U.S. patent number 3,711,794 [Application Number 05/191,216] was granted by the patent office on 1973-01-16 for surge suppression transmission means.
This patent grant is currently assigned to General Electric Company. Invention is credited to John D. Harnden, Jr., Francois D. Martzloff, Dante M. Tasca.
United States Patent |
3,711,794 |
Tasca , et al. |
January 16, 1973 |
SURGE SUPPRESSION TRANSMISSION MEANS
Abstract
In a coaxial connector a generally toroidal shaped member of
metal oxide varistor material is connected between the inner and
outer conductors of the connector. The metal oxide varistor
material has 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.
The spacing of the peripheral portions of the member is set so that
a high impedance is presented to normal applied voltage between the
peripheral portions. For voltages applied between the peripheral
portions progressively in excess of the normal voltage rapidly
decreasing impedance is presented by the toroidal member in
accordance with the alpha of the material thereby limiting the
variation in voltage between the peripheral portions of the
toroidal shaped member.
Inventors: |
Tasca; Dante M. (Philadelphia,
PA), Harnden, Jr.; John D. (Schenectady, NY), Martzloff;
Francois D. (Schenectady, NY) |
Assignee: |
General Electric Company
(N/A)
|
Family
ID: |
22704582 |
Appl.
No.: |
05/191,216 |
Filed: |
October 21, 1971 |
Current U.S.
Class: |
333/243; 333/81A;
338/21; 361/56; 439/181; 333/13; 333/260; 338/216; 361/111;
361/91.1 |
Current CPC
Class: |
H01C
7/12 (20130101); H02H 9/044 (20130101); H01R
24/48 (20130101); H03G 11/006 (20130101); H01R
24/44 (20130101) |
Current International
Class: |
H01C
7/12 (20060101); H01R 13/00 (20060101); H02H
9/04 (20060101); H03G 11/00 (20060101); H01R
13/646 (20060101); H01p 001/00 (); H01p 001/22 ();
H01c 007/12 () |
Field of
Search: |
;333/97R,81AB,17,13,24.2,2,96,97S ;338/20-21,216,220 ;317/61.5
;339/147 ;329/161-162 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
Gunn, M. W., "Wave Propagation in Rectangular Waveguide Containing
a Semiconducting Film" Proc. IEE. Vol. 114 2-1967, pp.
207-210..
|
Primary Examiner: Lieberman; Eli
Assistant Examiner: Punter; Wm. H.
Claims
What we claim as new and desire to secure by Letters Patent of the
United States is:
1. A section of transmission line comprising:
a generally cylindrical outer conductor,
an inner conductor within and in spaced relationship to said outer
conductor,
a member having a peripheral outer portion contacting said outer
conductor and a peripheral inner portion contacting said inner
conductor,
said member being constituted of a metal oxide varistor material
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, the
spacing of said peripheral portions being set so that a high
impedance is presented between said peripheral portions when normal
voltages appear between said peripheral portions and when voltages
in excess of the normal voltage progressively appear thereacross a
rapidly decreasing impedance is presented by said member in
accordance with the alpha of the material of said member thereby
limiting the variation in voltage between the peripheral
portions.
2. The combination of claim 1 in which said outer and inner
peripheral portions are cylindrical.
3. The combination of claim 1 in which said member is generally
toroidal shaped.
4. The combination of claim 1 in which said outer conductor is
concentric with said inner conductor.
5. The combination of claim 1 in which said inner conductor is
elongated.
6. The combination of claim 1 in which said member supports said
elongated inner conductor in spaced relation to said outer
conductor.
7. The combination of claim 1 in which said member is provided with
a side surface which is substantially planar, an inner conductive
ring and an outer conductive ring concentrically located on said
side surface, said inner conductive ring in conductive contact with
said inner conductor, said outer conductive ring in conductive
contact with said outer conductor, the adjacent edges of said ring
being uniformly spaced in said side surface.
