BACKGROUND OF THE INVENTION
1. Field of the Invention:
The present invention relates to a glow plug,
used for improving the start-up ability of a diesel engine or the
like.
2. Related Art:
From DE-A-40 01 296 a glow plug is known, which comprises a cylindrical
housing, a cup-shaped heater tube having a smaller diameter portion at a closed end and
a larger diameter portion at an open end, wherein the larger diameter portion is fixed to
an end of the housing. A first resistor is provided in the heater tube adjacent to the
closed end and a second resistor is provided adjacent to the first resistor within the
smaller diameter portion of the heater tube. This known glow plug also comprises a
third resistor, which is electrically connected in series with the second and first resistor,
wherein the resistance-temperature coefficient of the third resistor is larger than a
resistance-temperature coefficient of the first and second resistor, wherein the first
resistor and the second resistor are completely accommodated in the smaller diameter
portion of the heater tube, whereas the third resistor is completely accommodated within
the larger diameter portion of the heater tube.
Fig. 8 shows a further conventional glow plug 1 disclosed in Unexamined
Japanese Patent Application No. 3-99122, published in 1991. The conventional glow
plug 1 comprises a first resistor 4 and a second resistor 5 both serving as a heat
generating element. The second resistor 5 has a positive resistance-temperature
coefficient larger than that of the first resistor 4. Both the first resistor 4 and the second
resistor 5 are provided in a heater tube 3. With this arrangement, immediately after the
glow plug 1 receives electric power, a large amount of electric current flows across the
first resistor 4. The first resistor 4 generates heat. After an elapse of a predetermined
time, the second resistor 5 increases the temperature. The resistance value of the second
resistor 5 increases correspondingly. The electric power supplied to the first resistor 4 is
decreased due to the increase of the resistance of the second resistor 5. This prevents the
first resistor 4 from being burned out by excessive heat generation.
The diameter of the heater tube 3 is small at a portion accommodating the first
resistor 4 compared with at a portion accommodating the second resistor 5. With this
arrangement, the heat capacity of the heater tube 3 becomes small in the vicinity of the
first resistor 4 compared with in the vicinity of the second resistor 5. The temperature of
the first resistor 4 can be
quickly increased immediately after the glow plug 1 is activated. More specifically, an
ordinary engine requires a preheating. Within a given preheating time (for example, 5
seconds), the front end of the glow plug 1 reaches a predetermined temperature (e.g.,
800 °C).
However, according to the above-described conventional glow plug, the
temperature of the glow plug may increase excessively after the front end of the glow
plug reaches the above-described predetermined temperature. The maximum
temperature possibly increases up to 1,050 °C. The power supply voltage is set at
approximately 12 V during the above-described preheating. After the preheating is
finished, the engine is started. A generator voltage of a charging generator is controlled
to a predetermined voltage by a regulator. The setting voltage of the regulator is
increased to approximately 14 V, once the engine starts rotating. The increased voltage
is supplied to the sufficiently heated glow plug 1. Thus, the temperatures of the resistors
4 and 5 of the glow plug 1 further increase. Such an excessive temperature increase may
cause oxidation and burnout of the resistors 4 and 5. This possibly gives adverse,
influence to the durability of the glow plug 1.
SUMMARY OF THE INVENTION
In view of the foregoing problems, it is an object of the present invention to
provide a glow plug capable of maintaining quick heat generation ability as well as
suppressing the excessive temperature increase in the glow plug after finishing the
preheating.
According to the present invention, this object is solved by the features of claim 1.
An improved embodiment of the inventive glow plug results from the subclaim 2.
The present invention provides a glow plug having various aspects, which will be
described hereinafter. Reference numerals in parentheses, added in the following
description, show the correspondence to the components described in preferred
embodiments of the present invention. The reference numerals are thus merely
used for the purpose of expediting the understanding to the present invention
and not used for narrowly interpreting the scope of the present invention.
According to one embodiment of the present invention, a glow plug comprises
a cylindrical housing (2). An elongated cup-shaped heater tube (3) has a larger-diameter
portion (32) which is fixed to an end (2a) of the housing (2). A first
resistor (4) and a second resistor (5) are provided in an inside space of the
heater tube (3). The second resistor (5) has a positive resistance-temperature
coefficient larger than that of the first resistor (4). The second resistor (5) is
electrically connected in series with the first resistor (4). Furthermore, the
entire body of the first resistor (4) and at least part of the second resistor (5)
are accommodated in a smaller-diameter portion (31) of the heater tube (3).
