Terminal for telegraph and telephone systems

A terminal for telegraph and telephone systems using as an overvoltage/overcurrent protecting component a positive temperature coefficient thermistor element comprising a plate-shaped ceramic body made of a ceramic material whose Curie temperature is in the range of 60.degree. to 120.degree. C., and having a thickness of 2.5 to 5.0 mm, and electrodes formed on both major surfaces of the ceramic body.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates generally to a terminal for telegraph and 
telephone system, and more particularly, to a terminal for telegraph and 
telephone systems using a positive temperature coefficient thermistor 
element (hereinafter referred to as a PTC element) as an 
overvoltage/overcurrent protecting component. 
2. Description of the Prior Art 
Examples of terminals for telegraph and telephone systems includes a 
telephone set, a facsimile, a PBX (private branch exchange) and the like 
used on the subscriber's side. Many of the terminals contain a bell 
circuit and a speaking circuit. More specifically, a bell circuit 1 and a 
speaking circuit 2 are connected to subscriber's lines 4a and 4b through 
hook-switches 3a and 3b, as shown in FIG. 2. Reference numeral 5 denotes a 
surge absorbing element, which is constituted by, for example, a varistor 
and is connected so as to absorb a surge current. 
The hook-switches 3a and 3b are connected to the bell circuit 1, as shown 
in FIG. 2, at the on-hook time. At the off-hook time, the hook-switches 3a 
and 3b are switched so as to be connected to the speaking circuit 2. A 
voltage applied to the circuits in an interface portion 6 is generally 48 
volts. When the terminal starts reception in the connected state shown in 
FIG. 2, that is, the on-hook state, a bell voltage such as an AC voltage 
of 75 volts (in the case of Japan) or an AC voltage of 150 volts (in the 
case of U.S.) is applied, so that a bell begins to ring. When a user takes 
up a receiver to bring the terminal into the off-hook state, the 
hook-switches 3a and 3b are switched so as to be connected to the speaking 
circuit 2. Accordingly, the supply of the voltage to the bell circuit 1 is 
cut off, so that the bell stops ringing. Consequently, the speaking 
circuit 2 is connected to the subscriber's lines 4a and 4b, so that the 
terminal enters the speaking state. 
However, in the interface portion 6 in the above described terminal, a very 
large overvoltage may, in some cases, be applied due to a failure, an 
interconnection error or the like of the terminal. For example, the above 
described bell voltage may be erroneously applied to the speaking circuit 
2 due to a failure, or the interface portion 6 may be erroneously 
connected to a commercial power supply due to an interconnection error, so 
that an overvoltage of approximately 200 volts may, in some cases, be 
applied. In order to provide protection against such an overvoltage, 
therefore, an overvoltage protecting component as shown in FIGS. 3 and 4 
has conventionally been connected to the interface portion 6. 
More specifically, not only the surge absorbing element 5 but also a 
current fuse 7 serving as an overvoltage protecting component is connected 
to the interface portion 6 comprising the bell circuit and the speaking 
circuit, as in the construction shown in FIG. 3. Alternatively, a PTC 
element 8 serving as an overvoltage protecting component is connected to 
the interface portion 6, as in the construction shown in FIG. 4. 
In the construction shown in FIG. 3, when an overvoltage/overcurrent is 
applied, the current fuse 7 is fused, so that the interface portion 6 in 
the terminal is protected. Similarly, in the construction shown in FIG. 4, 
the interface portion 6 is protected by the current limiting function of 
the PTC element 8. 
In recent years, a protecting operation against a very large overvoltage of 
600 volts has been required for safety reasons for the terminal for 
telegraph and telephone systems. The reason for this is to provide 
protection against a case where a high-voltage line is brought into 
erroneous contact with a telephone line as a result of a tornado or an 
earthquake. 
In the terminal using the current fuse 7 as an overvoltage/overcurrent 
protecting component, as shown in FIG. 3, when the terminal is erroneously 
connected to the commercial power supply or the like due to an 
interconnection error, the current fuse 7 is fused, to protect the 
interface portion 6. In addition, when a large overvoltage of 600 volts is 
applied as described above, the current fuse 7 is also fused, to reliably 
protect the interface portion 6. Consequently, a requirement of UL1459, 
which is a standard related to a telegraph and telephone apparatus is 
satisfied, thereby making it possible to reliably protect the terminal for 
telegraph and telephone systems. 
