Time-current characteristics variable chip fuse

A chip fuse is disclosed which includes a rectangular parallelepiped box shaped main body with an opening on the upper side thereof, and a cover covering the opening. Two main terminals are disposed on opposite shorter side walls of the main body. A plurality of subsidiary terminals are located along inner faces of the opposite longer walls of the main body. A fusible element is extended, inside the main body, between one of the main terminals via a some or all of the plurality of subsidiary terminals and the other of the main terminals in a zigzag manner.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a subminiature chip fuse capable of being 
directly soldered to a printed circuit board for use. 
2. Description of the Prior Art 
In conventional chip fuses, a fusible element is stretched between metal 
terminal plates placed apart at a certain distance on a plane in such a 
manner that the end surfaces of the metal terminal plates oppose each 
other (for instance, refer to the official gazette of Japanese Utility 
Model Laid-Open No. 119546/1984) or wherein a pair of main electrodes are 
provided at ends of a flat chip board with a pair of auxiliary electrodes 
being provided at an intermediate portion between the main electrodes, and 
wires are connected between these main and auxiliary electrodes so as to 
be bonded thereto (for instance, refer to the official gazette of Japanese 
Patent Laid-Open No. 172626/1987). 
In the former type of conventional chip fuse described above, only a narrow 
gap is feasible between the terminals in which a fusible element is 
extended due to the small size of a fuse body which is required in a chip 
fuse. As a result, in relation to the time-current characteristics 
thereof, an excessively fast fuse response tends to be exhibited, thereby 
causing a risk of the fusible element being fused by a rush current 
generated by switching during a normal operation. 
In addition, the narrow gap necessitates the use of a short fusible 
element, which results in a fusible element having a low resistance. This 
requires a fusible element having a smaller diameter to be employed when 
attempting to produce a fuse with a lower current value. However, the 
extremely small diameter which is required for such a fusible element is 
impossible to manufacture. Even though a fusible element with such an 
extremely small diameter thereof may be manufactured, it would be 
extremely difficult for it to be assembled into a fuse. 
In the latter type of conventional chip fuse, the surface of the wires is 
covered with a transparent resin that is curved in a lens fashion, and 
this transparent resin deprives the wires of Joule's heat. This makes the 
fusing performance thereof unstable. In addition, since there are provided 
only a pair of auxiliary electrodes, as in the case of the former chip 
fuse, an excessively fast fuse response is exhibited, and it is not 
possible to modify an such response time. 
SUMMARY OF THE INVENTION 
In view of the above problems that are inherent in the prior art chip 
fuses, an object of the present invention is to provide a chip fuse that 
may be easily produced, and which has variable time-current 
characteristics and exhibits a stable fusing performance. 
This object is achieved by a time-current characteristics variable chip 
fuse comprising: a main body having a wall and a cavity defined by said 
wall in said main body; a pair of main terminals provided on said wall in 
such a manner that said pair of main terminals penetrate through said 
wall, and are opposed to eahc other, said pair of main terminals having 
end portions for electrical connection to an external circuit at the outer 
side of said wall; a plurality subsidiary terminals being located in said 
cavity at each of both sides of a line extending from one of said pair of 
main terminals to the other of said pair of main terminals; and a fusible 
element having both end portions which are respectively secured and 
electrically connected to said pair of main terminals, said fusible 
element being provided in said cavity in such a manner that said fusible 
element is extended between one of said pair of main terminal via some or 
all of said plurality subsidiary terminals and the other of said pair of 
main terminals in a zigzag fashion so as to be secured to said some or all 
of said plurality subsidiary terminals. 
In addition, said fusible element may comprise a section extending between 
said subsidiary terminals and/or a section extending between a said 
subsidiary terminal and said main terminal, the melting point, thickness 
and/or resistivity of said sections or portions of one of said sections 
being partially or wholly different from one another. 
Thus, the actual length of the fusible element can be made significantly 
greater than the distance between the main terminals. In addition, the 
length of the fusible element is determined depending on the number of 
subsidiary terminals used, and the time-current characteristics thereof is 
designed to vary depending on the length of the fusible element so 
determined. As a result of this, the time-current characterstics of the 
fusible elemtn can be designed in accordance with required circuit 
protection characteristics. Furthermore, it is possible to improve the 
time-current characteristics and to obtain a smaller rated current value 
by extending the fusible element in the space formed inside the main body. 
These and other objects and advantages will become clear by reading the 
following description of the invention with reference to the accommpanying 
drawing.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring to the drawings, the present invention will be described. FIG. 1 
is a drawing showing the appearance of a chip fuse according to the 
present invention that has an extremely small main body which is 2.6 mm 
wide, 5 mm long and 2 mm high. 
FIG. 2 shows the inside of the main body of the present invention with a 
cover being removed for good visibility. FIGS. 3 and 4 are plan and 
cross-sectional views of the chip fuse without a cover illustrated in FIG. 
