Surge protection device and system

A surge protector particularly adapted for use with telecommunication equipment is non-conductive during normal operation and becomes conductive upon surge conditions of a first level and provides a short circuit condition when subjected to surge condition above a second, greater level. The protector comprises a semiconducting chip (16) disposed between first (32, 32') and second (30, 30') electrodes and is provided with an environmental seal (60, 60') shrunk abut the outer peripheral surfaces of the electrodes while being non-adhering to both provide contaminant protection and free slidability of the electrodes within the sleeve.

BACKGROUND OF THE INVENTION 
The field of the invention is that of surge protection systems and the 
invention relates more particularly to systems having solid state surge 
protection elements. 
Solid state surge protection systems conventionally employ a surge 
protection device having a semiconducting element between a pair of 
electrodes. This element is arranged in the circuit to selectively conduct 
electrical energy between a portion of a telephone circuit and ground, for 
example, in the event the circuit experiences an electrical surge as a 
result of lightning or a short circuit or the like in the circuit, thereby 
to protect electrical components connected in the circuit from damage due 
to the surge condition. On occasion the circuit will experience an 
electrical surge which results in destruction of the semiconducting 
element so that the circuit components would not thereafter be protected 
against subsequently occurring electrical surge conditions. Accordingly, 
conventional surge protection systems typically incorporate means to move 
the electrodes into electrical engagement with each other following 
destruction of the semiconductor elements, thereby to maintain a short 
circuit condition between the electrodes to protect the circuit components 
against subsequently occurring surge conditions until such time as the 
semiconducting element is replaced. Frequently, difficulty is encountered 
in establishing and maintaining the desired short circuit condition 
between the electrodes following destruction of the semiconducting 
element. 
In U.S. Pat. No. 5,224,088, issued Jun. 29, 1993 and assigned to the 
assignee of the present invention, a surge protection system comprising a 
circuit and a device is disclosed and claimed which provides first and 
second levels of surge protection for the circuit and for electrical 
components in the circuit. The device comprises a semiconducting element 
having first and second electrodes disposed in electrically conductive 
relation with respective opposite sides of the element and the element is 
normally operable to selectively conduct electrical energy between the 
electrodes to provide a first level of surge protection for the circuit. 
For example, in one embodiment the electrodes are connected to a portion 
of the telephone circuit and to ground respectively, and the 
semiconducting element is non-conductive under normal telephone circuit 
voltage conditions. The element becomes conductive on occurrence of 
selected surge voltages in the circuit due to lightning or the like for 
connecting the circuit to ground to protect the circuit and circuit 
components from the voltage surge. When the surge condition has passed the 
semiconducting element returns to its non-conductive condition permitting 
resumption of normal circuit operation. In that way the surge protection 
device provides a first level of surge protection for the circuit or 
system. 
The electrodes are adapted to move toward each other to maintain a short 
circuit condition between the electrodes in the event the device is 
destroyed as a result of being subjected to a greater electrical surge 
exceeding the first level of surge protection as described above, thereby 
to provide a second level of surge protection. Typically for example, the 
device electrodes are resiliently engaged between spring-loaded electrical 
contact arms or the like to be biased together by the spring contact arm 
loading in the event the semiconducting element is destroyed by the noted 
greater surge conditions. A first one of the electrodes is disposed in 
electrical engagement with a limited portion of one side of the 
semiconducting element to selectively concentrate the electrical energy to 
the limited portion to ensure the establishment and maintenance of the 
desired short circuit condition between the electrodes in the event of 
element destruction. The first electrode comprises a disc part and post 
part of lesser cross-sectional size then the disc part. A distal end of 
the post part of the first electrode is secured in electrically conductive 
relation to the selected limited portion of the one side of the element 
whereby as destruction of the semiconducting element occurs the post part 
of the first electrode passes through the remains of the element to be 
electrically engaged in short circuit condition with the second electrode 
for providing the second level of surge protection. The second electrode 
has a recess therein and the opposite side of the semiconducting element 
is received within the recess ensuring alignment in predetermined relation 
to the second electrode. The disc part of the first electrode is 
coextensive with the second electrode and the semiconducting element is 
selected to be operable with either polarity. An environmental seal of an 
electrically insulative coating or the like is disposed over a portion of 
said one side of the semiconducting element around the post part of the 
first electrode. In one embodiment the seal is spaced from the disc part 
of the first electrode to facilitate movement of the electrodes into short 
circuit condition in providing the second level of surge protection. In 
another embodiment the seal is provided with a selected limited thickness 
and with brittleness to be readily broken by bias of the electrodes toward 
each other in the event of destruction of the semiconducting element to 
provide the second level of surge protection. In another embodiment the 
seal is selected to be destroyed by overheating of the semiconducting 
element resulting in destruction of the element thereby to facilitate 
movement of the electrodes short circuit condition. 
