Overcurrent preventing circuit

Abstract: An overcurrent detecting circuit provided within a battery pack detects any overcurrent flowing in a circuit electrically connecting the battery and connection terminals. Upon detection of any overcurrent by this overcurrent detecting circuit, a switch circuit intercepts the electric connection between the battery and the terminals, and opens the so far closed circuit.

Description: The present invention relates to an overcurrent preventing circuit, and more particularly to an overcurrent preventing circuit which, upon detection of an overcurrent, intercepts the shorted-out circuit. Description of the Related Art The connection terminals of the case for housing a battery are exposed so that they can be readily connected to their counterparts on the principal apparatus to which the battery supplies power. Therefore, if the connection terminals of the plus pole and the minus pole are accidentally short-circuited by coming into contact with an electroconductive object, a large current may flow in the shorted-out circuit, resulting in the overheating of the battery case. This might burn the user or even invite a fire. In view of this risk, various structures have been proposed to prevent short circuiting. For instance, the Japanese Laid-Open Patent Application No. 53873/1988 (JP-A-63-53873) discloses a connecting terminal arrangement which connects, via relay terminals, connection terminals provided on a box, such as a battery case, to those of the principal apparatus to which the battery supplies power. This connecting terminal arrangement further has partitioning boards on both sides of the relay terminals. In this manner, when the box is separated from the principal apparatus, the partitioning boards prevent the connection terminals from coming into contact with each other even if an electroconductive object approaches the box. The Japanese Laid-Open Utility Model Application No. 50447/1988 (JP-U-63-50447) discloses a structure to prevent short-circuiting by covering at least one of paired connection terminals when the battery case is not fitted to the principal apparatus to which the battery supplies power. Similarly, the Japanese Laid-Open Utility Model Application No. 62657/1990 (JP-U-02-62657) discloses a battery pack having a protective cover over the connection terminals. Furthermore, the Japanese Laid-Open Utility Model Application No. 14364/1992 (JP-U-04-14364) discloses a short-circuit preventing apparatus having a lead switch within the power supply circuit of the battery box and a magnet within the principal apparatus to which the battery supplies power. This lead switch is usually kept open and is closed when the principal apparatus is fitted to the box, i.e. when magnetic force is given by the magnet. Since all these examples of the prior art prevent the connection terminals from contacting each other by a mechanical structure, the two connecting terminals may become exposed if the contact preventing part is damaged or deformed. Therefore, accidental short-circuiting of the connection terminals cannot always be avoided. Moreover, they require such extra parts as the relay terminals, protective cover and magnet, which make it necessary to provide a protruding part on the battery box. Furthermore, the magnet may affect internal circuits of the principal apparatus to which the battery supplies power. A feature of embodiments to be described below, therefore, is to provide an overcurrent preventing circuit capable of detecting any overcurrent flowing in the internal circuit of a battery case. A particular embodiment to be described below, by way of example in illustration of the invention has an overcurrent preventing circuit which, upon detection of any overcurrent flowing between a battery and connection terminals, can interrupt the connecting path between the battery and the connection terminals. An overcurrent preventing circuit may have an overcurrent indicating circuit for detecting any overcurrent flowing in a closed circuit connecting a battery and connection terminals, and a switching circuit responsive to the detection output of the overcurrent detecting circuit for controlling the opening and closing of the closed circuit. Features of embodiments illustrative of the present invention to be described below by way of example are that if a pair of connection terminals is short-circuited by any cause, an overcurrent detecting circuit will detect the overcurrent due to that short-circuiting and open a switching circuit. Since this opening interrupts the circuit which electrically connects the battery and the connecting terminals, the overcurrent can be prevented from flowing in this circuit and it is made possible to avoid any accident which would otherwise result from the overcurrent. The determination of overcurrent includes a comparison of a voltage developed by a series resistor with a reference voltage proportional to the voltage of the battery. In the specification of U.S. Patent No. 4 992 340, there was proposed a safety device for a battery such that when contacts were shorted, a voltage drop across a sensing resistor turned-on an SCR, if the