Thermistor apparatus and manufacturing method thereof

Abstract: A positive-characteristics thermistor apparatus has an insulating case (1), positive-characteristics thermistor devices (5, 6), planar terminals (10, 11), and spring terminals (12, 13). Whichever thermistor device has a lower resistance of the two thermistor devices (5, 6) is trimmed to have a higher resistance which is near the resistance of the other thermistor device such that the two thermistor devices have substantially the same resistance (for example, within a difference of ±1 Ω). In other words, a part of an electrode of the thermistor device having a lower resistance is removed with a laser beam.

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to thermistor apparatuses and, more particularly, to an overcurrent-protection thermistor apparatus for protecting communications equipment such as telephone exchanges from overcurrent and to a manufacturing method thereof. 2. Description of the Related Art Generally, an overcurrent-protection, positive-characteristics thermistor apparatus has been known. The known apparatus has one case in which two positive-characteristics thermistor devices are housed in order to protect communications equipment such as telephone exchanges from overcurrent, caused by lightning surges, contact with power lines or the like, intruding from communication lines. It is preferable that the difference in resistance between the two positive-characteristics thermistor devices is close to 0 Ω. This is because resistance matching needs to be maintained between the transmission and receiving circuit lines in communication circuits in communications equipment such as telephone exchanges. In the conventional positive-characteristics thermistor apparatus, troublesome work has been required to make the difference in resistance between the two positive-characteristics thermistor devices close to 0 Ω. Selecting and pairing two positive-characteristics thermistor devices having substantially the same resistance among a number of positive-characteristics thermistor devices has been required. This work is made more complicated because positive-characteristics thermistor devices display large variations in resistance due to slight differences in manufacturing conditions. A method can be considered in which positive-characteristics thermistor devices are classified into groups according to their resistances and then thermistor devices in a certain group are paired. If the resistance of each of the two positive-characteristics thermistor devices is measured at different times, however, the measurement data may not be accurate due to a change in the ambient temperature at each measurement or a minute change due to aging of the resistance measuring instrument, thereby the difference in resistance between the two combined thermistor devices can become large. In the worst case, resistance matching between the transmission and receiving circuit lines cannot be maintained. Another method can be considered in which the resistance of each positive-characteristics thermistor device is measured and a device having a too-low resistance is trimmed to have a higher resistance such that all the thermistor devices have the specified resistance in the end. If the resistances of the two combined thermistor devices are measured at different times before they are trimmed, the measurement data may not be accurate due to the above-described reasons, making the difference in resistance measurements between the two thermistor devices inaccurate. Therefore, trimming cannot be conducted accurately and the resistance difference between the two thermistor devices can become large. SUMMARY OF THE INVENTION Accordingly, it is an object of the present invention to provide an easy-to-manufacture thermistor apparatus in which the two built-in thermistor devices have a small resistance difference and to provide a manufacturing method thereof. The foregoing object is achieved in one aspect of the invention, as will be described in Claim 1, through the provision of a thermistor apparatus comprising: an insulating case; two thermistor devices housed in the insulating case; and two pairs of terminals to bracket the two thermistor devices respectively, wherein either one having a lower resistance of the two thermistor devices is trimmed to have a higher resistance which is substantially the same as the resistance of the other thermistor device. The foregoing object is achieved in another aspect of the invention, as will be described in Claim 2, through the provision of a manufacturing method of a thermistor apparatus, comprising the steps of: providing an insulating case, two thermistor devices to be housed in the insulating case, and two pairs of terminals to bracket the two thermistor devices respectively; measuring the resistances of the two thermistor devices; and trimming whichever thermistor device has a lower resistance of the two thermistor devices to have a higher resistance which is substantially the same