While the surge protection device in accordance with the teachings of U.S. 
Pat. No. 5,224,008 is very effective in providing the desired first and 
second levels of surge protection there is a need to provide an 
environmental seal which is more easily installed having the desired 
operational parameters of protecting the semiconducting element and the 
interfaces of the electrodes from contamination including humidity, salt, 
dust particles and the like without interfering with the collapsing motion 
of the electrodes upon destruction of the semiconducting element in 
providing the second level of surge protection. 
BRIEF SUMMARY OF THE INVENTION 
It is an object of the invention to provide a novel and improved surge 
protection system having an environmental seal for a surge protection 
device in which first and second electrodes are adapted to collapse toward 
each other into electrically conductive relation with one another upon 
destruction of a semiconducting element disposed between the electrodes in 
providing a selected level of surge protection. 
Briefly, in accordance with the invention, the novel and improved surge 
protection system of the invention comprises a circuit having an 
environmental seal for a surge protection device providing first and 
second levels of surge protection for the circuit and for the electrical 
components in the circuit. The surge protection device comprises a 
semiconducting element having first and second electrodes disposed in 
electrically conductive relation with respective opposite sides of the 
element and the element is normally operable to selectively conduct 
electrical energy between the electrodes to provide a first level of surge 
protection for the circuit. For example, in one preferred embodiment, the 
electrodes are connected to a portion of a telephone circuit and to ground 
respectively, and the semiconducting element is non-conductive under 
normal telephone circuit voltage conditions. The element becomes 
conductive on occurrence of selected surge voltage in the circuit due to 
lightning or the like for connecting the circuit to ground to protect the 
circuit and circuit components from the voltage surge. When the surge 
condition has passed, the semiconducting element returns to its 
non-conductive condition permitting resumption of normal circuit operation 
thereby providing a first level of surge protection. 
In the event the device is subjected to a greater electrical surge 
exceeding the first level, a second level of protection is provided by the 
destruction of the semiconducting element and concomitantly the collapsing 
or moving together of the electrodes into electrical engagement with one 
another under the influence of external spring-loaded electrical contact 
arms engaging the terminals. 
The electrodes of the device have outer peripheral surfaces which are 
coextensive with one another and an environmental seal is provided 
comprising a sleeve formed of electrically insulative, heat shrinkable 
material shrunk onto the outer peripheral surfaces of the electrodes. The 
sleeve is in intimate sealing engagement with the outer peripheral 
surfaces of the electrodes without inhibiting sliding motion of the 
electrodes upon destruction of the semiconductor element under the 
influence of conventional contact arm loading. The length of the sleeve is 
selected so that it is shorter than the distance between the outer face 
sides of the electrodes following destruction of the semiconductor element 
to avoid any interference between the outer face sides of the electrodes 
and contact arms adapted for engagement therewith. In one preferred 
embodiment, the semiconductor chip is disposed in a recess formed in one 
of the electrodes which places the element in selected alignment with the 
other electrode. In another preferred embodiment, the semiconducting 
element is disposed on a flat electrode surface with alignment of the 
semiconducting element between the electrodes being effected through 
appropriate fixturing during the assembly process. The fixturing has a 
given tolerance resulting in the possibility of an edge portion of the 
semiconducting element protruding out beyond the outer peripheral surfaces 
of the electrodes. In order to protect the semiconducting element from 
mechanical damage due to handling and the like, the thickness of the 
sleeve is chosen to be greater, after shrinking, than the given tolerance 
so that the edge cannot protrude through the sleeve. As a result, the 
outer diameter of the electrodes can be reduced to thereby reduce the 
overall size of the device. According to a feature of the invention, the 
sleeve can be provided with different colors coded to provide product 
differentiation.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring to the drawings, numeral 10 in FIGS. 1, 2 and 4 indicates a 
preferred embodiment of the novel and improved surge protection device by 
itself and in FIG. 5 as used in a system. In FIG. 5 the system is shown to 
include a circuit 12 and a surge protector device 10 arranged to provide 
first and second levels of surge protection for the circuit 12. The surge 
protection device includes a semiconducting surge protection element 16 
(see FIG. 3) having a first terminal 18 on a bottom side 19 of the element 
and a second terminal 20 on a top side 21 of the element. The surge 
protecting element 16 is of a generally conventional type. Preferably the 
element comprises a silicon semiconducting body having a reverse breakdown 
PN junction. In a preferred embodiment of the invention the element 
comprises a bidirectional silicon semiconductor such as a TR730YQ element 
made and sold by Texas Instruments Incorporated having a critical energy 
conduction path through the element defined between a selected limited 
portion of side 21 and another opposite side 19. Terminal surface 18 on 
side 19 extends up over the sides to a point indicated by broken line 22 
spaced inwardly from the outer periphery of the element on the opposite 
side 21 and spaced from terminal surface 20 on the said opposite side. 
