Electrical fuse component and method of using same

An electrical component has a plurality of paired contacts with spaced inner contact sections bridged by a fuse element. The paired contacts have outer contact sections enabling mounting to a circuit panel to complete circuit paths thereon. The component is used as a fuse component to protect the circuit panel by preventing overload of a circuit path by the respective fuse element. When the circuit paths have different design in-service currents and different levels of excessive electrical currents, the corresponding fuse elements can have correspondingly different in-service current-carrying capabilities and different melting points, in the same component. The component can be surface mountable and can have DIP configuration.

The present invention is related to the field of electrical components, and 
more particularly is related to the field of components for circuit 
panels. 
BACKGROUND OF THE INVENTION 
Circuit paths of a circuit panel presently are protected in one manner 
against excessive electrical load by a fuse located in the power supply. 
It is a known problem that in such situations the circuit panel, or one of 
its paths, may receive an electrical load excessive to it which could not 
be protected against by the single fuse of the power supply, and the 
entire circuit panel becomes ruined and must be replaced. This is 
especially true where different paths of the same circuit panel have 
different design in-service load-carrying capabilities receiving voltage 
and current from the same power supply, such as -5 volts, +5 volts, +12 
volts and +24 volts so on, and 1 ampere, 1.2 amperes, 3 amperes and 0.5 
amperes. Another manner of protecting an individual circuit path presently 
known is by using a discrete fuse such as a Subminiature Picofuse (125 V), 
part number 275001 sold by Littelfuse, Inc. of Des Plaines, Ill. Such a 
fuse has pigtail leads allowing mounting to sockets of a circuit panel. 
Several circuit paths could be protected by a like plurality of such 
discrete fuses. But such fuses are relatively large considering the trend 
to very closely spaced centerlines such as 0.050 inches to conserve 
valuable real estate on a circuit panel. 
It is desirable to provide a fuse component mountable to a circuit panel to 
protect a plurality of circuit paths, which component is disposable and 
replaceable upon fuse failure after receipt of an excessive electrical 
load, allowing continued use of the circuit panel. It is also desirable to 
provide one component to protect several circuit paths having different 
in-service and maximum loads. 
SUMMARY OF THE INVENTION 
The electrical fuse component used in the method of the present invention 
has pairs of contacts having outer contact sections electrically 
engageable with contact sections (such as sockets or conductive pads) of 
circuit paths of a circuit panel such as a circuit board or a flexible 
panel. Inner contact sections are in spaced pairs in respective cavities 
of the housing and are bridged by a fuse element secured thereto in 
electrical engagement therewith. The fuse element for each pair of 
contacts has a selected in-service current-carrying capability 
corresponding with that of the circuit path whose circuit it completes 
upon the component being secured to the circuit panel, and also has 
characteristics selected to cause opening of the fuse by melting upon 
receipt of an electrical current excessive either in level or duration or 
both. 
According to the method of using the electrical component of the invention, 
the fuse component serves to protect the circuit paths of the circuit 
panel from excessive in-service electrical current. During normal 
in-service functioning the circuit panel has the fuse component mounted 
thereon bridging all of the circuit paths, and upon receipt of an 
electrical current excessive in level or duration on one of the circuit 
paths the fuse element normally completing the circuit of the one circuit 
path will melt and open, protecting the circuit panel and other electronic 
components on the circuit path. The entire fuse component is then removed 
and another is secured in its place. 
One embodiment of the electrical fuse component of the present invention 
provides fuse elements having different selected in-service 
current-carrying characteristics and different melting points, 
corresponding to different in-service capabilities of the various circuit 
paths to be protected, and different currents deemed excessive in level or 
duration or both.

DETAILED DESCRIPTION OF THE INVENTION 
FIGS. 1 and 2 show the electrical component 10 usable with the method of 
the present invention. Such an electrical component usable with the method 
of the present invention is disclosed in U.S. patent application Ser. No. 
857,204 filed Apr. 29, 1986. Electrical component 10 has a housing 12 and 
a plurality of pairs of contacts 14A,14B spaced therealong preferably in a 
dual in-line arrangement. Each contact 14A has an inner contact section 
16A spaced proximate an inner contact section 16B of its corresponding 
contact 14B. A fuse element 30 extends between the pair of inner contact 
sections 16A,16B and is mechanically secured thereto at joints 32 in 
electrical engagement therewith. Each of the pair of contacts 14A,14B also 
has an outer contact section 18A,18B respectively for electrical 
engagement with a corresponding contact means of an electrical circuit 
path on a circuit panel disposed in a component receiving region thereof. 
