U.S. patent number 3,768,058 [Application Number 05/165,001] was granted by the patent office on 1973-10-23 for metal oxide varistor with laterally spaced electrodes.
This patent grant is currently assigned to General Electric Company. Invention is credited to John D. Harnden, Jr..
United States Patent |
3,768,058 |
Harnden, Jr. |
October 23, 1973 |
METAL OXIDE VARISTOR WITH LATERALLY SPACED ELECTRODES
Abstract
A metal oxide varistor having an alpha in excess of 10 in the
current density range of from 10.sup..sup.-3 to 10.sup.2 amperes
per square centimeter is formed with laterally spaced electrodes
adjacent a first surface. A spaced third electrode may be
associated with the first surface or a second surface. To improve
the current carrying capacity of the varistor body the conduction
gap between the electrodes may extend along the first surface an
extended distance greater than the width of the surface. The
conduction gap width may be varied continuously or in discrete
steps.
Inventors: |
Harnden, Jr.; John D.
(Schenectady, NY) |
Assignee: |
General Electric Company
(Syracuse, NY)
|
Family
ID: |
22596985 |
Appl.
No.: |
05/165,001 |
Filed: |
July 22, 1971 |
Current U.S.
Class: |
338/20; 29/613;
361/56; 29/610.1; 257/1 |
Current CPC
Class: |
H01C
7/102 (20130101); Y10T 29/49087 (20150115); Y10T
29/49082 (20150115) |
Current International
Class: |
H01C
7/102 (20060101); H01c 007/10 () |
Field of
Search: |
;338/13,20,21 ;29/610
;317/238,234V |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Albritton; C. L.
Claims
What I claim and desire to secure by Letters Patent of the United
States is:
1. The combination comprising
a substrate having first and second opposed major surfaces
comprised of a metal oxide varistor body lying along at least said
first major surface and having an alpha in excess of 10 in the
current density range of from 10.sup.-.sup.3 to 10.sup.2 amperes
per square centimeter and
first and second electrodes lying in ohmic contact with said first
major surface and laterally spaced to form a conduction gap
therebetween along said first major surface having a minimum width
less than the thickness of said substrate between said major
surfaces.
2. The combination comprising
a substrate having first and second opposed major surfaces;
said substrate consisting of a metal oxide varistor body;
said varistor body having an electrical resistance which varies as
a function of applied voltage in accordance with the formula
I = (V/C).sup.alpha
where V is the voltage in volts applied to the body, I is the
current in amperes through the body resulting from such voltage,
and C and alpha are constants;
said body having an alpha in excess of 10 in the current density
range of from 10.sup.-.sup.3 to 10.sup.2 amperes per square
centimeter;
first and second electrodes lying in ohmic contact with said first
major surface and laterally spaced on said first major surface to
form a conduction gap therebetween along said first major
surface;
said conduction gap having a minimum width less than the thickness
of said varistor body measured normal to said first major
surface.
3. The combination according to claim 2 wherein said varistor body
comprises predominantly zinc oxide.
4. The combination according to claim 1 in which said substrate is
additionally comprised of a dielectric support associated with said
varistor body.
5. The combination according to claim 1 in which a third electrode
lies in ohmic contact with said first major surface, said third
electrode lying in laterally spaced relation with said second
electrode and laterally separated from said first electrode by said
second electrode, said second and third electrodes forming a
conduction gap therebetween along said first major surface having a
minimum width along said first major surface exceeding the maximum
width along said first major surface of the conduction gap between
said first and second electrodes.
6. The combination according to claim 1 in which said substrate is
formed entirely by said varistor body and additionally including a
third electrode lying in ohmic contact with said second major
surface.
7. The combination according to claim 1 in which adjacent edges of
said first and second electrodes are substantially parallel.
8. The combination according to claim 1 including
a metal oxide varistor body having an alpha in excess of 10 within
the current density range of from 10.sup.-.sup.3 to 10.sup.2
amperes per square centimeter and presenting at least one major
surface,
first and second electrodes lying in ohmic contact with said body
along said major surface and laterally spaced relatively to form a
conduction gap therebetween, and
dielectric means overlying said varistor body along the conduction
gap and cooperating with adjacent edges of said electrodes to
protect said varistor body against alteration of its electrical
characteristics.