Typically, the semiconductor element has a rectangular configuration as 
shown in FIG. 3 although elements of other outlines are also possible 
within the scope of the invention. Typically a portion of the 
semiconductor element such as the side inboard of the marginal surface 
indicated by broken line 22, requires some type of environmental seal to 
protect the portion from corrosion or the like or prevent shorting between 
the terminals of the element by moisture or the like without interfering 
with movement of the surge protector electrodes in providing the second 
level of surge protection to be described below. 
Surge protection device 10 has electrodes 30 and 32 disposed in 
electrically conductive relation with respective opposite sides 19, 21 of 
the element. The semiconductor element is normally non-conductive but is 
adapted to become conductive when a selected level of voltage is applied 
to the element across the electrodes, thereby to provide a first level of 
surge protection for the circuit. The semiconducting element is adapted to 
become non-conducting again following removal of the voltage surge 
condition to permit resumption of normal operation of the circuit 12. 
The electrodes are arranged to move toward each other and to be 
electrically engaged with each other in short circuit condition in the 
event the semiconductor element is destroyed by an electrical surge in the 
circuit exceeding the first level of surge protection, thereby to provide 
a second level of surge protection and assure that the circuit is 
protected against subsequent surge conditions until the device with the 
destroyed semiconducting element has been replaced. Typically, for 
example, contact arms 34, 36 in the circuit 12 are resiliently biased as 
indicated by arrows 38 by spring means or the like to provide a contact 
loading for example of 3-5 pounds. Contact arm 36 is shown connected in a 
line in a telephone system and contact arm 34 is shown being connected to 
ground as diagrammatically indicated in FIG. 5. In that way, device 10 is 
adapted to protect telephone circuit components 40 against a first level 
of surge protection with only temporary interruption of the telephone 
circuit but if a second relatively greater level of electrical surge 
occurs in the system such as would destroy the semiconducting surge 
protector element, the device provides a second level of surge protection 
to protect the circuit and circuit components until the device with the 
destroyed element is replaced. 
In accordance with the invention, the first electrically conductive 
electrode 32 through its geometric configuration is disposed in 
electrically conductive relation to a limited part of one side of the 
semiconducting element 16. Preferably, for example, the first electrode is 
provided with a disc part 42 having a centrally disposed node part 44 of 
lesser cross sectional size than the disc part depending downwardly from 
the disc part as seen in FIGS. 1 and 4. The lower surface of the node part 
is provided with a solder coating 46 and is secured in electrically 
conductive relation to the desired limited part of the element side 21. 
Preferably, for example, the lower surface of node part 44 is soldered to 
the terminal 20 to dispose the node part of the first electrode in 
electrically and thermally conductive engagement with the selected limited 
portion of the element side 21 which defines the critical energy 
conduction path as above described. Where the element is bidirectional the 
node part 44 and terminal 20 are arranged to overlap the preferential 
conduction path in each direction as noted above. The node part 44 of the 
first part extends away from the disc part 42 a distance which at least 
equals and which preferably exceeds the thickness t of the semiconductor 
element 16. 
The surge protector device 10 further comprises a second electrically 
conductive electrode 30 which is disposed in electrically conductive 
relation to the opposite side 19 of the semiconducting element. The second 
electrode 30 is provided with a recess 48 which receives a semiconductor 
element 16 therein for aligning the element in predetermined position 
relative to the second electrode. Preferably a layer of solder material 50 
is provided in recess 48 for receiving element 16 thereon and the solder 
is melted for securing the entire side 19 of the semiconducting element in 
secure, electrically and thermally conductive relation to the electrode. 