With this arrangement, the heat capacitors of both the first resistor (4)
and the second resistor (5) can be reduced. Not only the warm-up ability of the
second resistor (5) is improved, but also the resistance increasing ability of the
second resistor (5) is improved. Accordingly, the electric power supply to the
first resistor (4) can be quickly suppressed. In addition, the temperature
increase of the first resistor (4) can be quickly suppressed. Thus, it becomes
possible to prevent any excessive temperature increase of the glow plug (1)
after the temperature of the front end of the glow plug (1) reached the above-described
setting temperature (e.g., 800 °C).
Accordingly, even if the glow plug (1) receives the electric power
approximately 14 V, an excessive temperature increase of the glow plug (1) is
effectively prevented. The oxidation and burnout of the first and second
resistors (4, 5) can be suppressed. The durability of the glow plug (1) can be
adequately maintained.
As described above, the first resistor (4) is also accommodated in the
smaller-diameter portion (31). The first resistor (4) has a smaller heat capacity
comparable with that of the conventional one. Thus, the temperature of the first
resistor (4) can be quickly increased immediately after the glow plug (1) is
activated.
When installed in the combustion chamber of an internal combustion
chamber, the glow plug (1) is subjected to an explosive pressure caused in the
combustion chamber. The heater tube (3) may cause a displacement about a
cylindrical peripheral portion (34) facing to and brought into contact with an
inner cylindrical surface of one end (2a) of the housing (2). More specifically,
the closed end (3a) of the heater tube (3) may shift in a radial direction with
respect to the housing (2). In this respect, according to the present invention,
the larger-diameter portion (32) of the heater tube (3) is brought into contact
with the inner cylindrical surface of the housing (2). The connecting strength
in the vicinity of the cylindrical peripheral portion (34) can be maintained
sufficiently. It becomes possible to eliminate the possibility that the heater tube
(3) may be bent or broken at the above-described cylindrical peripheral portion
(34) during the repetitive operations of the glow plug (1) in the combustion
chamber.
According to the present
invention, more than half of the second resistor (5) is
accommodated in the smaller-diameter portion (31) of the heater tube (3). With
this arrangement, an excessive temperature increase succeeding the preheating
can be effectively suppressed compared with the previously described prior art.
This effect was confirmed based on a later-described experiment.
Furthermore, in one embodiment of the invention the larger-diameter portion (32) has an
outer diameter in a range of 4.5 mm to 6.0 mm. Too much reducing the outer
diameter of the large-diameter portion (32) is not preferable in that it becomes
difficult to maintain a satisfactory strength in the vicinity of the cylindrical
peripheral portion (34) where the heater tube (3) is brought into contact with
the inner cylindrical surface of the housing (2). Too much increasing the outer
diameter of the large-diameter portion (32) is not preferable in that it becomes
difficult to suppress an overall size of the glow plug (1) and assure a smooth
installation of the glow plug (1) to the engine.
According to the invention the outer diameter of the smaller-diameter
portion (31) is 0.6 to 0.9 times as large as the outer diameter of the
larger-diameter portion (32). Too much reducing the outer diameter of the
smaller-diameter portion (31) is not desirable in that the smaller-diameter
portion (32) becomes too thin to manufacture. Too much increasing the outer
diameter of the smaller-diameter portion (31) is not desirable in that the
smaller-diameter portion becomes too thick to sufficiently reduce the heat
capacity of the first and second resistors (4,5).
BRIEF DESCRIPTION OF THE DRAWINGS
The above and other objects, features and advantages of the present
invention will become more apparent from the following detailed description
which is to be read in conjunction with the accompanied drawings, in which:
Fig. 1A is a cross-sectional view showing an overall arrangement of a
glow plug in accordance with a first embodiment of the present invention; Fig. 1B is an enlarged view showing a first resistor and a second resistor
connected each other in the glow plug in accordance with the first embodiment
of the present invention; Fig. 2 is a diagram showing a current supply circuit of the glow plug in
accordance with the first embodiment of the present invention; Fig. 3 is a graph comparatively showing heat generation characteristics
at the front end of the glow plug in accordance with the present invention and
a prior art; Fig. 4 is a graph showing an overshoot temperature in relation to a
variation of an overlap ratio L1/L0 between a smaller-diameter portion of a
heater tube and the second resistor, obtained in an experiment conducted in the
resent invention; Fig. 5 is a graph showing a warm-up ability at the front end of the glow
plug in relation to a variation of the overlap ratio L1/L0 between the smaller-diameter
portion of the heater tube and the second resistor, obtained in an
experiment conducted in the resent invention; Fig. 6 is a cross-sectional view showing an overall arrangement of a
glow plug in accordance with a second embodiment of the present invention; Figs. 7A through 7C are enlarged views showing connecting structures
in accordance with third through fifth embodiments of the present invention;
and Fig. 8 is a cross-sectional view showing an overall arrangement of a
conventional glow plug.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Preferred embodiments of the present invention will be explained
hereinafter with reference to accompanied drawings. Identical parts are
denoted by the same reference numerals throughout the drawings.
