(1) Field of the Invention
This invention relates to a safety device of an electric circuit for preventing an accident due to an overcurrent from occurring and a process for producing the same and, more specifically, to a safety device of an electric circuit containing first and second protective devices, wherein an accident is securely prevented from occurring by using the second protective device such as a thermal fuse even when the first protective device such as an overcurrent protective element is extraordinarily overheated.
(2) Description of the Related Art
An overcurrent protector is employed in an electric circuit of a power unit and the like mounted on an electric vehicle in order to prevent an accident due to overcurrent from occurring. For example, a positive temperature coefficient (hereinafter, called PTC) element, a variable resistor (hereinafter, called varistor) and an aluminum electrolytic capacitor are used as the overcurrent protector that generates heat to increase its electrical resistance when an overcurrent flows therethrough and then, reduces or cuts off the overcurrent, thereby keeping the electric circuit in secure condition.
However, when the overcurrent is kept flowing to overheat the overcurrent protector extraordinarily and is left as it goes, the overcurrent protector might catch fire to cause a vehicle fire.
Therefore, various measures to cope with the extraordinary overheating of the overcurrent protector have hitherto been proposed.
For example, Japanese Patent Application Laid-Open No. H8-250304 has proposed a safety device 110 of an electric circuit, in which one of a pair of leads 112 and 113 soldered to a PTC element 111 having a spring characteristic with respect to a direction apart from the PTC element 111 as shown in FIG. 13.
According to the above construction, when the PTC element 111 is extraordinarily overheated due to overcurrent, the solder is brought into melted condition and one lead 112 is lifted up to be apart from the PTC element 111, thereby the overcurrent flowing through an electric circuit is cut off.
Japanese Utility Model Application Laid-Open No. H1-129744 has proposed a safety device 120 of an electric circuit, in which a voltage-dependent type varistor (hereinafter, called ZNR) 121 is employed as shown in FIG. 14A.
In the safety device 120, a lead 122 of the ZNR 121 is bended at right angles and soldered to one conductor pattern 123 and another conductor pattern 124 that are separated and insulated from each other on an circuit board, thereby the conductor pattern 123 is connected to the other conductor pattern 124.
According to the above construction, as shown in FIGS. 14A, 14B and 14C, when the ZNR 121 is extraordinarily overheated, solder for fixing the lead 122 melts to force the ZNR 121 to be left out by its own weight, thereby breaking an electrical connection between the conductor pattern 123 and the other conductor pattern 124 and also breaking the overcurrent flowing in the electrical circuit.
The above conventional safety devices 110 and 120 of an electric circuit can prevent an accident due to an extraordinary overheating of an overcurrent protector, however causing the following problems.
In the safety device 110 shown in FIG. 13, the soldering must be done against an elastic force of the lead 112, resulting in time consuming for soldering and that the safety device 110 can not be produced easily by employing an automatic soldering machine and the like.
In the safety device 120 shown in FIG. 14, the soldering must be done with holding the ZNR 121 so that the ZNR 121 is left out by its own weight upon the extraordinary overheating, also resulting in that a lot of time is required for soldering and that the safety device 120 can not be produced easily by employing an automatic soldering machine and the like.
As a result of study, the present inventors have reached a technological idea that a thermal fuse be situated in the vicinity of the overcurrent protector and the thermal fuse detects an extraordinary overheating of the overcurrent protector, thereby the electric circuit is cut off.
In the following, an ordinary thermal fuse is explained with reference to FIGS. 15A, 15B and 15C.
As shown in FIG. 15A, one conductor pattern 132 and another conductor pattern 133 are formed on a circuit board 131, then electrodes 132a and 133a of the conductor pattern 132 and other conductor pattern 133, respectively, are insulated from each other by a gap 134. Then, as shown in FIG. 15B, the electrodes 132a and 133a are connected by bridging the gap 134 with solder 135, thereby a thermal fuse 130 is formed.
According to the above construction, when a temperature of the conductor pattern 132, the other conductor pattern 133 or an atmosphere around the solder 135 rises high, the solder 135 completely melts.
Then, as shown in FIG. 15C, the solder 135 is repeled by an surface of the circuit board, which is hard to be wetted by solder, at the gap 134 and then, splits into two parts toward each electrode 132a and 133a, which are easily wetted by solder, by surface tension.
As a result, the electrical connection between the electrodes 132a and 133a is cut off and an accident due to an extraordinary overheating of the electric circuit can be prevented from occurring.
When the above conventional thermal fuse 130 is formed, the solder 135 must be put on the gap 134 between the electrodes 132a and 133a on such a condition that the solder 135 is nearly completely melted. However, at this time, the solder 135 splits into two parts toward each electrode 132a and 133a, that is, it is very hard to put the solder 135 that is nearly completely melted on the gap 134 between the electrodes 132a and 133a. 