2, respectively. Referring to FIGS. 2 to 4, the main body 1 is of a 
rectangular parallelepiped box shape and has an opening 2 on the upper 
side of the main body 1, opposite shorter side walls 3 and opposite longer 
side walls 4. A pair of main terminals 5 are disposed on the opposite 
shorter side walls 3 in such a manner that the terminals 5 penetrate 
through the opposite shorter side walls 3 so that both end portions of a 
fusible element 6 made of a fusible wire are respectively secured to both 
main terminals 5, 5 on the inner surface of the shorter side walls 3, 
while the main terminals 5 are directly soldered to an external circuit on 
the outer surfaces of the shorter side walls 3. 
A plurality of subsidiary terminals 7 are fixed to the inner surfaces of 
the opposite longer side walls 4 of the main body 1 of a rectangular 
parallelepiped box shape when the body 1 is formed by a molding process. 
The plurality of subsidiary terminals 7 are disposed on the inner surface 
of the longer side walls 4 at equal intervals. These subsidiary terminals 
7 serve as junction terminals when the fusible element 6 is extended 
between the pair of main terminals 5, 5. Thus, the fusible element 6 is 
extended from one of the pair of terminals 5 via the plurality of 
subsidiary terminals 7 to the other of the pair of terminals 5 in a zigzag 
fashion so as to be secured to the main terminals 5 and the plurality of 
subsidiary terminals 7. After the fusible element 6 has been extended 
between the main terminals 5 and secured thereto in this manner, an 
opening 2 of the box-shaped main body 1, which is the top side thereof, is 
hermetically sealed with a cover 8 and a chip fuse is thus completed. 
In a case where the fusible element 6 is extended between the main 
terminals 5 in a zigzag fashion by making use of the subsidiary terminals 
7, although the distance between the main terminals 5 is so short that 
they are disposed 5 mm apart from each other in order to meet the 
requirement for miniaturization of a chip fuse main body, the actual 
length of the fusible element is made equal to or greater than 20 mm, and 
thus, it is possible to obtain a fusible element length that is four or 
more times greater than that of a conventional chip fuse. As a result of 
this, the diameter of the fuse wire of the present invention may be 
increased by a factor of two or more when compared with that of a fuse 
wire used with the prior art chip fuse, and similarly, the time-lag 
characteristics of the chip fuse of the present invention can also be 
improved by a factor of two or more when compared with that of the prior 
art chip fuse. 
FIg. 5 shows another embdiment of the present invention in which the 
fusible element 6 is caused to transit the subsidiary terminals 7 in a 
fashion different from that described above with the thickness thereof 
being varied at an intermediate portion along the length thereof. In other 
words, a thick fusible element 6A is extended between the main terminal 5A 
and one of the subsidiary terminals 7A at one of the longer side walls 4 
of the main body 1 and in turn mechanically connected to both of the 
terminals 5A, 7A, a thin fusible element 6B is then extended between the 
subsidiary terminals 7A and the other of the subsidiary terminals 7B at 
the other of the longer side walls 4 of the main body 1, which is 
positioned opposite the subsidiary terminals 7A, and in turn mechanically 
connected to both the terminals 7A, 7B, and a fusible element 6C which is 
as thick as the fusible element 6A extended between the main terminal 5A 
and the subsidiary terminal 7A is extended between the subsidiary terminal 
7B and the other main terminal 5B and in turn mechanically connected to 
both terminals 7B, 5B. Thus, in case where a super fast response as the 
time-current characteristics of a chip fuse, are required to protect a 
semiconductor device with the present invention, the distance between the 
subsidiary terminals can be reduced to 0.6 mm, which is extremely short, 
and also the thickness of a fusible element used therebetween can be 
varied thereby making it possible to obtain superior super quick acting 
characteristics. 
On top of this, it is also possible to make a chip fuse function as 
high-performance fusing resistors by using a wire with high resistance at 
a portion along the length of a fusible element. Furthermore, such a 
portion along the length of a fusible element extended can be constructed 
by a fusible wire with a melting point which is different from the one of 
the remaining portion of the fusible element according to various 
applications. 
With prior art fusing resistors, it takes 60 to 90 seconds before fusion is 
effected under a load of ten times greater than the rated power therefor, 
and with some of those fusing resistors, the temperature at the time of 
fusion increase to become excessively high and reaches 200.degree. C., 
causing a major problem of putting a substrate and other peripheral 
components under stress. In contrast, however, with the fusing resistors 
according to the present invention, an abnormal current is cut off within 
a second even under a load four times greater than the rated power 
therefor, and the temparature at the time of fusion increases little. 
Thus, the present invention exhibits a superior performance in protecting 
a circuit. 
With the chip fuse of the present invention constructed as described above, 
various types of time-current characteristics, from time-lag 
characteristics to super quick acting characteristics, may be obtained 
with a single main body configuration, and a function as a fusing 
resistors may also be added thereto. Furthermore, with the present 
invention, fusible elements with smaller rated current values may easily 
be manufactured. 
The present invention has been described in detail with reference to 
certain preferred embodiments thereof, but it will be understood that 
various modifications can be effected within the spirit and scope of the 
invention.