Note: Descriptions are shown in the official language in which they were submitted.
~053731
THERMAL SWITCH DEVICE
AND METHOD OF MAKING
Background of the Invention
This invention relates generally to a switch which is
responsive to an ambient temperature level.
The invention more particularly relates to an unresettable
switch which will open a circuit when the ambient temperature
around the circuit is increased to a predetermined level.
Switches of the type described have become necessary to
protect various circuitry in devices such as appliances, etc.,
from the hazards of high temperatures generated therein. An in-
creasing awareness of the hazards that present themselves as a
result of a device which is capable of generating unchecked
levels of heat emphasizes the importance of incorporating thermal
switches in such devices. Not only destruction to the device
but to the immediate surroundings could possibly be eliminated
through the use of a switch which is capable of accurately sens-
ing the increase in ambient temperature level to a predetermined
amount and quickly and reliably opening the circuit to stop the
flow of current therein.
Prior art devices of the type described are generally
multi-piece units with a conductive casing. The multi-piece
devices of the prior art are inherently costly to produce with a
high level of quality control. One such prior art device utiliz-
es a pair of coaxially arranged conductor wires, one of which iselectrically connected to an outer conductive casing and the
second of which is placed in releasable contact with the con-
ductive casing through a thin washer member. The washer member
is urged into contact with the second conductor wire through a
spring and a thermal pellet. When a predetermined temperature
level is reached, the thermal pellet liquifies, thus releasing
the spring energy and allowing a secondary spring to force the
washer out of contact with the second conductor wire. Such a
-1-
~` ~o5373'1
device has approximately eight to ten different elements not
including the conductor wires.
It is the primary object of the invention to provide a
thermal switch device with a minimum of elements.
The invention in one of its broader aspects comprehends
a thermal switch device including a pair of electrical conductor
wire members in side by side relationship to each other and hav-
ing free ends extending generally in the same direction forming
switch sections and lead wire sections. The switch ~ection in-
cludes a localized contact region providing an electrically con-
ductive path between the wire members. At least one of the
conductor wire members in the switch section includes a spring
means biasing the contact regions and adjacent portions of the
pair of conductor wire members away from each other. The free
extremities of the wire members, the contact region and at least
a portion of the spring means are encapsulated with a nonconduc-
tive heat fusible material with the conductor wires being held,
against the spring bias, into electrical contact with each other,
by the encapsulating material. The nonconductive heat fusible
material and portions of the conductor wire adjacent the spring
means which extend out of the encapsulated material are con-
formally coated with a rigid insulating material to totally en-
capsulate the heat fusible material and space and electrically
insulate the extending lead wire sections from each other. The
conductor wires of the encased switch section are adapted to
spring away from and out of contact from each other at the con-
tact region when the heat fusible material is subjected to a
predetermined temperature level causing it to flow and release
the energy stored in the spring means.
Another aspect of the invention comprehends a method of
forming a thermal switch device of the type having a pair of
conductor wire members in side by side relationship and having
their free extremities extending generally in the same direction
and adapted to spring apart, out of electrical contact with each
-2-
`` ~OS373~
other when a predetermined temperature level is attained. The
method steps include forming an electrically conductive wire
member to include at least one first region wherein a portion of
the wire is spring biased out of line with the adjoining portion,
the first region also including a localized contact region. The
conductive wire member intermediate the extremities is then bent
to provide a generally U-shaped preform with the free extremities
extending generally in the same direction forming an open portion
at one end of the preform with a bight portion at the other end
of the preform, the lateral spacing between opposing wire sections
at a first end of the preform being greater than the lateral spac-
ing between opposing wire sections at a second end of the preform.
A first lateral compressive force is applied to the preform to
bring the localized contact region into electrical contact with
an opposing section, and the second end of the preform is sur-
rounded with a heat fusible material to retain the electrical
contacting configuration. A second lateral compressive force
is applied to the preform to load energy in the first region by
moving a portion of the wire inwardly against a spring bias.