9. The combination according to claim 8 in which said dielectric
means and said electrodes together completely envelop said varistor
body.
10. The combination according to claim 8 additionally including
packaging means cooperating with said dielectric means and said
electrodes to envelop said varistor body.
11. A varistor comprising
a metal oxide varistor body having an electrical resistance which
varies as a function of applied voltage in accordance with the
formula
I = (V/C).sup.alpha
where V is the voltage in volts applied to the body, I is the
current through the body in amperes resulting from such voltage,
and C and alpha are constants;
said body having an alpha in excess of 10 in the current density
range of from 10.sup.-.sup.3 to 10.sup.2 amperes per square
centimeter;
said body having at least one major surface;
first and second electrodes lying in ohmic contact with said major
surface and laterally spaced on said major surface to form a
conduction gap therebetween along said major surface;
said conduction gap having a minimum width less than the thickness
of said body measured normal to said major surface.
12. The combination comprising
a metal oxide varistor body having an alpha in excess of 10 in the
current density range of from 10.sup.-.sup.3 to 10.sup.2 amperes
per square centimeter and presenting first and second opposed major
surfaces,
first and second electrodes lying in ohmic contact with said first
major surface and laterally spaced to form a conduction gap
therebetween along said first major surface having a minimum width
less than the thickness of said body between said major surfaces,
and
a third electrode lying in ohmic contact with said second major
surface. pg,24
13. A varistor according to claim 11 wherein said varistor body
comprises predominantly zinc oxide.
Description
My invention is directed to a circuit component including a metal
oxide varistor having laterally spaced electrodes.
It can be generally stated that the current which flows between two
spaced points is directly related to the potential difference
between the points. For most known substances current conduction
therethrough is equal to the applied potential difference divided
by a constant, which has been defined by Ohm's law to be its
resistance. There are, however, a few known substances which have
been observed to exhibit non-linear resistances and which require
resort to the following equation (1) to relate quantitatively
current and voltage:
(1) I = (V/C).sup. alpha
Where V is the voltage between two points separated by a body of
the substance under consideration, I is the current flowing between
the two points, C is a constant, and alpha is an exponent greater
than 1. There are many known electrical circuits in which it is
quite desirable to incorporate one or more functional elements
having non-linear or exponential resistance characteristics. For
example, the non-linear resistance properties of silicon carbide
have been widely utilized in commercial silicon carbide varistors.
Typically silicon carbide varistors exhibit an alpha of no more
than 6.
It has been recently appreciated that varistors having alphas in
excess of 10 within the current density range of 10.sup.-.sup.3 to
10.sup.2 amperes per square centimeter may be made from bodies
which are comprised of metal oxides. The metal oxide body may be
formed predominantly of zinc oxide with small quantities of one or
more other metal oxides being present. Metal oxide varistors having
alphas in excess of 10 are disclosed in Canadian Pat. No. 831,691,
issued Jan. 6, 1970, for example. While the alphas of these metal
oxide varistors are identified by the current density range of
10.sup.-.sup.3 to 10.sup.2 amperes per square centimeter, which
characteristically exhibits substantially constant alphas, it is
appreciated that their alphas remain high also at higher and lower
currents, although some decline from maximum alpha values have been
observed.
The construction of a conventional metal oxide varistor having an
alpha in excess of 10 is shown in FIG. 1. The metal oxide varistor
1 is formed of a sintered ceramic metal oxide body 3. The body
includes a first major surface 5 and a second, opposed major
surface 7. The major surfaces are separated by a thickness X. First
and second electrodes 9 and 11 are associated with the first and
second major surfaces respectively, so that they lie in ohmic
contact therewith.
In placing the metal oxide varistor in use, when a potential
difference is placed across the electrodes 9 and 11, a current is
conducted through the bulk of the metal oxide body 3. Since the
distance between the electrodes along the surface of the metal
oxide varistor body is greater than through the bulk of the body,
little, if any, current is conducted along the surface of the body.
