U.S. patent number 3,711,668 [Application Number 05/210,979] was granted by the patent office on 1973-01-16 for switch with surge protection.
This patent grant is currently assigned to General Electric Company. Invention is credited to John D. Harnden, Jr..
United States Patent |
3,711,668 |
Harnden, Jr. |
January 16, 1973 |
SWITCH WITH SURGE PROTECTION
Abstract
An insulating member of metal oxide varistor material separates
the two conductive parts of a hermetically sealed enclosure to
which the electrodes of the switch are attached. The metallic oxide
varistor material has a voltage versus current characteristic such
that when normal voltage appears across the electrodes, a high
impedance is presented by the insulating member and when voltages
in excess of normal voltage appears across the electrodes, a
rapidly decreasing impedance is presented by the insulating member,
thereby limiting the voltage which is sustainable across the
electrodes of the switch.
Inventors: |
Harnden, Jr.; John D.
(Schenectady, NY) |
Assignee: |
General Electric Company
(N/A)
|
Family
ID: |
22785125 |
Appl.
No.: |
05/210,979 |
Filed: |
December 22, 1971 |
Current U.S.
Class: |
200/266; 200/222;
361/13; 200/302.1 |
Current CPC
Class: |
H01H
29/24 (20130101); H01H 13/06 (20130101); H01H
13/48 (20130101) |
Current International
Class: |
H01H
29/00 (20060101); H01H 29/24 (20060101) |
Field of
Search: |
;1h/160
;200/222,166C,168G |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Jones; H. O.
Claims
What I claim as new and desire to secure as Letters Pat. of the
U.S. is:
1. A switch comprising:
a sealed enclosure having a pair of conductive portions and an
insulating member including an annular portion,
said annular portion of said insulating member having a peripheral
outer portion contacting one of said conductive portions and an
inner peripheral portion contacting the other of said conductive
portions,
a pair of spaced electrodes each included within said enclosure and
each connected to a respective one of said conductive portions,
externally located portions of said conductive portions providing
terminals for connecting said switch in circuit,
the annular portion of said insulating member being constituted of
a metal oxide varistor material.
2. The combination of claim 1 in which said metal oxide varistor
material has an alpha in excess of 10 in the current density range
of 10.sup..sup.-3 to 10.sup.2 amperes per square centimeter.
3. The combination of claim 1 in which the portions of said annular
portion of said insulating member in contact with said conductive
portions are spaced to provide a current flow between said
conductor portions which is low when normal operating voltage
appears across said conductive portions and when voltages in excess
of normal voltage progressively appear thereacross a rapidly
decreasing impedance is presented by said annular portion of said
insulating member in accordance with the alpha of the material,
thereby limiting the voltage appearing between said conductive
portions.
4. The combination of claim 1 in which said outer and inner
peripheral portions of said annular portion are cylindrical.
5. The combination of claim 1 in which said annular portion of said
insulating member is provided with a side surface which is
substantially planar, a conductive ring concentrically located on
said side surface, said conductive ring being in conductive contact
with one of said conductive portions, the edge of said conductive
ring adjacent the other of said conductive portions being uniformly
spaced with respect thereto in said side surface.
6. The combination of claim 1 in which said annular portion of said
insulating member is provided with a side surface which is
substantially planar, an outer conductive ring and an inner
conductive ring concentrically located on said side surface, said
outer conductive ring in conductive contact with said one
conductive portion, said inner conductive ring in conductive
contact with said other conductive portion, the adjacent edges of
said ring being uniformly spaced in said side surface.
7. The combination of claim 1 in which said annular portion of said
insulating member has a pair of major opposed faces, a layer of
metal oxide varistor material bonded to one of said faces and outer
peripheral portion of said layer conductively connected to said one
conductive portion and an inner peripheral portion of said layer
conductively connected to the other of said conductive portions,
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 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 of the alpha of the material thereof, thereby limiting
the variation in voltage between said conductive portions.
8. The combination of claim 1 in which said sealed enclosure
includes a resilient portion for enabling said electrodes to be
brought into contact with one another.