First embodiment
A glow plug 1 of the present invention shown in Fig. 1 is provided in
each of a plurality of (e.g., four) cylinders (not shown) of a diesel engine. The
glow plug 1 has a function of promoting firing and combustion of fuel during
an engine start-up operation.
The glow plug 1 comprises a cylindrical hollow housing 2 made of iron-group
material. The housing 2 has a screw portion 21 which is detachably
engageable with the cylinder of the engine. An elongated cup-shaped heater
tube 3 is inserted into one end 2a of the housing 2. The heater tube 3 is fixed
to the housing 2 by soldering. The heater tube 3 is made of a conductive
member (such as stainless material) which is excellent in the heat durability
and the resistivity to oxidation. The heater tube 3 has a smaller-diameter
portion 31 at a closed end 3a. A larger-diameter portion 32 is integral with the
smaller-diameter portion 31 and provided at an opened end 3b of the heater
tube 3. A diameter of the larger-diameter portion 32 is larger than a diameter
of the smaller-diameter portion 31. The closed end 3a of the heater tube 3
extends outward from the one end 2a of the housing 2 and exposed to the
outside of the housing 2. The larger-diameter portion 32 of the heater tube 3
fits the inner cylindrical wall of the one end 2a of the housing 2. The larger-diameter
portion 32 is fixed to the one end 2a of the housing 2.
The heater tube 3 comprises a tapered portion 33 connecting the
smaller-diameter portion 31 and the larger-diameter portion 32. The tapered
portion 33 has a diameter increasing gradually from the smaller-diameter
portion 31 to the larger-diameter portion 32. From the requirements in the
manufacturing, the tapered portion 33 is inclined at approximately 15° with
respect to the larger-diameter portion 32.
The heater tube 3 has an inner hollow space which accommodates first
and second coil- like resistors 4 and 5. These resistors 4 and 5 extend in an
axial direction of the heater tube 3. The first resistor 4 is adjacent to the closed
end 3a of the heater tube 3. The second resistor 5 is closer to the opened end
3b of the heater tube 3 than the first resistor 4. The entire body of the first
resistor 4 and at least part (e.g., 3/4 the length of the second resistor 5) of the
second resistor 5 are accommodated in the smaller-diameter portion 31.
The first resistor 4 has one end 41 electrically connected to the closed
end 3a of the heater tube 3. The other end 42 of the first resistor 4 is
electrically connected to one end 51 of the second resistor 5. The other end 52
of the second resistor 5 is electrically connected to one end 61 of an
intermediate shaft 6. The intermediate shaft 6 is inserted in the housing 2 and
fixed to the housing 2.
Insulating powder 30, comprising heat-resistive material (e.g.,
magnesia), is provided in the heater tube 3. The above-described one end 61
of the intermediate shaft 6, the first resistor 4 and the second resistor 5 are
buried in this insulating powder 30. With this arrangement, the one end 61 of
the intermediate shaft 6, the first resistor 4 and the second resistor 5 are
electrically isolated from the heater tube 3.
The first resistor 4 is made of a first conductive member (e.g., iron-chrome
alloy or nickel-chrome alloy) which has a resistance change rate of
approximately 1. The resistance change rate is defined as a ratio of a resistance
change to a temperature change. For example, the resistance change is a
difference between a resistance value at 1,000 °C and a resistance value at 20
°C. The temperature change is a difference between 1,000 °C and 20 °C,
wherein 20 °C represents a room temperature and 1,000 °C represents a
temperature of the first resistor 4 during the preheating of the glow plug 1. The
second resistor 5 is made of a second conductive member (e.g., nickel, low
carbon steel, or cobalt-iron alloy) having a resistance change rate in a range of
5 to 14. The present invention refers the above-described resistance change as
a resistance-temperature coefficient which is represented by a gradient or
inclination of a plotted line in a graph with an abscissa representing a
temperature and an ordinate representing a resistance value. Accordingly, the
second conductive member has a larger resistance-temperature coefficient than
the first conductive member.
