# Patent

## Title
Plug-in fuse

## Abstract

A plug-in fuse 200 comprises a body 201 with side indented portions 203 which allow secure gripping by the thumb and finger of a hand. The side indented portions define upper and lower extended portions 319, 321 allowing insertion and retraction forces to be exerted on the fuse. The improved body shape allows quick and easy insertion of the fuse in high-density fuse holders. A thermally-degradable material 323 in close proximity to the burnout portion of the fuse element provides visual and/or tactile indication of fuse condition.

## Background
This application is a continuation-in-part of U.S. patent application Ser. No. 09/638,636, filed Aug. 14, 2000 now U.S. Pat. No. 6,522,234, entitled “Plug-In Fuse”.


BACKGROUND OF THE INVENTION
I. Field of the Invention
The present invention relates generally to the field of electrical fuses and, more particularly, to plug-in fuses with simplified methods to determine fuse condition.
II. Description of the Related Art
Electrical fuses provide reliable and economical protection for electrical wiring and components. Because of their small size and low cost, plug-in fuses have found wide use in industrial and consumer products such as automobiles, aircraft, air conditioning and heating equipment.
Due to the complexity of such applications, manufacturers often utilize fuse blocks or holders having multiple fuse receptacles. In order to minimize space requirements, fuse receptacles are densely packed, allowing minimal space for inspection, insertion and removal of the fuses. Component packaging often dictates placement of the fuse holders in inaccessible areas, increasing the problem of accessing individual fuses for troubleshooting, testing, and replacement.
In the past, inspection of a fuse to determine its condition required removal of the fuse and subsequent testing of the fuse with a meter such as a volt-ohm meter. The removal, testing and re-insertion of a fuse, especially if the fuse holder is in an inconvenient location, can become a tedious chore, increasing the time and cost for troubleshooting and repair of equipment. Close spacing of fuses often requires use of a tool to aid in the removal and/or insertion of the fuse. Such requirement for specialized tools and equipment prevents inexperienced users from performing an otherwise simple task.
Various fuse designs have been introduced to address the problem of inspecting the condition of a fuse, and to make the fuse easier to remove and re-install. U.S. Pat. No. 4,604,602 discloses a plug-in fuse with an overhanging upper portion to improve grasping of the fuse. U.S. Pat. No. 4,670,729 discloses a plug-in fuse with a transparent case to allow visual determination of the condition of the fuse link. U.S. Pat. No. 4,308,516 discloses a plug-in fuse with a high-resistance, heat responsive member which changes color upon heating. U.S. Pat. No. 5,598,138 discloses a fault-indicating blade fuse utilizing a LED or incandescent bulb which lights upon opening of the fuse element.
Despite a significant number of fuse designs introduced, there remains a significant need for improvement in fuse condition monitoring and fuse removal/insertion features.
SUMMARY OF THE INVENTION
In accordance with the present invention and the contemplated problems which have and continue to exist in this field, an object of the present invention is to provide a plug-in fuse which can be easily removed and inserted into a high-density fuse holder without the need for special tools or equipment.
A further object of the present invention is to provide a plug-in fuse for automotive, aircraft, industrial equipment and consumer equipment with multiple means to quickly determine the fuse condition without special tools or equipment.
A further object of the present invention is to provide a plug-in fuse which utilizes the heat produced by the burnout of the burnout portion to produce a visual indication of fuse condition.
A further object of the present invention is to provide a plug-in fuse which utilizes the heat produced by the burnout of the burnout portion to produce a tactile indication of fuse condition.
A further object of the present invention is to provide a plug-in fuse with a transparent body to provide a visual indication of fuse condition.
Another object of the present invention is to provide a fuse in which the condition can be determined, and replacement made if necessary, by a person of limited experience.
The fuse of the current invention comprises a fusible link or burnout portion, and at least two terminals connected to the burnout portion by connecting portions. The burnout portion is enclosed in a fuse body and the two terminals extend from the bottom of the fuse body in the preferred embodiments. The shape of the fuse body is necked between the top and bottom of the fuse body by a side-indented portion on each side of the fuse.
The side-indented portions provide grip surfaces for the thumb and finger of a hand.
Side-extended portions above and below the side-indented portions provide ledge surfaces which allow push forces to be exerted by the thumb and finger to insert the fuse in a fuse holder, and for pull forces to be exerted by the thumb and finger to remove the fuse from the holder. Similarly, the front and back faces of the fuse may be necked to allow gripping of the fuse body by the faces.
In the preferred embodiments, the top portion of the fuse contains a heat or thermally-degradable material in close proximity to the burnout portion. Heat from burnout of the burnout portion conducted to the thermally-degradable material raises the temperature of the thermally-degradable material sufficiently to provide visual and/or tactile changes in the thermally-degradable material. The thermally-degradable material may be disposed as a layer on the upper portion of the body or, alternatively, the upper portion of the body, a substantial portion, or the entire body may be made of the thermally-degradable material. In other embodiments, the burnout portion may be in contact with the thermally-degradable material. In still other embodiments, a heat-conductive material may be disposed between the burnout portion and the thermally-degradable material, hi the preferred embodiments, the body portions comprising the thermally-degradable materials are on the top of the fuse, or another external surface of the fuse which is easily seen, and/or felt by, a user when installed.
The thermally-degradable material may be a material which degrades to produce tactile changes on the surface of the body of the fuse. Or, the thermally-degradable material may be a material which decomposes to form discolored gaseous or particulate products observable in a chamber enclosing the burnout portion of the fuse element.
In the preferred embodiments, a cavity in the body encloses the burnout portion. The body of the fuse is made of a transparent or translucent material to allow visual observation of the condition of the burnout portion or discolored burnout components or gas. External electrical contacts, connected to the connecting portions of the burnout portion, allow electrical testing of the fuse without removal of the fuse from the fuse holder.
Other objects, advantages and capabilities of the invention will become apparent from the following description taken in conjunction with the accompanying drawings showing the preferred embodiment of the invention.



