Note: Descriptions are shown in the official language in which they were submitted.
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BACKGROUND t~F THE INVENTI ON
The present invention relates to a temperature responsive
safety switch, fuse, or circuit breaker, of the type having
a meltable insert melting of which opens the switch.
Temperature responsive safety switches including a melt-
able solder insert are known in general and are disclosed,
for example in German Auslegeschrift [Published Patent Appli-
cation] No. 2,012,426. In another prior art temperature
responsive safety switch of the type under consideration, the
meltable solder insert comprises a meltahle solder in the
form of a round disc or wafer which is fastened in a recess
of a housing of insulating material and bridges an opening
in a heat tran~fer plate made of a material having good
thermal conductivity. The diameter of this opening is
larger than the diameter of an actuating plunger which rests
on the upper side of the disc. When there is enough heat
to cause the solder to melt such melting has the result of
eliminating the cover over the opening, since that cover was
constituted by the solder disc. Due to the force of the
contact spring and simultaneous interruption of the electric
current path, the actuating plunger is then pressed into the
opening in the heat transfer plate~
The drawback of this prior art temperature responsive
safety switch is its poor service life. This has a disad-
vantegous effect on its use as a temperature responsive fuse.
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Although the particular alloy composition of the melting solders
employed leads to the expectation that a defined melting point
exists, the solder will soften somewhat before such melting
temperature is reached. That means that if there is a long
duration heating influence, but at a temperature below the
intended actuation temperature, the melting solder will lose
firmness, as a result of which the force exerted by the
contact spring on the actuating plunger may lead to pre~ature
opening of the temperature responsive fuse. Moreover that
the melting solder is essentially exposed to the atmosphere.
which is a drawback since the presence of oxygen may change
the melting point of some solders.
SUMMARY OF THE INVENTION
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It is an,object of the present invention to provide a
temperature responsive safety switch of the above-mentioned
type which avoids the above-described drawbacks.
A more specific object of the invention is to construct
the temperature responsive safety switch in such a way that
variations in its response temperature are reduced and the
; 20 design temperature is maintained over a long period of time.
These and other objects are achieved, according to the
invention, in a temperature responsive safety switch including
an insulating housing provided with a recess, a thermally
conductive heat transfer plate mounted on the housing, a
pair of electrically conductive contact elements mounted in
the housing and defining an electric current path extending
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through the housing, one o~ the contact elements being a spring
element which is internally stressed to tend to separate from
the other contact element, a body of meItable material mounted
on the heat transfer plate, and a force-transmitting plunger
displaceably mounted in the housing and having one end in engage-
ment with the body of meltable material and its other end in
engagement with the spring element to maintain the contact
elements in contact with one another when the body of meltable
material has not melted, by the provision of a hollow cylindrical
sleeve member in which the body of meltable material is enclosed
and which is oriented with its longitudinal axis extending in the
displacement direction of the plunger, the sleeve member and
plunger being so positioned and dimensioned relative to one
another that the end of the plunger remote from the contact spring
extends into the sleeve member with a degree of play which is
sufficient to create a space large enough to permit melted
solder to pass out of the sleeve in the direction perpendicular
to the longitudinal axis of the sleeve member, the sleeve member
being mounted in the recess in the housing, and the housing
recess being dimensioned to present a space around the sleeve
member sufficient to receive the entire mass of the solder body
when the material melts and is forced outof the sleeve member
by displacement of the plunger.
The present invention assures that solder, upon becoming
softened or melted, cannot escape directly in the radial
direction, i.e. perpendicular to the longitudinal axis of
the actuating plunger. Rather it must first leave its
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994
sleeve at both ends in axial directions, i.e. parallel to the
longitudinal axis and the direction of movement of the actua-
ting plunger, before it can be displaced radially and will
only then release the opening path for the actuating plunger
or for the contact spring, respectively. Thus the melting
solder, if it should soften due to long-lasting thermal
stresseswillbe prevented by the sleeve from undergoing a
reduction in its effective height as a result of a slow
radial escape and from thus at least initiating the opening
process. Before it reaches the temperature of its final
melting point, it is essentially impossible for the melting
solder to leave the two ends of the sleeve.
~ ccording to preferred embodiments of the invention, the
recess in the housing is further dimensioned for presenting
a space adjacent the end of the sleeve member which is
directed toward the plunger. This makes it possible for
the sleeve member to be automatically raised slightly from
its support on the heat transfer plate so that the melting
solder after having been liquefied can exit from both ends
of the sleeve member and can be displaced in the radial
direction out of the opening path of the actuating plunger,
~ According to a further feature of the invention, the
sleeve member is formed to present an axial passage having a
reduced cross section at the end thereof remote from the
plunger. This offers the advantage that the effective
passage cross sections for the liauefied melting solder at
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the upper and lower exits of the sleeve member can be matched
to one another in such a way that the discharge of the lique-
ied melting solder from both sleeve ends begins ~ractically at
the same time. In this way, the melting solder is prevented
from prematurely leaving through the lower end of the sleeve
member, which is adjacent to the heat transfer plate.
