 |
Description  |
|
|
BACKGROUND OF THE INVENTION
The present invention relates to a chip-type varistor (nonlinear resistor)
and, more particularly, to a chip-type varistor in which the shape of the
varistor has been improved.
Recently, demand has increased for small varistors such as chip-type
varistors which can be mounted on printed circuit boards without using
lead wires or external terminals.
FIGS. 1-3 show a conventional chip-type varistor. FIG. 1 is a plan view of
the varistor, FIG. 2 is a sectional view of the varistor, taken along line
A--A' in FIG. 1, and FIG. 3 is also a sectional view, taken along line
B--B' in FIG. 1. As shown in these figures, the chip-type varistor
comprises a thin, rectangular varistor element 11, and two electrodes 12a
and 12a' formed on one of the two major surfaces of element 11, two
electrodes 12b and 12b' formed on the other major surface of element 11.
The varistor further comprises two electrodes 13 and 13' covering the ends
of element 11. Electrode 13 also covers parts of electrodes 12a and 12b,
thus electrically connecting these electrodes. Similarly, electrode 13'
also covers parts of electrodes 12a' and 12b', electrically connecting
electrodes 12a' and 12b'. Portion W of element 11, which is sandwiched by
the overlapping portions of electrodes 12a and 12b' provides varistor
characteristic.
As shown in FIGS. 2 and 3, edges 14 and 14' of element 11 form 90.degree.
corners. Electrodes 13 and 13' are formed on these edges. Electrodes 13
and 13' are formed by dipping the ends of varistor element 11 in a paste
of an electrode material such as Ag or Ag-Pd and then drying and baking
the material attained to the ends of element 11. As element 11 is pulled
up from the paste, the molten material runs off the corners. As a result,
those portions of electrodes 13 and 13' overlying the 90.degree. corners
are invariably too thin.
When a surge current flows through electrodes 13 and 13', its generated
Joule heat will cause melting of the thinest portions of the electrodes
because of the high electric resistance of the thinnest portions.
Another problem is that cracks 15 are made in the thinest portions disposed
over the corners of element 11 since stress concentrates on the thin
portions of electrodes 13 and 13' in the directions shown in FIGS. 4 and 5
when the volume of organic solvent within the electrode material
evaporates during the drying and baking process. In this case, more Joule
heat will be generated at those thin portions of electrode 13 and 13' as a
surge current flows through the electrodes, and the thin portions thereof
will be more easily melted away.
To prevent said portions of electrodes 13 and 13' from being melted away or
from having cracks 15, element 11 can be dipped in a paste of electrode
material, dried and baked several times. This, however, reduces the
efficiency with which varistors can be manufactured. In addition,
electrodes 13 and 13' thus formed on the ends of element 11, become too
thick. Furthermore, since it is difficult to control the surface evenness
of electrodes 13 and 13', varistors of a uniform shape cannot be obtained.
Consequently, the varistor produced through repeated dipping of element 11
in a paste of electrode material cannot be stably mounted on a printed
circuit board. This is a crucial drawback for any chip element.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a chip-type varistor
which has precise and therefore uniform dimensions and which can be stably
mounted on a printed circuit board.
It is another object of the invention to provide a chip-type varistor
having external electrodes with no cracks.
Still another object of this invention is to provide a chip-type varistor
characterized of such stability that its lifetime is prolonged as a result
of its capacity to withstand surge currents.
According to the present invention, there is provided a chip-type varistor
comprising: a platelike varistor element made of a sintered body having
rounded sides and corners; a pair of platelike electrodes of a first
polarity and different lengths formed on two major surfaces of said
varistor element and electrically insulated from each other; a pair of
platelike electrodes of a second polarity and different lengths formed on
two major surfaces of said varistor element and electrically insulated
from each other, one of said electrodes partially overlapping one of the
electrodes of the first polarity; a first external electrode formed on one
of the two ends of said varistor element and connecting the electrodes of
the first polarity; and a second external electrode formed on the other
end of said varistor element and connecting the electrodes of the second
polarity. In this varistor, that portion of said varistor element which is
sandwiched by the overlapping portions of the electrode of the first
polarity and electrode of the second polarity provides varistor
characteristics.
In the present invention, a platelike varistor element with rounded sides
and corners is used. Therefore, the electrodes formed on the ends of this
element have no cracks at these corners because these electrodes are then
made sufficiently thick at these corners even when they are formed by
dipping the ends of the varistor element only once in a paste of electrode
material. The varistor of this invention, therefore, has precise and
uniform dimensions and can be stably mounted on a printed circuit board
and has a long lifetime since the electrodes withstand surge currents.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a plan view of a conventional chip-type varistor;
FIG. 2 is a sectional view of the conventional varistor, taken along line
A--A' in FIG. 1;
FIG. 3 is a sectional view of the varistor, taken along line B--B' in FIG.
1;
FIG. 4 is an enlarged view of portion C (FIG. 2) of the varistor;
FIG. 5 is an enlarged view of portion D (FIG. 3) of the varistor;
FIG. 6 is a plan view of a chip-type varistor according to the present
invention;
FIG. 7 is a front view of this varistor;
FIG. 8 is a side view of the same varistor;
FIG. 9 is a sectional view of the varistor, taken along line A--A' in FIG.