For various voltage levels across the electrodes the current
follows equation 1. For a given cross-sectional area of the metal
oxide body measured normal to the direction of current flow
therethrough and for a given current level it has been observed
that the voltage across the electrodes is a function of the
thickness X.
For many circuit applications where relatively high voltage levels
are desired at a given current conduction level this relationship
is convenient, as it is quite simple to choose a thickness value X
to yield the desired voltage characteristic for the varistor. In
circuit applications where a relatively low voltage value is
desired, however, for a given current conduction by the varistor,
the value of X may become so small that it is quite difficult
either to form or to handle the metal oxide varistor body without
damage. For example, for comparatively low voltage applications a
thickness for the metal oxide varistor body of only 2 or 3 microns
may be indicated. Further, when comparatively low voltage
characteristics are desired, the correct dimensions of the metal
oxide body 3 can become quite important, as an error in thickness
of only a few microns might double or halve the desired voltage
characteristic.
It is an object of my invention to provide a metal oxide varistor
construction in which the voltage characteristic is independent of
the thickness of the metal oxide body. It is a more specific object
of my invention to provide a metal oxide varistor in which a rugged
and easily formed metal oxide body having no critical dimensions
can be employed for even the lowest voltage applications. It is
still another object to provide a metal oxide varistor according to
my invention which is self-protected from overloading. It is an
additional object to provide a metal oxide varistor capable of
clamping at multiple voltage levels.
In one aspect, my invention is directed to the combination
comprised of a substrate having first and second opposed major
surfaces comprised of a sintered ceramic metal oxide varistor body
lying along at least the first major surface and having an alpha in
excess of 10 in the current density range of 10.sup.-.sup.3 to
10.sup.2 amperes per square centimeter. First and second electrodes
lie in ohmic contact with the first major surface and are laterally
spaced to form a conduction gap therebetween along the first major
surface having a minimum width less than the thickness of the
substrate between the major surfaces.
My invention may be better understood by reference to the following
detailed description considered in conjunction with the drawings,
in which
FIG. 1 is a schematic sectional view of the conventional metal
oxide varistor discussed above;
FIG. 2, 3, and 4 are schematic sectional views of separate
embodiments according to my invention;
FIG. 5 is a schematic circuit diagram;
FIGS. 6 and 7 are schematic sectional views of additional
embodiments according to my invention;
FIG. 8 is a schematic circuit diagram utilizing the embodiment of
FIG. 7;
FIG. 9 through 12 inclusive are plan views of additional
embodiments according to my invention; and
FIG. 13 is a schematic sectional view of a packaged unit
incorporating the varistor of FIG. 3.
In FIG. 2 a varistor 20 is shown formed according to my invention.
The varistor includes a metal oxide varistor body 21 having an
alpha as defined by equation 1 in excess of 10. The metal oxide
varistor body may be formed according to the teaching of the
Canadian patent cited above or in any other known manner. The body
is provided with a first major surface 22 and a second, opposed
major surface 23. The second major surface is shown to be parallel
to the first major surface, but may take any geometrical form
convenient for the specific application to which the varistor is to
be placed. The thickness of the varistor body measured normal to
the major surfaces is not critical and may vary widely. The
varistor body thickness is in most instances chosen so that the
varistor body is rugged enough to avoid damage both in fabrication
and handling. For example, the varistor body will normally exhibit
a thickness of at least 25 microns. In theory there is no limit to
the maximum thickness of the varistor body, except that excessive
thicknesses may unnecessarily add to the bulk and cost of the
varistor as well as lengthening the thermal impedance path through
the varistor body.
Mounted on the first major surface is a first electrode 24 and a
second electrode 25. The electrodes may be ohmically conductively
associated with the major surface in any convenient conventional
manner. The electrodes are laterally separated by a width Y,
referred to as the conduction gap width. In the varistor 20 the
conduction gap extends linearly across the first major surface and
is of uniform width throughout. The conduction gap width determines
the voltage level to be observed across the electrodes for a given
current conduction level. Accordingly, it is desirable in most
instances to precisely control this width. This can be accomplished
by positioning the electrodes using known masking techniques to
assure that they are accurately spaced or by initially forming a
single electrode and thereafter relieving an intermediate portion
of the electrode in a controlled manner to leave the first and
second electrodes in spaced relation.