However, the current fuse 7 used as an overvoltage/overcurrent protecting 
component has the disadvantage of having no restoring characteristics for 
the protecting operation. More specifically, every time the current fuse 7 
is fused, the current fuse 7 must be replaced with a new current fuse. 
Consequently, complicated maintenance work must be performed. 
On the other hand, in the terminal using the PTC element 8 shown in FIG. 4, 
the PTC element 8 is an overvoltage/overcurrent protecting component 
having restoring characteristics for the protecting operation. 
Accordingly, the above described complicated maintenance work can be 
omitted. However, the overvoltage/overcurrent protecting component using 
the conventional PTC element can only protect the interface portion 6 
against the erroneous application of the bell voltage and the erroneous 
connection of the terminal to the commercial power supply of approximately 
200 volts. It cannot reliably protect the interface portion 6 against a 
vary large overvoltage of 600 volts. More specifically, when a very large 
overvoltage of 600 volts is applied, the PTC element 8 may, in some cases, 
be short-circuited and destroyed, so that a very large current is applied 
to the speaking circuit, causing a serious accident such as ignition of 
the terminal. Consequently, the terminal using the conventional PTC 
element does not satisfy the requirement of a standard for requiring 
protection against an overvoltage of 600 volts, for example, a standard of 
UL1459, CSA or Bellcore. 
SUMMARY OF THE INVENTION 
An object of the present invention is to provide a terminal for telegraph 
and telephone systems comprising a protecting component having restoring 
characteristics for the protecting operation against a low overvoltage of 
not more than 200 volts and capable of reliably protecting a speaking 
circuit and the like without causing a serious accident such as ignition 
even when a very large overvoltage of 600 volts is applied. 
The present invention provides a terminal for telegraph and telephone 
comprising an overvoltage/overcurrent protecting component which is 
constituted by a PTC element, which is characterized in that the above 
described PTC element comprises a plate-shaped ceramic body made of a 
ceramic material whose Curie temperature is in the range of 60.degree. to 
120.degree. C., and having a thickness of 2.5 to 5.0 mm; and electrodes 
formed on both major surfaces of the above described ceramic body. 
Lead terminals are generally connected by solders to the electrodes on both 
the major surfaces of the PTC element in the terminal for telegraph and 
telephone systems according to the present invention so as to make 
electrical connection to the exterior. In addition, portions of the PTC 
element, other than the regions of the forward ends of the lead terminals 
that are led out, are preferably coated with insulating resin. 
According to the present invention, a PTC element used as an 
overvoltage/overcurrent protecting component in a terminal for telegraph 
and telephone systems is made of a ceramic material having a Curie 
temperature in the above described particular range, and is constructed so 
as to have a thickness in the above described particular range. 
Accordingly, when an overvoltage of 600 volts is applied, the PTC element 
is reliably destroyed in a layer shape. Consequently, the circuits are in 
their opened state, thereby protecting the terminal. That is, the circuits 
are reliably brought into their opened state by destroying the PTC element 
in a layer shape, thereby making it possible to reliably prevent the 
occurrence of a serious accident such as ignition. 
On the other hand, when an overvoltage of approximately 200 volts or an 
overvoltage below 200 volts is applied, the circuits are protected by the 
current limiting function of the PTC element, similarly to the case of the 
conventional PTC element. The protection by the current limiting function 
has restoring characteristics, so that no complicated replacing work is 
required. 
The foregoing and other objects, features, aspects and advantages of the 
present invention will become more apparent from the following detailed 
description of the present invention when taken in conjunction with the 
accompanying drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 is a perspective view for explaining a PTC element used in the 
present embodiment and a state where the PTC element is destroyed. A PTC 
element 11 has a structure in which electrodes 13 and 14 are formed on 
both major surfaces of a plate-shaped ceramic body 12. The ceramic body 12 
is made of a ceramic material whose Curie temperature is in the range of 
60.degree. to 120 .degree. C. and has a thickness in the range of 2.5 to 
5.0 mm. 
In a terminal for telegraph and telephone systems according to the present 
embodiment, the above described PTC element 11 is used as an 
overvoltage/overcurrent protecting component. Specifically, the PTC 
element 8 in the conventional terminal shown in FIG. 4 is replaced with 
the above described PTC element 11. Consequently, other circuit portions 
of the terminal for telegraph and telephone systems are the same as those 
in the conventional terminal described with reference to FIGS. 2 to 4 and 
thus, the description thereof is not repeated by incorporating the 
description of the conventional terminal. 
