Thermal fuse

A resistor with a thermal fuse includes: a substrate consisting of an insulating material; a wire pattern formed on a surface of the substrate; a gap for electrically cutting off the wire pattern; a plate spring consisting of an electrically conductive material arranged across the gap; an electrically conductive member fixed to one end of the plate spring on a side of the wire pattern and having approximately the same linear conduction coefficient as that of the substrate; a first low melting point alloy for welding the electrically conductive material and the wire pattern provided on one side of the gap; a second low melting point alloy for welding the other end of the plate spring and the wire pattern provided on the other side of the gap; wherein, when the first low melting point alloy is melted, the electrically conductive member is released from the first low melting point alloy so that the plate spring is also released from the wire pattern.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a resistor with a thermal fuse. The 
resistor according to the present invention is advantageous for a resistor 
used as a speed control device provided for a blower in a vehicle. 
2. Description of the Related Art 
Recently, a resistor with a thermal fuse has been widely used in various 
fields, such as a home electronics equipment, an alarm system in a vehicle 
and buildings, and the like. As is known, the thermal fuse operates when 
peripheral temperature abnormally rises and a low melting point alloy, 
which holds the thermal fuse, melts. In this case, before the low melting 
point alloy is melted, i.e., before the thermal fuse operates normally, a 
crack may occur in the low melting point alloy caused by different thermal 
expansion between structural elements, such as a thermal fuse, a wire 
pattern and a substrate. This crack results in disconnection between the 
thermal fuse and the wire pattern. 
Accordingly, it is desired to provide an improved resistor with a thermal 
fuse which can solve the problem of the different thermal expansion 
between the structural elements. 
SUMMARY OF THE INVENTION 
The object of the present invention is to provide an improved resistor with 
a temperature fuse which can suppress occurrence of a crack in the low 
melting point alloy caused by different thermal expansions between 
structural elements. 
In accordance with the present invention, there is provided a thermal fuse 
including: a substrate consisting of an insulating material; a wire 
pattern formed on a surface of the substrate; a gap for electrically 
cutting off the wire pattern; a plate spring consisting of an electrically 
conductive material and arranged across the gap; an electrically 
conductive member fixed to one end of the plate spring on a side of the 
wire pattern and having approximately the same linear expansion 
coefficient as that of the substrate; a first low melting point alloy for 
welding the electrically conductive material and the wire pattern provided 
on one side of the gap; a second low melting point alloy for welding the 
other end of the plate spring and the wire pattern provided on the other 
side of the gap; wherein, when the first low melting point alloy is 
melted, the electrically conductive member is released from the first low 
melting point alloy so that the plate spring is also released from the 
wire pattern. 
In a preferred embodiment, the plate spring has a shape for absorbing 
thermal expansion thereof at a position between the first and second low 
melting point alloys. 
In another preferred embodiment, the electrically conductive member has a 
hairpin-like shape, and is caulked to one longitudinal end of the plate 
spring. 
In still another preferred embodiment, the electrically conductive member 
has a hairpin-like shape so as to have an elastic force, and is fixed to 
one longitudinal end of the plate spring based on its elastic force. 
In still another preferred embodiment, the electrically conductive member 
has a rivet-like shape, is inserted into a hole provided on the end of the 
plate spring, and is pressed and fixed to one end of the plate spring so 
as to form a flat portion on the side of the wire pattern. 
In still another preferred embodiment, the electrically conductive member 
is fixed to the plate spring and is able to thermally expand independently 
of the plate spring.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Before describing the preferred embodiments, a conventional art and its 
problem will be explained in detail below. 
FIGS. 1A and 1B are sectional views of a resistor with a thermal fuse 
according to a conventional art. This conventional art has been disclosed 
in Japanese Unexamined Patent Publication No. 3-280318. 
In FIGS. 1A and 1B, reference letter A denotes a thermal fuse, 8 a 
substrate, 9 (9') a wire pattern, 10 a gap, 11 a plate spring, 11a and 11c 
end portions of the plate spring, and 13 (13') a low melting point alloy. 