8. The combination of claim 1 in which said member is provided with
a side surface which is substantially planar, an inner conductive
ring and an outer conductive ring concentrically located on said
side surface, said inner conductive ring in conductive contact with
said conductor, said outer conductive ring in conductive contact
with said outer conductor, said inner conductive ring having a
plurality of projections, each extending radially outward and
terminating in a straight edge, said outer conductive ring having a
plurality of straight edges located in the inner edge thereof, each
of said straight edges of said outer conductive ring parallel to
and equally spaced from a respective straight edge of said inner
conductive ring.
9. The combination of claim 8 in which said projections are four in
number equally spaced about the periphery of said inner conductive
ring.
10. A section of transmission line comprising:
a generally cylindrical outer conductor,
a pair of inner conductors within and in spaced relationship to
said outer conductor,
a pair of toroidal shaped members, each having a peripheral outer
portion contacting said outer conductor and each having a
peripheral inner portion contacting a respective one of said inner
conductors,
said members being constituted of a metal oxide varistor material
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, the
spacing of said peripheral portions of each of said members being
set so that a high impedance is presented between the peripheral
portions of each of said members when normal voltages appear
between the peripheral portions and when voltages in excess of
normal voltages progressively appear thereacross progressively
decreasing impedance is presented by said member in accordance with
the alpha of the material of said member thereby limiting the
variation in voltage between the peripheral portions.
11. A section of transmission line comprising:
a generally cylindrical outer conductor,
an inner conductor within and in spaced relationship to said outer
conductor,
a support member of insulating material having a cylindrical outer
portion and a cylindrical inner portion and a pair of major opposed
faces, a layer of metal oxide varistor material bonded to one of
said faces, an outer peripheral portion of said layer conductively
connected to said outer conductor and an inner peripheral portion
of said layer conductively connected to said inner conductor,
said layer being constituted of a metal oxide varistor material
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, the
spacing of said peripheral portions being set so that a high
impedance is presented between said peripheral portions when normal
voltages appear between said peripheral portions and when voltages
in excess of normal voltage progressively appear thereacross, a
rapidly decreasing impedance is presented by said layer in
accordance with the alpha of the material thereof thereby limiting
the variation in voltage between the peripheral portions.
Description
The present invention relates to lines for the transmission of
electrical signals from one point to another point in a system and
in particular to parts of such lines referred to a connectors.
Connectors are commonly used for interconnecting electrical devices
or apparatus, for example, for interconnecting a source of
electrical signal to a utilization device, such as a metering or
visualization device. The utilization device may include elements
which are sensitive to voltages or surges of voltage exceeding a
predetermined limit and susceptible to damage thereby. It is common
to provide in such systems, preferably in or adjacent to the
connector, filter elements for limiting device. Such filters are
commonly referred to as bulkhead filters and usually include
discrete elements which are coupled to the conductors of a
transmission line to provide the necessary attenuation of low and
high frequency voltage surges or spurious signals. Such prior art
means of surge suppression have a number of disadvantages.
Substantial capacitance and inductance are introduced into the
transmission line system which affects the performance of the
system. The capacitance and inductance deteriorates the high
frequency performance. Expressed in other words, substantial
insertion loss or attenuation is introduced into the transmission
line system by the discrete elements coupled into the system.
Accordingly, an object of the present invention is to provide a
transmission line element which provides surge suppression without
introduction of significant series inductance or shunt capacitance
or signal attenuation into the transmission path of the
element.
Another object of the present invention is to provide a
transmission line connector which does not require any additional
elements, yet which provides surge suppression in addition to its
usual electrical coupling function.
Another object of the present invention is to provide a surge
suppression connector which is nonresonant in operation and
functions by dissipation and not by storage of surges of electrical
energy.
Another object of the present invention is to provide a surge
suppression connector which provides a high shunting impedance for
voltages below a certain value and for progressively higher
voltages rapidly progressive lower impedances.
Another object of the present invention is to provide a connector
which introduces a minimum of loading of a transmission system over
a broad band of frequencies yet which dissipates the energy of
unwanted signals exceeding a predetermined amplitude appearing in
the system.
Another object of the present invention is to provide a surge
protection connector which has substantially negligible time delay
in the operation thereof in the suppression of surges.
A further object of the present invention is to provide a simple
surge protection connector with capabilities of absorbing surges of
considerable energy.