In the terminal for telegraph and telephone systems according to the 
present embodiment, the above described PTC element 11 is used. 
Accordingly, protection can be repeatedly provided against an overvoltage 
of approximately 200 volts, and protection is provided by the destruction 
of the PTC element 11 into separate layers against a very large 
overvoltage of 600 volts. This will be described more specifically. 
Description is now made of protection against an overvoltage in a case 
where a bell voltage such as an AC voltage of 75 volts or 150 volts is 
erroneously applied to a speaking circuit or a case where the terminal is 
erroneously connected to a commercial power supply of 100 volts or 200 
volts. In this case, the speaking circuit and the like are reliably 
protected by the current limiting function of the PTC element 11, 
similarly to the case of the conventional PTC element 8. Since the 
protection by the current limiting function of the PTC element 11 has 
restoring characteristics, the speaking circuit and the like can be 
reliably protected many times without replacing the PTC element 11. 
Consequently, it is possible to protect the terminal without performing 
complicated maintenance work by the restoring characteristics for the 
protecting function of the PTC element 11 against an error which is liable 
to relatively frequently occur such as erroneous connection to the 
commercial power supply or an interconnection error. 
When an overvoltage of 600 volts is applied, the PTC element 11 is 
destroyed into separate layers, i.e., divided into destroyed pieces 11a 
and 11b, as shown in the lower part of FIG. 1. The reason why the PTC 
element 11 is thus destroyed into separate layers is that the temperature 
of the PTC element 11 is rapidly raised if the overvoltage is applied, so 
that a very large temperature difference appears between the surface and 
the center of the PTC element 11, resulting into destruction into separate 
layers due to the difference in thermal expansion therebetween. 
As described above, the PTC element 11 is broken into separate layers in a 
state where it is divided into the destroyed pieces 11a and 11b. 
Accordingly, when an overvoltage of 600 volts is applied, the circuits are 
in their opened state by the destruction of the PTC element into separate 
layers, so as to protect the terminal. In this case, since the PTC element 
11 is destroyed, the PTC element 11 cannot be used again. However, it is 
very rare that such a very large overvoltage is applied, and the other 
components cannot usually perform their functions in many cases after such 
a large overvoltage is applied. Consequently, if a large overvoltage of 
600 volts is applied, the other components are also forced to be replaced, 
so that the restoring characteristics for the protecting function of the 
PTC element 11 are not particularly required. Accordingly, the terminal is 
sufficiently protected by the above described destruction of the PTC 11 
into separate layers. 
As described in the foregoing, according to the present embodiment, the 
protection against an overvoltage of 600 volts is provided by reliably 
destroying the PTC element 11 into separate layers. Consequently, when a 
very large overvoltage of 600 volts is applied, the PTC element 11 must be 
reliably destroyed into separate layers, as shown in the lower part of 
FIG. 1. In order to thus destroy the PTC element 11 into separate layers, 
the ceramic body of the PTC element has a thickness in the range of 2.5 to 
5.0 mm and the Curie temperature of the ceramic material composing the 
ceramic body is in the range of 60.degree. to 120.degree. C., in the 
present invention. The basis for these aspects of the present invention 
will be described with reference to FIGS. 5 to 7. 
FIG. 5 is a diagram showing the relationship between the thickness of the 
PTC element 11 and a withstand voltage. (1), (2) and (3) in FIG. 5 
respectively indicate a region where the PTC element 11 is liable to be 
short-circuited and destroyed, a region where the PTC element 11 is 
destroyed by separating into layers by an overvoltage of 600 volts, while 
performing the protecting operation by the current limiting function at an 
overvoltage of 200 volts, and a region where the PTC element 11 is liable 
to be destroyed into separate layers. As represented by a solid line A in 
FIG. 5, the larger the thickness of the PTC element 11 is, the higher the 
static withstand voltage of the PTC element 11 is. In addition, when the 
thickness of the PTC element 11 is less than 2.5 mm, the static withstand 
voltage is significantly reduced. If an overvoltage of 600 volts is 
applied, the PTC element 11 is liable to be short-circuited and destroyed. 
Consequently, in order to prevent the PTC element 11 from being 
short-circuited and destroyed when an overvoltage of 600 volts is applied, 
the thickness of the PTC element 11 is not less than 2.5 mm in the present 
invention. 