The method further includes the steps of surrounding the heat
fusible material and the adjacent portions of the second end of
the preform with an additional amount of heat fusible material
to retain the second end in spring loaded configuration, and
coating the heat fusible material with non-conductive rigid in-
sulating material. Thus the opposing wire sections are electric-
ally insulated except at the contact regions with the first end
of the preform forming terminal wires and the second end forms
a spring loaded thermal switch encased within heat fusible
material and rigid insulating material.
Thus the invention provides a thermal switch device which
is capable of quickly and reliably opening a circuit at a pre-
determined temperature level, a device which reduces arcing
between contact points as the switch is opened, a device which
~ 53731
provides a generally constant cross-sectional area in the cur-
rent carrying elements to minimize hot spots in the circuit and
a device which is encased with a nonconductive material which
facilitates the x-ray inspection of such a device.
More particularly the present invention basically pro-
vides for the encapsulation of contact regions of eonduetor wires,
when spring loaded into contact with eaeh other, with a heat
fusible material, such as an appropriate organic mixture, follow-
ed by a eoating or eneapsulation of a rigid insulating material
providing a struetural and proteetive eneasement for th~ circuitry
while insulating the conductor wires from one another except at
the contact region. The spring energy is provided in the system
by forming a section of at least one of the eonduetor wires, ad-
jaeent the terminal extremity and below a spring hinge region,
away from and out of line with the conductor wire section above
a spring hinge region. A localized contact region is provided
below the spring hinge region so that contact regions may be forc-
ed together into elecrical contact by a slight, lateral compres-
sion force and dipped or encapsulated in a heat fusible material,
followed by a second compressive force to load the spring while
dipping or encapsulating the contact region in a similar heat
fusible material. The heat fusible material will be encapsulated
with a rigid insulating material to structurally support the
circuit. The encapsulated region will thus provide a elosed eir-
cuit until the fusible material flows releasing the energy stored
in the spring.
Brief Description of the Drawings
Fig. 1 is an elevation of the completed thermal switch
being drawn generally to the same scale and size as an actual
switch constructed in accordance with the invention.
Fig 2. is an elevationa-l view of a conductive wire preform
used in the construction of a preferred embodiment of the inven-
tion.
Fig. 3 is a side view of the preform shown in Fig. 2.
--4--
``` ` 105373~
Fig. 4 is a partial elevational view of the preform
during a first step in the manufacture of a switch in accordance
with the invention.
Fig. 5 is a partial elevational view of the preform dur-
ing a second step in the manufacture of a device in accordancewith the invention.
Fig. 6 is a cross-sectional view of the switching
portion of the device of the invention following its final man-
ufacturing step and in a loaded condition.
Fig. 7 is a cross-sectional view of the device similar
to that shown in Fig. 6 after the circuit has opened.
Fig. 8 is a partial elevational view of a secondary em-
bodiment of the invention.
Fig. 9 is a partial elevational view of the preform of
the embodiment in Fig. 8 following the first step in the man-
ufacture of a device in accordance with the invention.
Fig. 10 is a partial elevational view of the switch por-
tion of the embodiment of Fig. 8 following a second step in the
construction of the device.
Fig. 11 is a cross-sectional view of the alternate em-
bodiment of the switch following the last construction step and
showing it in a loaded position.
Fig. 12 is a cross-sectional view of the device shown
in Fig. 11 showing the circuit in an open condition.
Fig. 13 is an elevational view of an alternate conduc-
tive wire preform used in the construction of a thermal switch
according to this invention.
Fig. 14 is a side view of the preform shown in Fig. 13.
3~)5~731
Fig. 15 is a partial elevational view of the preform of
Fig. 13 during a first step in the manufacture of a switch.
Fig. 16 is a view similar to that of Fig. 15 following a second
step in the manufacture of a switch in accordance with the invention.
Fig. 17 is a partial elevational view of the preform of Fig. 13
during a third step in the manufacture of a device in accordance with
the invention.
Fig. 18 is a cross-sectional view of the switching portion of
the device of the invention in a loaded condition following its final manu-
facturing step.