In carrying out the invention in one illustrative embodiment
thereof, there is provided a generally cylindrical outer conductor
and an inner conductor within and in spaced relationship to the
outer conductor. A member is provided having a peripheral outer
portion contacting the outer conductor and a peripheral inner
portion contacting the inner conductor. The member is constituted
of a metal oxide varistor material 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. The spacing of the peripheral
portions of the member is set so that a high impedance is presented
to normal applied voltage between the peripheral portions.
Accordingly, for voltages applied between the peripheral portions
progressively in excess of the normal voltage applied thereto
rapidly decreasing impedance is presented by the toroidal member in
accordance with the alpha of the material thereby limiting the
variation in voltage between the peripheral portions of the
toroidal shaped member.
The features of the invention which are believed to be novel are
set forth with particularity in the appended claims. The invention
itself, however, both as to its organization and method of
operation together with further objects and advantages thereof may
best be understood by reference to the following description taken
in connection with the accompanying drawings in which:
FIG. 1 is a block diagram of a signal transmission system useful in
explaining the manner in which the invention may be used.
FIG. 2 is a sectional view of the connector of the transmission
system of FIG. 1 incorporating an embodiment of the present
invention.
FIG. 3 is a graph of applied voltage versus current of a specific
active surge suppression element of the connector of FIG. 2 plotted
on log-log coordinates.
FIG. 4 shows a front view partially in section of another
embodiment of the present invention.
FIG. 5 is an end view of the embodiment of FIG. 4.
FIG. 6 shows a front view partially in section of another
embodiment of the present invention.
FIG. 7 is an end view of the embodiment of FIG. 6.
FIG. 8 is a graph in log-log coordinates of the voltage versus
current characteristic of a specific surge suppression element of
the connector FIGS. 6 and 7.
FIG. 9 shows a series of graphs in log-log coordinates of the
connectors of FIG. 2 and FIG. 6 showing the impedance versus
frequency characteristics thereof as well as the impedance versus
frequency characteristics of a conventional connector not
incorporating the surge suppression element of the present
invention.
FIG. 10 shows a front view partially in section of a further
embodiment of the present invention.
FIG. 11 is a sectional view of a still further embodiment of the
present invention incorporating several inner conductors.
FIG. 12 is an end view of the embodiment of FIG. 11.
FIG. 13 shows graphs of the electrical characteristics of three
metal oxide varistor materials suitable for use in the connector
devices of the present invention.
Reference is now made to FIG. 1, which shows a transmission system
10 including a source 11 of electrical signal, a utilization device
or load 12 and a transmission line 13 with a connector 14 connected
between the source and the load to supply electrical signal
thereto. The source 11, for example, may represent an antenna which
picks up a high frequency electrical signal. The transmission line
13 may be a coaxial transmission line having an outer conductor and
a concentrically located inner conductor. The connector 14 is a two
element device, one element 15 of which is connected to the
terminal end of the transmission line 13 and the other element 16
of which is connected to the input terminals of the load 13. The
connector assembly 14 includes the male member 15 and the female
member 16 which are separable one from the other to facilitate
making and breaking connections in the system. The load 12 may be a
circuit including voltage surge sensitive transistor elements. The
system 10 may be situated in the field in the presence of spurious
signals of large voltage amplitude in excess of the voltage
amplitude in which the voltage sensitive devices may be safely
operated. In such environment, the source 11 may pick up such
voltage surges and pass them on through to the load 12 with
resultant damage or destruction of the voltage sensitive elements
thereof. Accordingly, it is highly desirable to provide somewhere
along the transmission path protective means for filtering out or
dissipating the surges of electrical energy that are passed on to
the load.