Preferably, the sleeve member includes a radially
inwardly directed collar which defines the reduced cross
section of the axial passage. This represents a particularly
advantageous structural arrangement for achieving the desired
reduced cross section. Moreover, this feature takes account
of the fact that a temperature gradient is present between
the end of the solder facing the heat transfer plate and the
end facing the,actuating plunger so that the solder is always
~omewhat ~o,lder and firmer in the region of the actuating
plunger than in the region of the heat transfer plate.
The present invention thus accomplishes that the solder,
upon melting, leaves both ends of the sleeve only after it has
been substantially liquefied so that the contact spring
performs its circuit opening movement with greater speed.
The structure according to the invention further encapsu-
lates the melt body solder essentially against attacks of
oxidation from the outside. ~oreover, the heat transfer plate
no longer need be provided with an opening for the passage of
the actuating plunger.
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BRIEF DESCRIPTION OF THE DRAWING
Figure 1 is a cross-sectional, elevational view of a
preferred embodiment of a temperature responsive safety
switch according to the invention with the current path closed.
Figure 2 is a view similar to that of Figure 1 showing
the same switch with the current path opened.
Figure 3 is a cross-sectional detail view to an enlarged
scale of the switch portion enclosed by circle III in Figure 1.
~: ~ DESCRIPTION OF THE PREFERRED EMBODIMENTS
The switch illustrated in Figure 1-3 includes an insula-
: ting housing 1 in which the electrical terminals 2 and 3 of
:a current path are fixed by rivets 4 and 5. The inner end 6
of the left-hand terminal 2 carries a stationary countercontact 7
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which p:rojects in the direction toward the right-hand terminal
3. Opposite thereto, the inner end 8 of the right-hand terminal
3 is provided with a contact spring 9 whose free end 10 lies
below the countercontact 7. The contact spring 9 is under the
influence of an internal deflecting force which urges its free
end 10 downwardly into the open position shown in Figure 2 r
in which the free end 10 uould break contact with the counter-
contact 7. Contact spring 9 and countercontact 7 are here
accommodated in an upper recess 11 in the insulating housing 1.
~t its underside, the insulating housing is provided with
a lower recess 12 whose lower opening is covered, or closed,
by a heat transfer plate 13 having good thermal conductivity.
The recesses 11 and 12 communicate via a channel 14 which is
penetrated, or'traversed, by the actuating plunger 15. The
actuating plunger 15 has a substantially greater length than
the channel 14 ~o.that both its ends protrude far out of the
channel. The lower end 16 of the actuating plunger 15 .
projects into the upper end of a sleeve 17 which, as best
seen in Figure 3, is filled with a mass of meltable solder 18
most of which is located below the lower end 16 of the plunger
15. The lower end 20 of solder mass 18 rests upon the inside
19 of the heat transfer plate 13. The upper end 21 of the
sleeve 17 thus encloses the lower end 16 of the actuating
plunger 15 with play.
When the solder mass 18 is still cold, as shown in Figures
1 and 3, the upper end 2,2 of the act,uating plunger 15 rests against
the underside of the contact spring 9 and holds its free end
10 in the closed position against the countercontact 7 of
the left-hand terminal 2. Channel 14 has such dimensions that ,
the actuating plunger 15 can be freely displaced in the insula-
ting housing 1 in the direction of the plunger longitudinal
~axis 23. Between the periphery of the lower end of the actua-
! ting plunger 15 and the inner wall 24 of the sleeve 17, there
'remains an annular gap 25 which is sufficiently large to
10 ; permit the liquefied, melted solder 18 to pass out of the
8 leeve.
Between the outer wall 26 of the sleeve 17 and the inner
~wall 2-70f the recess 12 facing wall 26 there is sufficient
space 28 to completely acc,o,m~oda,te thQsolder 18 after it has
melted and been displaced out of the sleeve 17. A sufficient
space 30 is likewise provided between the upper end 2i of the
sleeve 17 and the upper delimiting wall 29 of the recess 12
to allow sleeve 17 to be automatically lifted up while plunger
15 is pressing downwardly on molten solder, to thereby enable
solder to also exit via the bottom of sleeve 17.
At its lower end 20, the sleeve 17 is provided with an
annular collar 31 which is fixed to or integral with its
inner wall2~ and projects radially inwardly therefrom. Thus
the passage cross section for molten solder at the lower e~d
20 of the ~leeve 17 is reduced in size.
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If, due to heating, particularly of the heat transfer
plate 13, the melting point of the solder 18 is reached, the
solder :L8 liquefies. Under the influence of the internal
force contact spring 9, the actuating plunger 15 is pressed
downwardly, penetrating deeper into the sleeve 17. This
causes the low viscosity, flowing molten solder 18 to be
displaced and to escape past the upper end 21 and the lower
end 20 of the sleeve, initially axially downwardly and
upwardly, and then radially into the free space of the recess
12 while slightly raising the sleeve from the heat transfer
plate. The stroke of the actuating plunger 15 is here
dimensioned in such a manner that opening of the current path
between the electric terminals 2, 3 is assured with certainty,
According~to one typical embodiment, the ~older body
can consist of 82.5% lead and 17.5 cadmium and have a melting
point o~ 248C.
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It will be understood that the above description of the
present invention is susceptible to various modifications.
changes and adaptations, and the same are intended to be
comprehended within the meaning and range of equivalents of
the appended claims.
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