[0001] This invention relates to a thermal fuse which can be constructed easily and automatically
and yet works accurately.
[0002] Conventionally, a thermal fuse is connected to an electric circuit in order to act
as a safety means. The fuse is so designed that when a preset temperature is reached,
a fusible alloy in it fuses and the electric circuit is opened, thereby protecting
electric appliances and preventing a fire. In this thermal fuse, two lead wires which
in their normal state are separated from each other are brought compulsorily into
contact with each other and deposited with fusible alloy to close the circuit between
the lead wires. Therefore, when the temperature of the protected electronic circuit
itself rises to the preset temperature due to an excess current (at overload) or when
the ambient temperature rises excessively, the fusible alloy fuses and thus the circuit
is opened. However, in manufacturing this thermal fuse it is required to fuse the
fusible alloy and to keep the two lead wires in a compressed state until the fused
fusible alloy solidifies. Thus, it takes much trouble for deposition of lead wires.
[0003] In accordance with the present invention there is provided a thermal fuse characterised
in that it comprises a cylindrical conductive case with a lead wire at the bottom
side thereof, another lead wire in the case, a solid fuse alloy piece put in the inner
bottom part of the case and ball contacts pressed by springs from both sides between
the fuse alloy piece and the other lead wire, wherein an electric circuit is formed
between the two lead wires through the medium of the ball contacts and the case, the
arrangement being such that, when the fuse allow piece fuses at a preset temperature,
the ball contacts under spring pressure are released from pressing on the other lead
wire and the electric circuit is thus opened.
[0004] The thermal fuse according to the present invention is manufactured in the following
way.
[0005] In a cylindrical conductive case with a lead wire at its bottom side, a fuse alloy
piece which fuses at the preset temperature is inserted. Then, a compressed spring
and an alloy flowing out preventive plate are fitted in. An insulating lid is fixed
at the open end of the case and a further lead wire is inserted in the case through
the lid. A spring is put on the end portion of the further lead wire in such a fashion
that it engages with a protrusion made at the outer periphery of the further lead
wire. By compressing the spring an insulating plate is fitted to the lead wire. A
plurality of ball contacts an electric conductive wire which connect the case and
the further lead wire are electrically interposed between the insulating plate and
the alloy flowing out preventive plate. While the fuse alloy piece is in solid condition,
the ball contact is pressed by the opposing two springs but when the fuse alloy piece
fuses, the ball contacts and the insulating plate are separated compulsorily from
the further lead wire by the action of the spring and thus the electric circuit is
opened.
[0006] In order that the invention may be better understood, an embodiment thereof will
now be described by way of example only and with references to the accompanying drawings
in which:-
Figure 1 is a cross sectional view of an embodiment of a thermal fuse according to
the present invention in its normal condition;
Figure 2 is a cross sectional view of a thermal fuse according to the present invention
when it worked; and
Figure 3 is a cross sectional view of the part taken along the line III - III in figure
1.
[0007] Figure 1 shows the thermal fuse according to the present invention in its normal
condition and Figure 2 shows the same thermal fuse after operation.
[0008] Reference numeral 1 designates a cylindrical case formed of nickel plated copper
or iron or formed of electric conductive metal such as aluminium. A lead wire L1 is
fitted to the outer surface of the bottom of the case 1 by deposition or other means.
Reference numeral 2 designates a fuse alloy piece to be put in the inner bottom of
the case 1. It is made of fusible alloy which fuses at a preset temperature. This
fuse alloy piece 2 is put in the inner bottom by using a press or the like.
[0009] Reference numeral 4 designates an alloy flowing out preventive plate to keep fuse
alloy piece 2, when fused, from flowing out toward ball contacts 5 and toward the
side of the lead wire L2. It is formed of copper or other metal or ceramics and is
slidably inserted in the case 1. Between the fuse alloy piece 2 and the alloy flowing
out preventive plate 4, a spring 3 is inserted in compressed state. A lid 8 made of
ceramic or other insulating material is fixed to an open end portion of the case 1.
An opening end edge 11 of the case 1 is bent inwardly to hold a flange 81 of the lid
8 and to fix the lid 8 to the case 1. A hole 82 through which the lead wire L2 is
put is made at the centre of lid 8. A protrusion L21 is made at the outer periphery
of the lead wire L2 and a spring 7 is put on the outer periphery of the lead wire
L2 in such a fashion that it engages with the protrusion L21. The spring 7 should
be weaker in resilience than the spring 3 which presses the alloy flowing out preventive
plate 4 and the other end of the spring 7 is supported by one side surface of the
insulating plate 6 of washerlike shape fitted to the lead wire L2. The insulating
plate 6 is forced to be pushed out toward the fuse alloy piece 2 by the resiliency
of the spring 7.
[0010] The insulating plate 6 is made of ceramic, ebonite or other insulating material to
withstand the specific temperature and a hole 61 through which the lead wire L2 is
put in is made at the centre thereof. A plurality of ball contacts 5 are interposed
between the insulating plate 6 and the alloy flowing out preventive plate 4. The ball
contact 5 has a ball-like shape and is made of copper or metal of good conductivity.
While the fuse alloy piece 2 is in solid state, both the spring 3 and the spring /
are in compressed state and by this spring force, the alloy flowing out preventive
plate 4 and the insulating plate 6 are pressed from both directions and a forward
end L22 of the lead wire L2 makes contact with the ball contacts 5, which make contact
with the inner peripheral surface of the case 1. Thus, between opposing two lead wires
L1 and L2, an electric circuit is formed through the medium of ball contacts 5 and
the case 1.
[0011] When an excess current flows through the thermal fuse shown in Figure 1 or when the
ambient temperature rises up to the preset temperature, the fuse allow piece in solid
condition fuses. The fuse alloy piece 2' thus fused flows and moves from its original
position at the inner bottom of the case as shown in Figure 2, whereupon the spring
3 is released from its compressed state and stretches. By the stretching of the spring
3, the spring 7 which has been in compressed state by stronger resilience of the spring
3 stretches. The stretching of the spring 7 pushes the insulating plate 6, ball contacts
5 and the alloy flowing out preventive plate 4 toward the side of the lead wire L1,
whereby contact between the lead wire L2 and the ball contacts 5 is released and the
electric circuit is opened. Since the fused fuse alloy 2' is kept from flowing out
toward the side of the lead wire L2 by the alloy flowing out preventive plate 4, there
is no fear that the electric circuit is closed again after the fuse alloy fused.
[0012] In the above-described embodiment, fuse alloy in solid state is put in the case and
an electric circuit is formed between two lead wires, through the medium of ball contacts
which are under spring pressure from both sides and the case. Therefore, there is
no need of fusing fusible alloy for deposition as in the case of the conventional
thermal fuse. Thus, the thermal fuse is easy to construct and the automation is possible
for its construction.
1. A thermal fuse characterised in that it comprises a cylindrical conductive case
with a lead wire at the bottom side thereof, another lead wire in the case, a solid
fuse alloy piece put in the inner bottom part of the case and ball contacts pressed
by springs from both sides between the fuse alloy piece and the other lead wire, wherein
an electric circuit is formed between two lead wires through the medium of the ball
contacts and the case, the arrangement being such that, when the fuse allow piece
fuses at a preset temperature, the ball contacts under spring pressure are released
from pressing on the other lead wire and the electric circuit is thus opened.