Fig. 19 is a cross-sectional view of the device similar to that
shown in Fig. ~ 8 after the circuit has opened.
Fig. 20 is an elevational view of an alternate embodiment of a
conductive wire preform used in the construction of a further embodi-
ment of the invention.
Fig. 21 is a partial elevational view of the preform of Fig. 20
during a first step in the manufacture of a switch in accordance with
the invention.
Fig. 22 is a partial elevational view of the preform of Fig. 20
during a second step in the manufacture of a device in accordance with
the invention.
105373~
Fig. 23 is a cross-sectional view of the switching portion of
the device of Fig. 22 in a loaded condition following its final manu-
facturing step.
Fig. 24 is a cross-sectional view of the device similar to that
shown in Fig. 23 after the circuit has opened.
Fig. 25 is a partial elevational view of the switch section of
a preform during a first step in another alternate configuration of
the manufacture of a switch in accordance with the invention.
Fig. 26 is a perspective of the heat fusible annular ring utilized
10 in the embodiment shown in Fig. 25.
Description of the Preferred Embodiment
. .~
The thermal switch device 10 shown in Fig. 1, and in more
detail in Fig. 6, will first be described relative to the various steps
to manufacture a preferred embodiment thereof and with particular
reference to Figs. 2-5.
A length of conductive wire is first formed into a generally
U-shaped configuration, such as shown in Fig. 2, to include an open
portion and a closed bight portion at opposing ends of the preform.
The closed portion, in the preferred embodiment, will be constructed
of a lead wire sections 14 which are spaced from one another a
distance greater than the switch sections 15 of the wire adjacent the
open end of the preform. The switch sections 15 adjacent the open end
will generally consist of two subsections 16 and 18. The lower sub-
section 18 will include portions extending laterally outwardly from the
upper subsection 16 such as resulting from being formed at an angle
1053731
to one another and interconnected by a spring-like hings 26. A
localized contact region 28 is included in the switch section and is
preferably positioned intermediate the extremities 22 of the conductor
wire and the associated spring region 2ff. The cross-sectional
configuration of the switch sections 15 are deformed from the generally
circular cross section in the lead portions 14 to a substantially flat
configuration for a purpose to be described later herein.
The contact region 28 may advantageously be formed in the
switch section 15 as a protuberance or protuberances extending
toward opposing faces of the conductor wire.
The preform in the condition shown in Fig. 2 may be subjected
to a slight compressive force Fl, preferably in the upper region of the
preform and exerted on the lead wire portions 14. This initial force
brings the opposing switch sections 15 together so that the contact
point or points 28 are in physical electrical contact with each other.
In this position, the extremities 22 of the conductor wire are dipped
in a heat fusible material, such as an organic, having a predetermined
melting or flowing temperature. The dipping is, of course, done while
the organic is in a liquid state followed by cooling to room temperature,
subsequent to the dipping, to produce an intial encapsulation 30 which
will hold the contacts together in the position shown in Fig. 4.
~ith the preform in the retained position shown in Fig. 4, a
subsequent lateral force F2 is applied to the upper region of the
preform. This force F2 will be of a greater force than the initial
--8--
~OS373~
force and of a value great enough to overcome the spring bias of the
spring means 28 and bring the upper section 16 into alignment with
the lower section 18. With the spring loaded in this manner, the switch
section is subjected to a second dip of heat fusible organic material to
provide a complete covering 32 of the spring region 26, the contact
point 28, portions of upper subsection 26, all of lower subsection 18,
as well as the initial encapsulation 30. The two dips of fusible organic
material may, of course, be of exactly the same material and may become
essentially homogeneous. Upon cooling of the heat fusible material
following this second dip, the circuit will be closed and the spring
loaded in a subassembly shown in Fig. 5.
Attention is now directed to the preform shown in Fig. 2.
The opposing terminal wire portions 14 will be severed at the bight
portion to provide two leads for the component. The bight portion
is preferably severed prior to the second dipping step in order to
reduce the shear stress on the contact region. Notches 38 inter-
connecting terminal wire portions 14 facilitate such a severing.