The conduction gap width Y may be of any desired value, depending
upon the voltage desired for a given level of current conduction.
The lateral spacing of the electrodes of the varistor 20 is,
however, particularly advantageous when the conduction gap width Y
is less than the thickness of the varistor body, as would be the
case in comparatively low voltage applications. To illustrate this,
it is merely necessary to observe that if an electrode spacing of 2
microns between electrodes is indicated to yield the desired
current and voltage characteristic for a varistor, it would be
necessary to form the varistor body 3 with the thickness X being a
value of only 2 microns. However, in my varistor 20 the varistor
body 21 can be formed of any convenient thickness. It is only the
conduction gap width Y that must be controlled at 2 microns. By
comparison to forming the varistor body itself of this small
thickness, like spacing of the electrodes is quite simple to
accomplish employing techniques well known to the art.
The operation of the varistor 20 differs from that of a
conventional varistor as shown in FIG. 1. When a potential is
impressed across electrodes 24 and 25, the current that is
conducted between the electrodes is along or immediately beneath
the surface of the varistor body within the conduction gap Y. This
is in direct contrast to the conventional varistor in which the
current is more or less uniformly distributed within the bulk of
the varistor body. Of course, in the varistor 20 there will be some
fraction of the current that will be carried through the bulk of
the varistor body beneath the surface of the body, particularly as
higher voltages are reached, but this should still be only a small
proportion of the total current and may under most circumstances be
considered negligible. Hence, while the varistor 2 will follow
equation 1 similarly as varistor 1, its internal conduction mode is
quite dissimilar.
In addition to the varistor 20 I have also invented various
alternative embodiments differing in one or more functional and
structural aspects. Except for the specific differing features
noted and discussed, the remaining embodiments of my invention
should be understood to employ structural characteristics identical
to those of the varistor 20.
In FIG. 3 a varistor 30 is shown, which is a modified form of my
invention. A sintered ceramic metal oxide varistor body 31 is
provided having a first major surface 32 and a second major surface
33. Electrodes 34 and 35, identical to electrodes 24 and 25, are
associated with the first major surface and are separated by
conduction gap width Y. A dielectric support 36 is associated with
the second major surface. The dielectric support may be chosen from
any one of a variety of electrically insulative, comparatively
inert materials, such as, but not limited to, known glass, ceramic,
and polymeric insulators. The advantage of using the support 36 is
that the thickness X3 of the varistor body can now conveniently be
reduced, since the ruggedness of the varistor body itself is
supplemented to a considerable extent by the support. It is still a
uniquely advantageous feature of my invention that the combined
thickness of the varistor body and support, which together form a
common substrate, can be greater than the conduction gap Y,
although this is not absolutely essential to all applications of my
invention. It is recognized that in some circumstances,
particularly when the support is a ceramic, it may be advantageous
to form the varistor body as a coating on the upper surface of the
support. The varistor body and dielectric support can be bonded
together to form a unitary substrate by conventional bonding
techniques.
In FIG. 4 a varistor 40 is shown provided with a varistor body 41,
which may be identical to 21, having a first major surface 42 and a
second, opposed major surface 43. A first electrode 44 is ohmically
conductively associated with a portion of the first major surface.
A second electrode 45 is provided with a portion 45A ohmically
conductively associated with the first major surface and laterally
spaced from the first electrode by conduction gap width Y. A
remaining portion 45B of the second electrode is associated with
the second major surface, and an intermediate portion 45C ohmically
conductively connects the portions 45A and 45B of the second
electrode. It is to be noted that the first and second major
surfaces of the varistor body and, hence, the first electrode and
the portion 45B of the second electrode are separated by a
thickness X2, which exceeds the conduction gap width Y.