as the resistance of the other thermistor device. An advantageous embodiment comprises the steps of: providing an insulating case, two thermistor devices to be housed in the insulating case, and two pairs of terminals to bracket the two thermistor devices respectively; measuring the resistances of the two thermistor devices substantially at the same time; and trimming whichever has a lower resistance of the two thermistor devices to have a higher resistance which is substantially the same as the resistance of the other thermistor device. A further advantageous embodiment is described in Claim 4, through the provision of a manufacturing method of a thermistor apparatus according to Claim 3, wherein, in a condition in which the two thermistor devices are housed in the insulating case, the resistances of the two thermistor devices are measured at substantially the same time, and whichever has a lower resistance of the two thermistor devices is trimmed to have a higher resistance which is substantially the same as the resistance of the other device having a higher resistance between the two thermistor devices. A still further advantageous embodiment of the invention is described in Claim 5, through the provision of a manufacturing method of a thermistor apparatus according to Claim 3, wherein, in the condition in which the two thermistor devices are housed in the insulating case, the resistances of the two thermistor devices are measured at substantially the same time, and whichever has a lower resistance of the two thermistor devices is trimmed using a high-energy beam incident through an opening of the insulating case to have a higher resistance which is substantially the same as the resistance of the other thermistor device. In the thermistor apparatus according to Claim 1 and the manufacturing method of a thermistor apparatus according to Claim 2, trimming is only applied to one of the two thermistor devices and the other thermistor device needs not to be trimmed. Therefore, trimming work is halved compared with the conventional thermistor apparatus. In the manufacturing method of a thermistor device according to Claim 3, the resistances of the two thermistor devices are measured nearly at the same time, hence such measurement is unlikely to be adversely influenced by effects caused by a change in the ambient temperature at the time of resistance measurement and a minute change by aging of the resistance measuring instrument. Therefore, the difference in resistance between the two thermistor devices is accurately measured and accurate trimming is applied to whichever thermistor device has a lower resistance. In the manufacturing method of a thermistor device according to Claim 4, since trimming as well as measuring resistance substantially at the same time is conducted in the condition in which the two thermistor devices are housed in the same case, smooth assembling is performed and the occurrence of cracks or chips on the thermistor devices is reduced, preventing a change in resistance. In the manufacturing method of a thermistor device according to Claim 5, foreign matter is unlikely to enter the case since a high-energy beam is used in trimming, improving the reliability of the thermistor apparatus. As a result of the present invention, the easy-to-manufacture thermistor apparatus having a small difference in resistance between the two built-in thermistor devices can be consistently obtained. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a partially broken elevation illustrating a first embodiment of a thermistor apparatus and a manufacturing method thereof according to the present invention. Fig. 2 is a perspective view of one of two thermistor devices used in the thermistor apparatus shown in Fig. 1. Fig. 3 is a perspective view of the other one of the two thermistor devices used in the thermistor apparatus shown in Fig. 1. Fig. 4 is a plan illustrating a second embodiment of a thermistor apparatus and a manufacturing method thereof according to the present invention. Fig. 5 is a partial cross section taken along line V-V of Fig. 4. Fig. 6 is a plan showing processes of a manufacturing method of the thermistor apparatus illustrated in Fig. 4. Fig. 7 is a partial cross section showing processes of the manufacturing method subsequent to those shown in Fig. 6. Fig. 8 is a partial cross section showing processes of the manufacturing method subsequent to those shown in Fig. 7. Fig. 9 is a perspective view of a thermistor device used for a thermistor apparatus according to another embodiment. Fig. 10 is a perspective view of a thermistor device used for a thermistor apparatus according to still another embodiment. Fig. 11 is a perspective view of a thermistor device used for a thermistor apparatus according to yet another embodiment. Fig. 12 is a perspective view of a thermistor device used for a thermistor apparatus according to a further embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Preferred embodiments of a thermistor apparatus and a