In accordance with one embodiment of the present invention, there
is shown in FIG. 2, a connector element 20 corresponding to the
connector element 16 of FIG. 1. The connector element 20 includes a
generally cylindrical outer conductor 21 or shell which is threaded
at one end 22 to engage mating threads in the chassis of the
utilization apparatus 12. The other end of the outer conductor 21
is provided with a pair of projections 23 to engaged latches on a
mating member corresponding to element 15 of FIG. 1 and is entirely
conventional. Also, provided is an elongated inner conductor 25
located within and having a longitudinal axis which is concentric
with the axis of the outer cylindrical conductor 21. One end of the
elongated inner conductor includes a plurality of fingers 26
defining an opening into which a corresponding inner conductor of a
mating connector member (not shown) may be inserted. The other end
27 of the elongated conductor 25 is connected to a terminal of the
utilization apparatus 12. The inner conductor 25 is spaced and
positioned within the outer conductor 21 by means of an insulating
insert 28 which, for example, may be made of a material such as
teflon. The insert 28 is conventional and functions to maintain the
inner conductor 25 concentrically located with respect to the outer
conductor 21 determines the characteristic impedance of the
connector. On the shoulder 29 of the inner conductor 25 is inserted
a toroidal shaped member 30 having a peripheral outer portion 31
and a peripheral inner portion 32 and also a pair of opposed sides
34 and 34. Preferably, the peripheral outer portion 31 is provided
with a conductive coating 35 bonded to the member 30 and similarly
the inner peripheral portion is provided with a conductive coating
36 bonded thereto. The toroidal shaped member 30 is inserted in the
connector such that the inner peripheral cylindrical surface
thereof conductively engages the outer cylindrical surface of the
shoulder 29 and the outer cylindrical surface of the toroidal
shaped member 30 conductively contacts the inner cylindrical
surface of the outer conductor 21. The toroidal member 30 is held
in axial position by means of a circular shoulder 40 formed in the
inner portion of the outer conductor 21 which engages planar side
34 of the toroidal member and by means of the teflon insert 28
which engages the opposite planar side 33 of the toroidal member.
The toroidal member 30 is constituted of a metal oxide varistor
material having an alpha (.alpha.) in excess of 10 in the current
density range of from 10.sup..sup.-3 to 10.sup.2 amperes per square
centimeter. The spacing of the conductive peripheral portions 35
and 36 of the toroidal member is set so that a high impedance is
presented to a normal applied voltage between the conductive
peripheral portions whereby when voltages are applied between the
peripheral portions progressively in excess of a predetermined
normal voltage, progressively and rapidly decreasing impedance is
presented by the toroidal member 30 in accordance with the alpha
(.alpha.) of the material thereof thereby limiting the variation
voltage between the conductive peripheral portions 35 and 36 and
hence, between the inner and outer conductors of the connector
20.
The toroidal shaped member 30 is constituted of a metal oxide
varistor material such as described in Canadian patent 831,691,
which has a non-linear current versus voltage characteristic. The
material described in the aforementioned patent is constituted of
fine particles of zinc oxide with certain additives which have been
pressed and sintered at high temperatures to provide a composite
body of material. The current versus voltage characteristics of the
composite body is expressed by the following equation:
I = (V/C).sup..alpha. (1)
where
V is voltage applied across a pair of opposed surfaces or
planes,
I is the current which flows between the surfaces,
C is a constant which is a function of the physical dimensions of
the body as well as its composition and the process used in making
it,
.alpha. is a constant for a given range of current and is a measure
of the non-linearity of the current versus voltage characteristic
of the body.
In equation (1), when V is used to denote voltage between opposed
planes of a unit volume of material, or voltage gradient, current
flow through the unit volume of material in response to the voltage
gradient becomes current density. For the metal oxide varistor
material for current densities which are very low, for example, in
the vicinity of a microampere per square centimeter, the alpha
(.alpha.) is relatively low, i.e., less than 10. In the current
density range of from 10.sup..sup.-3 to 10.sup.2 amperes per square
centimeter, the alpha is high, i.e., substantially greater than 10
and relatively constant. In the current density range progressively
in excess of 10.sup.2 amperes per square centimeter, the alpha
progressively decreases. When the current versus voltage
characteristic is plotted on log-log coordinates, the alpha is
represented by the reciprocal of the slope of the graph in which
current density is represented by the abscissa and voltage gradient
is represented by the ordinate of the graph. For a central range of
current densities of from 10.sup..sup.-3 to 10.sup.2 amperes per
square centimeter, the reciprocal of the slope is relatively
constant. For current densities below this range, the reciprocal of
the slope of the graph progressively decreases. Also, for current
densities above this range, the reciprocal of the slope of the
graph progressively decreases.