6;
FIG. 10 is a sectional view of the varistor, taken along line B--B' in FIG.
6;
FIGS. 11 to 14 are plan views of four other chip-type varistor according to
the invention, not provided with electrode 4 or 4'.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
One embodiment of the invention will be described in detail with reference
to the accompanying drawings.
FIGS. 6-10 show the first embodiment, i.e., a chip-type varistor. As shown
in these figures, the varistor comprises a thin, rectangular varistor
element 2 made by pressing a ceramic powder mixture containing zinc oxide
as a major component and several other metal oxides as minor components
and by drying and sintering the pressed mixture. Element 2 has rounded
sides and corners 1 and 1' formed during the pressing of the ceramic
powder mixture. Two platelike electrodes 3a and 3a' are formed on one of
the two major surfaces of element 2, and two platelike electrodes 3b and
3b' are formed on the other major surface of element 2. The varistor
further comprises two electrodes 4 and 4' covering the ends of element 2.
Electrode 4 also covers the parts of electrodes 3a and 3b, electrically
connecting these electrodes. Similarly, electrode 4' also covers the parts
of electrodes 3a' and 3b', electrically coupling electrodes 3a' and 3b'.
Portion L of element 2, which is sandwiched by the overlapping portions of
electrodes 3a and 3b' provides varistor characteristics.
As shown in FIGS. 6 to 10, the edges of element 2 form rounded sides and
corners 1 and 1'. Electrodes 4 and 4' are formed on these edges by dipping
the ends of varistor element 2 in a paste of an electrode material such as
Ag or Ag-Pd alloy and then drying and baking the material attached to the
ends of element 2. Those portions of electrodes 4 and 4' which are formed
over the rounded sides and corner 1 and 1' are as thick as the other
portions formed on the sides and major surfaces of element 2, as shown in
FIGS. 9 and 10. In other words, external electrodes 4 and 4' are uniformly
coated on sides and corners 1 and 1' of element 2 through a single dipping
process because of the provision of rounded sides and corners 1 and 1'.
Therefore, a great deal of Joule heat is not generated in those portions
of either external electrode, which are formed over sides and corners 1
and 1', to melt away these portions. Also, the uniformly coated electrode
4 or 4' do not develop cracks since stress on electrodes 4 and 4' can be
uniformly dispersed when the coating is dried and baked. As a result,
variations in the dimensions of the varistors are eliminated, facilitating
reliable mounting of the varistors on a printed circuit board.
In the above embodiment, varistor electrodes 3a and 3b have right-angled
corners. Electrodes 3a and 3b can be replaced by the electrode shown in
FIG. 11 which has rounded corners 5, the electrode shown in FIG. 12 which
has parabolic corners 6, the electrode shown in FIG. 13 whose right-angled
corners have been cut off, or the electrode shown in FIG. 14 which has an
arcuate end. Note that the same numerals are used in FIGS. 11 to 14,
designating the same components as in the embodiment shown in FIGS. 6 to
10, and a detailed description of these components is omitted.
Referring to FIG. 9, thickness t1 of element 2 is less than distance t2
between electrodes 3a and 3a'. If the portion of element 2, between
electrodes 3a and 3a' is highly resistant, however, t1 can be greater than
t2. Also in this case, portion L of element 2, which is sandwiched by the
overlapping portions of electrodes 3a and 3b', provides varistor
characteristics.
The characteristics of the chip-type varistor embodying the present
invention (shown in FIGS. 6-10), and those of the conventional chip-type
varistor (shown in FIGS. 1-5) were tested in applying a surge current of a
standard waveform (8.times.20 .mu.sec.), and the lifetime of the
respective varistors were determined. Surge test was applied to 30 pieces
of zinc oxide-based varistor of the type shown in FIGS. 1-5, and also to
30 pieces of zinc oxide-based varistor of the type shown in FIGS. 6-10.
The conventional varistors were damaged at 500 to 1,000 A, whereas the
varistor of the invention were damaged until at 2,000 to 4,000 A. As the
results demonstrate, the lifetime of the varistor of the present invention
can be significantly improved due to the capacity of the varistor to
withstand surge currents.
The present invention can be applied to varistors other than zinc
oxide-based ones. When it is applied to varistor elements made of
strontium titanate-based, iron oxide-based, barium titanate-based, and
silicon carbide-based varistors, it can achieve the same advantages as
when applied to zinc oxide-based varistors.
In the embodiments shown in FIGS. 6 to 10 and in FIGS. 11 to 14, the
rounded sides and corners of the varistor elements were formed during the
pressing of ceramic powder mixture. Instead, they can be formed after the
sintering, by barell-grinding platelike varistor elements.
Furthermore, though the varistor elements of the above embodiments have no
outer coating, the outer surfaces of a chip-type varistor and the surfaces
of its electrodes can be coated with an insulating material (e.g., glass)
if necessary.
* * * * *
|
|
 |
|
 |
Description |
|