9. A switch comprising:
a cylindrical metal shell having an open end and a closed end,
a cover plate hermetically sealed to close said open end, said
plate including an annular insulator having a terminal pin sealed
to an inner peripheral portion thereof and a metal retaining ring
sealed to the outer peripheral surface thereof, said retaining ring
conductively sealed to said shell,
an insulating liner in said shell having a transverse partition
dividing said shell into two chambers,
said chambers containing a quantity of mercury occupying less than
half the volume thereof,
said partition having an opening therethrough for the passage of
mercury between said chambers, said partition having a well for
mercury diametrically opposite said opening, said partition 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 the bottom
of said well in respect to the opposed surface of said partition
being set so that a high impedance is presented therebetween when
normal voltages appear between the pools of mercury in said
chambers and when voltages in excess of normal voltage
progressively appear thereacross, a rapidly decreasing impedance is
presented by said partition in accordance with the alpha of the
material of said partition, thereby limiting the variation in
voltage between the the pools of mercury.
Description
The present invention relates in general to electrical switches and
in particular to button-type switches in which is included
electrical protection for devices such as lamps and appliances
connected in circuit with such switches from surge and other
spurious electrical voltages.
Electrical switches, commonly referred as button switches, because
of their "button-like" appearance are used as wall switches to
provide electrical power to the lamps, appliance motors and the
like. One such switch is a mercury button switch in which pools of
mercury are utilized to make and break electrical contact. The
pools are located in a pair of chambers separated by an apertured
partition within the switch housing. In one position, or
orientation about a rotational axis, the pools of mercury are
separated by the partition. In another axial position, an aperture
in the partition is positioned to allow the pools of mercury to
merge, thereby providing conductive connection between a pair of
terminals conductively juxtaposed to the pools of mercury. Such
switches have a number of advantages such as quiet operation,
minimal actuating force, and are compact and small in size. In some
applications, such switches may present a disadvantage. The quick
breaking action of the switch produces high induced voltages in the
circuits in which the switch is connected, particularly in circuits
with significant inductance. For example, in ordinary house wiring,
when the only inductive load connected in circuit is the inductance
of the house wiring, the rapid action of the mercury button switch
may produce voltages in the circuit as high as twice the normal
operating voltages. Of course, when appliances and devices which
have higher inductance are connected in circuit, the rapid breaking
of the circuit thereof by the switch produces much higher inductive
voltages. Voltages in excess of 2000 volts have been measured under
such circumstances. Such voltages have an adverse effect on the
electrical devices connected to the house lines such as filamentary
lamps, clock motors and the like. Increasing the operating voltage
on filamentary lamps rapidly deteriorates the performance thereto
and reduces the life thereof as well. Also, high inductive voltages
may cause the insulation of such devices as clock motors to
fail.
Accordingly, an object of the present invention is to provide a
switch of the kind described in which the magnitude of the
electrical surges produced thereby is substantially reduced.
Another object of the present invention is to provide fast acting
switches in which the amplitude of the voltage surge produced
thereby is limited to a predetermined value.
Another object of the present invention is to provide a switch for
electrical devices which in addition to providing fast action, also
provide protection against electrical pulses without the need for
separate or additional elements to provide such function.
A further object of the present invention is to provide a switch of
long life.
In carrying out the present invention, in one illustrative
embodiment thereof, there is provided a sealed enclosure having a
pair of conductive portions and an insulated member including an
annular portion. The annular portion of the insulating member has a
peripheral outer portion contacting one of the conductive portions
and has an inner peripheral portion contacting the other of said
conductive portions. A pair of electrodes are provided each
included within the enclosure and each connected to a respective
one of the conductive portions. The conductive portions include
externally located portions providing terminals for connecting the
switch in circuit. The annular portion of the insulating 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 are set so that a high impedance is presented
between the peripheral portions when normal voltages appear
therebetween and when voltages in excess of normal voltages
progressively appear thereacross, a rapidly decreasing impedance is
presented by the annular portion in accordance with the alpha of a
material thereof, thereby limiting the variation in the voltage
between the peripheral portions.
