56, in series with each other, is connected in
parallel with a circuit branch comprising the contacts 38, the
bimetallic arm 36, the resistor 34 of the ambient-responsive
control 32, the heater 20, and the safety thermostats 22, in
series with each other, whereby the circuit branch comprising
the resistor 50 and other series components is enabled, although
it may not conduct as explained herein, whether the contacts
38 and the safety thermostats 22 are opened or closed~ Alter-
natively, as shown in Figure la, the circuit branch comprising
the resistor 50, the conductor 54, the layer 58, and the con-
ductor 56, in series with each other, is connected in parallel
with a circuit branch comprising-the heater 20 and the safety
thermostats 22, in series with each other, but not the ambient-
responsive control 32, whereby the circuit branch comprising the
resistor 50 and other series components is disabled when the
contacts 38 are opened.
r As shown in Figure 1, the conductor 56 and the conductor
54 through the resistor 50 thus are maintained at relatively
different electrical potentials so long as the plug 48 is con-
nected to a source of electrical power and the switch 46 is
closed, whether or not the contacts 38 and the safety thermo-
stats 22 are closed so as to enable the heater 20 to operate.
As shown in Figure la, the conductor 56 and the conductor 54
through the resistor 50 thus are maintained at relatively different
electrical potentials when the heater 20 operates.
As shown either in Figure 1 or in Figure la, the circuit
branch comprising the resistor 50, the conductor 54, the layer 58,
and the conductor 56, in series with each other, conducts little
current except when some part of the layer 58 is heated so as
to act as an electrical conductor having a low impedance as in an
overheated condition of some part of the electric bedcover 10,
rather than as an electrical insulator having a high impedance,
1~15317
because of the negative temperature coefficient of resistance of
the material chosen for the layer 58 as discussed above. There-
fore, said circuit branch dissipates little power through the
resistor 50, or through other inherent resistances, except when
some part of the layer 58 thus acts as a shunt between the con-
ductor 54 and the conductor 56.
As shown either in Figure 1 or in Figure la, the circuit
branch comprising the resistor 50, the conductor 54, the layer
58, and the conductor 56, in series with each other, conducts
sufficient current to cause the resistor 50 to dissipate sufficient
heat to cause the bimetallic element 36 of the ambient-responsive
control 32 to open the contacts 38 when some part of the layer 58
is heated so as to act as an electrical conductor having a low
impedance as in an overheated condition of at least part of the
electric bedcover 10, rather than as an electrical insulator
having a high impedance, again because of the negative temperature
coefficient of resistance of the material chosen for the layer 58
as discussed above. Such part of the layer 58 thus acts as a
shunt between the conductor 54 and the conductor 56,
As shown in Figure 1, the resistor 50 continues to heat
the bimetallic arm 36 so long as some part of the layer 58
between the conductor 54 and the conductor 56 continues to act as
a conductor as discussed above, and thus so long as an overheated
condition is sensed by some part of the sensor 52 so as to heat
some part of the layer 58. As shown in Figure la, the resistor
50 is allowed to cool once the bimetallic arm 36 opens the con-
tacts 38, but the ambient-responsive control 32 cycles over a
period that is shorter than normal if an overheated condition
continues to be sensed by some part of the sensor 52 so as to
heat some part of the layer 58 whenever the contacts 38 are
closed.
As shown either in Figure 1 or in Figure la, the pro-
tective circuit provided by this invention supplements the safety
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thermostats 22 so as to sense an overheated condition that may
obtain in a location away from any safety thermostat 22, as in
the location 60 indicated in Figure 2~ As mentioned above,
the protective circuit provided by this invention dissipates
little power during normal operation of the electric bedcover
10, and no separate relays or separate thermostatic devices are
required.
A protective circuit according to this invention may be
used independently, as sole protective circuit for the electric
bedcover 10, whereupon the safety thermostats 22 are omitted.
However, it is preferable to include the safety thermostats 22,
as mentioned above.
In the alternative embodiment of Figure 3, the electric
bedcover 10 comprising the heater 20, the safety thermostats 22,
the connector 24, and the sensor 52, as shown in Figure 2, may
be used without any change. Thus, in the sensor 52, one con-
ductor 54 is spaced from another conductor 56 by a layer 58 of
material having a negative temperature coefficient of resistance,
as discussed above.
As shown in Figure 3, a solid-state control 100 and the
heater 20 may be connected, by means of a conventional plug 102,
to an alternating current source ~not shown) of line voltage
(100-120 VAC, 60 Hz). The solid-state control 100 is similar
to prior solid-state controls used in electric bedcovers sold
~y Northern Electric Company, a division of Sunbeam Corporation,
5224 North Kedzie Avenue, Chicago, Illinis 60625, except for
additlonal ~atter including a resistor 200 and its physical and
electrical relationships to other components as described below.
Other solid-state controls having similar features, except for
such additional matters, are disclosed in U~ S. Patent No~
3,588,446 to E. R. Mills et al, and U. S. Patent No. 3,58~,447
to E. R. Mills et al
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As in prior solid-state controls as mentioned above,
a bidirectional triode thyristor (triac) 104 is connected, as
a solid-state switch, in series with the heater 20. The gate
of the triac 104 is connected, through a diac 106, to a timing
circuit comprising a timing capacitor 108, a coupling resistor
110, a filtering capacitor 112, a trim potentiometer 114, a
current-limiting resistor 116, and a thermistor 118 having a
positive temperature coefficient of resistance, all connected as
shown.
