Note: Descriptions are shown in the official language in which they were submitted.
The invention relates to fuses, particularly for responding
to external -temperatures to interrupt an electrical circuit~ and is
especially, but not exclusively, applicable -to fuses for protecting
battery Feed resistors connected to telephone lines.
Various protection arrangements are provided to protect
equipmen-t terminating a telephone line against voltage and current surges
on the line~ Nevertheless it is possible for a current overload ~o be
sustained long enough to cause overheating of the battery Feed resistor
which is connected in series with the line~ There is a need, therefore,
for a fuse which will interrupt the line circuit when the temperature of
the battery feed resistor becomes excessive.
An object of the present invention is to provide a Fuse
which is suitable for such a purpose.
According to the present invention a fuse comprises:-
(i) a substrate;
~ii) a pair of electrodes disposed upon the substrate so as to define a
gap ~herebetween;
(iii) a fuse link extending across said gap to interconnect said
electrodes electrically;
0 ~iv) a film of solder overlying said fuse link and overlapping at least
partly at least one of said pair of electrodes;
the fuse link being formed of a material which is soluble in the
molten solder;
the arrangement being such that when its melting point is exceeded
the solder melts, dissolves the fuse link~ and retreats from the
gap, thereby separating the electrodes electrically, Preferably
the film overlaps part of each of said electrodes~
In its app'lication to protecting a resistor~ such as a
battery feed resistor, the fuse would be connected with its e'lectrodes in
series with the resistor and the substrate would be arrangecl to sense the
temperature of the resistor.
The fuse and the resistor may be formed, conveniently using
-thick Film techniques, upon the same substrate, either on the same surface
or on opposite surfaces.
In preferred embodiments the fuse link is a layer of gold,
conveniently deposited onto a substrate of ceramic material, For example
alumina.
A protective film, for example an ultraviolet curable
organic overcoat may be provided over the deviceS especially to retain the
solder pas~e in position and protect it from being removed by subsequent
cleaning processes. The Fuse may be combirled on d single substrate with a
resistor, the temperature of which is to be sensed.
An embodiment of the invention will now be described by way
oF example only and with reference to the accompanying drawing9 in
which:~
Figure 1 shows a plan view oF a fuse operable by indirect
heating rather than internally-generated hea-ting of its fuse
link;
Figure 2 is a perspective view of such Fuses and resistors
combined on a single substrate and;
Figure 3 is a perspective view of simi'lar combination having
a fuse and a resistor on each side of the substrate.
In the drawing a fuse comprises a substrate 10 of ceramic
material, such as about 96% alumina~ The substrate 10 supports two
electrodes 12 and 149 respectively of Pd/Ag alloy thick film paste, l2 to
15 microns Fired -thickness, (e.gO Dupont 6130-Trade Mark) The electrodes
12 and 14 comprlse rectangular portions 16 and 18~ respec-tively, disposed
side-by-side with a gap 20 between them, In a practical ernbodiment the
gap was 0~040 inches wide. Extension arms 22 and 24, respeclively,
project parallel to each o-ther from the outer parts o-F the rectangular
portions 16 and 18 and serve as terminals for connectiny the fuse into an
electrical circuit,
The two electrodes 12 and 14 are interconnecte~ by a fuse
link in -the ~orm of a strip of gold 26 which bridges the gap 20 at its
middle. The gold strip 26 is conveniently deposited as a thick film paste
abou-t 5 to 8 microns fired thickness (e.g. Dupont 4019~Trade Mark), by
conventional techniques to extend across the ceramic substra-te in the gap
20 and to overlie at each end -the upper surface of the respective one of
elec-trodes 12 and 14.
A layer of solder paste 28, about 140 microns thick overlies
the fuse link 26, the gap 20 and a substantial part (typîcally half) of
each of electrodes 12 and 14 adjacent the gap 200 A U.VO curable organic
overcoat 30 overlies at least the solder paste 28, and preferably all of
the fuse area, to protect it.
The solder paste 28 must be chosen to melt at the
temperature at which the Fuse is required to operate, and so that, when
molten, it will leach the fuse link. In the case of a gold fuse link, a
tin content solder is appropriate, for example 62% tin~ 36% lead~ 2%
silver which will melt at about 190C. A flux is also provided
conveniently as a constituent of the paste~ so that the molten solder
~7~1L'7
~"ill Flow onto the electrodes 12 and 14 and hence separate at the gap 20
due to the non~wetting nature of the ceramic in the gap 20~
In operation, the fuse is located near to the device which
is expected to overheat and its terminals 22 and 24 are connected in the
electrical circuitO ~hen the device ge~s sufficien-tly hotg ~he solder
paste melts, leaches the gold fuse link, separates into a globule on each
electrode, and thus interrupts the electrical circuit.
Figures 2 and 3 illustrate how the fuse may be combined with
a resistor, such as a battery feed resistorO In Figure 2, two resistors
30 are prGvided on one side of a ceramic or other substra-te 32. Two fuses
34 are aligned, one with the middle of each resistor on the other side at
the substrate 32. Each fuse 34 is connected in series with the associated
resistor by a link 36 extending around the edge oF the substra-te 32.
Terminals 38 and 40 connected to the free ends oF the resistor and fuse,
respectively, serve to connect the device externally.
Figure 3 shows an alternative configuration in which a
resis-tor and a serial fuse are deposited on each side of the substrate 41.
As shown the resistor 42 is generally rectangular except for an indent
area 46 at one corner. The fuse 44 is deposited in this indent area 46
and has one electrode connected to an edge connector 48, and the other
electrode connected to both a second edge connector 50 and one edge of the
resistor 42. The opposite edge of the resistor 42 is connected to an edye
connector 52. This "three lead" connection is particularly useful for
battery feed applications for telephone circuits.
Preferably each resistor 30 is deposited upon the support as
a thick Film~ Such an arrangement provides good thermal colnrnunication
between the resistor and the fuse.
It should be noted that the resistors and fuses have been
duplica-ted so -that one can be connected in each line oF a supply.
However, it is envisaged that a device rnight comprise a single resistor
and Fuse~
It will be appreciated that the fuse might be modified
without departing from the scope of -the invention. For example, a
diFferent material might be used for the fuse link, for example silver,
provided a suitable solder is also selected which will leach -the fuse link
and s-till melt at the required temperature.
An advantage of embodiments oF the present invention is that
their operating temperature can readily be varied, during manuFacture,
merely by varying the proportions of the constituents oF the solder, which
of course alters its melting point~
It should also be appreciated that although Fuses embodying
the invention are especially suitable for battery feed applications they
can also be applied elsewhere, generally where there is a need for a fuse
which interrupts an electrical circuit at a given temperature rather than
at a given current load.
