Electrical resistors – Resistance value responsive to a condition – Current and/or voltage
Reexamination Certificate
1998-04-21
2001-02-13
Easthom, Karl D. (Department: 2832)
Electrical resistors
Resistance value responsive to a condition
Current and/or voltage
C338S195000, C338S225000, C338S235000
Reexamination Certificate
active
06188307
ABSTRACT:
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 &OHgr;. 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 &OHgr;. 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, 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, through the provision of a manufacturing method of a thermistor apparatus, comprising the steps of: preparing 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.
The foregoing object is achieved in still another aspect of the invention, through the provision of a manufacturing method of a thermistor apparatus, comprising the steps of: preparing 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.
The foregoing object is achieved in a further aspect of the invention, through the provision of a manufacturing method of a thermistor apparatus according to the preceding paragraph, 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.
The foregoing object is achieved in a still further aspect of the invention; through the provision of a manufacturing method of a thermistor apparatus according to the paragraph preceding the paragraph, 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 and the manufacturing method of a thermistor apparatus, 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, the resistances of the two thermistor devices can be 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, since trimming as well as measuring resistance substantially at the same time are conducted when the two thermistor devices are stored 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, 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, an easy-to-manufacture thermistor apparatus having a small difference in resistance between the two built-in thermistor devices can be consistently obtained.
REFERENCES:
patent: 2010814 (1935-08-01), Ellis
patent: 2382024 (1945-08-01), Priessman
patent: 2609644 (1952-09-01), Brown, Jr. et al.
patent: 3768157 (1973-10-01), Buie
patent: 3914727 (1975-10-01), Fabricius
patent: 4024427 (1977-05-01), Belhomme
patent: 4031499
Katsuki Takayo
Shikama Takashi
Burns Doane , Swecker, Mathis LLP
Easthom Karl D.
Murata Manufacturing Co. Ltd.
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