Electrical resistors – Resistance value responsive to a condition – Current and/or voltage
Reexamination Certificate
1994-03-30
2001-03-13
Easthom, Karl D. (Department: 2832)
Electrical resistors
Resistance value responsive to a condition
Current and/or voltage
C338S225000, C338S328000, C338S324000
Reexamination Certificate
active
06201464
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a thermistor device, which comprises a thermistor element body and an ohmic electrode provided on its major surface.
2. Description of the Background Art
In a positive characteristic (PTC) thermistor and a negative characteristic (NTC) thermistor, a structure is known, comprising a thermistor element body and ohmic electrodes, mainly composed of silver or the like, which are provided on both major surfaces thereof.
FIGS. 10 and 11
are respectively a perspective view and a sectional view showing a conventional thermistor device having such a structure. Referring to
FIGS. 10 and 11
, a PTC thermistor comprises a thermistor element body
1
which is made of a ceramic material such as barium titanate, and ohmic electrodes
2
and
3
, mainly composed of silver or the like, which are provided on both major surfaces thereof. In such a thermistor device, the ohmic electrodes
2
and
3
are directly exposed to the outside air. When potential difference is developed across the ohmic electrodes
2
and
3
, therefore, electrode components forming the ohmic electrodes
2
and
3
may deposit on an outer peripheral surface
1
c
of the thermistor element body
1
and move to approach each other, to cause shorting across the electrodes
2
and
3
. Such a phenomenon is generally called migration, which is very likely to be prompted particularly in a hot and humid atmosphere.
FIGS. 12 and 13
are respectively a perspective view and a sectional view showing the structure of another conventional thermistor device which is known in the art. Referring to
FIGS. 12 and 13
, this conventional thermistor device comprises a thermistor element body
1
and ohmic electrodes
2
and
3
, having smaller areas than the major surfaces of the thermistor element body
1
. The ohmic electrodes are provided on the major surfaces. Gap portions
1
a
and
1
b
are defined between outer peripheral edges of the ohmic electrodes
2
and
3
and those of the thermistor element body
1
. In the thermistor device having such a structure, it may be possible to prevent shorting caused by the aforementioned migration by defining sufficiently wide gap portions
1
a
and
1
b.
In this case, however, chipping results from a difference in stress between heating and non-heating portions of the thermistor element body
1
caused by thermal expansion, or stress caused by electric field concentration.
In order to prevent such shorting caused by migration, Japanese Utility Model Laying-Open No. 62-76504 (1987) proposes a structure of a thermistor device comprising a thermistor element body and resin coating layers provided on its outer peripheral surface.
FIG. 14
is a sectional view showing the thermistor device having such a structure. As shown in
FIG. 14
, a resin coating layer
4
is provided on an outer peripheral surface of a thermistor element body
1
.
However, this structure is not suitable for practical application since the application of such a resin coating layer provided on the outer peripheral surface of a thermistor element body requires a complicated step, to increase the cost.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a structure of a thermistor device which can prevent migration of an electrode material through a simple step while preventing chipping in the vicinity of an outer periphery of a thermistor element body.
The thermistor device according to the present invention is characterized in that an ohmic electrode has an outer peripheral edge that is positioned within that of a thermistor element body to define a gap portion between the outer peripheral edges of the ohmic electrode and the thermistor element, and an insulating coating portion is so formed as to cover a portion where the outer peripheral edge of the ohmic electrode is in contact with the thermistor element body.
According to the present invention, the ohmic electrode which is provided on a major surface of the thermistor element body can be made of a material which is generally employed for an ohmic electrode of a thermistor device, such as a metal or alloy material such as Ag, Ag+Zn/Sb, Ag+In or Al, for example.
According to the present invention, further, the material for the insulating coating portion which is so provided as to cover the contact portion between the outer peripheral edge of the ohmic electrode and the thermistor element body is not particularly restricted, as long as the same has electrical insulativity and is capable of covering the aforementioned portion, so that this material can be prepared from glaze (glass), epoxy resin, silicon resin or the like, for example.
In the thermistor device according to the present invention, the ohmic electrode is so provided that a gap portion is defined between the outer peripheral edges of the ohmic electrode and the thermistor element body. Due to such provision of the gap portion, the electrode material forming the ohmic electrode causes hardly any migration as compared with a structure provided with no gap portion. According to the present invention, further, the insulating coating portion is so formed as to cover the portion where the outer peripheral edge of the ohmic electrode is in contact with the thermistor element body. Therefore, occurrence of migration is further suppressed by the insulating coating portion.
In addition, it is possible to prevent chipping in the vicinity of the outer peripheral portion of the thermistor element body by the insulating coating portion covering the contact portion between the outer peripheral edge of the ohmic electrode and the thermistor element body.
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.
REFERENCES:
patent: 3296574 (1967-01-01), Tassara
patent: 5210516 (1993-05-01), Shikama et al.
patent: 972851 (1959-10-01), None
patent: 2351956 (1974-04-01), None
patent: 7507461 (1976-12-01), None
patent: 2901711 (1980-07-01), None
patent: 0229286 (1987-07-01), None
patent: 4139157 (1992-06-01), None
patent: 0443618 (1991-08-01), None
patent: 6276504 (1985-11-01), None
Easthom Karl D.
Murata Manufacturing Co. Ltd.
Ostrolenk Faber Gerb & Soffen, LLP
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