Therefore, there has been a problem that the thermal fuse 130 can not be easily formed by soldering simultaneously with soldering of other electric components by employing an automatic soldering machine and the like.
As shown in FIG. 15A, the gap 134 between the electrodes 132a and 133a extends in a direction along an arrow shown in the figure, in another word, there is no electrode that can stop the molten solder 135 along the direction, therefore, the molten solder 135 can not stay steadily on the surface of the circuit board at the gap 134, causing the above problem.
However, the nature that the molten solder 135 easily splits into two parts toward each electrode 132a and 133a essentially contributes to reliability of the thermal fuse upon the extraordinary overheating of the electric circuit, it is required that the molten solder 135 should not split into two parts toward each electrode 132a and 133a only when the thermal fuse 130 is formed.
In order to solve the above problem, Japanese Patent Application Laid-Open No. H4-56028 has proposed a thermal fuse, in which an electrode of one conductor pattern and an electrode of another conductor pattern, which are separated and insulated from each other on a circuit board, are connected by using porous solder layer in which openings among the solder grain remain.
According to the above idea, however, in order to form the porous solder layer, particular steps including printing solder cream on a circuit board and half-melting the solder cream are required, causing a problem that the thermal fuse can not be easily formed by soldering simultaneously with soldering of other electric components by employing an automatic soldering machine and the like.
It is therefore an objective of the present invention to solve the above problems and to provide a safety device of an electric circuit and a process for producing the same, the safety device containing first and second protective devices, wherein an accident is securely prevented from occurring by using the second protective device such as a thermal fuse even when the first protective device such as an overcurrent protective element is extraordinarily overheated, further, the thermal fuse can quickly operate when the overcurrent protector is extraordinarily overheated, and furthermore, the thermal fuse can be easily formed by employing an automatic soldering machine and the like.
In order to attain the above objective, the present invention is to provide a safety device of an electric circuit comprising: first protective means in an overcurrent protector for protecting the electric circuit from an overcurrent, which generates heat to increase its electrical resistance when an overcurrent flows in the electric circuit, whereby reducing or cutting off the overcurrent; and second protective means including a thermal fuse situated in the vicinity of the overcurrent protector comprising: a first electrode having a first conductor pattern connected to the overcurrent protector; a second electrode having a second conductor pattern insulated from the first electrode by a gap; and solder connecting the first and second electrodes and melting by heat from the overcurrent protector so as to split into two parts toward the first and second electrodes.
With the construction mentioned above, when the overcurrent protector is extraordinarily overheated due to the overcurrent, the thermal fuse operates responding to the heat and the electric circuit is cut off.
Therefore, an accident due to the extraordinary overheating of the overcurrent protector, such as a vehicle fire and the like, can be securely prevented.
Resist films around the thermal fuse can be peeled off by the heat from the overcurrent protector.
With the construction mentioned above, when the solder for forming the thermal fuse melts, the molten solder is sucked into a space between the peeled resist films and a surface of the circuit board, thereby an electrical connection between the first and second conductor patterns are cut off more quickly.
Therefore, the thermal fuse can quickly operate when the overcurrent protector is extraordinarily overheated, thereby an accident can be more securely prevented from occurring.
The thermal fuse is disposed at substantially the level of the overcurrent protector or a higher level than that of the overcurrent protector when a circuit board, on which the overcurrent protector and the thermal fuse are mounted, is set vertically.
With the construction mentioned above, thermal conduction from the overcurrent protector to the thermal fuse becomes good due to a positional relationship therebetween.
When the thermal fuse is situated nearly at a level of the overcurrent protector, the heat of the overcurrent protector is directly conducted to the thermal fuse. When the thermal fuse is situated at a higher level than that of the overcurrent protector, the heat of the overcurrent protector situated at a lower level can be efficiently conducted to the thermal fuse situated at a higher level.
In addition, since the circuit board is set vertically, the solder melted due to the heat of the overcurrent protector naturally can flow downward.
Therefore, when the overcurrent protector is extraordinarily overheated, an electrical connection between the. first and second conductor patterns are cut off more quickly, that is, the thermal fuse can quickly operate so as to prevent an accident from occurring more securely.
The gap between the first and second electrodes for forming the thermal fuse is provided with a hole, a lead of an electric component is inserted in the hole, and the first and second electrodes together with the lead are connected by solder.
With the construction mentioned above, since the solder for connecting the first and second electrodes adheres to the lead of an electric component, the solder is easily set between the first and second electrodes, therefore, the thermal fuse can be easily formed by soldering using ordinary solder simultaneously with soldering of the other electric components by employing an automatic soldering machine and the like.
Furthermore, since no additional separated thermal fuse is needed to be formed, a number of points for soldering on the ciucuit board becomes less, thereby improving workability and reliability.
The electric component is the overcurrent protector.
With the construction mentioned above, the heat of the overcurrent protector is conducted to the thermal fuse through the lead of the overcurrent protector, therefore, the thermal fuse can more quickly operate when the overcurrent protector is extraordinarily overheated.