Following the second dip step, the switch section of the device
and the heat fusible encapsulation 32 is totally and conformally coated
with a layer of rigid insulating material 34. This coating 34 may also
be done in a dipping process but conventional casing techniques may be
utilized as long as the casing intimately and conformally surrounds the
heat fusible material and provides a structure for protecting the switch
circuit while electrically insulating the lead wires 14 at their point of
entry into the switch section. Certain types of epoxy material are
capable of functioning as the rigid insulating encasing material as long
1~53731
as the material sufficiently resists cold flow responsive to the energy
stored in the spring sections. However, while the invention may be
herein described as utilizing epoxy as the encasing material, it should
be understood that epoxy is only representative of a suitable material
S and the invention should not be restricted to a particular rigid insulating
material .
The device shown in Fig. 6, thus, is representative of the switch
in its final loaded condition capable of transmitting current from one
lead wire 14 to the other. When the environmental or ambient tempera-
ture reaches a predetermined level, the heat fusible organic 30 and 32
will flow or liquify allowing the energy stored in the spring to be released
and thus providing the open circuit structure shown in Fig. 7. The
upper sections 16 will be locked in the epoxy so lower region 18 and
contacts 28 will spring laterally outwardly relative to the opposing wire
sections and resume a substantially relaxed position as the energy is
released in the spring. The cavity within the epoxy coating 34 will
contain generally only the heat fusible material so that when the organic
melts the displaced organic will flow between the lead wires and increase
the dielectric strength between the open contacts. This construction
also acts to preclude the formation of a corona discharge since no air
will be present to ionize.
The lead wire in the switch region 15 flattened to distribute
the contact reaction force and, thus, the tendency of the energy being
dissipated by cold flow of the organic. The switch section will also
be of generally constant cross-sectional configuration which tends to
-10-
~053731
eliminate hot spots in the electrical switch circuit. The spring sections
are generally very thin and are extruded laterally with a generally
concave surface formed in the outwardly facing portions of the wire.
No more heat is dissipated in the spring portions than in any other
unit length of the conductor wire. Since the spring section will have
greater area per unit length than a round wire, it follows that the
springs will be cooler than the remaining conductor in the event of a
very high sustained current. Further, the heated fusion of the organic
acts to heat sink the leads so that the leads will remain at approxima-
tely the organic melting point thus protecting the integrity of the spring.
Turning to Figs, 8-12, a further embodiment of the invention
is described which provides a continuing preloading force at the contact
region to further enhance the reliability of the device, The switch
section 15a of the embodiment differs from the device described above
in the preferred embodiment by the addition of a secondary spring in
the system at the point of contact, One of the conductor wire switch
sections will include an upper portion 16a and a primary spring
region 26a to load energy in the system in a manner similar to the
system described above relative to Figs, 1-7, The contact region 28a
includes a further spring by virtue of hinge 27a at the contact region
at the juncture of upper portion 16a and lower portion 18a,
Both of the opposing conductor wire portions in the switch
section 15a are flattened to a substantially continuous, thin cross-
sectional configuration from tips 22a to the point of intersection 17a
with the round lead wires 14a. Lower portior~l8a are bent outwardly
lOS373~
from the remaining, upper portions of the spring section. The bend
lines 27a effectively provide a secondary spring in the system. The
contact protuberances 28a are located substantially at bend lines 27a
and thereby serve as a fulcrum about which the springs in the system
are loaded.
Fig. 9 shows that the first dip of the heat fusible mater~al 30a
is taken while the unit has been subjected to a first compressive force
in a manner similar to that described above. A subsequent compressive
force caused energy to be stored in upper region 16a by virtue of the
10 bowing of region 26a above the contact points since the lower region is
retained from separation while the lead wires 14a are forced together.