Inner contact sections 16A,16B are preferably disposed in respective 
cavities 20 of housing 12. The electrical component is preferably sealed 
by securing a sealing member 22 to the top 24 of housing 12 and a sealing 
member 26 to the bottom 28 of the housing. Outer contact sections 18A,18B 
are vertical leg sections for insertion into holes 54 which form part of 
the circuit paths 52 on a rigid circuit panel such as a printed circuit 
board 50 with paths 52 shown in phantom in FIG. 1 and disposed on the 
bottom surface of board 50. The fuse element 30 is preferably secured for 
performance reasons to the top surfaces of inner contact sections 16A,16B 
and arced upwardly away from the ends of the contact sections. 
FIG. 3 illustrates an alternate embodiment where fuse element 30 is secured 
by joints 32 to bottom surfaces of the inner contact sections 46A,46B of 
contacts 44A,44B, in housing 42 of component 40. Because the walls of the 
housing must have a height sufficient to enable handling by automated 
handling apparatus for positioning on a circuit panel, it is easier during 
fabrication of the fuse shunt to secure the fuse element 30 to the bottom 
surfaces of contact sections 46A,46B which are proximate the bottom face 
of the fuse shunt. Contacts 44A,44B include outer contact sections 48A,48B 
which comprise a pair of coplanar horizontally extending sections, for 
surface mounting to conductive pads 64 of the circuit paths 62 on the top 
surface of a circuit panel such as flexible circuit panel 60. 
The fuse element is secured to the inner contact sections such as by 
conventional resistance welding or wire bonding techniques to form joints 
32. Another method of joining the fuse element is disclosed in U.S. patent 
application Ser. No. 857,209 filed Apr. 29, 1986. In that method the fuse 
element is a wire segment first disposed in a groove skived axially along 
the inner contact sections and then terminated by deforming portions of 
the inner contact sections forming sidewalls of the groove over the top of 
the wire at at least one location on each wire end section by means of a 
bifurcated terminating tool, or alternatively by a flat-ended terminating 
tool. Various methods based on conventional techniques may be used so long 
as heat is not generated in sufficient amounts to inadvertently open or 
damage the fuse element which is fragile requiring care in handling and 
processing. 
It is believed preferable to secure the fuse element to the pair of contact 
sections after first securing the contacts in the housing so that the 
housing provides mechanical stability and enhances physical protection of 
the fragile wire fuse element during fabrication as is shown in FIGS. 4A 
to 4C. While joined to a carrier strip 70, the contacts 14A,14B are 
preferably placed in a mold and a dielectric housing 12 molded thereto by 
conventional insert molding techniques as shown in FIG. 4B. Fuse elements 
30 are then secured to respective contact sections as in FIG. 4C. Thin, 
transparent sealing membranes 22,26 are then preferably adhered to the top 
and bottom surfaces 24,28 respectively of housing 12 completing the 
manufacture of the electrical component as in FIG. 4D. The completed 
components can then be severed from the carrier strip and the outer 
contact sections 18A,18B formed into the desired configuration. The 
contacts 14A,14B are preferably stamped from a strip of copper alloy, and 
outer contact sections 18A,18B may be tin-lead plated for solderability. 
Housing 12 may be formed of a thermoplastic material such as glass-filled 
polyester resin. Sealing membranes 22,26 may be MYLAR (trademark of E. I. 
du Pont de Nemours and Company). 
Fuse element 30 is preferably a wire segment of a selected very small 
diameter creating high resistance, and may be any of several conventional 
types of conductive metals such as high copper content alloy, aluminum, 
silver alloy, or constantan. The proper material to be used, and the 
actual diameter selected depend on the type of current desired to be 
carried by the fuse during normal in-service use and also the current 
deemed excessive for the circuit path, at which the fuse element is 
designed to open. For example, a satisfactory fuse element can be a short 
length of aluminum wire having a diameter of 0.0007 inches if it is 
desired that the fuse carry an in-service current of 0.1 amperes and open 
upon receiving a current of 1.0 ampere for 100 milliseconds or less. A 
satisfactory fuse element can be a short length of constantan alloy having 
a diameter of 0.0015 inches for the same in-service and excessive 
currents. 