The first resistor 4 and the second resistor 5 are connected by plasma
arc welding. A fused portion 45 is formed at a connecting portion of the first
resistor 4 and the second resistor 5. The other end 42 of the first resistor 4 is
overlapped with the one end 51 of the second resistor 5. The plasma arc is
applied to the overlapped portion between the first resistor 4 and the second
resistor 5 to form the fused portion 45.
The one end 61 of the intermediate shaft 6 extends into the heater tube
3 from the opened end 3b of the heater tube 3. The other end 62 of the
intermediate shaft 6 is fixed to the other end 2b of the housing 2 by a nut 9. An
O ring 7 and a resin bush 8 are interposed between the nut 9 and the housing
2. The O ring 7 is made of an insulating elastic member such as fluoro rubber.
Thus, the intermediate shaft 6 is electrically isolated from the housing 2.
Hereinafter, an arrangement of a current supply circuit of the above-described
glow plug 1 will be explained.
As shown in Fig. 2, there are a plurality of (e.g., four) glow plugs 1
connected in parallel with each other. A battery power source (i.e., power
supply source) 20 produces a power voltage of 12 V. Each glow plug 1
receives the power voltage of 12 V through a relay 201. The glow plug 1
causes heat and starts preheating. A start-up ability of the engine can be
improved.
The glow plug 1 has a body earth. In the drawing, reference numeral 202
denotes an engine key switch. Reference numeral 203 denotes a control unit
having a timer function. Reference numeral 204 denotes an engine cooling
water temperature sensor. Reference numeral 205 denotes a start-up timing
indicator. The control unit 203 receives a signal of the engine cooling water
temperature sensor 204 in response to ON (turned-on) of the engine key switch
202.
The start-up timing indicator 205 is activated in response to a signal of
the control unit 203. The relay 201 is activated. A predetermined voltage (e.g.,
12V) is applied from the power source 20 to the glow plug 1. The glow plug
1 starts preheating. There is a predetermined waiting time (e.g., 5 seconds)
provided immediately after the turning-on (ON) operation of the engine key
switch 202. After the waiting time has elapsed, the start-up timing indicator
205 is turned off in response to a signal sent from the control unit 203. The
turning-off of the start-up timing indicator 205 allows a driver to visually
recognize the termination of the preheating. Thus, the driver can start the
engine.
The cooling water temperature sensor 204 detects the temperature of the
engine cooling water. When the engine cooling water temperature is lower than
a predetermined value (e.g., 60 °C), electric power is continuously supplied to
the glow plug 1 to increase the temperature of the engine. This operation is
generally referred to as afterglow. When the engine cooling water temperature
is substantially equal to the above-described predetermined value, the glow
plug 1 is deactivated in response to a signal sent from the control unit 203.
Performing the above-described afterglow is effective to promote the
combustion of the fuel mixture in the cylinder. Vibrations of the engine can be
suppressed. And, the white smoke of the exhaust gas can be eliminated. Once
the engine starts rotating, a regulator increases the voltage level of the electric
power supplied to the glow plug 1. For example, the glow plug 1 receives 14
V which is higher than the above-described predetermined voltage (12 V).
According to the above-described arrangement, the entire body of the
first resistor 4 and 3/4 of the second resistor 5 are completely accommodated
in the smaller-diameter portion 31 of the heater tube 3. This arrangement is
effective to reduce the heat capacity of the second resistor 5 as well as the first
resistor 4. Not only the warm-up ability of the second resistor 5 but also the
resistance increasing ability of the second resistor 5 can be improved. As a
result, the electric power supply to the first resistor 4 can be quickly
suppressed. The temperature of the first resistor 4 can be sensitively controlled.
In other word, it becomes possible to effectively suppress the temperature
overshoot (i.e., excessive temperature increase) of the glow plug 1 after the
front end of the glow plug 1 reached the above-described setting temperature
(e.g., 800 °C).