Net resistance and net capacitance of the timing circuit
control the charging rate of the timing capacitor 108 so as to
control the time necessary for the timing capacitor 108 to be
charged to the breakover voltage of the diac 106~ When the
timing capacitor 108 is charged to the breakover voltage of
the diac 106, the diac 106 becomes conductive bidirectionally,
and the timing capacitor 108 discharges through the diac 106 to
trigger the gate of the triac 104, which thus becomes conductive
bidirectionally so as to allow the heater 20 to conduct~ Thus, `
the firing angle of the triac 104 may be controlled, for full-wave
phase control of delivery of power to the heater 20~
A triac, a diac, and associated components have been
used similarly in various applications and may be further under-
stood from standard references including General Electric Company,
SCR MANUAL (4th edition, 1967) at 9.4.2.
A radio-frequency choke 130 and an on-off switch 132,
which may be act~ated ~ a user, a~sD arP c~nnected ~n series
with the triac 104 and the heater 2~. The choke 130 cooperates
with a capacitor 134, which is connected across the choke 130
and the triac 104 as shown, to suppress emission of high-frequency
electromagnetic noise. A neon lamp 136 is connected in series
with a current-limiting resistor 138 across the lin-e and the
switch 132 as shown, so as to indicate, by energization of the
neon lamp 136, closure of the switch 132 for energization of
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the heater 20.
A resistor 142 is mounted for dissipation of heat tothe thermistor 118 and connected in series with an adJustable
potentiometer 144 as shown, so as to be energized whenever the
switch 132 is closed, even when the triac. 104 is nonconductive.
The adjustable potentiometer 144 may be manually adjusted, as
- by a knob (no~ shown) manipulated by a user, so as to control
current through the resistor 142, which thermally biases the
thermistor 118. The filtering capacitor 112 and the coupling
resistor 110 tend to minimize snap-on effects of hysteresis
in the adjustable potentiometer 144.
For purposes of this invention, the solid-state control
100 is modified by addition of the afor~mentioned resistor 200,
which also is mounted for dissipation of heat to the thermistor
118, and which is connected in a circuit branch comprising the
resistor 200, the conductor 56, the layer 5~, and the conductor
54, in series with each other, so as to conduct little current
except when some part of the layer 58 is heated so as to act as
an electrical conductor having a low impedance as in an overheated
condition of some part of the electric bedcover 10, rather than
as an electrical insulator having a high impedance, because of the
negative temperature coefficient of resistance of the material
chosen for the layer 58 as discussed above. Therefore, said
circuit branch dissipates little power through the resistor
200, or through other inherent resistances, except when some
part of the layer 58 thus acts as a shunt between the conductor
54 and the conductor 56.
When some part of the layer 58 thus acts as a shunt
between the conductor 54 and the conductor 56, the thermistor
118 receives substantial heatfrom the resistor 200, far more
heat than when little power is dissipated through the resistor
200 as mentioned above. As the temperature of the thermistor
118 thus is elevated, the time required for the timing capacitor
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108 to be charged to the breakover vol.tage of the diac 106 thus
is lengthened, and the firing angle of the triac 104 thus is
: advanced. When the thermistor 118 is heated sufficiently, its
resistance is increased sufficiently for deLivery of power to
the heater 20 to be blocked by the triac 104, over all but a
. negligible portion of each alternating-current cycle~
Therefore, when some part of the layer 58 is heated so
as to act as an electrical conductor having a low impedance as
in an overheated condition of some part of the electric bPdcover
10, rather than as an electrical insulator having a high impedance~
the thermistor 118 essentially terminates delivery of power to
the heater 20.
Exemplary specifications and component values for the
. alternative embodiment of Figure 3: triac 104, 4 amps, 200 volts;. diac 106, breakover voltage of 60 volts; capacitor 108, 0~047
microfarad; resistor 110, 12 kilohms; capacitor 112, 0~047
microfarad; trim potentiometer, 1 : megohm; resistor 116, 47 kil-
ohms; thermistor 118, 50 kilohms at 140F (60C), variation of
approximately 12% per F (21% per C); choke 130, 135 micro-
henry; capacitor 134, 0.1 microfarad; neon lamp 136, 0~25 watt;
resistor 138, 0.25 kilohms.
A sensor may be made as described above in a length
of about 80 feet, as for an electric bedcover for a standard bed,
so as to exhibit net resistance of about 200,000 ohms between
its conductors, if the sensor as a whole remains at a temperature
of about 95F, as in a normal operation of an electric bedcover,
and so as to exhibit net resistance of about 10-20,000 ohms, if
a~out a two-foot length of t~e sensor is heated to a temperature
of about 250F.
Thus, a protective circuit according to this invention
may be designed to deenergize a blanket heater if about a two-foot .
length of the sensor iæ heated to about 150F, as in an overheated
condition of an el~ectric bedcover. The resistor connected in
series with the conductors and intermediate layer of the sensor
,s,~
thus may have a resistance of about 10-20,000 ohms over a
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comparable range of temperatures~
It is to be understood that the embodiments described
above are exemplary and not intended to limit this invention
as covered by the claims below. As this invention may be applied
to other electric pads, blankets, and appliances, which are
equivalent to an electric bedcover, all references herein to an
electric bedcover are to be understood as to embrace such pads,
blankets, and appliances.
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