Ammunition and explosives – Igniting devices and systems – Electrical primer or ignitor
Reexamination Certificate
1999-08-03
2001-08-07
Jordan, Charles T. (Department: 3641)
Ammunition and explosives
Igniting devices and systems
Electrical primer or ignitor
C337S401000, C337S227000, C337S297000
Reexamination Certificate
active
06269745
ABSTRACT:
DESCRIPTION
The present invention relates to an electrical fuse element according to the preamble of claim
1
.
Fuse elements are used in large numbers for protecting electrical and electronic circuits from excessive currents. In such cases, they have to be adapted to the current ranges occurring in an application, by the tripping characteristics respectively required. The generally perceivable and ever increasing tendency for circuit components to be made smaller while having the same or even enhanced capabilities leads to considerable problems in the area of electrical fuse elements.
EP 0 515 037 A1 discloses a fuse located on the substrate of a hybrid circuit, where the fuse is supported on a thermally insulating layer and teaches to adjust the operating parameters of the fuse by varying e.g. the degree of thermal insulation about the fusible track. With using a support layer having a high thermal resistivity, the effect of raising the total thermal resistivity is not achievable. Thus a fuse cannot be reduced in size. Further very few fusing characteristics can be realized in this way only.
It is therefore the object of the present invention to provide a fuse element for all known tripping characteristics by a cost-effective production technique for the medium and low currant range. Furthermore, by having a smaller outer geometry, the fuse element is to be adaptable to modern methods of insertion.
According to the invention, this object is achieved by that
the carrier consists of a material of poor thermal conduction, in particular of a glass ceramic,
the fusible conductor is indirectly heated, preferably by at least one additional heating element, where
at least one heating element is arranged together with the fusible conductor jointly on the substrate and
a distance between the heating element and the fusible conductor is variable, in order to set the degree of thermal coupling with otherwise the same geometry of the circuit.
In the past there have been numerous known attempts to make the outer dimensions of electrical fuse elements considerably smaller while retaining their operational current range, their switching capacity and their specific tripping characteristics. However, these attempts resulted in failure because either the internal heating of the fuse element became too great and/or the desired tripping characteristic could not be achieved, or the fuse element became unsoldered at its contact points owing to increased self-heating.
By the use according to the invention of a carrier made of a material of poor thermal conduction, the present invention overcomes a widespread prejudice to the use of such materials in fuse construction. By the use of such a carrier material, the hot zone (hot spot) of the fuse element can be advantageously restricted to the core region of the carrier or of the housing, since the heat dissipation is very low. Thus, the heat removal by conduction via the external contacts is significantly less. Consequently, unsoldering of its own accord or inadmissible heating is no longer possible for a fuse element according to the invention. Furthermore, by concentrating the “hot spot” in a certain region, the entire power consumption of a fuse element according to the invention is lowered. Thus, a minimal power consumption also results in less of a retroactive effect on the surrounding electric circuit.
Among suitable materials of poor thermal conduction are ceramics, glass ceramics or glass. Glass ceramics are preferred, however.
For a cost-effective mass production of fuse elements according to the invention with their small geometrical sizes, formation of the carrier in sheet form is advantageous, preferably in the form of a sheet-like substrate. Thus, fuse elements according to the invention can be produced in a cost-saving manner in multiple repeats, for example in the size of customary service-mounted devices (SMDs) on a planar substrate.
In a fuse element according to the invention, the fusible conductor may act as a single heat source. However, to set different nominal currents and switching characteristics, an indirect heating of the fusible conductor is preferred.
At least one additional heating element serves for this purpose. In the case of some embodiments, two heating elements are used with preference, for example, as is shown below with reference to illustrations of a number of exemplary embodiments according to the invention. Cases with more than two heating elements are also conceivable. When reference is made below to a heating element, these possibilities are also intended to be included.
A fuse having an addition a heating element is known e.g. form AT-B 383 697. The fusible element is thermally coupled to a resistor where the components are located on the same sheet-like substrate. The substrate is made of a ceramic material. The resistor acts as a current sensor. An amount of thermal energy sent from the resistor to the fusible element is equivalent to the amount of current. But within the teaching of this document there is no way of changing the characteristics of the fuse.
According to the invention, the heating element is arranged together with the fusible conductor jointly on the substrate. In this case, the degree of thermal coupling between heating element and fusible conductor is influenced in each case by the distance from each other. The consequently achievable effects of shifting the characteristic curve of the fusible element are explained in more detail below with reference to exemplary embodiments.
Further the distance produced between the heating element and the fusible conductor is kept variable, in order to set the degree of thermal coupling and consequently the tripping characteristic of the fusible conductor and the nominal current while otherwise retaining the same materials and the same geometry of the circuit. With a fixed circuit geometry, setting of the characteristic is possible by simply shifting the individual production masks in relation to one another in a predetermined way and fixed amount.
In a development according to claim
2
the distance between the heating element and the fusible conductor assumes a minimal value when the heating element and the fusible conductor are arranged lying one over the other. This minimal value is in this case determined by the layer thickness of an electrical insulation, which may consist of a dielectric such as glass, but also a ceramic or a curable paste. The good thermal contact may take place over the entire base area of the fusible conductor. Preferably, the fusible conductor is arranged over the heating element, so that there is adequate space available for receiving the gases and particles released in the event of the fusible conductor tripping, as well as for pressure equalization.
According to the invention, the properties of the fusible conductor can be significantly influenced directly by the thermal coupling with the heating element. The thermal coupling is intensified in a simple way by the actual fusible conductor being applied to a thin layer, which preferably consists of silver and effects adhesive bonding with good conduction on the substrate surface. As a result, the characteristic can be reproduced even more exactly.
In the case of a fusible conductor formed as a multilayer arrangement, for example in the case of a material combination of a layer of silver and a covering layer of tin, an additional influencing of the tripping characteristic can be achieved by diffusion processes. Other material combinations with mutual solubility are also possible.
Furthermore, the fusible conductor may have a constriction or tapering in its central region. This reduction in cross-section increases the intrinsic resistance. What is more, the material of the fusible conductor is weakened at this notable point and correspondingly less material has to be melted during tripping. The constriction is advantageously in the “hot spot” of the fuse element.
Alternatively, the fusible conductor may, however, also be a wire, which has, for example, as described above, a sil
Cieplik Ingeborg
Frochte Bernd
Rupalla Manfred
Banner & Witcoff , Ltd.
Chambers Troy
Jordan Charles T.
Wickmann-Werke GmbH
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