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 perspective view of a mercury button switch made in
accordance with the present invention,
FIG. 2 is a sectional view of the switch shown in FIG. 1,
FIG. 3 is an end view of the switch of FIG. 1,
FIG. 4 shows graphs of the electrical characteristics of three
materials of differing voltage gradients and alphas suitable for
utilization in the switches of the present invention,
FIG. 5 is a side view in section of another embodiment of the
present invention,
FIG. 6 is an end view of the embodiment of FIG. 5,
FIG. 7 is a side view in section of another embodiment of the
present invention,
FIG. 8 is a side view in section of a further embodiment of the
present invention,
FIGS. 9, 10 and 11 are broken perspective views of the ceramic
barrier of the switch shown in FIG. 1 useful in explaining the
operation of the mercury button switch.
FIG. 12 is a still further embodiment of the present invention
showing a sectional view of a vacuum button switch.
FIG. 13 is one end view of the device of FIG. 12.
FIG. 14 is another end view of the device of FIG. 12.
The present invention is directed to providing improvements in
switches such as the mercury button switches described in U.S. Pat.
No. 3,229,354 Cook et al. Referring to FIGS. 1, 2 and 3 of the
drawing, there is shown a mercury button switch comprising a
cylindrical shell 11 or first conductive portion with a closed end
12 and an open end having an outwardly extending flange 13 with a
lip 14. The other solid elements of the switch comprise a terminal
metal pin 15 or second conductive portion centrally positioned with
respect to a retaining ring 16 by means of an insulating glass seal
17 of annular form. Directly bonded to the glass seal is a
cylindrical ceramic barrier or liner 18 which has an aperture (not
shown) through which mercury can flow as will be explained below.
When direct-bonded, the pin 15, glass seal 17, retaining ring 16,
and barrier 18 form an assembly which is sealed, as by welding of
the ring 16 to the flange 13 of the shell with the barrier on the
interior thereof. Bonded to the exterior portion of the annular
glass seal is an annular member 20 of metal oxide varistor
material.
Reference is now made to FIGS. 9, 10 and 11 which show the barrier
or liner 18 and the manner of its operation in the switch 10. The
ceramic barrier or line 18 has a transverse central partition 25
which serves to divide the interior of the switch into pools of
mercury 26 and 27. The partition 25 has a circular opening or port
28 and a reservoir cavity 29 in one side thereof. The through
opening 28 is designed to permit the mercury on both sides of the
partition to flow together at the center of the opening and
complete an electrical circuit between the metal shell 11 and the
head 23 of the terminal pin 15. It is important that the switch be
capable of operation with a rotational angle of 20.degree. or less.
Accordingly, the through opening is located as far as possible from
the geometric axis of the metal shell. The reservoir or cavity 29
is positioned with its lower edge on a line drawn through the
center axis of the shell and the center axis of the through opening
28. Referring particularly to FIGS. 10 and 11, as the switch is
rotated in a counter clockwise direction, as indicated by arrows
24, the through opening will begin to sink below the top level of
the mercury pools 26 and 27. This begins to remove the obstruction
between the two pools and they enter the through opening 28 with
rounded frontal surfaces 30 and 31. As these two bodies of liquid
mercury come into contact, a high inrush current will immediately
vaporize the frontal surfaces of the mercury bodies. Final circuit
enclosure might occur only after two or more such vaporizations or
explosions. The purpose of the reservoir 29 is to dump the last
traces of mercury into the mercury stream at the opportune moment
when the two pools begin to merge at the center of the through
opening 28. This action increases the kinetic energy of the moving
mercury bodies and minimizes the local explosive effects of the
contacting surfaces of mercury. When the switch is turned to open
circuit position, the reservoir 29 again serves to increase the
kinetic energy of the moving mercury bodies as they begin to
separate. The metal shell 11 has an indexing notch 19 which is
utilized for positioning and rotating the switch. A complementary
notch on the surface of the ceramic barrier 18, insures the proper
orientation of the barrier 18 within the metal shell 11. Prior to
the final sealing of the pin 15, ring 16 and barrier 18 assembly
and mercury within the shell 11, a non-oxidizing atmosphere is
provided within the switch. Preferably, this atmosphere is
predominantly argon with hydrogen added for alternating current
operation. For direct current operation, the atmosphere is
preferably predominantly hydrogen and both alternating and direct
current operation may be at a pressure above atmospheric, for
example, 40 pounds per square inch.