manufacturing method thereof according to the present invention will be described below by referring to the drawings. First embodiment (Figs. 1 to 3) As shown in Fig. 1, a positive-thermistor apparatus comprises an insulating case 1, a lid member 2, two positive-characteristics thermistor devices 5 and 6, two planar terminals 10 and 11, two spring terminals 12 and 13, and an insulating plate 15. The insulating case 1 is closed at the left-hand opening with the lid member 2. Materials used for the insulating case 1 and the lid member 2 include thermosetting resin such as phenol and thermoplastic resin such as polyphenylene sulfide. The positive-characteristics thermistor devices 5 and 6 have circular shapes as shown in Figs. 2 and 3 and are made from ceramics such as BaTiO3. The thermistor devices 5 and 6 have electrodes 5a, 5b, 6a, and 6b at the respective front and rear surfaces. Whichever has a lower resistance of the two thermistor devices is trimmed to have a higher resistance such that its resistance is near the resistance of the other device, i.e., such that the two thermistor devices 5 and 6 have substantially the same resistance (for example, within a difference of ±1 Ω). In the first embodiment, part of the electrode 6a of the thermistor device 6 is removed by laser trimming as shown in Fig. 3. The insulating plate 15 is interposed between the two thermistor devices 5 and 6 and it is made from a material having a good thermal conductivity and is formed integrally with the insulating case 1, for example. The planar terminals 10 and 11 are disposed between the insulating plate 15 and the thermistor device 5, and between the insulating plate 15 and the thermistor device 6, respectively. One planar terminal 10 touches a major surface of the insulating plate 15 and the electrode 5b of the thermistor device 5. The other planar terminal 11 touches the other wall surface of the insulating plate 15 and the electrode 6a of the thermistor device 6. One end 10a and 11a of both of the planar terminals 10 and 11 protrudes from the case 1 as shown at the right of Fig. 1. The spring terminals 12 and 13 are disposed between the case 1 and the thermistor device 5, and the case 1 and the thermistor device 6, respectively. The spring terminal 12 touches an inner surface of the case 1 and the electrode 5a of the thermistor device 5, and the spring terminal 13 touches inner surface of the case 1 and the electrode 6b of the thermistor device 6. One end 12a and 13a of both of the spring terminals 12 and 13 protrudes from the case 1 as shown at the right of Fig. 1. The two thermistor devices 5 and 6 are held by the terminals 12 and 13 with pressure applied in the devices' thickness direction. When the case 1 is sealed with the lid member 2, the thermistor devices 5 and 6 sandwich or bracket the planar terminals 10 and 11 and the insulating plate 15. The thermistor devices 5 and 6 are electrically insulated from each other by the insulating plate 15. The thermistor devices 5 and 6 are closely thermally-connected to each other through the insulating plate 15 and the planar terminals 10 and 11. A procedure for reducing the difference in resistance between the two positive-characteristics thermistor devices 5 and 6 in the positive-characteristics thermistor apparatus having the structure described above will be described in detail below. Among a plurality of prepared positive-characteristics thermistor devices, two positive-characteristics thermistor devices 5 and 6 are selected and their resistances are measured with a resistance measuring instrument. It is preferred that the resistances of the two thermistor devices 5 and 6, which are to be housed in the same case, are measured at nearly the same time. This avoids adverse effects caused by a change in the ambient temperature at the time of resistance measurements and by a minute change by aging of the resistance measuring instrument, thereby the difference in resistance between the two thermistor devices 5 and 6 is accurately measured to conduct accurate trimming in a subsequent process. The accurately measured resistance data is sent to a calculation processing unit. An electrode area to be removed from whichever thermistor device has a lower resistance between the two thermistor devices (e.g., in the first illustrated embodiment, the thermistor device 6) is calculated from the resistance difference between the two thermistor devices 5 and 6. Then, according to the electrode area to be removed, a drive signal is sent from the calculation processing unit to a laser trimming unit. The laser trimming unit emits a laser beam to trim the thermistor device 6, which has a lower resistance in this example. In other words, a part of the electrode 6a is removed and the whole area of the electrode is reduced by the specified area. The thermistor device 6 in which part of the electrode 6a has been removed has a higher resistance than before, and is now substantially the same as that of the other thermistor device 5. Trimming can be conducted