Overload protected solid state varistors

A fail-safe varistor includes either a fail-short or a fail-open device. Both devices include a fusible, electrically conductive material that melts before the varistor fails due to overvoltage. In the fail-open device, the fusible, electrically conductive material joins separated portions of the leads. The material also may join at least one of the leads directly to a ceramic disk of the varistor. Upon reaching the predetermined temperature, the varistor melts causing a circuit including the varistor to open. In the fail-short device, the material melts creating a short between the leads. This short causes a fuse or a breaker to open the circuit.

BACKGROUND OF THE INVENTION 
The present invention relates to a solid state varistor, and, more 
particularly, to a solid state varistor having a fail-safe feature to 
protect against destructive failure of the varistor due to overheating. 
Solid state varistors are normally comprised of metal oxides. This type of 
varistor is characterized by a highly non-linear current-voltage 
relationship governed by I.varies.V.sup..alpha., where 
2.ltoreq..alpha..ltoreq.6. At relatively low voltage values, the 
relationship is nearly linear. However, as the voltage value increases, 
the current increases exponentially. See Lionel M. Levinson & H. R. 
Philipp, The Physics of Metal Oxide Varistors, Journal of Applied Physics, 
March 1975, 1332-1341, the subject matter of which is incorporated by 
reference. 
A metal oxide varistor operating under sustained AC overvoltage conditions 
and unlimited current flow shorts out in a few seconds due to excessive 
heating (I.sup.2 R losses). Immediately thereafter, AC follow current may 
cause the varistor to explode. An explosion opens the circuit terminating 
the dangerous conditions. This failure mechanism is considered "safe" 
because it quickly opens the circuit before a fire or personal safety 
hazards develop. 
In another scenario, other circuit elements (loads) may limit the current 
flowing through the varistor to a few amperes or less. The solid state 
varistor again overheats to a limit determined by the current flow and the 
resistance of the varistor. The varistor may even reach red heat. The heat 
may ignite the organic coating of the varistor causing obnoxious fumes, 
open flames, and shock hazards. After the organic coating burns completely 
away, if the lead wires maintain contact with the ceramic disk of the 
varistor, the varistor will remain in an overheated state and continue to 
present a hazard. Both Underwriters Laboratories and the Canadian 
Standards Association have developed safety standards requiring the 
addition of "fail-safe" provisions to all listed transient voltage surge 
protectors, especially those employing solid state varistors. 
Some manufacturers of surge protectors have devised strategically located 
"board level" fusible links and thermal cut-off devices for circuits. 
SUMMARY OF THE INVENTION 
The advantages and purpose of the invention will be set forth in part in 
the description which follows, and in part will be obvious from the 
description, or may be learned by practice of the invention. The 
advantages and purpose of the invention will be realized and attained by 
the elements and combinations particularly pointed out in the appended 
claims. 
To attain the advantages and in accordance with the purpose of the 
invention, as embodied and broadly described herein, a solid state 
varistor of the invention comprises leads connected to the varistor, at 
least one of the leads has a fusible link. The fusible link melts when 
heated to a predetermined temperature to produce an open circuit in the 
lead. 
In a second aspect of the invention the advantages and purpose of the 
invention are attained by a method of manufacturing a solid state varistor 
having thermal overload protection. The method comprises the steps of 
connecting leads to a ceramic disk; separating at least one of the leads 
into separated portions; and forming a fusible link connecting the 
separated portions, the link being meltable when heated to a predetermined 
temperature creating an open circuit between the separated portions. 
In another aspect of the invention, a fusible link joins at least one of 
the leads to the varistor. Upon reaching the predetermined temperature, 
the link melts opening the circuit between the lead and the varistor. 
In yet another aspect of the invention, a metal oxide varistor has an 
opening therethrough; leads are connected to the varistor; and fusible, 
electrically conductive material is located in or adjacent the opening. 
The material melts upon reaching a predetermined temperature creating a 
short circuit between the leads. This short causes a device elsewhere in 
the circuit to open the circuit. 