In FIG. 1A, the plate spring 11, made of a metal, is previously bent as 
shown by a chain dotted line (see the reference number 11'). Then, the 
plate spring 11 is formed straight by using the low melting point alloy 13 
(13') as shown by a solid line, and arranged across the gap 10 which is 
formed by cutting a part of wire pattern 9 (9') provided on the substrate 
8. Both of ends 11a and 11c of the plate spring 11 are electrically 
connected to the wire pattern 9 (9') by welding the low melting point 
alloy 13 and 13'. 
Further, the wire pattern 9 (9') includes a resistance element (not shown) 
therein. When an over-current flows through the resistor, the resistance 
element becomes hot due to Joule heating. As a result, the low melting 
point alloy 13 is heated and melted so that the end portion 11a of the 
plate spring 11 is released from the low melting point alloy 13 and 
returned to the original position shown by the chain doted line 11' in 
accordance with an elastic force of the spring itself. Accordingly, the 
connection between the wire patterns 9 and 9' is broken (i.e., a circuit 
is turned off). In general, in order to perform this operation, the end 
portion 11a of the plate spring 11 is arranged in the vicinity of the 
position where the temperature becomes the highest on the substrate 8. 
However, in the above explained conventional resistor, when the resistor 
has been used for a long time, a crack(s) 15 may occur in the low melting 
point alloy 13 as shown by the drawing. When this crack 15 grows large, 
the low melting point alloy 13 is gradually destroyed and completely cut 
off from the wire pattern after a short time. Accordingly, there are 
problems that the thermal fuse cannot operate normally and a heating of 
the resistance element cannot be detected correctly. 
The crack in the low melting point alloy will be explained in detail below. 
In the conventional resistor with the thermal fuse, the resistance element 
which is provided within the wire pattern 9 becomes hot in a normally 
activated state even if an over-current does not flow therethrough. In 
this case, the heating of the resistance element is not high compared to 
the heating in the abnormal state so that the low melting point alloy 13 
is not melted. However, the substrate 8, the wire pattern 9 (9'), the 
plate spring 11, and the low melting point alloy 13 thermally expand due 
to this heating (i.e., thermal expansion). At the same time, the low 
melting point alloy 13 becomes soft. 
In this case, since the wire pattern 9 (9') is very thin compared to the 
substrate 8 and strongly fixed to the substrate 8, the thermal expansion 
of the wire pattern 9 (9') becomes equivalent to that of the substrate 8. 
Further, the linear expansion coefficient of the plate spring 11 
consisting of the metal spring member, such as a beryllium copper or a 
phosphor bronze spring, is larger than that of the substrate 8 consisting 
of an insulating material. Accordingly, although the plate spring 11 
expands in the longitudinal direction caused by the heating (i.e., 
expanded to the right of the plate spring 11), the wire pattern 9 (9') is 
only slightly expanded in the same direction compared to the plate spring 
11. 
As a result of the different thermal expansion between the plate spring 11 
and the substrate 8, the low melting point alloy 13 receives a stress and 
is deformed in such a way that the upper side of the low melting point 
alloy 13 (i.e., a side of the plate spring 11 indicated by (a) in FIG. 1B) 
is deformed in the longitudinal direction caused by the thermal expansion 
of the plate spring 11. 
However, a lower side of the low melting point alloy 13 (i.e., a side of 
the substrate 8 indicated by (b) in FIG. 1B) is slightly deformed. As a 
result, a thermal stress is applied to the end portion 13a of the low 
melting point alloy 13 so that the crack 15 occurs at that portion. 
Further, since the turning on/off operation is repeated in the electric 
circuit, the above deformation of the low melting point alloy 13 is 
repeated so that the crack grows larger. 
As a countermeasure to the above problem, the inventor of the present 
application had proposed a U-shaped portion provided in the vicinity of a 
center of the plate spring 11 in order to absorb the thermal expansion 
between the low melting point alloy 13 and 13'. However, this was 
insufficient to clearly suppress the crack in the low melting point alloy 
13. 