in two or more steps. The resistances of the thermistor devices may be measured subsequent to a first trimming, and trimming may be conducted again depending on the results of the first measurement. The two positive-characteristics thermistor devices 5 and 6 which have a small resistance difference are thus obtained. Since trimming is only applied to the thermistor device 6, which has a lower resistance, trimming work is halved compared with the conventional method in which trimming is applied to both thermistor devices. Second embodiment (Figs. 4 to 8) As shown in Figs. 4 and 5, a positive-characteristics thermistor apparatus comprises an insulating case 21, two positive-characteristics thermistor devices 25 and 26, two protruding terminals 30 and 31, and two spring terminals 32 and 33. The insulating case 21 has a partition 21c at its center and two circular cavities 21a and 21b disposed to the left and to the right of the partition within a plane. The thermistor devices 25 and 26 have circular shapes and are provided with electrodes 25a, 25b, 26a, and 26b at the respective front and rear surfaces. Whichever has a lower resistance of the two thermistor devices is trimmed to have a higher resistance. The higher resistance of the trimmed thermistor is near the resistance of the other device such that the two thermistor devices 25 and 26 have substantially the same resistance (for example, within a difference of ±1 Ω). The protruding terminals 30 and 31 are insert-molded in the case 21 and are provided with protrusions 30a and 31a at their centers. The protrusions 30a and 31a project through holes 21d and 21e provided at the bottom of the case 21, and touch electrodes 25b and 26b of the thermistor devices 25 and 26, respectively. The other ends of the protruding terminals 30 and 31 are extended along the left and right side faces of the case 21 and folded on the surface of the case 21 to form external-connection portions 30b and 31b. The spring terminals 32 and 33 comprise electrodes 32a and 33a and external-connection portions 32b and 33b. The electrodes 32a and 33a are disposed on the upper surface of the case 21 and cover the openings of the cavities 21a and 21b. The external-connection portions 32b and 33b are folded along the surface of the case 21 to extend to the bottom surface by way of the left and right side faces of the case 21. To increase the seal at the openings of the cavities 21a and 21b, another lid may be used to cover the openings. The two thermistor devices 25 and 26 are sandwiched or bracketed by the protruding terminals 30, 31 and the spring terminals 32, 33 in the cavities 21a and 21b, respectively, and held with pressure in the direction of the thickness of the thermistor devices. A procedure for manufacturing the thermistor apparatus having the structure described above will be described by referring to Figs. 6 to 8. A hoop material 40 on which the protruding terminals 30 and 31 are connected is prepared by punching a strip-shaped metal plate as shown in Fig. 6. The hoop material 40 is provided with feed holes 41 at both edges and transferred using these holes in the direction indicated by arrow "a" to each process. Therefore, assembling and trimming can be conducted in one line as will be described later, thereby facilitating automation of the manufacturing process. The protruding terminals 30 and 31 are insert-molded with resin. The case 21 is formed, with the protrusions 30a and 31a and the external-connection portions 30b and 31b being exposed. The thermistor devices 25 and 26 are inserted horizontally into the cavities 21a and 21b of the case 21, as shown in Fig. 7. One measuring terminal 45a of a resistance measuring instrument 45 is inserted into a first hole 21d of the case 21 to touch a first protruding terminal 30. The other measuring terminal 45b is also inserted into a first cavity 21a to touch the first spring electrode 25a. In the same way, one measuring terminal 46a of a second resistance measuring instrument 46 touches a second protruding terminal 31 and the other measuring terminal 46b touches a second electrode 26a. Then the resistances of the thermistor devices 25 and 26 are measured at the same time to avoid adverse effects caused by a change in the ambient temperature at resistance measurement and a minute change by aging of the resistance measuring instruments 45 and 46. Therefore the difference in resistance between the two thermistor devices 25 and 26 is accurately measured to conduct accurate trimming in a subsequent process. The measured, accurate resistance data is sent to a calculation processing unit 47 and an electrode area to be removed from whichever has a lower resistance between the two thermistor devices 25 and 26 (in the second embodiment, the left thermistor device 25 as shown in Fig. 4) is calculated from the resistance difference between the two thermistor devices. Then, according to the electrode area to be removed, a drive signal is sent from the calculation processing unit 47 to a laser trimming