When the varistor 40 is called upon to conduct low current levels,
its operation is identical to that of varistor 20. That is, current
is conducted almost exclusively across conduction gap Y, and a
relative stable low level voltage range (compared to that
obtainable using a resistor) is maintained across the electrodes.
Should, however, the voltage level continue to rise across the
first and second electrodes, as might occur in the case of a high
power surge requiring current conduction beyond the capacity of the
conduction gap at the first major surface, the voltage across the
electrodes can be stabilized again at a somewhat higher voltage
level determined by the spacing X2 between the first electrode and
the portion 45B of the second electrode. This will become more
apparent when it is recognized that the conduction gap width Y,
though lower in value than the thickness X2, relies for current
conduction upon a relatively restricted area of the varistor body
lying adjacent or immediately below the surface of the conduction
gap, and for this reason its current conducting capabilities are
limited. By contrast the somewhat more widely spaced first
electrode and portion 45B of the second electrode are capable of
conducting current therebetween through the bulk of the varistor
body over a relatively extended area. In this instance it can be
seen that the varistor 40 combines the very low voltage
characteristics of the varistor 20 while also incorporating as an
added feature the larger power handling capability of a
conventional varistor, such as shown in FIG. 1, which also offers a
second range of voltage stabilization.
Each of the varistors 20, 30, and 40 can be placed in an electrical
circuit to provide a shunt path around a high voltage degradable
circuit unit, as is illustrated in FIG. 5. The varistor is
connected in the circuit to selectively shunt current around the
degradable unit in proportion to the voltage across the terminals
50 and 51. The current through the varistor rises exponentially
with any increase in voltage and hence serves to stabilize the
voltage across the terminals.
In FIG. 6 a varistor 60 is illustrated. The varistor includes a
varistor body 61 having a first major surface 62 and a second major
surface 63 opposed thereto. Associated with the first major surface
are first, second, and third electrodes 64, 65, and 66,
respectively. The electrodes are each laterally spaced with the
second electrode being interposed between the first and third
electrodes. The first and second electrodes are separated by a
conduction gap width Y1 and the second and third electrodes are
separated by a conduction gap width Y2, which exceeds conduction
gap width Y1 in value. The varistor 60 possesses all the advantages
of the varistor 20 plus the added advantage that the first and
third electrodes can be simultaneously and independently referenced
to the second electrode. Further, by controlling the placement of
the second electrode 62 with respect to the first and third
electrodes, the resistance to current flow between the first and
second electrodes can be related to the resistance to current flow
between the second and third electrodes to provide any desired
ratio of these resistances. For certain applications the gap width
Y1 and Y2 may be equal in value.
In FIG. 7 a varistor 70 is illustrated which is provided with a
varistor body 71 that may be identical to varistor body 41.
Adjacent first major surface 72 first and second electrodes 74 and
75 are located separated by conduction gap width Y. A third
electrode 76 is associated with the second major surface 73. The
third electrode is separated from the first and second electrodes
by a thickness X2 of the varistor body. The thickness X2 exceeds
the gap width Y. Both the first and second electrodes can be
referenced to the third electrode while at the same time being
referenced at a lower voltage range with respect to each other.
A specific application for the varistor 70 is shown in FIG. 8.
Circuit terminals 80 and 81 are shown. These terminals may be
connected to a series related electrical load and power source. The
anode terminal 82 and the cathode terminal 83 of an SCR 84 are
shown connected to the terminals 80 and 81, respectively. Gate
terminal 85 of the SCR is connected to the cathode of a diode 86
and the anode of the diode is connected to other conventional
trigger circuitry 87 which is in turn electrically connected to the
terminals 80 and 82. The first electrode 74 of the varistor is
connected to the gate terminal 85. The second electrode 75 of the
varistor is connected to the SCR anode terminal 82, and the third
electrode 76 of the varistor is connected to the cathode terminal
83 of the SCR.
It can be seen that in circuit operation the varistor 70 acts as a
shunt across the SCR 84. Should a voltage surge develop across the
SCR it would be shunted through the varistor body between the
second and third electrodes 75 and 76. At the same time the
varistor 70 is also capable of shunting a lower voltage that might
develop across the diode 86 and coventional trigger circuitry 87.