It is to be understood that both the foregoing general description and the 
following detailed description are exemplary and explanatory only and are 
not restrictive of the invention, as claimed. 
Additional objects and advantages of the invention will be set forth in 
part in the description which follows, and in part will be obvious from 
the description, or may be learned by practice of the invention. The 
objects and advantages of the invention will be realized and attained by 
the elements and combinations particularly pointed out in the appended 
claims.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Reference will now be made in detail to the present preferred embodiments 
of the invention, examples of which are illustrated in the accompanying 
drawings. Wherever possible, the same reference numbers will be used 
throughout the drawings to refer to the same or like parts. 
In accordance with the invention, the varistor of the present invention 
includes a ceramic disk, leads, and means for opening a circuit if the 
temperature of the varistor rises above a predetermined level. 
Preferably, the varistor is a metal oxide varistor and said means comprises 
a mass of fusible, electrically conductive material which melts causing 
the circuit including the varistor to open. 
The invention will be further clarified by the following examples, which 
are intended to be purely exemplary of the invention. 
First, second, and third embodiments of the invention are all directed to 
varistors having various fail-open devices. These embodiments are 
illustrated in FIGS. 1-6. A solder mass completes a circuit including 
leads and a ceramic disk. When there is an overvoltage, the temperature of 
the varistor rises. This event causes the solder mass to melt, creating an 
open circuit. 
Fourth and fifth embodiments of the invention are directed to varistors 
having various fail-short devices. These embodiments are illustrated in 
FIGS. 7 and 8. A solder mass is located on or in the ceramic disk of the 
varistor between the leads. This mass does not complete a circuit. When 
there is an overvoltage, the temperature of the varistor rises causing the 
solder mass to melt and flow, creating a short between the leads. This 
short causes a separate fuse or breaker elsewhere in the circuit to open 
the circuit. 
The first embodiment of a varistor having a lead with a fusible link is 
illustrated in FIGS. 1 and 2. A varistor 100 includes metallic wire 
electrical leads 110 attached to each side of a ceramic disk 120. The 
leads 110 extend distally from the disk. At least one of the leads is 
separated into proximal and distal portions. The proximal portion includes 
a proximal straight portion 111 and a proximal bent portion 112 extending 
outwards (away from the opposite lead) approximately 90 degrees from a 
distal end of the proximal straight portion. The distal portion includes a 
distal straight portion 113 and a distal bent portion 114 extending 
outwards approximately 90 degrees from a proximal end of the distal 
straight portion. Bent portions 112 and 114 are parallel with one another. 
A fusible, electrically conductive material 130 joins the bent portions 
112 and 114. The fusible, electrically conductive material or solder 130 
melts upon reaching a predetermined temperature creating an open circuit. 
It is understood that one as well as both leads may be formed having the 
above-described fusible link. 
A method of manufacturing a varistor according to the first embodiment of 
the invention is described hereupon. FIGS. 1A, 1B, 1C, and 1D illustrate 
intermediate and final products after some method steps have been 
performed. Kinks 115 are formed along the length of leads 110. The kinks 
are formed by bending out the leads 110. The fusible, electrically 
conductive material 130 is introduced within the kinks 115. The material 
130 has a wetting affinity for the leads 110, thus allowing application of 
the material 130 within the kink by a solder-immersion assembly operation. 
Solder 135 is also applied to the faces of the ceramic disk for attaching 
the leads 110. After withdrawal from the solder bath and cooling, a 
fusible solid solder mass remains within the kinks. An epoxy coating 125 
is applied such that the meniscus on the leads does not extend into the 
kink area. In a final step, the ends 116 of the kinks have been removed. 
It is understood that this method of manufacturing may be applied to one 
as well as both leads. 
The second embodiment of a varistor 200 having a lead with a fusible link 
is illustrated in FIGS. 3C. The varistor 200 includes leads 210. At least 
one of the leads has proximal and distal separated portions 211, 212 
separated by a hole fusible, electrically conductive material 230 joins 
the proximal and distal separated portions 211, 212. As in the first 
embodiment, the material 230 melts upon reaching a predetermined 
temperature creating an open circuit. 
