Note: Descriptions are shown in the official language in which they were submitted.
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RESISTOR AND PROTECTOR FOR TRANSMISSION LINE
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a resistor and protector for protecting a
transmission line. More particularly, the present invention pertains to a
protector for
protecting transmission equipment, such as switching equipment, from abnormal
currents
flowing through a transmission cable due to contact between the transmission
cable and a
power line (or due to other sources of abnormal current), and to a resistor
used for this
protector.
2 Description of Related Art
Fig. 5 shows an equivalent circuit of a protector for a transmission line.
This protector, generally indicated by reference numeral 40, comprises a
series
combination of resistors 41a and 41b for protecting the transmission line and
thermistors
42a and 42b having positive temperature coefficients. First ends of these two
series
combinations are connected with input terminals 43a and 43b, respectively, of
the
transmission line. Second ends of these two series combinations are connected
with output
terminals 44a and 44b, respectively, of equipment to be protected (referred to
henceforth
as "protected equipment"). Thus, two protector circuits for protecting the
transmission
line are formed. These protector circuits are not electrically connected.
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The structure of one known transmission line protecting resistor for use in
the
transmission line protector 40 shown in Fig. 5 is shown in Fig. 6. The
transmission
line protecting resistor, generally indicated by reference numeral 41a,
comprises an
alumina plate 45 that forms an insulating substrate and a serpentine resistive
component 46 formed on the alumina plate 45 by thick-film printing technology.
The
resistor 41b for protecting the transmission line is constructed similarly and
so
description of the structure of this resistor 41b is omitted.
Referring back to Fig. 5, the operation of the transmission line protector 40
is
described. Generally, the thermistors 42a and 42b having positive temperature
coefficients exhibit nonuniform impedances at room temperature. This adversely
affects the characteristics of the transmission line. Accordingly, the line
protecting
resistors 41a and 41b set at appropriate values are connected in series with
the
positive temperature coefficient thermistors 42a and 42b, respectively, thus
rendering
the impedances of the two transmission line protector circuits substantially
uniform.
If contact between a transmission cable and a power line induces an abnormal
current flowing from the transmission cable into the transmission line
protector 40
constructed as described above, the current causes the thermistors 42a and 42b
having
positive temperature coefficients to increase in temperature (e.g., to get
hot). The
resulting heat rapidly increases the resistances of the thermistors 42a and
42b. This,
in turn, decreases the current flowing through the thermistors. In this
manner, the
transmission equipment is protected from abnormal currents; otherwise, the
equipment may be destroyed.
Even with this transmission line protector 40, if a transient overcurrent
takes
place due to a surge caused by an electrical storm (e.g., a "lightning
surge"), for
example, the thermistors 42a and 42b having positive temperature coefficients
simply
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act as resistors. Therefore, diodes, varistors, and other devices for
absorbing
lightning surges are also fitted to the transmission equipment to supplement
the
protection.
In the aforementioned transmission line protecting resistors 41a and 41b (each
of which is referred to generically as resistor 41), if a high-voltage
overcurrent flows
due to a lightning surge, for example, and if the bent portions of the
serpentine
resistive component 46 have a small radius, an electric field is concentrated
in the
inner portions of the bent portions, which may produce a spark. This creates
the
possibility of destruction of the serpentine resistive component 46. To avoid
this, the
radius of the bent portions of the serpentine resistive component 46 is set
large to
make the currents flowing in the inner and outer radial portions of the bent
portions
as uniform as possible. However, this increases the spacing between the
adjacent
straight portions, thus increasing the whole area of the resistive component
46.
Hence, miniaturization of the transmission line protecting resistor 41 is
hindered.
In recognition of these problems, a resistor structure as shown in Fig. 7 is
proposed in Japanese Unexamined Utility Model No. 172901/ 1981. This resistor,
generally indicated by numeral 50, comprises an alumina plate 51 that forms an
insulating substrate, straight resistive strips 52 formed on the alumina plate
51 in
parallel relation to each other, and rectangular connector electrodes 53 for
connecting
adjacent ones of the resistive strips 52 at their opposite ends. As a whole, a
serpentine resistive component is formed.