This could occur, for example, if a reverse voltage were applied to
the SCR well within its voltage blocking capability, but
approaching the voltage blocking capability of the diode 86. In
this instance the diode is protected by the varistor's voltage
clamping ability through conduction gap width Y between the first
and second electrodes. It is to be further noted that the portion
of the varistor having the highest power handling capability is
used to protect the power handling portion of the circuit, namely
the SCR, while the portion of the varistor having a lower power
handling capability, the first major surface associated conduction
gap, protects the signal portion of the circuit. It is to be still
further noted that excessive gate voltages are prevented by the
varistor, since in this instance conduction can occur through the
varistor body between electrodes 74 and 76. It is recognized that
the varistor 60 could be substituted for the varistor 70 in the
circuit shown iwth first electrode 62 being connected to gate
terminal 85, second electrode 65 connected to anode terminal 82,
and third electrode 66 connected to cathode terminal 83.
While I recognize that the limited current carrying capacity of my
varistors may be a disadvantage in certain applications requiring
substantial power handling capabilities, their current handling
capabilities may be enhanced by increasing the distance traversed
by the conduction gap over the major surface so that it exceeds the
maximum dimension of the major surface. In other words, the
conduction gap need not extend linearly across the major surface as
described for simplicity in the foregoing embodiments.
A simple approach for increasing the distance traversed by the
conduction gap on a major surface of a varistor according to my
invention is best appreciated by reference to FIG. 9. In this
figure is shown a varistor 90 having a circular first eleectrode 91
and an annular second electrode 92 which is concentric with the
circular electrode and which is uniformly spaced from the circular
electrode by a conduction gap width Y. It may be readily observed
that the distance traversed by the conduction gap exceeds the outer
diameter of the annular electrode 92. In this way the current
carrying area is increased over what would be present if two
semicircular electrodes were employed in association with the same
underlying varistor body.
In FIG. 10 an approach for further increasing the area available
for current conduction is illustrated. A varistor 100 is provided
with a central first electrode 101 having a plurality of regularly
spaced fingers 102 extending radially outwardly. An outer electrode
103 is provided with a plurality of radially inwardly spaced
fingers 105 interdigitated with the fingers 103. In this
arrangement a variable spacing between the inner and outer
electrodes is required if an equal amount of current is to be
conducted throughout the conduction gap, as the different
curvatures presented by the different portions of the fingers will
produce differing electrical fields if a uniform spacing is
employed. Where unequal stresses can be tolerated on the fingers,
it may be most convenient to provide a uniform spacing between the
fingers or an approximately uniform spacing.
In FIG. 11 a varistor 110 is illustrated which is provided with a
first electrode 111 and a second electrode 112 associated in
laterally spaced relation to an underlying varistor body. The
electrodes are formed so that they are laterally separated by a
minimum conduction gap width Y3 and progressively diverge to a
maximum conduction gap width Y4. The effect of varying the
conduction gap width in this manner is to cause the varistor to
present a somewhat lower alpha than should be present based upon
the characteristics of the varistor body, per se. This approach is
particularly useful in using metal oxide varistors incorporating
varistor bodies having an alpha in excess of 10 in the current
density range of from 10.sup.-.sup.3 to 10.sup.2 amperes per square
centimeter to replace previously utilized varistors, such as
selenium and silicon carbide varistors having alphas appreciably
below 10.
In FIG. 12 a varistor 120 is illustrated having a first electrode
121 and a second electrode 122. The two electrodes are laterally
spaced on a varistor body in two discrete stepped increments. The
left hand portion of each electrode is laterally spaced by a
conduction gap width Y5 which is less than the conduction gap width
Y6 of the right hand portion of each electrode. It has been
observed that with a constant direct current bias placed across the
electrodes of a metal oxide varistor a gradual increase in the
voltage level across the electrodes can occur as an aging function,
particularly where the device is biased at near its power handling
capacity. In the varistor 120 the voltage between the electrodes
will initially be determined by the conduction gap width Y5. As the
varistor ages in use it is possible that the voltage across the gap
width Y5 may approach the voltage level in which the right hand
portion of the device becomes active. In this way an aging device
is protected against runaway voltages developing for a period of
time permitting replacement before uncontrolled voltage increase
occurs.