Further, the inventor found the following fact. That is, as explained 
above, the end portion 11a of the plate spring 11 is arranged in the 
vicinity of the position where the temperature becomes the highest in the 
substrate 8. Accordingly, even if the U-shaped portion is provided in the 
vicinity of the center of the plate spring 11, as shown in FIG. 1B, the 
plate spring 11 is expands in the longitudinal direction from the vicinity 
of the center of the low melting point alloy 13. 
Accordingly, as shown by the chain dotted line in FIG. 1B, the upper side 
(a) of the low melting point alloy 13 is deformed in the longitudinal 
direction (see an arrow EX) from the vicinity of the center. However, the 
lower portion (b) is only slightly deformed in the same direction from the 
vicinity of the center. As a result, the thermal stress is applied to the 
end portion 13a of the low melting point alloy 13, and the deformation of 
the low melting point alloy 13 is repeated in accordance with the turning 
on/off of the electric circuit so that the crack 15 occurs in the end 
portion 13a. Further, the crack 15 grows larger when the deformation is 
repeated so that the low melting point alloy 13 is cut off. 
The present invention aims to suppress occurrence of the crack in the low 
melting point alloy 13 as explained in detail below. 
FIG. 2 is a sectional view of a resistor with a thermal fuse according to 
the present invention. In FIG. 2, the reference numbers used in FIGS. 1A 
and 1B are attached to the same elements in this drawing. 
According to the present invention, one feature lies in that an 
electrically conductive member 12, which has approximately the same linear 
expansion coefficient as that of the substrate 8, is provided to an inner 
surface (i.e., the side of the low melting point alloy 13) of the end 
portion 11a of the plate spring 11. 
As a result, when the wire pattern 9 (9') is heated in the normally 
activated state, the electrically conductive member 12 is expanded in the 
longitudinal direction by approximately the same amount as the substrate 
8. In this case, the wire pattern 9 (9') is also expanded to the 
longitudinal direction by approximately the same amount as the substrate 
8. Accordingly, the deformation of the plate spring 11 becomes 
approximately the same as the deformation of the low melting point alloy 
13 at the side of the wire pattern 9 (9'). Therefore, the thermal stress, 
which is applied to the end portion of the low melting point alloy 13, 
becomes very small so that it is possible to easily prevent the cracking 
which occurs in the low melting point alloy 13. 
Further, as another feature, the plate spring 11 has a shape which absorbs 
the deformation caused by the thermal expansion, between a first low 
melting point alloy 13 and a second low melting point alloy 13'. According 
to this shape, the thermal expansion of the plate spring 11 in the 
longitudinal direction becomes small so that it is possible to suppress 
the deformation of the longitudinal direction of the plate spring 11. As a 
result, the thermal stress, which is applied to the end portion of the low 
melting point alloy 13, becomes very small so that it is possible to 
prevent the cracking which occurs in the low melting point alloy 13. 
Still further, as still another feature, a mechanically fixing means, for 
example, a rivet (see FIGS. 6A to 6C) is used for fixing the electrically 
conductive member 12 to the end portion 11a of the plate spring 11, it is 
possible to easily mount the electrically conductive member 12 to the 
plate spring 11. 
The preferred embodiments of the present invention will be explained in 
detail below. 
FIG. 3 shows one example of a resistor with the thermal fuse, which is 
applied to a resistor used as a speed control device (below, speed control 
resistor) provided for a blower of a vehicle. In this drawing, reference 
number 1 denotes a speed control resistor, 2 an air duct, 3 a fan, 4 a 
damper, 7 a blower motor, and 14 a bracket. The speed control resistor 1 
is provided downstream of the fan 3 and fixed to an inner surface of the 
air duct 2 through the bracket 14 which is fixed to an outer surface of 
the duct 2 by using screws. Accordingly, the speed control resistor is 
always cooled by the air from the fan 3. In the drawing, the damper 4 is 
provided for switching the air from the outside and the air from the 
inside. 