The voltage gradient versus current density characteristics of
three types of material in log-log coordinates are set forth in
FIG. 13. Graphs 105 and 106 are materials of high voltage gradient
material and graph 107 is a graph of low voltage gradient material.
For all of the graphs in the current density range from
10.sup..sup.-3 to 10.sup.2 amperes per square centimeter, the alpha
is high and is substantially greater tan 10 and relatively
constant. For densities progressively greater than 10.sup.2 amperes
per square centimeter, the alpha progressively decreases. For
current densities progressively less than 10.sup..sup.-3, the alpha
also progressively decreases.
As the metal oxide varistor material is a ceramic material, the
surfaces thereof may be metallized for facilitating electrical
connections thereto in a manner similar to the manner in which
other ceramic materials are metallized. For example, Silver Glass
Frit, Du Pont No. 7713, made by the Du Pont Chemical Company of
Wilmington, Delaware, may be used. Such material is applied as a
slurry in a silk screening operation and fired at about
550.degree.C to provide a conductive coating on the surface. Other
methods such as electroplating or metal spraying could be used as
well.
The non-linear characteristics of the material results from bulk
phenomenon and is bi-directional. The response of the material to
steep voltage wave fronts is very rapid. Accordingly, the voltage
limiting effect of the material is practically instantaneous. Heat
generation occurs throughout the body of material and does not
occur in specific regions thereof as in semiconductor junction
devices, for example. Accordingly, the material has good heat
absorption capability as the conversion of electrical to thermal
energy occurs throughout the material. The specific heat of the
material is 0.12 calories per degree Centigrade per gram.
Accordingly, on this account, as well, heat absorption capability
of the material is advantageous as a surge absorption material. The
heat conductivity of the material is about one-half the heat
conductivity of alumina. Accordingly, any heat generated in the
material may be rapidly conducted from the material into
appropriate attached heat sinks.
The material, in addition to the desired electrical and thermal
characteristics described above, has highly desirable mechanical
properties. The material has a fine grain structure, may be readily
machined to a smooth surface and formed into any desired shape
having excellent compressive strength. The material is readily
molded in the process of making it. Accordingly, any size or shape
of material may be readily formed for the purposes desired.
Accordingly, the toroidal member 30 is formed to the desired
dimensions by either molding or by machining from a larger body of
material and electrodes or conductive coatings applied as described
above. The electrodes are of a size and are spaced apart so as to
provide the desired characteristic of voltage versus current. A
connector of the design or configuration shown in FIG. 2 for use
with a line having a characteristic impedance of 50 ohms in which
the toroidal member 30 was provided with an outer diameter (surface
31) of 7 millimeters, an inner diameter (surface 32) of 2.5
millimeters and a distance between opposed sides 33 and 34 of 2.5
millimeters was connected in a system such as shown in FIG. 1 with
a load impedance of 50 ohms. The voltage versus current
characteristic shown in graph 41 of FIG. 3 was obtained in which
voltage between inner and outer portions of the toroidal member 30
is plotted along the ordinate of the graph and current flow between
these portions is plotted along the abscissa. In the graph of FIG.
3, for a signal having an amplitude of 70 volts, a current of one
microampere flows. At this value of current the effective alpha of
the connector is substantially less than 10 and as a matter of
fact, is approximately 4. As signals of greater amplitude are
applied, the current increases and correspondingly, the alpha of
the material in that current range increases. When a current of
10.sup..sup.-3 amperes flows as a result of a voltage of
approximately 120 volts being applied between the electrodes on the
peripheral portions of the toroidal member, the alpha of the
material is approximately 15. A continuous signal of such magnitude
would generate 0.12 of a watt in the toroidal member. Such power
would be readily conducted through the material to the shell or
outer conductor 21 and dissipated. The higher voltage signals
appearing would be substantially limited to less than 200 volts as
the graph of the characteristic extends toward higher currents with
only a slight rise in voltage. The important consideration to note
in graph 41 is that for signals of 70 volts or less, dissipation in
the connector is less than 70 microwatts which represents low
standby power dissipation. If however, surges should occur of a
value of 150 volts, dissipation would rise to 1.5 watts, provided
of course, the surges had low driving point impedance. Such power
would be dissipated in the toroidal member and conducted to the
outer conductor 21 where it would be dissipated. Unless surges were
of very high power, occurred over a long interval of time and were
obtained from a source of negligible impedance, any surge even in
excess of approximately 200 volts would be essentially limited to
the 200 volt range of the characteristic of the toroidal
member.