The second conductor pattern for forming the thermal fuse is formed on one side of the circuit board and connected to the second electrode having an island shape formed on an opposite side of the circuit board through a through-hole, the first electrode continuously surrounds the second electrode with a gap between the first and second electrodes on the opposite side of the circuit board, and solder connects the first and second electrodes.
With the construction mentioned above, since nearly completely molten solder situated inside of the continuous first electrode can be stopped by the first electrode, the molten solder can stay stably on the gap between the first and second electrodes.
Consequently, soldering using ordinary solder simultaneously with soldering of the other electric components by employing an automatic soldering machine and the like can easily form the thermal fuse.
Further, when the overcurrent protector is extraordinarily overheated, the nearly completely molten solder splits into two parts toward the first and second electrodes, then an electrical connection between the first and second conductor patterns is cut off, thereby reliability of the thermal fuse is improved.
The second electrode for forming the thermal fuse is formed to have an island shape at the periphery of a hole for inserting a lead of an electric component therein, the first electrode continuously surrounds the second electrode with a gap between the first and second electrodes, and solder connects the first electrode, the second electrode and the lead of the electric component inserted in the hole.
With the construction mentioned above, the solder connecting the first and second electrodes adheres to the lead of the electric component, so that the solder can be much more easily put on the gap between the first and second electrodes.
Further, the thermal fuse can be formed only on one side of the circuit board, thereby simplification of the circuit board is attained.
The first electrode for forming the thermal fuse is formed to have an island shape at the periphery of a hole for inserting a lead of the overcurrent protector therein, the second electrode continuously surrounds the first electrode with a gap between the first and second electrodes, and solder connects the first electrode, the second electrode and the lead of the overcurrent protector inserted in the hole.
With the construction mentioned above, the heat of the overcurrent protector is conducted to the thermal fuse through the lead of the overcurrent protector, therefore, the thermal fuse can much more quickly operate when the overcurrent protector is extraordinarily overheated.
The overcurrent protector is a positive temperature coefficient element.
A PTC element, specifically a low resistance-type PTC element is a suitable overcurrent protector for an electric circuit of a power unit and the like mounted on an electric vehicle. By employing the PTC element as an overcurrent protector that constructs the safety device of an electric cuicuit, the safety device becomes specifically suitable for use in an electric vehicle.
Further, the present invention is to provide a preocess for producing the safety device of an electric circuit, characterized in that soldering for connecting the first and second electrodes to form the thermal fuse is performed simultaneously with soldering of electric components including the overcurrent protector on the circuit board by employing an automatic soldering machine.
According to the process mentioned above, the thermal fuse can be easily formed by soldering using ordinary solder, which is used for connecting other electric components to a circuit board, simultaneously with soldering of the other electric components by employing an automatic soldering machine and the like, thereby enabling a production of the safety device of an electric circuit at a low cost without employing a conventional specific production step.
A through-hole of a circuit board is filled with solder, the solder has a concave shape at one opening of the through-hole and a convex shape at an opposite opening of the through-hole, and the convex-shape solder connects the first and second electrodes.
With the construction mentioned above, when the solder melts due to an extraordinarily high temperature, solder at the convex shape portion moves into the through-hole (that is, toward the concave shape portion), thereby an electrical connection between the first and second electrodes is cut off quickly and securely.
An inner diameter of the through-hole is larger than a thickness of the circuit board.
With the construction mentioned above, the solder at the convex shape portion can move more quickly into the through-hole, thereby the interruption of the circuit is attained more quickly. Further, in a process for producing the safety device, the concave shape portion and convex shape portion are formed easily and securely due to the solder""s own weight.
An outer diameter of an exposed conductor on the circuit board at the concave-shape solder side is larger than an outer diameter of the second electrode at least near to the through-hole.
With the construction mentioned above, when the molten solder melted due to an extraordinarily high temperature moves toward the concave shape portion, the molten solder is sucked by an exposed conductor having good wettability and therefore moved quickly, thereby the interruption of the circuit is attained quickly and more securely.
The circuit board is arranged vertically or arranged so as to set the convex-shape solder up.
By arranging the circuit board vertically, the molten solder melted due to an extraordinarily high temperature is securely sucked into the through-hole. By arranging the circuit board so as to set the convex shape portion up, the solder at the convex shape portion quickly moves toward the concave shape portion due to the solder""s own weight, thereby the interruption of the circuit is attained quickly and more securely.
Furthermore, the present invention is to provide a preocess for producing the safety device of an electric circuit, characterized in that the circuit board is arranged horizontally and the through-hole extending vertically is filled with molten solder, whereby the concave shape and the convex shape are formed at respecitve sides with its own weight.
According to the process mentioned above, the concave shape portion and convex shape portion of the solder are easily formed due to the solder""s own weight, resulting in a low cost of producing the thermal fuse.