FIG. 4 is an electric circuit including the speed control resistor, and 
FIG. 5 is a perspective view of a mechanical structure of the electric 
circuit of FIG. 4. 
As shown in FIG. 4, the electric circuit includes the speed control 
resistor 1 consisting of a thermal fuse A, resistance elements 41 to 43, 
and terminals 51 to 54; a movable member 6 provided to a switch; and the 
blower motor 7. Further, reference letter P denotes a power source for the 
blower motor 7. 
The movable member 6 is switched to any one of contact points (1) to (4) in 
order to change the current amount flowing through the blower motor 7. As 
a result, the rotational speed of the blower motor 7 is changed in 
accordance with the change in the current so that it is possible to change 
the amount of air (i.e., an airflow amount) from the fan 3. 
In actual use, a driver of the vehicle operates a switch knob (not shown) 
which is provided in the vehicle, for changing the airflow amount so as to 
obtain the maximum airflow amount from the fan 3 by connecting the movable 
member 6 to the contact point (1). 
In this case, since the current is directly supplied to the blower motor 7 
through the movable member 6 and the contact point (1), and since 
resistance elements 41 to 43 are not included in a current path to the 
blower motor 7, the current flowing through the blower motor 7 becomes 
maximum. On the contrary, when the movable member 6 is connected to the 
contact point (4), since the current flows through all resistance elements 
41 to 43, the current flowing through the blower motor 7 becomes minimum 
so that it is possible to obtain the minimum airflow amount. 
In an actual structure, as shown in FIG. 5, the speed control resistor 1 is 
mounted on the inner surface of the air duct 2 by using the bracket 14 as 
shown in FIG. 4. In FIG. 5, resistance elements 41 to 43 are provided on 
the front and back surfaces of the substrate 8 (see the solid line 41 and 
the dotted lines 42 and 43), and the thermal fuse A is provided on the 
front surface of the substrate 8. Further, terminals 51 to 54 are fixed to 
the bracket 14 and each connected in series through the resistance 
elements 41 to 43 (see the solid line 41 and the dotted lines 42 and 43). 
The material of the thermal fuse A will be explained in detail with 
reference to FIG. 2 below. 
First, as shown in FIG. 2, the substrate 8 consists of an insulating 
material, such as a ceramic having a linear expansion coefficient 
.alpha.=7.times.10.sup.-6 (/.degree.C.). Further, the wire pattern 9 (9') 
including the resistance elements 41 to 43 is formed on the surface of the 
substrate 8. The wire pattern 9 (9') consists of a corrosion-resisting 
electrical conductive material, such as a copper or a silver, and, 
preferably, its thickness is about 15 to 20 .mu.m. Further, the gap 10 is 
provided between the wire pattern 9 and 9' in order to cut off an 
electrical connection therebetween. Preferably, the width of the gap 10 is 
about 18 to 20 mm. 
As shown in FIG. 2, the plate spring 11 is provided across the gap 10, and 
a U-shaped portion 11b is provided in the vicinity of the center thereof. 
The plate spring 11 consists of a metal material having a highly elastic 
characteristic, such as a beryllium copper (the linear expansion 
coefficient .alpha.=17.8.times.10.sup.-6 (/.degree.C.)) or a phosphor 
bronze. The original length before it is bent is about 35 mm, the width is 
about 3 mm and the thickness is about 0.1 to 0.15 mm. After the U-shaped 
portion was formed, the length of the plate-like spring 11 becomes about 
26 mm. 
Further, in this embodiment, the plate-like electrically conductive member 
12 is fixed to the inner surface of the end portion 11a of the plate 
spring 11 by using a welding technique. In this case, the electrically 
conductive member 12 is formed by the metal having a small linear 
expansion coefficient as well as the substrate 8, such as a molybdenum 
(.alpha.=5.1.times.10.sup.-6 (/.degree.C.)), an iron-nickel alloy 
(Fe--42Ni) (.alpha.=4.4.times.10.sup.-6 (/.degree.C.)) or an iron 
(.alpha.=12.times.10.sup.-6 (/.degree.C.)). 