By constructing the resistor SO as described above, the resistor is free of
bent
portions. The connector electrodes 53 existing at the bent portions of the
serpentine
resistive component have low resistance and so the electric field is less
concentrated
in these portions. However, the end portions of the resistive strips 52 are
made
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square, and the connector electrodes 53 are shaped rectangularly. The electric
field is
concentrated in these corners. An electric discharge easily occurs from these
corners.
For instance, the portion consisting of two resistive strips 52 and one
connector
electrode 53 is equivalently expressed in Fig. 8, where the resistance RO of
each
resistive strip 52 and the resistance R 1 of the connector electrode 53 are
arranged
alternately and connected in series. The resistive strip 52 and the connector
electrode
53 differ widely in resistivity. A strong electric field is applied to the
junction of
these two kinds of conductors. Again, an electric discharge easily takes place
from
this junction.
SUMMARY OF THE INVENTION
The present invention is intended to solve at least the foregoing problems.
It is an exemplary object of the present invention to provide a resistor which
acts to protect a transmission line and can be miniaturized, and in which
neither
electric field concentration nor an electric discharge easily takes place.
It is another object of the invention to provide a transmission line protector
using the resistor described in the immediately preceding paragraph.
A resistor for protecting a transmission line in accordance with the present
invention comprises an insulating substrate having bent portions, a serpentine
resistive component formed on the substrate, and electrodes (which comprise
any
low-resistivity components) having a resistivity lower than that of the
serpentine
resistive component and formed at least over the inner portions of the bent
portions of
the serpentine resistive component.
A transmission line protector in accordance with the present invention
comprises two transmission line protector circuits each consisting of the
transmission
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line-protecting resistor described in the immediately preceding paragraph and
a
positive temperature coefficient thermistor connected in series with the
transmission
line protecting resistor.
In the transmission line protecting resistor constructed in this manner,
electric
field concentration is less likely to occur in the bent portions of the
serpentine
resistive component. Therefore, an electric discharge is less likely to occur.
Furthermore, destruction of the serpentine resistive component is less likely
to occur.
Hence, miniaturization can be accomplished by decreasing the radius of the
bent
portions. Additionally, the transmission line protector in accordance with the
present
invention can be fabricated in smaller size.
BRIEF DESCRIPTION OF THE DRAWINGS
The foregoing and other objects, features and advantages of the present
invention will be more readily understood upon reading the following detailed
description in conjunction with the drawings in which:
Fig. 1 is a view illustrating an exemplary transmission line protecting
resistor
in accordance with the invention;
Fig. 2 is an equivalent circuit diagram of a bent portion and the vicinity
portions thereof of the resistor shown in Fig. 1;
Fig. 3 is a view similar to Fig. I , but showing another exemplary
transmission line protecting resistor in accordance with the invention;
Fig. 4 is a view illustrating an exemplary transmission line protector in
accordance with the invention;
Fig. 5 is an equivalent circuit diagram of a transmission line protector;
Fig. 6 is a view illustrating the structure of a known transmission line-
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protecting resistor;
Fig. 7 is a view illustrating the structure of another known transmission line-
protecting resistor; and
Fig. 8 is an equivalent circuit diagram of a connector electrode portion and
the vicinity portions thereof of the resistor shown in Fig. 7.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring to Fig. 1, there is shown a resistor for protecting a transmission
line, the resistor being built in accordance with one exemplary embodiment of
the
present invention. This resistor, generally indicated by reference numeral l,
comprises an alumina plate 2 that forms an insulating substrate, a serpentine
resistive
component 3 formed on the alumina plate 2 by thick-film printing technology or
other
suitable technology, and substantially semicircular electrodes 4 formed over
the inner
portions of the bent portions of the serpentine resistive component 3 by thick-
film
printing technology or other suitable technology. More specifically, the term
"inner
portions" refers to a portion of the bent portions which is radially inward
from the
outer circumference of the bent portions. The resistivity (e.g., 0.00001 S~ ~
cm) of
the electrodes 4 is sufficiently lower than the resistivity (e.g., 0.001 S2 ~
cm) of the
serpentine resistive component 3. Here, the resistivity of the resistive
component 3 is
100 times greater than the resistivity of the electrodes 4, but those skilled
in the art
will recognize that the resistances can vary by different amounts. The term
"electrode" as used here refers generally to any member having a resistivity
which is
less than the resistivity of the resistive component 3.