The above-described increased power voltage (14 V) is applied to the
glow plug 1 during the afterglow operation. However, the arrangement of the
present embodiment can prevent the temperature of the glow plug 1 from
increasing extraordinarily. The oxidation and burnout of the resistors 4 and 5
can be suppressed. The durability of the glow plug 1 is adequately maintained.
As the first resistor 4 is also accommodated in the smaller-diameter
portion 31, the first resistor 4 has a smaller heat capacity comparable with that
of a conventional one. Thus, the temperature of the first resistor 4 can be
quickly increased immediately after the glow plug 1 is activated.
Hereinafter, an experimental result will be explained. An experiment
was conducted to evaluate the effect of an overlap length L1 between the
smaller-diameter portion 31 and the second heater 5. Experimental data
relating the temperature overshoot and the quick warm-up ability were
obtained by changing the value of the overlap length L1. Detailed content and
result will be explained hereinafter. The tapered portion 33 is not included in
the smaller-diameter portion 31.
In this experiment, the first resistor 4 is made of a 80wt%Ni-20wt%Cr
alloy. The second resistor 5 is made of a 92wt%Co-8wt%Fe alloy. In an
assembled condition, the first resistor 4 and the second resistor 5 have a wire
diameter of 0.35 mm. The first resistor 4 is 6 mm in length, 2.5 mm in
diameter, and 0.60 mm in coil pitch. The second resistor 5 is 22 mm in length
(L0), 2.5 mm in diameter, and 0.50 mm in coil pitch. The smaller-diameter
portion 31 has an outer diameter of 4.3 mm. The larger-diameter portion 32 has
an outer diameter of 5.0 mm. A saturated temperature of the glow plug 1 is set
to 900 °C as a result of a combination of dimensions and materials of the first
and second resistors 4 and 5 and the heater tube 3.
Based on the above-described glow plug 1, the overlap length L1
between the second resistor 5 and the smaller-diameter portion 31 was
variously changed. A total of five samples of the glow plug 1 were fabricated
to set an overlap ratio (= L1/L0) to each of 0, 1/4, 1/2, 3/4 and 1. Each sample
of the glow plug 1 is subjected to a power voltage of 11 V. A temperature
change on the surface of the heater tube 3 was measured in relation to an
elapsed time. Fig. 3 shows the measured results of two samples, one sample
having the overlap ratio (L1/L0) of 0 corresponding to the prior art and the
other sample having the overlap ratio of 1/2 corresponding to the present
invention.
From the measurement result, an overshoot temperature T (°C) was
obtained. And, a time t (sec) was obtained as a warm-up period required for
the front end of the glow plug 1 to reach 800 °C from the application of the
power voltage. In each of Figs. 4 and 5, a curve plotted by white round points
shows the measured values of the overshoot temperature T and the warm-up
period t with respect to all of the above-described samples. The overshoot
temperature T is defined as a difference between the maximum temperature
and the saturated temperature of the glow plug 1.
Furthermore, a similar experiment was conducted on the glow plug 1
having different dimensions. The outer diameter of the smaller-diameter
portion 31 is set to 3.5 mm. In an assembled condition, the first and second
resistors 4 and 5 have a wire diameter of 0.30 mm. A total of five samples of
the glow plug 1 were fabricated to set an overlap ratio (= L1/L0) to each of 0,
1/4, 1/2, 3/4 and 1. The experiment was conducted in the same manner.
Experimental result is shown by a curve plotted by black round points in Figs.
4 and 5.
As understood from Fig. 4, the overshoot temperature T decreases with
increasing overlap ratio L1/L0. Especially, the overshoot temperature T greatly
decreases when the overlap ratio L1/L0 exceeds 1/2, compared with the value
obtained when the overlap ratio L1/L0 is 0.
Accordingly, having the overlap ratio L1/L0 larger than 0 is effective to
prevent the temperature of the glow plug 1 from excessively increasing. This
effect is realized when the smaller-diameter portion 31 accommodates at least
part of the second resistor 5. Oxidation and burnout of the first and second
resistors 4 and 5 can be suppressed. Accordingly, the glow plug 1 can maintain
appropriate durability.
As understood from Fig. 5, all samples of the glow plug 1 reached 800
°C within 5 seconds which corresponds to an ordinary preheating time. Thus,
the first resistor 4 can be quickly warmed up. In other words, the warm-up
ability of the glow plug 1 can be maintained adequately.