In operation, a switch handle (not shown) which, however, may be
part of an assembly such as shown in the above-mentioned U.S. Pat.
No. 3,229,354, is seated on the outer cylindrical side of the shell
11 in the indexing notch 19 and actuated to impart rotary motion
within a restricted arc to the mercury button switch. Rotation of
the switch produces the opening and closing of the electric circuit
from the terminal pin 15 to the shell 11 by means of the mercury
pools 26 and 27 as described above with reference to FIGS. 9, 10
and 11.
The annular member 20 of metal oxide varistor material has a
peripheral outer portion 21 and an inner peripheral portion 22. The
insulating glass seal member 17 has an exterior oriented surface
which is recessed from the plane of the exterior side of the
retaining ring 16 to form a recess into which the annular member 20
of metal oxide varistor material fits. Annular member 20 is bonded
to the glass seal 17. The outer peripheral portion 21 of the metal
oxide varistor member is metallized and conductively secured to the
inner surface of the annular retaining ring 16. The inner
peripheral portion 22 of the metal oxide varistor member is also
metallized and bonded to the terminal portion of the pin 15.
The annular member 20 is constituted of a metal oxide varistor
material such as described in Canadian Pat. No. 831,691, which has
a nonlinear voltage versus current characteristic. The metal oxide
varistor 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 or wafer 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 nonlinearity 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 the
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. 3. Graphs 35 and 36 are materials of high voltage gradient
material and graph 37 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 than 10 and relatively
constant. For current 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
amperes per square centimeter, 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, duPont No. 7713, made by the duPont Chemical Company of
Wilmington, Delaware, may be used. Such material is applied as a
slurry pg,13 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 nonlinear 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 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.
The closed portion 12 of the shell 11 of the switch of FIGS. 1, 2
and 3, serves as one electrode or terminal of the switch and the
exterior terminal portion of the pin 15 serves as the other
electrode of the switch. When the switch is opened, the voltage of
the line source would appear across the outer peripheral portion
21, and the inner peripheral portion 22 of member 20. Accordingly,
to provide an appropriate low current drain through the member 20
under normal operating voltages for the switch, the metal oxide
varistor material with the appropriate voltage gradient versus
density characteristics is selected. The normal operating voltage
rating of the switch of FIGS. 1, 2 and 3 may be altered without use
of a different material by the provision of electrodes which extend
along one of the opposed surfaces of the wafer toward one another,
thereby applying a higher voltage gradient to the wafer for a given
applied voltage between the electrodes such as shown in FIGS. 5 and
6.
Reference is now made to FIGS. 5 and 6 which show a modification of
the switch of FIGS. 1, 2 and 3. The elements of the embodiment of
FIGS. 5 and 6 which are identical to the elements of an embodiment
of FIGS. 1, 2 and 3 are identically designated. The embodiment of
FIGS. 5 and 6 include an outer conductive ring 40 and inner
conductive ring 41 useful in setting the voltage versus current
characteristics of the metal oxide varistor material connected in
shunt between the electrodes 12 and 15 of the switch. The outer
ring 40 is substantially a flat ring having a circular inner edge
42. The outer conductive ring is bonded to the retaining ring 16
and to the external surface of the member 20 of the metal oxide
varistor material. Similarly, the inner conductive ring 41 has an
outer edge 43 which is circular in outline and is bonded along one
face thereof to the external face of the member 20 of metal oxide
varistor material and the inner edge of the ring 40 is bonded to
the terminal pin 15. The inner edge 42 of the outer conductive ring
40 and the outer edge 43 of the inner conductive ring 41 are spaced
so that the uniform distance exists therebetween around the
periphery thereof. The spacing of the inner edge of the outer ring
and the outer edge of the inner ring is set to produce the desired
voltage versus current characteristics in the material. The closer
the spacing, the lower the voltage at which substantial increase
current flow is obtained.