Referring now to FIG. 9, there are shown graphs in log-log
coordinates of the impedance as a function of frequency for various
connectors. Graph 45 represents the impedance of a conventional
connector such as shown in FIG. 2 without the surge suppression
member in the absence of surge suppression members such as member
30, as a function of frequency. Graph 46 shows the impedance
characteristic of connector 20 of FIG. 2 with the surge suppression
member having dimensions and characteristics set forth above. As
metal oxide varistor material has a relatively high dielectric
constant (approximately 1800), the toroidal member 30 increases the
capacitance at the point of its insertion in the connector 20.
Accordingly, the graph 46 is shifted to the left and is essentially
parallel with the graph 45. That is, the impedance of the connector
20 is lower in view of the fact that the capacitance thereof is
higher. A certain amount of series inductance is also introduced
into the connector with the inclusion of the toroidal member 30.
Accordingly, at higher frequencies, resonance effects predominate
and in the particular example of connector 20 described above, the
resonance peak occurs near 100 Megahertz. The graph 46 instead of
being a straight line, has a gradually increasing negative slope
which reaches its maximum value at approximately 100 Megahertz.
When the connector 20 is connected to a 50 ohm transmission line
for which it was designed, and is terminated with a 50 ohm
impedance, the graph 47 is obtained. Graph 47 essentially
represents the paralleling of the 50 ohm terminal impedance with
the impedance of the connector 20 as represented in graph 46.
The embodiment shown in FIG. 4 is directed to improve the impedance
versus frequency response of the connector of FIG. 2 and still
provide the desired surge protection. To this end, the shunting
capacitance provided by the high dielectric constant of the metal
oxide varistor material is reduced. The connector of FIG. 4 is
essentially the same as the connector of FIG. 2, with the exception
that a toroidal member 51 thereof is provided with electrodes on a
side thereof, thereby substantially reducing the capacitance
appearing in the shunt between the inner and outer conductor of the
connector 50 in the vicinity of the toroidal member 51. The
connector 50 includes an outer shell member 52 and an inner
conductor 53 member. A toroidal member 51 of metal oxide varistor
material is provided on one side 55 thereof with an inner
conductive ring 56 and an outer conductive ring 57. The outer
conductor member 52 is provided with a circular ridge 58. Inner
conductor is also provided with a cylindrical ridge 59 or shoulder.
The toroidal member 51 is restrained in movement in one direction
along the axis of the connector by the ridges 58 and 59. It is
restrained in movement in the other direction along the axis of the
connector by the insulating member 60 which also supports the inner
conductor 53 in relation to the outer conductor 52. The outer ring
57 abutts against the shoulder 58 and inner ring 56 abutts against
shoulder 59 on the inner conductor. In the connector of FIGS. 4 and
5 as a minimum of cross-section area is presented by the adjacent
edges of the concentric ring conductors 56 and 57 and as the
current flow occurs between such edges near the surface of the
metal oxide varistor body 51, the capacitance at that planar region
in the connector is substantially reduced, thereby substantially
improving the impedance versus frequency characteristic of the
connector while at the same time providing the desired protection
against voltage surges from appearing at the output terminals of
the connector.