Considering the experimental results, it is preferable that more than half
of the second resistor 5 is accommodated in the smaller-diameter portion 31
of the heater tube 3. With this arrangement, a temperature overshoot (i.e., an
excessive temperature increase) succeeding the preheating can be effectively
suppressed. Furthermore, it is preferable that the larger-diameter portion 32 has
an outer diameter in a range of 4.5 mm to 6.0 mm. Too much reducing the
outer diameter of the large-diameter portion 32 is not preferable in that it
becomes difficult to maintain a satisfactory strength in the vicinity of the
cylindrical peripheral portion 34 where the heater tube 3 is brought into contact
with the inner cylindrical surface of the housing 2. Too much increasing the
outer diameter of the large-diameter portion 32 is not preferable in that it
becomes difficult to suppress an overall size of the glow plug 1 and assure a
smooth installation of the glow plug 1 to the engine.
Furthermore, it is preferable that an outer diameter of the smaller-diameter
portion 31 is 0.6 to 0.9 times as large as the outer diameter of the
larger-diameter portion 32. Too much reducing the outer diameter of the
smaller-diameter portion 31 is not desirable because the smaller-diameter
portion 32 becomes too thin to manufacture. Too much increasing the outer
diameter of the smaller-diameter portion 31 is not desirable because the
smaller-diameter portion becomes too thick to sufficiently reduce the heat
capacity of the first and second resistors 4 and 5.
Second Embodiment
As shown in Fig. 6, the diameter of the inner cylindrical wall of the
housing 2 is differentiated at both ends. More specifically, the inner diameter
of the housing 2 is larger at one end 2a of the housing 2 than at the other end
2b. A thin cylindrical portion 22 is provided at the one end 2a of the housing
2. The thin cylindrical portion 22 spatially surrounds the heater tube 3 so as to
define a so-called pocket space 10 between them. Thus, the heat conduction
between the heater tube 3 and the thin cylindrical portion 22 is worsened due
to air intervening in the pocket space 10.
With this arrangement, it becomes possible to prevent the heat of the
heater tube 3 from leaking to the housing 2 at the region corresponding to the
pocket space 10. Thus, the larger-diameter portion 32 keeps a significant
amount of heat. Not only the warm-up ability of the second resistor 5 but also
the resistance increasing ability of the second resistor 5 can be improved. As
a result, the temperature of the first resistor 4 can be sensitively controlled. In
other word, it becomes possible to effectively suppress the excessive
temperature increase of the glow plug 1 after the front end of the glow plug 1
reached the above-described setting temperature (e.g., 800 °C). The heater
tube 3 is brought into contact with and fixed to the inner cylindrical wall of the
housing 2 at a portion 34 adjacent to the one end 2a of the housing 2. The
portion 34 serves as a bottom of the pocket space 10.
Third through Fifth Embodiments
Third through fifth embodiments relate to the connecting structure
between the first resistor 4 and the second resistor 5. Fig. 7A shows the
connecting structure in accordance with the third embodiment. The other end
42 of the first resistor 4 is overlapped with the one end 51 of the second
resistor 5 at their distal ends. Using the plasma arc welding, the first resistor
4 and the second resistor 5 are fixed each other at the overlapped portion.
Fig.7B shows the connecting structure in accordance with the fourth
embodiment. Both the other end 42 of the first resistor 4 and the one end 51
of the second resistor 5 extend in the axial direction. The other end 42 of the
first resistor 4 is overlapped with the one end 51 of the second resistor 5 at
their distal ends. Using the plasma arc welding, the first resistor 4 and the
second resistor 5 are fixed each other at the overlapped portion. Each length
of the first and second resistors 4 and 5 is defined by the length of their coil
portions. The portion A extending in the axial direction is not included in the
length of the first and second resistors 4 and 5.
Fig.7C shows the connecting structure in accordance with the fifth
embodiment. The other end 42 of the first resistor 4 extends parallel to the one
end 51 of the second resistor 5 in the axial direction. The other end 42 of the
first resistor 4 is overlapped with the one end 51 of the second resistor 5
entirely along the axially extending portions thereof. The axially extending
overlapped portions are welded at a plurality of points (e.g., three points) by
laser welding. In Figs. 7A through 7C, reference 45 denotes a fused portion
between the first resistor 4 and the second resistor 5.
Other Modifications
According to the above-described embodiments, the first resistor 4 is
made of the conductive member having a small positive resistance change rate.
It is possible to fabricate the first resistor 4 by a conductive member having a
negative resistance-temperature coefficient.