Electrical resistors – Resistance value responsive to a condition – Current and/or voltage
Reexamination Certificate
1996-01-24
2001-01-23
Easthom, Karl D. (Department: 2832)
Electrical resistors
Resistance value responsive to a condition
Current and/or voltage
C338S0220SD, C338S324000, C338S328000
Reexamination Certificate
active
06177857
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to thermistor devices and, more particularly, to a positive-characteristic thermistor device used in a demagnetizing circuit incorporated in a TV receiver and also to a negative-characteristic thermistor device used in a temperature-compensating circuit or the like.
2. Description of the Prior Art
A known thermistor device having a positive or negative temperature coefficient is shown in
FIGS. 7 and 8
. The body of the thermistor is indicated by numeral
30
. Electrodes
31
and
32
made from a conductive material consisting mainly of silver (Ag) are formed on the front and back surfaces, respectively, of the thermistor body
30
. The electrodes
31
and
32
are in ohmic contact with the thermistor body
30
.
In the thermistor device of this construction, if a potential difference is developed between the electrodes
31
and
32
, some Ag atoms forming the material of the electrodes
31
and
32
migrate across the surface of the thermistor body
30
, thus deteriorating the insulating performance. In the worst case, the electrodes
31
and
32
are shorted together. Referring to
FIG. 8
, A and D refer to the outer ends of the electrodes
31
and
32
, respectively, and B and C refer to the left and right edges, respectively, of the outer end surface of the thermistor body
30
. Because of the resistive component of the thermistor body
30
, potential differences are produced between A and B, between B and C, and between C and D on the surface of the thermistor body
30
. These potential differences cause migration of the Ag atoms forming the electrodes
31
and
32
.
Another thermistor device equipped with means for reducing or slowing this problem has been proposed, and is shown in
FIGS. 9 and 10
. This thermistor device is similar to the known thermistor device already described in conjunction with
FIGS. 7 and 8
except that the surface of the thermistor body
30
, excluding the portions covered by the electrodes
31
and
32
, is coated with an insulating film
33
made of a resin, glass, or the like. As shown in
FIGS. 9 and 10
, the Ag migration entails the movement of metal caused by a potential difference between A and B, between B and C, and between C and D. In addition, if there is a potential difference, the migration velocity is accelerated when the thermistor device is operated in a moist atmosphere, and the electrolytic ion such as chloric ions, sulfurate ions, or the like are absorbed onto the thermistor surface on operating. Coating the thermistor body with resin or glass will prevent water and the electrolytic ions from being absorbed onto the thermistor surface, thus maintaining the migration at a low velocity.
However, it is costly to fabricate this thermistor device shown in
FIGS. 9 and 10
, because it is cumbersome to coat the outer surface of the thermistor body
30
with the insulating film
33
made of a resin or glass.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the present invention to provide a thermistor device which is economical to fabricate and is free or substantially free from migration of Ag atoms.
This object is achieved in accordance with the invention by a thermistor device comprising a thermistor body, first electrodes formed in peripheral portions of the front and back surfaces, respectively, of the thermistor body, and second electrodes formed at least in central portions of the front and back surfaces, respectively, of the thermistor body. The first electrodes are made from a conductive material not containing silver (Ag). The second electrodes are made from a conductive material principally including silver (Ag).
In this construction, the outer surface of the thermistor body is not required to be coated with an insulating film. Even if a potential difference is produced between the second electrodes formed on the front and back surfaces, respectively, of the thermistor body, the first electrodes made from the conductive material not containing Ag prevents migration of Ag atoms from the second electrodes for reasons explained below.
REFERENCES:
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patent: 4053864 (1977-10-01), Rodriguez et al.
patent: 4251792 (1981-02-01), Ball, Jr.
patent: 4431983 (1984-02-01), Rodriguez
patent: 4635026 (1987-01-01), Takeuchi
patent: 4801784 (1989-01-01), Jensen et al.
patent: 4924204 (1990-05-01), Uchida
patent: 5210516 (1993-05-01), Shikama et al.
patent: 5289155 (1994-02-01), Okumura et al.
patent: 5337038 (1994-08-01), Taniguchi et al.
patent: 5557251 (1996-09-01), Takaoka
patent: 1-318 202 (1989-12-01), None
patent: 403174701 (1991-07-01), None
patent: WO95/24046 (1995-09-01), None
patent: 95/24046 (1995-09-01), None
Burns Doane , Swecker, Mathis LLP
Easthom Karl D.
Murata Manufacturing Co. Ltd.
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