voltage drop exceeded the firing threshold of the SCR. The SCR of this earlier proposal also needs unlatching after having been activated. The following description and drawings disclose, by means of examples, the invention which is characterised in the appended claims, whose terms determine the extent of the protection conferred hereby. In the drawings:- FIG. 1 is a block schematic diagram, FIG. 2 is a perspective schematic diagram illustrating a switch configuration for the switching circuit shown in FIG. 1; and FIG. 3 is another perspective diagram illustrating an alternative switching arrangement. In the drawings, the same reference numerals denote the same structural elements. FIG. 1 is a block diagram of a preferred embodiment of the present invention. In FIG. 1, a case 1 for a battery pack or the like has a battery 2, a pair of connection terminals 3 and 4, an overcurrent detecting circuit 5, and a switching circuit 6. The connection terminals 3 and 4, electrically connected to the batteries 2, are exposed on the surface of the case 1. The connection terminals of this case 1, when fitted to a principal apparatus (not shown), e.g. a portable telephone, to which the power is to be supplied, come into contact with the power supply circuit of the principal apparatus. This causes electric power to be supplied to the power supply circuit of the principal apparatus. The overcurrent detecting circuit 5 detects any overcurrent flowing between the pair of connection terminals 3 and 4. The switching circuit 6 controls the electric connection between the battery 2 on the one hand and the connection terminals 3 and 4 on the other according to the detection result of the overcurrent detecting circuit 5. Thus the switching circuit 6, upon detection of any overcurrent by the overcurrent detecting circuit 5, intercepts the electric connection between the battery 2 and the connection terminals 3 and 4. The overcurrent detecting circuit 5 preferably consists of resistors r1, r2 and r3 and a comparator 13. The resistors r1 and r2 are connected in series to the potentials of the two ends of the battery 2, and divides the two-end potentials. The resistor r3 is inserted between the connection terminal 4 of the minus pole and the battery 2. The uninverted input terminal of the comparator 13 is connected between the resistor r3 and the connection terminal 4 of the minus pole, and the inverted input terminal of the comparator 13 is connected to the connection point, or junction, between the resistors r1 and r2. The comparator 13 compares the fall voltage of the resistor r3 and the divided voltage of the resistors r2 and r3 and, if the former surpasses the latter, supplies the detection output signal to the switching circuit 6. In the above-described embodiment, the voltage divided by the resistors r2 and r3 is used with a view to saving power consumption, but it is not necessarily required to use a divided voltage, but the battery voltage may as well be used as it is. If the battery voltage is supplied to the inverted input of the comparator 13, the resistance of the resistor r3 will increase. Therefore, the power consumption will increase. The switching circuit 6 preferably has an inverter 14 and an analog switch 15. The analog switch 15 consists of a P-channel transistor and an N-channel transistor. The gate of the P-channel transistor is connected to the output end of the comparator 13, and that of the N-channel transistor, to the output end of the comparator 13 via the inverter 14. The sources and drains of the two channel transistors are connected between the battery 2 and the resistor r3. Next will be described the operation of the present invention embodied in such a configuration. If short circuiting takes place between the connection terminals 3 and 4 and an overcurrent flows in the overcurrent detecting circuit 5, the battery 2 and the connection terminals 3 and 4 will constitute a closed circuit. The overcurrent detecting circuit 5 detects the overcurrent flowing in this closed circuit, and supplies the detection output signal to the switching circuit 6, which, in response to this detection output signal, intercepts the electric connection between the battery 2 and the connection terminal 4. If no overcurrent is detected, the switching circuit 6 will maintain the closed circuit. Now will be described the operation which takes places when the current flowing in the resistor r3, at a battery voltage of 5 V, is 1 A or less, for example. Supposing the resistances of the resistors r1, r2 and r3 are 4.89 MΩ, 0.11 MΩ and 0.1 Ω, respectively, when the case 1 is connected to the principal apparatus to which the power is to be supplied. The uninverted input voltage of the comparator 13 will be 0.1 V or less, and the inverted input voltage, 0.11 V. Accordingly, the output of the comparator 13 will be "Low", and that of the inverter 14, "High". At this time, the P-channel transistor is turned ON, and so is the N-channel transistor. Therefore the analog switch 15 is brought into continuity, and thereby enabled to supply a voltage externally from the connection terminals 3 and 4. On the other hand, if the connection terminals 3 