The fuse element opens by melting upon sufficient heat buildup resulting 
from occurrence of a designed excessive current passing through its very 
small diameter and limited length for a sufficient time. It is possible to 
estimate an appropriate small diameter for the fuse element after the 
following items are selected or determined: the excessive level of current 
(I.sub.e) and duration of excessive current (t), length of the fuse (L), 
ambient temperature (T.sub.a), and metal alloy being used for the fuse. 
Characteristic properties of the metal alloy are ascertained; melting 
temperature (T.sub.m), specific heat (Cp), latent heat of fusion 
(Q.sub.f), resistivity (.rho.), and specific gravity (SG). 
The heat required to melt the fuse is related to the fuse element 
dimensions and properties as follows: 
EQU HEAT=MASS.times.[(T.sub.m -T.sub.a)Cp+Q.sub.f ] (1) 
where the mass of the fuse element is 
##EQU1## 
The power generated by the current through the fuse is: 
EQU POWER=I.sub.e.sup.2 R (3) 
where 
##EQU2## 
and the power and heat to melt are related to each other as: 
EQU HEAT=POWER.times.t (5) 
EXAMPLES 
Where the metal alloy considered is constantan, its characteristic values 
are as follows: 
TABLE 1 
______________________________________ 
Constantan 
______________________________________ 
SG (specific gravity) 
0.323 lb/ft.sup.3 
T.sub.m (melting temperature) 
2210.degree. F. 
Cp (specific heat) 0.098 Btu/lb/.degree.F. 
Q.sub.f (latent heat of fusion) 
100 Btu/lb 
.rho. (resistivity) 374 .OMEGA. mil.sup.2 /ft 
______________________________________ 
Typical values for the remaining variables relevant to the present 
invention and its purpose and typical enviroment, are: 
TABLE 2 
______________________________________ 
t (duration of excessive current = 
100 milliseconds 
until designed melting of fuse at l.sub.e) 
L (fuse length) = 0.10 inches 
T.sub.a (ambient temperature) = 
75.degree. F. 
______________________________________ 
Because adjacent contact structure at the terminations 32 of the ends of 
fuse element 30 is at theoretical ambient temperature, the contacts act as 
heat sinks and absorb some of the heat from the fuse element when current 
passes therethrough, effecting the occurrence of melting. 
It was desired to find the satisfactory diameters of constantan alloy wires 
designed to melt upon receipt of electrical currents of 1.0 ampere and 2.0 
amperes respectively for 100 milliseconds or less which would be typical 
excessive current levels and duration for circuit paths having designed 
in-service currents of 100 milliamperes and 200 milliamperes respectively 
while such fuse elements carry the designed in-service currents. 
It is believed that satisfactory diameters for a constantan fuse element 
are: 
Where I.sub.e =1.0 ampere, D.sub.1 =0.0015 inches 
Where I.sub.e =2.0 amperes, D.sub.2 =0.0021 inches 
It should be noted that fuse melting will also occur where over a long 
period of time, an electrical current is carried by the fuse element 
substantially higher than the in-service current which it is designed to 
carry but less than the current (I.sub.e) at which it was designed to open 
or melt in 100 milliseconds or less. Values for such intermediate 
excessive current levels and time-to-melt are dependent on variables such 
as heat transfer, tolerances in wire diameter and length due to 
manufacturing and termination respectively, and surface contamination on 
the fuse element. Heat transfer refers to the removal or dissipation of 
heat being built up in the fuse element because of resistance during the 
receipt of electrical current, by reason of adjacent contact structure, 
nearby component structure and the circuit panel, and possible cooling 
procedures utilized on the circuit panel. 
To avoid interfering with the opening of a fuse element, upon occurrence of 
the excessive current for which it was intended to melt, the element 30 
should be preferably spaced away from the ends of the inner contact 
sections 16A,16B and also from any of the structure of the housing 12 or 
seals 22,26 which would act to dissipate heat otherwise needed to melt the 
fuse. The seals 22,26 serve to physically protect the fragile fuse 
elements 30, and contan any vapors given off during the fuse melting and 
avoid possible contamination of nearby circuitry or components outside of 
component 10. 
FIG. 5 illustrates a circuit panel 80 having several segments 82 each 
having a set of circuit paths 84. Separate components 40 are mounted to 
panel 80, one such component 40 completing the set of circuit paths 84 of 
each segment 82 at a component-receiving region 86 thereof. This 
arrangement allows for ease of adaptation of existing circuit 
configurations to accommodate the fuse components of the present invention 
.