The equivalent circuit of a transmission line protector having the
transmission
line protecting resistor 1 is the same as the equivalent circuit of the known
structure
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shown in Fig. 5. Therefore, description of this equivalent circuit is omitted
here.
In the transmission line protecting resistor 1 constructed as described above,
the electrodes 4 are formed on top of the inner portions of the bent portions
of the
serpentine resistive component 3. Therefore, the resistivity of the inner
portions of
the bent portions is made lower than that of the straight portions. Therefore,
if an
overcurrent flows due to a lightning surge or the like, electric field
concentration in
the inner portions of the bent portions is reduced or suppressed. As a
consequence,
the possibility of creation of a spark or destruction of the serpentine
resistive
component 3 is made lower than the above-described conventional resistive
component. Since the bent portions of the serpentine resistive component 3 and
the
electrodes 4 are shaped substantially semicircularly, any corner at which
electric field
concentration would normally take place does not exist. Hence, the possibility
of
occurrence of an electric discharge is low. An equivalent circuit of a portion
of the
serpentine resistive component 3 is shown in Fig. 2, which includes
resistances R0,
R2, and RO connected in series. More specifically, the two resistances labeled
RO
designate the resistances of two immediately parallel resistive "straight
portions." The
resistance R2 designates the resistance of the bent portion between the two
immediately parallel resistive straight portions. The resistance R3 of the
electrode 4
is added as distributed resistance connected in parallel with the resistance
R2.
Therefore, any portion in which a strong electric field would normally be
concentrated does not exist between the serpentine resistive component 3 and
the
electrode 4. The possibility of an electric discharge at this junction is low.
As a
result, the radius of the bent portion can be reduced. As a consequence, the
spacing
between the adjacent straight portions of the serpentine resistive component 3
can be
decreased. Hence, the whole area of the transmission line protecting resistor
1 can be
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redUCed.
Referring next to Fig. 3, there is shown another exemplary resistor for
protecting a transmission line. This resistor, generally indicated by
reference
numeral 10, comprises an alumina plate 11, a serpentine resistive component 12
formed on the alumina plate 1 I by thick-film printing technology or other
suitable
technology, and arc-shaped electrodes 13 formed over the bent portions of the
serpentine resistive component 12 again by thick-film printing technology or
other
suitable technology. The electrodes 13 are similar in shape to the shape of
the bent
portions. The resistivity of the electrodes 13 is set lower than that of the
serpentine
resistive component 12.
By fabricating the transmission line protecting resistor 10 in this manner,
the
resistance of the bent portions of the serpentine resistive component 12 is
reduced. If
an overcurrent is induced by a lightning surge or the like, electric field
concentration
in the inner portions of the bent portions is suppressed. The possibility of a
spark or
destruction of the serpentine resistive component 12 is lowered. Furthermore,
since
the bent portions including the electrodes 13 are shaped into arcs, any corner
at which
electric field concentration would otherwise take place does not exist or is
at least
reduced. Hence, the possibility of occurrence of an electric discharge is low.
As a
result, the radius of the bent portions can be reduced. The whole area of the
transmission line protecting resistor can be decreased by narrowing the
spacing
between the adjacent straight portions of the serpentine resistive component
12.
In the above embodiments, the semicircular electrodes 4 and the arc-shaped
electrodes 13 are formed over the bent portions of the serpentine resistive
component
3 or 12. Alternatively, the electrodes 4 and the electrodes 13 may be
previously
formed under the bent portions. Generally speaking, the bent portions and the
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electrodes overlap each other, wherein the term "overlap" is intended to
encompass
the case where the electrodes are disposed over the bent portions, and the
case where
the electrodes are disposed under the bent portions.
In the above embodiments described above, an alumina plate is used as an
insulating substrate. Instead, other substrates, such as a ceramic substrate
or resinous
substrate, may be employed.
Further, although the resistive component is shown having a back-and-forth
(zig-zag) shape, the term "serpentine path" is used herein to designate any
elongate
path having curved portions, including various types of meandering paths and
spiral
paths.