BRIEF DESCRIPTION OF THE DRAWINGS
These and other features, aspects and advantages of the present invention will become better understood with regard to the following description, appended claims and accompanying drawings where:
FIG. 1 is a front elevation drawing of a plug-in fuse of a prior art design having a burnout portion enclosed by a body with a single gripping ledge;
FIG. 2 is a perspective drawing of a plug-in fuse of the present invention having side-indented portions for gripping with a thumb and finger of a hand, the plug-in fuse being inserted into a fuse holder;
FIG. 3A is a top view of an embodiment of the plug-in fuse of the present invention having side-indented portions, top and bottom side-extended portions, and a burnout portion enclosed by the body of the fuse, the figure showing the relative widths of the side indented portions, the side extended portions, and the thickness;
FIG. 3B is a front elevation drawing of the plug-in fuse of FIG. 3A;
FIG. 3C is a side elevation drawing of the plug-in fuse of FIGS. 3A and 3B;
FIG. 4 is a front cross-sectional view of an alternative embodiment of the fuse of the present invention showing the proximity of the burnout portion to the thermally-degradable material to provide visual and tactile indication of the fuse condition, and,
FIG. 4A is a detail cross-sectional drawing of the burnout portion and thermally-degradable material portion of FIG. 4 and showing the optional thermal coupling element coupling heat from the burnout portion to the thermally-degradable material.