~ince the force of the bowed spring region 26a as well as the secondary
spring 27a react at the contact region 28a, a continuous preloading
condition is thereby established. Fig. 10 shows the second dipping
15 step providing coating 32a during the loading of the spring region 26a
to store energy. Fig. 11 shows the loaded switch encased in epoxy 34a
to insulate and lock the leads in a manner such as described relative
to the preferred embodiment above.
It has been found to be advantageous to provide an outer coating
20 32a having a slightly lower melting temperature than the inner coating
30a. This allows the contacts to be held together longer, creating a
snap opening when coating 30a does melt.
Fig. 12 shows the relationship of the switch section 15a in an
open condition when a predetermined ambient temperature level has
25 been attained. Since the regions of the lead wire 14a adjacent the
1053731
regions 16a are locked and insulated from one another in the epoxy
coating 34a, the regions 16a and 18a will resume substantially the
relaxed position once the energy has been dissipated. The relaxed
position will thus open the electrical contact between fulcrum point
28a and the opposing wire surface.
With reference to Figs. 13-27, alternate methods of producing
switches of the type contemplated by this invention and alternate
configuratiorsof such switches are shown with like reference numerals
used throughout the views intended to designate similar elements or
components .
An alternate method of making such a switch contemplates a
length of conductive wire first formed into a generally U-shaped
configuration, such as shown in Fig. 13, to include an open portion
and a closed bight portion at opposing ends of the preform, The
closed portion will be located in the switch section 15 and will be
constructed so that the wire portions in the switch section are spaced
from one another a distance less than the lead wire sections 14 at the
open end of the preform. The switch sections 15 adjacent the bight
portion 24 of the preform will generally consist of two subsections
16 and 18. The lower subsection 18 will include portions extending
laterally outwardly from the upper subsection 16, such as resulting
from being formed at an angle to one another and interconnected by a
spring-like hinge 26. A localized contact region 28 is included on at
least one of the opposing conductor wires in the switch section and is
positioned intermediate the bight portion 24 of the conductor wire and
-13-
1053731
the associated spring region 26. The cross-sectional configuration
of the switch sections 15 are deformed from the generally circular
cross-section in the lead portions 14 to a substantially flat configuration
for a purpose to be described later herein.
The preform in the condition shown in Fig. 13 may be subjected
to a compressive force Fl, preferably in the upper region of the
preform, exerted on the lead wire portions 14. This force brings
the switch sections 15 together so that the opposing conductor wire
portions are in physical electrical contact with each other at the
contact point region. The force Fl will also be great enough to load
the device by overcoming the spring bias of region 26 by bringing upper
section 16 generally into alignment with the lower section 18. Since
the contact regions 28 are spaced upwardly from the extremities of
the preform, the contact region will serve as a fulcrum for this appli-
cation of force. A slight preload is provided at contact regions 28 due
to the force F1 and the retention of the ends of the conductor wires
spaced downwardly from the fulcrum 28. In the position shown in
Fig. 15, the bight portion 24, subsection 18, spring ~neans 26 and
portions of subsection 16 are dipped in a heat fusible material, such
as an organic, having a predetermined melting or flowing temperature.
The dipping is, of course, done while the organic is in a liquid state
followed by cooling to room temperature, subsequent to the dipping,
to produce an initial encapsulation 30, which will hold the contacts
together in the position shown in Fig. 15.
-14-
1053731
With the preform in a retained position shown in Fig. 15, a
second manufacturing step is performed thereon in order to sever the
bight portion 24 so that the only remaining conductive path between
opposing wire sections is at contact region 28. The severing may be
accomplished by grinding the end of the preform removing the lower-
most portions of the encapsulation 30 as at 31 in order to sever the
bight portion 24. The resulting configuration of the preform is shown
in Fig. 16.
With the spring loaded in the manner shown in Fig. 16, the
exposed ends of the switch section are subjected to a second dip of
heat fusible organic material to provide a complete insulating
covering 32 of these exposed ends. The two dips of fusible organic
material may, of course, be of exactly the same material and may
become essentially homogeneous. Upon the cooling of the heat
fusible material following this second dip, the circuit will be closed
and the spring loaded in a preassembly shown in Fig. 17.