Referring to Fig. 4, there is shown a transmission line protector in
accordance with exemplary embodiments of the invention. This protector,
generally
indicated by reference numeral 20, comprises an alumina plate 21 (or other
suitable
plate) that forms an insulating substrate, serpentine resistive components 22a
and 22b
formed on the alumina plate 21 by thick-film printing technology or other
suitable
technology, substantially semicircular electrodes 23a and 23b formed over the
inner
portions of the bent portions of the serpentine resistive components 22a and
22b,
respectively, by thick-film printing technology or other suitable technology,
positive
temperature coefficient thermistors 24a and 24b, interconnect electrodes 25a,
25b,
26a, 26b, 27a, 27b, metal parts 28a and 28b for connecting a first terminal of
the
positive temperature coefficient thermistors 24a and 24b with the electrodes
26a and
26b, respectively, input terminals 29a and 29b of the transmission line, and
output
terminals 30a, 30b of the protected equipment. The resistivity of the
electrodes 23a
and 23b is sufficiently lower than that of the serpentine resistive components
22a and
22b. The input terminal 29a of the transmission line is connected with the
output
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terminal 30a of the protected equipment via the interconnect electrode 27a,
the
serpentine resistive component 22a, the interconnect electrode 25a, the
positive
temperature coefficient thermistor 24a, the metal part 28a, and the
interconnect
electrode 26a, in this order. Thus, one transmission line protecting circuit
is thereby
formed. Similarly, the input terminal 29b of the transmission line is
connected with
the output terminal 30b of the protected equipment via the interconnect
electrode 27b,
the serpentine resistive component 22b, the interconnect electrode 25b, the
positive
temperature coefficient thermistor 24b, the metal part 28b, and the
interconnect
electrode 26b in this order. In this manner, another transmission line
protecting
circuit is formed.
An equivalent circuit of the transmission line protector 20 constructed in
this
manner is identical with the known structure shown in Fig. 5 and so
description of
the protector 20 is omitted. The resistors for the transmission line
protecting circuits
are the same as the resistor described already in conjunction with Fig. 1. The
area of
the transmission line protecting resistors can be reduced in the same way as
in the
embodiment illustrated in Fig. 1. Consequently, the transmission line
protector 20
itself can be miniaturized.
As mentioned, the transmission line protector 20 uses the same construction
as the transmission line protecting resistor 1 described in connection with
Fig. 1. The
transmission line protector 20 may also use the same construction as the
transmission
line protecting resistor 10 described previously in connection with Fig. 3.
A transmission line protecting resistor in accordance with the present
invention comprises an insulating substrate, a serpentine resistive component
formed
on the substrate, and electrodes formed over at least the inner portions of
the bent
portions of the serpentine resistive component.
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As a consequence, the resistivity of the inner portions of the bent portions
of
the serpentine resistive component is lowered. If an overcurrent is induced
due to a
lightning surge, for example, electric field concentration in the inner
portions of the
bent portions is suppressed. The possibility of production of a spark or
destruction of
the serpentine resistive component is made lower than in the above-described
conventional device. The bent portions and the overlying (or underlying)
electrodes
are shaped semicircularly or arc-shaped. Therefore, any corner at which
electric field
concentration would normally take place does not exist. Consequently, the
possibility
of an electric discharge is low. The electrodes overlying (or underlying) the
bent
portions are added in parallel with the resistance of the bent portions of the
serpentine
resistive component and distributed as a low-resistivity resistor. Therefore,
no strong
electric field concentration occurs in the junction of the serpentine
resistive
component and the electrode. An electric discharge is unlikely to occur. As a
result,
the radius of the bent portions can be made small. The spacing between the
adjacent
straight portions of the serpentine resistive component can be decreased. As a
consequence, the whole area of the transmission line protecting resistor can
be
reduced. Thus, the transmission line protector can be miniaturized.
The above-described exemplary embodiments are intended to be illustrative in
all respects, rather than restrictive, of the present invention. Thus the
present
invention is capable of many variations in detailed implementation that can be
derived
from the description contained herein by a person skilled in the art. All such
variations and modifications are considered to be within the scope and spirit
of the
present invention as defined by the following claims.