and 4 are short-circuited, for instance if the current flowing in the resistor r3 is 1.5 A, the uninverted input voltage of the comparator 13 will become 0.15 V, and accordingly the output of the comparator 13 will turn "High". Accordingly, the P-channel transistor is turned OFF, and so is the N-channel transistor because a "Low" level signal is supplied to its gate via the inverter 14. Therefore, the analog switch 15 will become open. This electrically intercepts the connection terminal 4 from the battery 2 to avoid short circuiting. It is desirable that, as in the above cited example, the resistances of the resistors r1 and r2 be relatively high, and that of the resistor r3, relatively low. Other examples of the switching circuit 6 include a delayed self-restoration type switch or a manipulated restoration type switch. The former would intercept the closed circuit in response to the detection output signal of the overcurrent detecting circuit 5 and, after the lapse of a predetermined duration following the extinction of the detection output signal, automatically close the switch. The latter would be caused by a restoring manipulation from outside to close the switch. FIG. 2 illustrates the configuration of the delayed self-restoration type switch and FIG. 3, that of the manipulated restoration type switch. In FIG. 2, metallic leads 23 and 24, respectively having contacts 22a and 22b, are arranged within a glass tube 21. These metallic leads 23 and 24 are connected to the battery 2 and the minus pole connection terminal 4, respectively. Around the circumference of the glass tube 21 is wound a coil 25 which would generate a magnetic field with an overcurrent. One end of the coil 25 is connected to the output end of the comparator 13, while the other end is either grounded or connected to the connection terminal 4. The contacts 22a and 22b are normally in contact, and warped outward by the generation of heat to become open. Thus, the metallic leads 23 and 24 in the glass tube 21 are heated by the overcurrent, and this heat warps the metallic leads. At the same time, a current flowing in the coil 25, i.e., the detection output signal, generates a magnetic field to separate the metallic leads 23 and 24 from each other. The resultant open state between the contacts 22a and 22b intercepts the electrically connected circuit between the battery 2 and the connection terminals 3 and 4. After the extinction of the detection output signal, i.e. when there is no longer an overcurrent, the magnetic field generated by the coil 25 disappears. As the heat is radiated from the glass tube 21 and the coil 25, the metallic leads 23 and 24 are restored after the lapse of predetermined duration. This brings the contacts 22a and 22b into contact with each other, and the closed circuit is automatically restored. The "predetermined duration" here means the time required for heat radiation. In FIG. 3, metallic leads 33 and 34, respectively having contacts 32a and 32b, are arranged within a switch body 31. The contacts 32a and 32b are normally in contact. Around the circumference of the switch body 31 is wound a coil 35 which would generate a magnetic field with an overcurrent. The metallic leads and the coil are connected in the same manner as in the switch of FIG. 2. A manipulating button 37 is fitted via a spring 36 to the switch body 31. This manipulating switch 37 is normally kept away from the contact 32b of the metallic lead 34 by the elasticity of the spring 36. Upon generation of heat by the metallic leads 33 and 34 in the switch body 31 due to an overcurrent, this heat warps the metallic lead 34 as shown by the waved line in the diagram. At the same time, a current flowing in the coil 35 generates a magnetic field to separate the metallic leads 33 and 34 from each other. The resultant open state between the contacts 32a and 32b intercepts the closed circuit. As the interception eliminates the overcurrent, the magnetic field generated by the coil 35 disappears. As there also works a force to restore the metallic lead, pressing the manipulating button 37 will return the metallic lead to its original position to bring the contacts 32a and 32b into contact with each other again to restore the closed circuit. Incidentally, the restoration can as well be automatically accomplished by a timer or the like after the lapse of the predetermined duration. As hitherto described, according to the present invention, even if a pair of connection terminals are short-circuited, the electric connection between the battery and the terminals is immediately intercepted to prevent an overcurrent from being supplied to the closed circuit connecting the battery and the terminals and thereby to avoid the occurrence of any accident which would otherwise result from short circuiting. While the invention has been described with reference to specific embodiments thereof, it will be appreciated by those skilled in the art that numerous variations, modifications and embodiments are possible and, accordingly, all such variations, modifications and embodiments are to be regarded as being within the scope of the invention, as defined in the appended claims.