DETAILED DESCRIPTION OF THE INVENTION
Referring to the drawings wherein like reference numerals designate corresponding parts throughout the several figures, reference is made first to a plug-in fuse with improved grip for quick insertion and removal in fuse holders.
FIG. 1 is an elevation drawing of a typical plug-in fuse 100 of prior art commonly used in automotive and commercial applications. The prior art fuse comprises a body 101, terminals 103 and a fusible link or burnout portion 105. Body 101 comprises a single ledge surface 107, used to grip the fuse for removal and insertion. Fuses of this general type are difficult to remove from a fuse block, especially if the fuse packing density is large.
FIG. 2 is a perspective drawing of a preferred embodiment of a plug-in fuse 200 of the present invention. Side-indented portions 203 of body 201 provide recesses in the side of fuse body 201, allowing secure gripping of fuse 200 for insertion and removal of fuse 200 in holder 205. FIGS. 3A, 3B and 3C are top, front and side views, respectively, of the fuse of FIG. 2. Fuse 200 comprises body 201 having an upper body portion 301 and a lower body portion 303. Fuse element 305 comprises a burnout portion 307, two terminals 309 extending downward from lower body portion 303, and fuse connecting portions 311 connecting terminals 309 with burnout portion 307.
In the preferred embodiments, fuse burnout portion 307, fuse connecting portions 311, and the upper portions of fuse terminals 309 are encapsulated in fuse body 201. Body 201 may be molded or die cast around fuse element 305, or, body 201 may be a multi-part structure assembled with is fuse element 305 interior to body 201.
Body 201 comprises a front face portion 313 and back face portion 315 defining thickness 317. Thickness 317 may be a uniform thickness over the front and back faces, or it may be a maximum thickness as shown in FIG. 3C.
As best seen in FIG. 3B, sides 329 of body 201 comprise upper side extended portions 319, side indented portions 203, and lower side extended portions 321. The width 323 of side indented portion 203 is less than the width 325 of upper side extended portion 319 and the width 327 of lower side extended portion 321. The maximum width dimension, normally width 327 of lower extended portion 321, is greater that maximum body thickness 317.
Side indented portion 203, having a width less than upper side extended portion 319 and lower side extended portion 321 provides secure grip surfaces for the thumb and finger of a hand in inserting and removing fuse 200 from a fuse holder. Extended portion 319 provides an extension or ledge for the thumb and finger to exert a pulling or removal force on body 201. Extended portion 321 provides an extension or ledge for the thumb and finger to exert a push or insertion force to be exerted on fuse body 201. In other embodiments, face indented portion 327 of FIG. 3 has reduced thickness as compared to upper body portion 301 and lower body portion 303 to aid in insertion and removal of fuses when face-to-face clearance allows.
A condition-indicating layer 323, adjacent to burnout portion 307 provides a thermally induced indication of burnout or opening of burnout portion 307. Heat generated by excessive current through burnout portion 307 is transmitted to layer 323 by thermal conduction of body portion 326 and proximity of burnout portion 307 to layer 323.
Condition-indicating layer 323 may be a material which provides a thermally induced coloration or light transparency change in the material. Thermally-degradable materials which degrade to produce permanent color changes upon over-temperature are known in the art and include thermosetting plastics such as epoxies. An example of materials which provide coloration change upon over-temperature are acrylic enamels and lacquers.
Condition-indicating layer 323 may also be a thermally-degradable material which produces changes in the layer that can be detected tactilely, for example by the fingers of one's hand. In a preferred embodiment, layer 323 is a polymeric material with a thermal degradation temperature less than, or equal to, the temperature attained in layer 323 due to burnout of burnout portion 307. The thermally-degradable layer is a material such as a thermally-degradable material selected to produce bubbling, wrinkling, or other roughness or surface irregularities (451 of FIG. 4A) upon reaching the temperature resulting from fusible link burnout, hi this manner, touching of the layer 323 on the top of the fuse 200 provides an indication of the condition of the fuse.
In the preferred embodiments, the thermal degradation temperature of the material of layer 323 is chosen to be less than the thermal degradation temperature of body portion 326. In still other embodiments, body 201 is made of the thermally-degradable material and layer 323 is integral to body 201. Thermosetting polymers have been found to produce a tactile-detectable roughness when exposed to the heat of fuse link burnout. An example of materials which produce tactile-detectable roughness when exposed to over-temperature resulting from fuse link burnout include acrylic lacquers and epoxies. Other paint and coating materials have been found to provide tactile-detectable roughness when exposed to elevated temperatures.
FIG. 4 is a side elevation drawing of fuse 400 showing fuse body 401 with upper side extended portion 419, side indented portion 403, and lower side extended portion 421. In the preferred embodiments, fuse 400 comprises an air space or chamber 441 internal to fuse body 401 which encompasses burnout portion 407. Said burnout portion 407 is connected to terminals 409 via connecting portions 411. Said connecting portions 411 are in contact with thermally-degradable material 423. Chamber 446 provides volume for fusible link 407 material when melted and reduces heat transfer to body 401. Chamber 441 also provides an expanded volume for visual materials resulting from burnout of burnout portion 407 such as burned particulate matter and gasses. Close proximity of burnout portion 407 to thermally-degradable material 423 provides sufficient temperature change in material 423 to provide the desired visual or tactile response discussed in the previous embodiment. In other embodiments, burnout portion 407 contacts thermally-degradable material 423. In still other embodiments, most or all of body 401 is made of a thermally-degradable material.
As best seen in FIG. 4A, spacing 443 between burnout portion 407 and thermally-degradable material 423 is chosen to provide sufficient transfer of thermal energy to material 423 to provide the desired effect, either visual or tactile, or both. In the preferred embodiments, spacing 443 is less than 0.20′′. In more preferred embodiments, spacing 443 is less than 0.10′′, and in still more preferred embodiment, spacing 443 is less than 0.05′′. In an alternative embodiment, burnout portion 407 contacts thermally-degradable material 423, or it may be partially or fully embedded in material 423. In still other embodiments, a thermal-coupling material 445 such as a metallic, ceramic or polymeric material with a high thermal conductivity may be used to transfer heat from burnout portion 407 to thermally-degradable material 423. Thermal-coupling material 445 allows placement of a thermally-degradable material indicator on another portion of the fuse, such as the side of the fuse body, etc.
Still another condition-indicating feature of the plug-in fuse of the present invention is a “clouding” or discoloration within chamber 441 due to gaseous and particulate matter formed upon decomposition of thermally-degradable material 423. Material 423 may be a thermally decomposable material chosen to produce discolored gasses and particulate matter when heated due to the proximity of burnout portion 407 to surface 444 of thermally-degradable material 423. Production of discolored particulate material results in condensing of the material as a discolored film 446 on the surface of chamber 441. The discolored film 446 is visible through transparent or translucent body material 448. Discolored gasses, trapped in chamber 441 also produce a “clouding” of the chamber visible through material 448. In an alternative embodiment, a thermally-degradable material 423A producing discolored gasses and/or particulate matter may be disposed on burnout portion 407 as shown in FIG. 4A. Material 423 and 423A may be organic material comprising carbon to produce the desired discoloration by-products. Examples of materials producing desired discoloration by-products upon degradation of heat resulting from burnout of the burnout portion are acrylic lacquers and epoxies.
Body 401 may be molded, cast, formed or fabricated from appropriate materials such as plastic. In the preferred embodiments, body 401 material is a transparent plastic material tinted to indicate fuse rating. Use of a transparent or translucent plastic allows visual inspection of burnout portion 407 and observation of burned materials resulting from burnout of burnout portion 407. External electrical contacts 447, extending from either side of burnout portion 407 provide an electrical method to check burnout portion 407. A sharp corner 453 at the upper extended portion 419 of fuse body provides improved grip of the fuse. Included angle 455 of corner 451 may be chosen less than 90 degrees to further improve grip.
Accordingly the reader will see that the PLUG-IN FUSE of the current invention provides a fuse for quick and convenient removal and insertion in high-density fuse holders. The device provides the following additional advantages:

    
    
        (1) The fuse provides both visual and tactile indication of fuse burnout, reducing time required to determine the fuse condition;
        (2) No special tool or equipment are needed to remove, insert, or check indication of the fuse;
        (3) The fuse replaces existing plug-in fuses; and
        (4) The fuse is simple and low in cost.
    
    


Although the description above contains many specifications, these should not be construed as limiting the scope of the invention but as merely providing illustrations of some of the presently preferred embodiments of this invention. Thus the scope of the invention should be determined by the appended claims and their legal equivalents, rather than by the examples given.
Therefore, the foregoing is considered as illustrative only of the principles of the invention. Further, various modifications may be made of the invention without departing from the scope thereof and it is desired, therefore, that only such limitations shall be placed thereon as are imposed by the prior art and which are set forth in the appended claims.

## Claims

1. A plug-in fuse for protecting electrical circuits, the fuse insertable and removable from a fuse holder, the fuse comprising:
a main fuse body comprising a first body material allowing light transmission through at least a portion of the body;
a fuse element disposed in an enclosed chamber in the body, the fuse element comprising a burnout portion, at least two terminals and at least two connecting portions connecting said at least two terminals to said burnout portion,
the chamber enclosing the burnout portion of the fuse element; and
a thermally-degradable material disposed on a portion of the body, the material being separated from the burnout portion by a thermally conducting airspace, the airspace being of a preselected distance sufficient to allow transfer of heat created upon the burnout of burnout portion, the heat causing the thermally degradable material to change color and cause dimensional irregularities on the material, the thermally degradable material further being in partial contact with the coupling portions away from the burnout portion,
wherein a portion of the thermally degradable material, the airspace and the burnout portion are generally straight and parallel with each other.

2. The plug-in fuse of claim 1 wherein the thermally-degradable material is disposed on at least a portion of a surface of the chamber.

3. The plug-in fuse of claim 1 wherein the first body material is a transparent material.

4. The plug-in fuse of claim 1 wherein the first body material is a translucent material.

5. The plug-in fuse of claim 1 further comprising a thermal conducting material in thermal contact with the thermally degradable material, the thermal conducting material coupling the thermally degradable material to additional thermally degrading material disposed in other portions of the body.

6. The plug-in fuse of claim 5 wherein the thermally degradable material is disposed on an outer surface of the body, the thermally degradable material showing color changes and a rippling upon blowout of the fuse.

7. The plug-in fuse of claim 6 wherein the additional thermal degradable material is disposed on an inside surface of the enclosed chamber in the fuse, the thermally-degradable material producing discolored by-products upon transfer of heat from burnout of the burnout portion of the fuse element whereby the condition of the fuse may be determined by a change in light transmission through the first body material.

8. The plug-in fuse of claim 1 further comprising a thermal coupling material disposed between and in thermal contact with the burnout portion and the thermally-degradable material.

9. The plug-in fuse of claim 8 wherein the thermal coupling material is metallic.

10. The plug-in fuse of claim 8 wherein the thermal coupling material is ceramic.

11. The plug-in fuse of claim 8 wherein the thermal coupling material is polymeric.

12. The plug-in fuse of claim 1 further comprising electrodes protruding from the body adjacent the thermally-degradable material, and being connected to the connecting portions, the electrodes providing an external connection points for testing the burnout portion.