As described above the varistors formed according to my teachings
are free of any protective packaging or external lead connections.
In the form shown the varistors may be utilized in protected
environments without additional packaging. For example, the
varistors could be incorporated in a hermetically sealed housing
alone or in combination with other electrical components. For most
applications it will be desirable to attach terminal leads to the
electrodes and to encapsulate the varistors in a dielectric
material to assure protection from environmentally encountered
substances altering their electrical characteristics.
To illustrate the packaging of a varistor formed according to my
invention, the varistor 30 shown in FIG. 3 is illustrated as the
packaged varistor 130 shown in FIG. 13. Elements of the varistor
130 corresponding to those of the varistor 30 are assigned like
reference characters and are not redescribed. Terminal leads 134
and 135 are soldered or otherwise suitably attached in low
impedance relation to the electrodes 34 and 35, respectively. A
substantially impervious dielectric body 136, preferably formed of
a dielectric glass of a type conventionally employed in the
passivation and/or packaging of semiconductor crystals, is shown
overlying the conduction gap and the adjacent edges of the
electrodes. Inasmuch as the conduction characteristics of the
varistor are most appreciably influenced by the conduction
properties present at or near its surface along the conduction gap,
this is the area to which maximum protection should be given. Note
should be taken of the fact that this relationship is directly in
contrast to that for the varistor 1 in which conduction occurs
through the bulk of the varistor body.
It is anticipated that for many applications the only protective
packaging needed or desired for the varistor will be the dielectric
body covering the conduction gap. For more general applications,
however, it is normally desirable that an additional dielectric
covering 137, which may take the form of any conventional plastic
or glass semiconductor packaging composition, be used to cover the
remaining exterior surfaces of the varistor body and, optionally,
its electrodes. It is further anticipated that the packaging
dielectric 137 may be used alone with the dielectric body 136 being
omitted. As shown, the dielectric package cooperates with the
dielectric substrate 36 to completely cover the exterior surfaces
of the varistor body. Where the varistor is of a form lacking a
dielectric substrate, it is appreciated that the package dielectric
137 may also completely envelop the varistor body and, optionally,
its attached electrodes.
While I have described my invention with reference to certain
preferred embodiments, it is appreciated that numerous variations
in form will readily occur to those skilled in the art. For
example, while I have disclosed the varistor bodies to be of
limited and regular lateral extent, it is appreciated that the
lateral extent of the varistor body beyond the conduction gap width
is not critical to its current conduction capabilities. For this
reason I contemplate that varistors according to my invention may
be formed with the varistor body extending laterally well beyond
(or short of) the electrode outer edges, if desired, and utilizing
lateral outline geometries of any convenient regular or irregular
configuration. In the forms of my varistors with electrodes
attached to only one major surface it is appreciated that it is
unnecessary to provide a second major surface of any regular
geometrical form or to have the second major surface parallel to
the first major surface. While in most instances it will be most
convenient to form the major surfaces so that they lie in a single
plane, it is anticipated that the major surfaces may, if desired,
be curved or bent, so that they lie in more than one plane. While I
have shown a varistor for purposes of illustration to be packaged
as a lead mounted device, it is anticipated that the varistors
formed according to my invention may be attached to electrical
terminals of various configurations. It is specifically
contemplated that the terminals attached may also serve to draw
heat from the varistor body, as is well understood in the
fabrication of electronic components. In this regard it is noted
that the varistor body is itself a fairly good thermal conductor
and will dissipate heat readily from the area immediately
underlying the conduction gap. While a specific form of electrode
interdigitation has been shown for purposes of illustration, it is
appreciated that electrode interdigitation is per se well known in
the electronic components arts and that many alternate forms of
electrode interdigitation could be easily substituted.
Still other variations are contemplated. It is accordingly intended
that the scope of my invention be determined by reference to the
following claims.
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