Claims: An overcurrent preventing circuit including means (r3) for developing an overcurrent voltage indicative of any overcurrent flowing in a current path in which a battery (2) and connection terminals (3) (4) are electrically connected together; a switching circuit (15, 25, 35) responsive to an output of a comparator (13) indicative that a reference voltage level has been exceeded by the overcurrent voltage connected to an input of the comparator (13) for controlling the opening and the closing of the current path, characterized in that the reference voltage level is arranged by circuit means (r1, r2) to be proportional to the voltage of the battery (2). An overcurrent preventing circuit, as claimed in claim 1, wherein the reference voltage is generated by a first (r1) and a second resistor (r2), which are connected in series to potentials of the two ends of the battery (2), for dividing those potentials of the two ends to generate a divided voltage as the reference voltage, a third resistor (r3) in the current path serving for developing the overcurrent voltage, and wherein the comparator circuit compares the voltage drop in the third resistor (r3) with the divided voltage, and supplies the indicative output signal to the switching circuit in accordance with a result of comparison. An overcurrent preventing circuit, as claimed in claim 1 or 2, wherein the switching circuit (15,25) interrupts an electric connection between the battery (2) and the connection terminal (4), and closes the current path upon a cessation of the indicative output signal. An overcurrent preventing circuit, as claimed in claim 3, wherein the switching circuit (15) includes: an analog switch connected in the current path, in which switch a P-channel transistor and an N-channel transistor are connected in parallel, wherein the indicative output signal from the comparator (13) is supplied to the gate of the P-channel transistor; and an inverter (14) for supplying the comparator output signal, after it has been inverted, to the gate of the N-channel transistor. An overcurrent preventing circuit, as claimed in claim 3, wherein the switching circuit (15,25) automatically closes the current path after the lapse of a predetermined duration following the cessation of the indicative output signal. An overcurrent preventing circuit, as claimed in claim 3, wherein the switching circuit (33,35) closes the current path by a restoring manipulation (37) after the cessation of the indicative output signal. An overcurrent preventing circuit, as claimed in claim 5, wherein the switch circuit includes: a pair of metallic leads (23,24), inserted into the current path, each having a contact (22a,22b) at one end; a glass tube (21) covering the metallic leads (23,24), and a coil (25) provided around the circumference of the glass tube (21) and connected between the output end of the comparator (13) of the overcurrent indicative circuit and ground; the contacts (22a,22b) being opened and closed in response to the indicative output signal. An overcurrent preventing circuit, as claimed in claim 6, wherein the switching circuit includes: a pair of metallic leads (33,34), inserted into the current path, each having a contact (32a,32b) at one end; a glass tube (31) covering the metallic leads, a coil (35) provided around the circumference of the glass tube and connected to the comparator output of the overcurrent indicative circuit and to ground, and a manipulating button (37) in contact with one of the contacts; wherein the contacts (32a,32b) are opened in response to the indicative output signal and closed by the manipulation of the manipulating button (37). An overcurrent preventing circuit, as claimed in claim 2, wherein the resistances of the first (r1) and second (r2) resistors are greater than that of the third resistor (r3). An overcurrent preventing circuit, as claimed in claim 2, wherein the comparator circuit (13) interrupts the electric connection path when the voltage drop is higher than the divided voltage. An overcurrent preventing circuit, as claimed in any of claims 1-10, wherein the battery (2) and the connection terminals (3,4) are provided within a battery pack (1). An overcurrent preventing circuit, as claimed in claim 11, wherein the battery pack (1) is to supply power to a portable communication apparatus. An overcurrent preventing circuit, as claimed in claim 1, wherein the over current voltage developing means detects any overcurrent generated by the short circuiting of a pair of connection terminals of the battery (2) . A method for controlling an overcurrent prevention, the method comprising the steps of: detecting overcurrent flowing in a current path in which a battery and a pair of connection terminals are electrically connected; developing an overcurrent indicative output in response to such a detection; comparing the overcurrent indicative output with a voltage reference level characterized by arranging this level to be proportional to the battery voltage; and opening the current path upon detection of overcurrent by the output of the comparator indicating that the level has been passed. A method for controlling as claimed in claim 14, the method further comprising the step of: closing the current path automatically when overcurrent is no longer detected.

Inventors: HIRATA MASARU, HIRATA, MASARU,, HIRATA, MASARU, C/O NEC CORPORATION
Applicants: NIPPON ELECTRIC CO, NEC CORPORATION
Application year: 1995
Main classification: H02H7
IPC codes: G01R19, H02H7, G01R31, H01M2, H02J7