The length of the electrically conductive member 12 is about 8 mm, the 
width is about 4 mm, and the thickness is about 0.25 mm. 
The electrically conductive member 12 is fixed to the plate spring 11 at 
the position B in the vicinity of the center of the end portion 11a. 
Further, the electrically conductive member 12 and the other end portion 
11c are fixed to the wire patterns 9 and 9' by using the low melting point 
alloys 13 and 13', respectively. 
The low melting point alloy 13 (13') consists of a solder (SL) formed of 
tin-silver alloys having various compositions, for example; 
the first SL is formed by Sn-96.5 and Ag-3.5 (.alpha.=30.2.times.10.sup.-6 
(/.degree.C.)), and its melting point is T.sub.m =221 (.degree.C.)); 
the second SL is formed by Sn-63 (.alpha.=25.5.times.10.sup.-6 
(/.degree.C.)) and its melting point is T.sub.m =183 (.degree.C.)); and 
the third SL is formed by Sn-100 (.alpha.=26.6.times.10.sup.-6 
(/.degree.C.)) and its melting point is T.sub.m =238 (.degree.C)). 
In this case, the length of the solder is about 3 mm, and the thickness is 
0.15 mm. The width of the solder is the same as that of the plate spring 
11. 
The plate spring 11 is previously bent before the end portions 11a and 11c 
are fixed to the wire pattern 9 (9') as shown by the chain dotted line 
11'. When the end portions 11a and 11c are fixed by using the low melting 
point alloys 13 and 13', the plate spring 11 is formed approximately 
straight as shown by the solid line. 
Further, in FIG. 5, the end portion 11a of the plate spring 11 is arranged 
in the vicinity of the position where the temperature becomes the highest 
on the substrate 8. Concretely, as shown by the number 11a in the drawing, 
the end portion 11a is arranged in the vicinity of the center 8a of the 
substrate 8. According to this position, the largest influence of the heat 
generated by the resistance elements 41 to 43 is applied to the end 
portion 11a. 
The operation of the present invention will be explained in detail with 
reference to FIGS. 2 to 5. 
As mentioned above, the U-shaped portion 11b is previously formed in the 
vicinity of the center of the plate spring 11. Further, each end portions 
11a and 11c of the plate spring 11 is fixed to the end of the wire pattern 
9 (9') by welding the low melting point alloy 13 (13'). According to this 
structure, the thermal expansion of the plate spring 11 can be absorbed by 
the U-shaped portion 11b between the low melting point alloys 13 and 13'. 
When the driver turns on an ignition switch (not shown) and operates an 
airflow adjusting switch (not shown) so as to obtain the minimum airflow 
from the fan 3 (see FIG. 3), the movable member 6 is connected to the 
contact point (4). In this case, since the speed control resistor 1 is 
arranged down-stream of the fan 3 in the air duct 2 as shown in FIG. 3, 
the speed control resistor 1 can be always cooled by the wind from the fan 
3. 
However, when an abnormal state, for example, a state which the blower 
motor 7 is locked, occurs in the vehicle, an over-current flows through 
the electric circuit including the speed control resistor 1 so that the 
resistance elements 41 to 43 are abnormally heated. Accordingly, as 
explained above, the temperature of the center 8a (see FIG. 5) on the 
substrate 8 rises to the highest state. 
As a result, when the low melting point alloy 13 which is used for fixing 
the end portion 11a to the wire pattern 9, is heated and its temperature 
reaches a predetermined melting point T.sub.m (about 200.degree. C.), the 
low melting point alloy 13 starts to melt. When the low melting point 
alloy 13 is melted, the end portion 11a of the plate spring 11 is released 
from the low melting point alloy 13 based on its own elastic force as 
shown by the chain dotted line in FIG. 2. Accordingly, since the thermal 
fuse A is disconnected from the resistance elements 41 and 42 in the 
resistor 1 in FIG. 2, the electric circuit can be completely cut off. 