Reference is now made to FIGS. 6 and 7 which show another
embodiment of a connector 65 in accordance with the present
invention for substantially reducing the capacitance introduced
into the connector by the metal oxide varistor protective body
thereof. In the connector 50 of FIGS. 4 and 5, unless the spacing
between the adjacent edges of the ring conductors 56 and 57 is
maintained to close tolerances, current conduction between the ring
conductors 56 and 57 is non-uniform with resultant inefficient use
being made of metal oxide varistor material. That is, certain
sectors of the space between the conductors will conduct
substantially more current than other sectors. This will be readily
understood by considering the voltage versus current
characteristics of the material. Let it be assumed that for two
sectors of the metal oxide varistor material between the ring
conductors of FIGS. 4 and 5, the spacing between the inner and
outer concentric ring is different. For one sector, a voltage
versus current graph such as shown in FIG. 3 would exist. For the
other sector for which the spacing is less, a voltage versus
current graph substantially identical to the voltage versus current
graph of the first sector would exist, however, it would be
displaced downward along the ordinate of the coordinate system from
the graph of the first sector. Accordingly, for a given voltage
appearing between the inner and outer ring conductors 56 and 57,
the sector with the smaller spacing would carry substantially
greater currents in a region where the graphs are relatively flat
as a line drawn through the given voltage and parallel to the
abscissa would be nearly parallel to graphs. To maintain uniform
current distribution while at the same time maintain reduced
capacitance, the inner and outer ring conductors have the form
shown in FIGS. 6 and 7 to which reference is now particularly made.
Elements of FIGS. 6 and 7, identical to elements of FIGS. 4 and 5
are identically designated. The inner ring conductor 66 is provided
with four projections 67, 68, 69 and 70, equally spaced about the
ring conductor. The edges of each of the projections are of equal
length, equally spaded from the central axis of the connector and
terminated in straight edges. The inner edge of outer is conductor
75 provided with four straight edges 76, 77, 78 and 79, each
parallel to a straight edge of a respective projection and equally
spaced from the central axis of the connector. Accordingly, a ring
conductor arrangement is provided in which over each of four
quadrants or sectors of the metal oxide varistor body, the
identical spacing occurs between inner and outer ring conductors 66
and 75.
A connector assembly such as shown in FIGS. 6 and 7 utilizing metal
oxide varistor material wafer having a voltage gradient of 200
volts per millimeter at a current density of one milliampere per
square centimeter was constructed. The connector was provided with
terminal straight edges of one millimeter in length for the
projections from the inner conductive ring. A gap spacing of 1
millimeter between a straight edge of a projection and a
corresponding straight edge of an outer conductive ring was also
provided. The voltage versus current response of the connector is
shown in graph 80 of FIG. 8 to which reference is now made. In this
figure, voltage between inner conductor 53 and outer conductor 52
is plotted along the ordinate and current flow through the toroidal
member 54 is plotted, along the abscissa of log-log coordinates.
For signals of 100 volts or less, the current flow through the
toroidal member 54 is less than one-hundredth of a microampere. As
the voltage and energy of the signal increases, current increases,
and for a voltage of 400 volts across the toroidal member, a
current of 1 microamperes flows. For voltages across the toroidal
member in excess of 400 volts, the current increases rapidly for
slight increases of voltage. For a voltage of 600 across the
toroidal member one-hundredth of an ampere flows through the
member. The alpha of graph 80 in the range from 1 microampere to
0.01 amperes is of the order of 20. Accordingly, it is readily
apparent that for surges of signal even of considerable power and
energy content, the voltage appearing at the output of the
connector is essentially limited to a low value. In connection with
the cross-pattern design of the inner conductive rings of the
toroidal member, should the spacing of one of the four gaps between
the inner ring conductor 66 and outer ring conductor 75 be 10
percent less than the others, the metal oxide varistor material in
that quadrant would take 90 percent of the total current flow,
assuming, of course, that the region of the current characteristic
in which the material is operated is in the range in which the
alpha is of the order of 20. It should be noted that a discrepancy
such as 10 percent in the spacing in one of the gaps does not
materially effect the overall capacitance between the inner ring
conductor and the outer ring conductor. However, unless the gaps
are all spaced to close tolerances, the advantage of such an
arrangement in equalizing current flow is illusory. It should be
recognized that while four gaps have been shown, other electrode
configurations are readily apparent in which two gaps are used or
three or a greater number then four. However, when a larger number
of gaps are utilized, it should be noted that unless all of the
gaps are precisely dimensioned, equalization of current flow in the
metal oxide varistor member will not be achieved. For the larger
number of gaps, inner ring conductor 66 and outer ring conductor 75
must be formed to closer tolerances than for a smaller number of
gaps. With a large number of gaps, the problem of maintaining close
tolerances in the gaps to achieve uniform current becomes as
difficult as with the concentric ring conductors of FIGS. 4 and
5.