On the other hand, if the thickness of the PTC element 11 exceeds 5.0 mm, 
the PTC element 11 is liable to be destroyed into separate layers. If the 
thickness of the PTC element is too large, however, the PTC element 11 is 
destroyed into separate layers even at a voltage significantly lower than 
600 volts. More specifically, as obvious from the broken line B in FIG. 5, 
if the thickness of the PTC element 11 exceeds 5.0 mm, the PTC element 11 
is destroyed into separate layers even when an overvoltage of 200 volts is 
applied. In the present invention, therefore, the thickness of the PTC 
element 11 is not more than 5.0 mm so as not to destroy the PTC element 11 
into separate layers at an overvoltage of approximately 200 volts. 
FIG. 6 is a diagram showing the relationship between a current flowing 
through the PTC element and the ambient temperature. A solid line C 
indicates protecting current characteristics in a case where the Curie 
temperature of the PTC element is 60.degree. C., and a broken line D 
indicates protecting current characteristics in a case where the Curie 
temperature of the PTC element is 120.degree. C. Respective regions below 
the solid line C and the broken line D are non-operating regions of the 
PTC element, and respective regions above the solid line C and the broken 
line D are operating regions of the PTC element. The operating regions and 
the non-operating regions of the PTC element 11 are determined by plotting 
the crest of a voltage/current characteristic curve for each ambient 
temperature. 
A temperature at which the terminal for telegraph and telephone systems is 
used, that is, a temperature at which the use must be ensured is generally 
in the range of -10.degree. to 50.degree. C. Consequently, when the Curie 
temperature of the PTC element is less than 60.degree. C., the difference 
from the ambient temperature becomes small. Accordingly, a non-operating 
current value is liable to be affected by the ambient temperature, as 
obvious from FIG. 6. Meanwhile, the non-operating current value means the 
maximum current value at which the current limiting function is not 
exhibited even if the PTC element 11 is energized. 
Consequently, the higher the ambient temperature is, the lower the 
non-operating current value is, as shown in FIG. 6. That is, the PTC 
element 11 has such a nature that the non-operating current value is 
lowered as the ambient temperature approaches the Curie temperature. 
On the other hand, the higher the Curie temperature of the ceramic material 
composing the PTC element 11 is, the higher the temperature of the PTC 
element 11 is at the time of applying a voltage. Consequently, when the 
PTC element 11 is made of a ceramic material whose Curie temperature 
exceeds 120.degree. C., the difference between the ambient temperature and 
the interior temperature of the PTC element 11 becomes large, so that the 
PTC element 11 is liable to be broken into separate layers. Accordingly, 
when the Curie temperature exceeds 120.degree. C., the PTC element 11 is 
liable to be destroyed into separate layers even at a voltage below 600 
volts. In addition, considering a case where an overvoltage of 600 volts 
is applied, if the temperature of the surface of the PTC element 11 is 
extraordinarily high, solders bonded to the PTC element 11 are melted, 
resulting in the possibility that the solders are brought into contact 
with the other components. Consequently, the Curie temperature of the 
ceramic material composing the PTC element 11 is not more than 120 
.degree. C. in the present invention. 
As described in the foregoing, according to the present invention, the 
thickness of the PTC element 11 used and the Curie temperature of the 
ceramic material composing the PTC element 11 are respectively set to the 
above described particular ranges, thereby making it possible to reliably 
destroy the PTC element 11 into separate layers against an overvoltage of 
600 volts, while repeatedly protecting the circuits by the current 
limiting function of the PTC element 11 against an overvoltage of 
approximately 200 volts. 
Although the PTC element 11 shown in FIG. 1 is illustrated as one example 
of PTC elements used in the present invention, the PTC elements used in 
the present invention may have shapes other than the plate shape. 
Furthermore, the PTC element 11 shown in FIG. 1 is generally used in the 
form of a component with leads in which lead terminals 15 and 16 are 
joined to electrodes 13 and 14 on both major surfaces by solders 17 and 
18, as shown in FIG. 8. In addition, portions of the PTC element 11, other 
than the regions of the forward ends of the lead terminals 15 and 16 that 
are led out, are preferably coated with insulating resin 19 (the portions 
where the insulating resin 19 is formed is represented by a one-dot and 
dash line). 
Although the present invention has been described and illustrated in 
detail, it is clearly understood that the same is by way of illustration 
and example only and is not to be taken by way of limitation, the spirit 
and scope of the present invention being limited only by the terms of the 
appended claims.