Even if the over-current does not flow through the electric circuit, the 
resistance elements 41 to 43 become hot in the normally activated state so 
that the center portion 8a of the substrate 8 is heated. In this case, the 
low melting point alloy 13 is heated based on the heat from the center 
portion 8a to about 140.degree. C. Accordingly, the low melting point 
alloy 13 becomes soft. 
However, in this embodiment, as shown in FIG. 2, the electrically 
conductive member 12 having the linear expansion coefficient which is very 
close to that of the substrate 8, is fixed to the end portion 11a of the 
plate spring 11 by welding the electrically conductive member 12 in the 
vicinity of the center portion B. In this case, since the thermal 
expansion coefficient of the plate spring 11 is very large, it is 
considerably expanded in the vicinity of the center portion B. On the 
other hand, since the linear expansion coefficients of the electrically 
conductive member 12 and the substrate 8 are small, the electrically 
conductive member 12 is slightly expanded and the expanded amount is very 
close to that of the substrate 8. 
Accordingly, the upper portion (see (a) in FIG. 2) of the low melting point 
alloy 13 is deformed in accordance with the thermal expansion of the 
electrically conductive member 12, i.e., approximately the same amount as 
the thermal expansion of the substrate 8. Further, since the wire pattern 
9 (9') is very thin compared to the substrate 8 and fixed thereto, the 
thermal expansion of the wire pattern 9 (9') is approximately the same 
amount as that of the substrate 8. Accordingly, the lower portion (see (b) 
in FIG. 2) of the low melting point alloy 13 is deformed in accordance 
with the thermal expansion of the substrate 8. As a result, when the 
electrically conductive member 12 is provided to the end portion 11a as 
shown in FIG. 2, the thermal stress, which occurs in the end portion 13a 
of the low melting point alloy 13, becomes very small so that it is 
possible to suppress occurrence of cracking in the low melting point alloy 
13. 
The inventor performed a test, in order to confirm the effect of this 
invention, as follows. 
The first test sample was a conventional type. A conventional speed control 
resistor having the thermal fuse A formed by the plate spring 11 was 
assembled in the speed control resistor of FIG. 5. In this case, the 
thermal fuse A was a straight shape as shown in FIG. 1A (i.e., no U-shaped 
portion and no electrically conductive member). Further, the gap 10 
between the low melting point alloy 13 and 13' was set to 20 mm. The plate 
spring 11 consists of the beryllium copper, and its length was 26 mm, the 
width was 2.5 mm, and the thickness was 0.1 mm. The low melting point 
alloy 13 (13') consists of the solder formed of tin (Sn) 96.5% and the 
silver (Ag) 3.5%, and its length was 3 mm, the width was 2.5 mm, and 
thickness was 0.15 mm. 
The second test sample incorporated the present invention. The speed 
control resistor 1, according to the present invention, having the thermal 
fuse A formed by the U-shaped portion 11b in the vicinity of the center of 
the plate spring 11, and the electrically conductive member 12 at the end 
portion 11a was assembled as shown in FIG. 2. In this case, the gap 10 
between the low melting point alloy 13 and 13' was set to 20 mm. The plate 
spring 11 consisted of beryllium copper, and the length was 35 mm, the 
width was 2.5 mm, and the thickness was 0.1 mm. The actual length of the 
plate spring 11 was about 26 mm after the U-shaped portion 11b was formed. 
Further, the electrically conductive member 12 consists of an iron-nickel 
alloy (Fe--42Ni), and the length was 8 mm, the width was 4 mm, and the 
thickness was 0.25 mm. Still further, the low melting point alloy 13 (13') 
consisted of the solder formed by tin (Sn) 96.5% and the silver (Ag) 3.5%, 
and the length was 3 mm, the width was 4.0 mm, and the thickness was 0.15 
mm. 
In an actual test, the first and second samples were alternately dipped 
during five minutes into two kinds of silicon oil having the temperature 
of minus (-) 40 (.degree.C.) and plus (+) 150 (.degree.C.). After these 
steps were repeated 1,200 times, the configuration (state) of the low 
melting point alloy 13 was compared between the first and second samples. 