It should be noted that since the contact region 28 is spaced
upwardly from the terminal extremity of subsection 18, a lever arm
is included thereby serving to minimize the unit force which is necessary
to establish firm electrical contact at the contact region. This con-
figuration in addition to the flat surface of the wire in the switch section
15 substantially reduces the unit pressure on the organic material
resulting from the energy loaded in the spring section. This, of course,
becomes important since material such as the encapsulating heat fusible
material will tend to be subjected to cold flow under the continuing forces
exerted in the spring section.
R~ -15-
l~?S373~
Following the second dipping step, the switch section of the
device and the heat fusible encapsulations 30 and 32 are totally and
conformally coated with a layer of rigid insulating material 34. The
device shown in Fig. 18 is representative of the switch in its final
loaded condition capable of transmitting current from one lead wire 14
to the other and Fig. 19 is representative of the switch in the npen
condition.
Of course, it is understood that a switch having the continuing
preloading as just described can be obtained by two discrete spring
regions with the contact region intermediate thereof. Figs. 20-24 show
such an embodiment and the assembly technique and functions of each
of the elements shown therein are identical to Figs. 2, 4-7 and a
detailed description of such an embodiment is therefore not necessary.
It suffices to state that switch section ]5 of Figs. 20-24 is comprised
of three subsections, i. e., 16, 18 and 20, with an upper spring
section 26 functioning as spring section 26 in Fig. 2 and a lower spring
section 27 causing the contact regions 28 to abut under a predetermined
preload.
While the preferred embodiment describes the initial step of
manufacturing to be that of a dipping process, Fig. 25 shows an alternate
embodiment to this aspect of the invention. An annular ring type of
pellet 30a may surround the lower subsection 20a in a manner similar
to the dip 30 shown in Fig. 4. Following the positionment of the ring
type preform in this manner, the switch may be completed in a manner
similar to that described relative to the primary embodiment in that the
-16-
1053731
switch sections 16a, 18a and 20a are brought generally into alignment
against the bias of the spring regions 26a and 27a. With an external
force exerted on the preform in the manner described previously, the
switch region may be then encapsulated with a heat fusible material such
as that shown and described relative to Fig. 5 of the primary embodiment.
However, the dipping of the organic may be replaced with a cap of solid
carbon dioxide or dr~y iceJ or any equivalent material which is solid at a
low temperature, This secondary surrounding of heat fusible material
may then be coated with rigid insulating material, such as epoxy, and
cured. When the epoxy has cured, the device may approach room
temperature and the gas within the switch section allowed to escape as
it expands. The epoxy coating or cap will then be sealed with a drop
or drops of epoxy. A further alternate approach to the manufacture
of the device described herein contemplates a cylindrical type of heat
fusible organic generally of a length consistent with the length of the
switch section 15a. The upper regions of such a preform may be
placed surrounding the subsections 20a dn the loading force applied
to the upper regions of the preform 14a, After this loading force is
so applied, the preform may be slid up to entirely surround the switch
section 15a and retain it in a loaded condition in a manner very similar
to that shown in the embodiment described as Fig. 5.
This invention thus provides a spring loaded switch which
encapsulates a pair of conductor wires in a first, contacting position
under bias of a spring region in at least one of the conductor wires.
The conductor wires are held in contact with each other against the
bias of a hinge-like spring region until -the encapsulating heat fusible
-17-
~053731
material is subjected to a predetermined temperature level. At such
a temperature level, the conductor wires move to a second, relaxed
position, out of contact with each other thus opening the circuit.
Thus, it is apparent that there has been provided in accordance
with the invention a thermal switch device that fully satisfies the
objects, aims and advantages set forth above. While the invention
has been described in conjunction with specific embodiments thereof,
it is evident that many alternatives, modifications and variations will
be apparent to those skilled in the art in light of the foregoing description.
Accordingly, it is intended to embrace all such alternatives, modifica-
tions and variations as fall within the spirit and broad scope of the
appended claims.
-18-