The impedance versus frequency characteristic for the specific
connector of FIGS. 6 and 7 described above is shown in graph 81 of
FIG. 9. As the capacitance of the connector of FIGS. 6 and 7 was
substantially reduced over the capacitance of the connector of
FIGS. 4 and 5, the essentially flat characteristic of impedance
versus frequency was extended out to close to 100 Megahertz.
Reference is now made to FIG. 10 which shows another embodiment of
the connector similar to the connectors of FIGS. 4 and 6. Elements
of FIGS. 6 and 7, identical to elements of FIG. 4 are identically
designated. In order to reduce the capacitance introduced by the
connector and improve the high frequency response of the connector,
a minimum of metal oxide varistor material is utilized. In this
embodiment, a toroidal ceramic substrate 85 is provided made of a
material such as alumina which has a low dielectric constant,
substantially lower than that of the metal oxide varistor material.
A toroidal shaped wafer 86 of metal oxide varistor material is
bonded on the toroidal substrate by a suitable bonding agent such
as epoxy. Concentric ring conductors are provided as shown in FIGS.
4 and 5. In other respects, the connector of FIG. 10 is identical
to the connector of FIG. 4. The cross pattern of the conductive
rings of FIGS. 6 and 7 could, of course, also be applied to the
wafer 86. As pointed out, the advantage of providing a toroidal
member in which the bulk of the toroidal member is a material of
low dielectric constant is that the capacitance is substantially
reduced, thereby further improving the high frequency response of
the connector, while at the same time, preserving the surge
protection characteristics of the device. With a reduced amount of
metal oxide varistor material being utilized in the toroidal
member, it should be noted that the dissipation capabilities of the
toroidal member of metal oxide varistor material are reduced. While
the composite toroidal member will reduce the voltage appearing at
the output of the connector, it will not handle as much energy as a
larger body of metal oxide varistor material.
Reference is now made to FIGS. 11 and 12 which show a connector 90
in which a plurality of inner conductors are provided. The
connector includes an outer shell 91 and a pair of elongated inner
conductors 92 and 93. A circular metal plate 94 is provided with a
pair of openings 95 and 96. Metal plate 94 is supported in the
outer shell. A pair of toroidal members 97 and 98 of metal oxide
varistor material are provided. Each of the toroidal members 97 and
98 has an outer metallized peripheral surface and an inner
metallized peripheral surface. The outer peripheral surfaces of
each toroidal members 97 and 98 are dimensioned to snugly fit into
a respective aperture in the plate 94. The inner surface of each of
the metal oxide varistor members 97 and 98 is dimensioned to
receive a respective one of the elongated inner conductors 92 and
93. The plate member 94 may be soldered in place in the connector
or may be appropriately positioned by means of the insulator
members 101 and 102. One insulating member 101 contacts one of the
opposed sides of the plate 94 and the other insulating member 102
contacts the other of the opposed sides of the plate. The material
of the plate is selected to have a coefficient of thermal expansion
similar to the coefficient of expansion of glass. Accordingly, many
of the materials utilized for making glass-to-metal seals and
having coefficients of expansion comparable to that of glass would
be suitable for use in the connector. For example, the plate 94 and
the conductors 92 and 93 may be made of molybdenum or of any of a
number of metal alloys of iron, nickel and cobalt which have a
coefficient of expansion equal or similar to the coefficient of
expansion of glass. In the arrangement of FIGS. 11 and 12, the
metal oxide varistor toroidal members provide the surge protection
of voltage surges occurring between each of the elongated
conductors and the outer conductor and of course, between the
elongated conductors as well. It will be understood that connectors
with more than two inner conductors could be readily provided in
accordance with the construction set forth in the embodiment of
FIGS. 11 and 12.
While the invention has been described in specific embodiments, it
will be appreciated that modifications may be made by those skilled
in the art and we intend by the appended claims to cover all such
modifications and changes as fall within the true spirit and scope
of the invention.
* * * * *