Where the electrodes are formed of copper or copper alloy having high 
thermal conductivity, they serve to rapidly dissipate heat from the 
semiconducting element in providing the noted first level of surge 
protection. The first and second electrodes are coextensive as shown in 
FIG. 1 so that disposition of the electrodes aligned in coextensive 
relation with each precisely positions node 44 of electrode 32 with the 
critical energy conduction path defined under terminal 20 of the element. 
Preferably, the element 16 is selected to be bidirectional or operable 
with alternate polarity and the coextensive electrodes permit the device 
10 to be accommodated between arms 34, 36 or other contact engagement 
means with alternate polarity and without requiring any particular 
orientation as it is installed in a system as shown in FIG. 5. 
In accordance with the invention, a seal is provided for device 10 
comprising a sleeve 60 formed of electrically insulating, heat shrinkable 
material. Sleeve 60 is chosen having a length, after shrinking, of less 
than the outer surfaces of electrodes 30, 32 with the semiconducting 
element 16 removed so that the length of the sleeve will not interfere 
with collapsing movement of the electrodes under the influence of the 
spring-loaded contact arms upon destruction of semiconducting element 16 
in providing the second level of surge protection. Sleeve 60 is formed of 
material which conforms closely to the coextensive outer peripheries of 
electrodes 30, 32 without adhering to the electrode surfaces. The close 
conformance of the sleeve to the electrode surfaces prevents entry of 
contaminants, moisture, salt, dust particles and the like from entering 
the space between disc part 42 of electrode 32 and electrode 30 while the 
non-adherence of the sleeve to the electrode surfaces allows the 
electrodes to slide, without interference, toward each other within the 
sleeve into short circuit condition as shown in FIG. 4 upon the occurrence 
of a surge condition which results in destruction of semiconductor element 
16. Polyvinylidene fluoride/polyolefin blend such as KVF 100 3/16 BLK SPL 
having 120 .degree. C. shrink temperature has been found to be a suitable 
material being both semi-rigid and not adherent to the electrodes. Other 
heat shrinkable materials could be employed, if desired, such as mylar, a 
trademark of DuPont de Nemours for polyethylene terephthalate, polyolefin, 
PVC, teflon, a trademark of DuPont de Nemours for polytrafluorethylene, 
elastomers and silicon rubber. 
In assembling the device, after the semiconducting element 16 has been 
soldered to electrodes 30, 32, a sleeve of suitable heat shrinkable 
material, such as polyvinylidene fluoride/polyolefin blend mentioned 
above, having an inside diameter slightly larger than the diameter of the 
outer peripheral surface of the electrodes is loosely placed over the 
electrodes and heat in the form of IR light, forced hot air, hot air 
convection, UV light or the like is applied to the sleeve causing it to 
shrink tightly about the electrodes. 
In operation, where device 10 is subjected to the above noted surge 
conditions exceeding the first level of surge protection semiconductor 
device 10 is destroyed and the electrodes 30, 32 slide together within 
sleeve 60 under the influence of the spring-loaded contact arms 34, 36 
with node 44 electrically engaging terminal 30 through the melted solder 
thereby maintaining a short circuit condition, as shown in FIGS. 4 and 7. 
Node 44 pierces through the shattered material of element 16 which is 
pushed into the remaining space between the electrodes around node 44. 
In another preferred embodiment electrode 30' is formed with a flat 
electrode surface 31, that is without a recess to align semiconductor 
element 16. In this embodiment appropriate fixturing tools are relied upon 
to align the semiconductor element and the electrodes. As a result the 
device can be made having a smaller outside diameter and can more easily 
be fit into the limited space available in many applications. For example, 
a device made in accordance with this embodiment can have electrodes 
having an outer diameter in the order of 0.200 inches. Due to tolerance 
requirement of the fixturing tools, an edge of semiconductor element 16 
could end up protruding beyond the outer peripheral surfaces of the 
electrodes by up to 0.003 inches; however, sleeve 60' is chosen to have a 
wall thickness greater than that, e.g., 0.006-0.008 after shrinkage so 
that the element is protected by the sleeve from damage due to handling 
and the like while at the same time providing a device with minimized 
package dimensions. 
Another advantage offered by the structure of the present invention is the 
improved over surface clearance between one electrode to another to reduce 
the leakage path in the presence of moisture versus prior art approaches 
which employ a seal disposed between the electrodes. 
The present invention also provides a convenient mechanism for providing 
product differentiation by using different selected colors as a code for 
devices having different electrical characteristics. 
It should be understood that although particular embodiments of the 
invention have been described by way of illustrating the invention, the 
invention includes all modifications and equivalents of the disclosed 
embodiment within the scope of the appended claims.