unit 50. The laser trimming unit 50 emits a laser beam L to trim the thermistor device 25, which has a lower resistance. In other words, a part of the electrode 25a, which is exposed through the opening portion of the cavity 21a, is removed and the whole area of the electrode is reduced by the specified area. The thermistor device 25 in which part of the electrode 25a has been removed has a higher resistance than before, the higher resistance being substantially the same as that of the other thermistor device 26. The two positive-characteristics thermistor devices 25 and 26 which have a small resistance difference are thus obtained. Since trimming is only applied to the thermistor device 25, which has a lower resistance, trimming work is halved compared with that for the conventional method in which trimming is applied to both thermistor devices. Since trimming as well as measuring resistance is conducted in the condition in which the thermistor devices 25 and 26 are housed in the case 21, smooth assembling can be performed and changes in resistance of the thermistor devices 25 and 26 due to cracks or chips occurring when the devices are handled can be prevented. Furthermore, foreign matter is unlikely to enter the case 21 since laser trimming is used. The spring terminals 32 and 33 are disposed at the openings of the cavities 21a and 21b in the case 21. Their external-connection portions 32b and 33b are folded along the surface of the case 21. Then, the positive-characteristics thermistor apparatus is taken out of the hoop material 40 by cutting the hoop material along a dot-and-dash line C shown in Fig. 6. The external-connection portions 30b and 31b of the protruded terminals 30 and 31 are folded along the surface of the case 21 to finish assembling the apparatus. A thermistor apparatus and a manufacturing method thereof according to the present invention is not limited to the foregoing embodiments. Within the scope of the invention, they can be modified in various manners. For instance, the thermistor apparatus using the positive-characteristics thermistor devices is described in the foregoing embodiments. The thermistor apparatus may use negative-characteristics thermistor devices. An area removed from an electrode of the thermistor in trimming can have any shape. As shown in Fig. 9, for example, a circumferential area of the electrode 6a may be removed. Part of the upper electrode 6a and part of the lower electrode 6b may be removed as shown in Fig. 10. Alternatively, the electrode 6a may be divided into two sections as shown in Fig. 11. Part of the thermistor body can be removed together with the upper and lower electrodes 6a and 6b. A laser beam is used in trimming in the foregoing embodiments. A high-energy beam, such as an electronic beam or an ion beam, can be used instead of the laser beam. The electrodes have a single layer in the foregoing embodiments. However, the electrodes may have multiple layers. The above described embodiments are illustrative of the present invention which is not limited to these embodiments. The scope of the invention is to be determined by the claims appended hereto.

Claims: A thermistor apparatus comprising: an insulating case (1; 21); two thermistor devices (5, 6; 25, 26) housed in said insulating case; and two pairs of terminals (10, 11, 12, 13; 30, 31, 32, 33) to bracket said two thermistor devices (5, 6; 25, 26) respectively, wherein either one having a lower resistance of said two thermistor devices (5, 6; 25, 26) is trimmed to have a higher resistance which is substantially the same as the resistance of the other thermistor device. A manufacturing method of a thermistor apparatus, comprising the steps of: providing an insulating case (1; 21), two thermistor devices (5, 6; 25, 26) to be housed in said insulating case (1; 21), and two pairs of terminals (10, 11, 12, 13; 30, 31, 32, 33) to bracket said two thermistor devices (5, 6; 25, 26) respectively; measuring the resistances of said two thermistor devices (5, 6; 25, 26); and trimming whichever thermistor device has a lower resistance of said two thermistor devices to have a higher resistance which is substantially the same as the resistance of the other thermistor device. The manufacturing method of claim 2, wherein the resistances of said two thermistor devices (5, 6; 25, 26) are measured substantially at the same time. The manufacturing method of Claim 3, wherein said two thermistor devices (5, 6; 25, 26) are housed in said insulating case (1; 21) during measuring and trimming. The manufacturing method of Claim 3, wherein said trimming step involves a high-energy beam incident through an opening of said insulating case (1; 21).

Inventors: KATSUKI TAKAYO, SHIKAMA TAKASHI, KATSUKI, TAKAYO, SHIKAMA, TAKASHI, Katsuki, Takayo c/o Murata Manufacturing Co. Ltd., Shikama, Takashi c/o Murata Manufacturing Co. Ltd.
Applicants: MURATA MANUFACTURING CO, MURATA MANUFACTURING CO., LTD.
Application year: 1996
Main classification: H01C17
IPC codes: H01C17, H01C7, H01C1