Reference is now made to FIG. 7 which shows a sectional view of
another embodiment of the present invention. The elements of the
embodiment of FIG. 7 which are identical to the elements of the
embodiment of FIGS. 1 and 2 are identically designated. In the
embodiment of FIG. 7, the glass seal member 17 has been replaced
with an annular insulating member 50, made of metal oxide varistor
material to which the pin 15 is bonded. The annular retaining ring
16 is bonded to an outer peripheral portion of the member 50. In
the switch structure of FIG. 7, the element 50, constituted of
metal oxide varistor material, provides both the sealing and
voltage limiting functions for the switch and simplifies the
construction and the processing of the switch.
Reference is now made to FIG. 8 which shows another embodiment of
the present invention. The elements of FIG. 8 identical to the
elements of FIGS. 1, 2 and 3 are similarly designated. In this
figure, members 20 of metal oxide varistor material secured to the
glass seal 17 has been eliminated and the barrier or liner member
18 thereto is constituted of metal oxide varistor material and is
designated barrier 55. As the geometrical form of the barrier 55 is
identical to the geometrical form of barrier 18, the same numerals
are utilized to designate the geometric elements thereof. The
spacing of the bottom of the well 29 is set in respect to the
opposed face of the central partition 25 so that a high impedance
is presented between such faces when normal voltages appear on the
switch terminals and hence between the pools of mercury 26 and 27.
When voltages in excess of normal voltage appear thereacross as
occurs on opening of the switch, a rapidly decreasing impedance is
presented across the partition in accordance of the alpha of the
material of the partition 25. Accordingly, the variation in voltage
between the pools of mercury 26 and 27 and hence between the
terminals of the switch is limited.
Reference is now made to FIGS. 12, 13 and 14 which show another
embodiment of the present invention. The switch 60 includes a cup
shaped enclosure member 61 of a conductive material having an
opening 62 in the base portion 63 thereof and an open portion
opposite the base portion on which circular flange 64 is located.
An insulating member 65 of metal oxide varistor material annular in
form is provided in which the outer peripheral portion 66 thereof
is secured to the one end of the member 61 and in which the inner
peripheral portion 67 thereof is conductively secured to a
conductive pin 68 having a cylindrical terminal portion 69 and a
contact element 70. A flexible bellows member 71 of metallic
material such as phosphor bronze, for example, circular in outline,
and having a conductive contact element 72 secured conductively at
a central region thereof is provided. The outer peripheral portion
of the bellows member 71 is conductively secured to the flange 64
of the cup member 61 by welding, so as to register axially the
contact element 72 with respect to the contact element 70 of the
pin. The sealed enclosure may be evacuated or may be backfilled
with an appropriate gas such as hydrogen or nitrogen, for example,
if desired. For this purpose, a tubulation 75 is provided in the
side cylindrical surface of the cup member 61. The contact elements
may be made of any of a number of materials such as copper alloys,
nickel alloys, molybdenum, tungsten and the like.
The aperture 62 in the base of the cup is circular in outline and
the terminal portion 63 to which the inner peripheral surface of
the metal oxide varistor 67 material is secured, is centrally
located so that the distance between a radial point on the pin 68
and a radial point on the inner edge 62 of the aperture is
identical in order to ensure that all of the metal oxide varistor
material is utilized in effecting a voltage limiting and electrical
energy absorption function as explained above.
The switch of FIGS. 12, 13 and 14 described above may be actuated
manually or by means of any of a number of mechanical assemblies
well-known to those skilled in the art. In the embodiment of FIGS.
12, 13 and 14, the bellows element 71 is flexible and in the
absence of force supplied to the contact element 72 is spaced from
contact element 70 when a mechanical force is applied between the
contact elements 70 and 72 to overcome the spring force of the
bellows, the contact elements engage. Upon removal of the force,
the contact elements return to their disengaged positions.
While the invention has been described in the specific embodiments,
it will be appreciated that modifications may be made by those
skilled in the art and I intend by the appended claims to cover all
such modifications and changes as fall within the true spirit and
scope of the invention.
* * * * *