A method of manufacturing a varistor according to the first embodiment of 
the invention is described hereupon. FIGS. 3A, 3B, and 3C illustrate 
intermediate and final products after some method steps have been 
performed. The fusible, electrically conductive material 230 is formed 
around a portion of at least one of the leads 210. Epoxy 225 is applied to 
the varistor. The hole 216 is punched through the portion of the lead 
surrounded by the material 230. 
A heat sensitive elastic member 160, 260, illustrated in FIGS. 4 and 5, may 
be used with the varistors of the first and second embodiments of the 
invention. The member 160,260 comprises a tubing placed over the leads 
110, 210. Upon reaching a predetermined temperature, the member contracts 
significantly and pulls the separated portions away from each other. 
As illustrated in FIG. 5, the leads 110, 210 may be bent over such that the 
member 160, 260 contacts the varistor 100 or 200 providing a greater 
contact area for thermal transfer. This accelerates the melting of the 
fusible, electrically conductive material 130, 230 and the contraction of 
the member 160, 260 producing a more responsive "fail-safe" event. 
The third embodiment of the invention, as illustrated in FIGS. 6A and 6B, 
includes a varistor 300 having a fusible, electrically conductive material 
disk joining at least one of the leads with a ceramic disk of the 
varistor. Silver electrodes 321 are printed on both sides of the ceramic 
disk 320 of the varistor 300. A fusible, electrically conductive material 
disk 331 contacts with at least one of the silver electrodes 321. A silver 
electrode 322 is printed on the outward surface of the fusible, 
electrically conductive material disk 331. One of the leads 310 touches 
the silver electrode 322. The other lead touches the silver electrode 321 
on a side of the ceramic disk opposite from disk 331. Upon reaching a 
predetermined temperature, the disk 331 melts within the epoxy containment 
325, creating an open circuit. In another variation, if the molten 
material expands sufficiently, it may erupt from the epoxy containment and 
flow out of position between the lead and the ceramic disk again creating 
an open circuit. It is understood that fusible, electrically conductive 
material disk may be located on one or both sides of the ceramic disk. 
The fourth embodiment of the invention including a varistor 400 with a 
through hole and a fusible, electrically conductive material pellet in the 
hole and is illustrated in FIGS. 7A and 7B. Silver electrodes 421 are 
printed on both sides of the ceramic disk 420 of the varistor 400. The 
hole 423 extends through the ceramic disk 420 and holds the fusible, 
electrically conductive material pellet 432. The electrodes are screen 
printed in a toroidal pattern such that there is a sufficient margin 
around the perimeter of the hole. This allows the pellet 432 to be 
inserted without creating a metal-to-metal short. Upon reaching a 
predetermined temperature, the pellet 432 melts within the hole, creating 
a short circuit between the leads 410. 
The fifth embodiment of the invention including a varistor 500 with a 
through hole and a fusible, electrically conductive material disk adjacent 
the hole is illustrated in FIGS. 8A and 8B. Silver electrodes 521 are 
printed on both sides of the ceramic disk 520 of the varistor 500. The 
fusible, electrically conductive material disk 531 contacts silver 
electrode 521 of varistor 500. The hole 523 extends through the ceramic 
disk 520. A silver electrode 522 is printed on the outward surface of the 
fusible, electrically conductive material disk 531. One of the leads 510 
contacts the silver electrode 522 and the other lead contacts the silver 
electrode 521 on the opposite side of the ceramic disk 520 from the disk 
531. Upon reaching a predetermined temperature, the disk 531 melts. The 
molten material flows into the hole 523 creating a short circuit between 
the leads. A second fusible, electrically conductive material disk also 
can be located on the opposite side of the ceramic disk 520. It is 
understood that fusible, electrically conductive material disk may be 
located on both sides of the ceramic disk. 
Other embodiments of the invention will be apparent to those skilled in the 
art from consideration of the specification and practice of the invention 
disclosed herein. It is intended that the specification and examples be 
considered as exemplary only, with a true scope and spirit of the 
invention being indicated by the following claims.