As a result of comparison, in the first sample (i.e., conventional type), a 
large crack occurred in the low melting point alloy 13 in longitudinal 
direction of the plate spring 11 so that the low melting point alloy 13 
was cut off. On the other hand, in the second sample (i.e., the present 
invention), no cracking occurred in the low melting point alloy 13. 
Accordingly, it was obvious that the present invention, having the 
electrically conductive member 12 and the U-shaped portion 11b, was very 
effective for prevention of cracking. 
FIGS. 6A to 6C show electrically conductive members according to another 
examples of the present invention, and FIG. 7 shows a thermal fuse 
according to a second embodiment of the present invention. 
The second embodiment will be explained in detail with reference to FIGS. 
6A to 6C and 7. In the first embodiment mentioned above, the electrically 
conductive member 12 is fixed to the end portion 11a by welding it to the 
end portion 11a. In this second embodiment, three examples of fixing the 
plate spring 11 and the electrically conductive member 12 are shown in 
FIGS. 6A to 6C. 
In FIG. 6A, the electrically conductive member 12 has a hairpin-like shape. 
This hairpin-like conductive member 12 is inserted to the end portion 11a 
of the plate spring 11, and these parts 11 and 12 are pressed (caulked) 
from the perpendicular direction (see arrow line F) so that the 
electrically conductive member 12 can be fixed to the end portion 11a. 
In FIG. 6B, the electrically conductive member 12 has a rivet-like shape. A 
hole is provided for the end portion 11a of the plate spring 11. The rivet 
is inserted into the hole and pressed from the perpendicular direction so 
that it is possible to obtain a flat portion of the electrically 
conductive member 12. 
In FIG. 6C, the conductive member 12 has a C-shaped shape so as to have an 
elastic force. The C-shaped conductive member 12 is inserted to the end 
portion 11a of the plate spring 11 so that the electrically conductive 
member 12 can be fixed to the end portion 11a of the plate spring 11 based 
on its own elastic force. 
According to the above second embodiment, since no welding portion is 
provided between the electrically conductive member 12 and the end portion 
11a of the spring 11, it is easy to reduce the manufacturing cost of the 
speed control resistor and to assemble these parts 11 and 12. 
Still further, in the examples shown in FIGS. 6A to 6C, since the 
electrically conductive member 12 is not welded to the end portion 11a and 
only strongly held by the end portion 11a,the plate spring 11 and the 
electrically conductive member 12 can thermally expand independently each 
other by overcoming a friction force which occurs between them. 
In the above embodiments, although the U-shaped portion is provided in the 
vicinity of the center of the plate spring 11, the present invention is 
not limited to this U-shape. That is, a M-shaped portion can be used 
instead of the U-shaped portion since the M-shaped portion also can absorb 
the thermal expansion of the plate spring 11. 
Still further, in the above embodiments, the U-shaped portion 11b and the 
electrically conductive member 12 are provided in order to prevent 
cracking in the low melting point alloy 13. However, only the electrically 
conductive member 12 is provided to the end portion 11a of the plate 
spring 11, it is possible to considerably suppress occurrence of cracking 
in the low melting point alloy 13 compared to the conventional art shown 
in FIGS. 1A and 1B. 
FIG. 7 is another example of the thermal fuse according to the present 
invention. In this example, the plate spring 11 is previously bent as 
shown by the chain dotted line 11' at a point X, and the U-shaped portion 
is not provided in this example. The end portion 11a having the 
electrically conductive member 12 is forcedly fixed to the wire pattern 9 
by welding the low melting point alloy 13 as shown by the solid line. As 
explained above, when the low melting point alloy 13 is melted, the end 
portion 11a of the plate spring 11 is released from the low melting point 
alloy 13 and returned to the original position shown by the dotted line. 
Still further, the plate spring 11 is not limited to this shape. That is, 
another shape can be used instead of the plate spring 11 if the function 
explained in the first and second embodiments can be obtained.