Patent Publication Number: US-2017358915-A1

Title: Interrupting device

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is the U.S. national stage of PCT/JP2016/050230 filed Jan. 6, 2016, which claims priority of Japanese Patent Application No. JP 2015-008792 filed Jan. 20, 2015. 
    
    
     TECHNICAL FIELD 
     The present invention relates to an interrupting device that interrupts a current flowing in a current path in order to prevent overcurrent from flowing in the current path. 
     BACKGROUND 
     A vehicle includes a power source device that, by switching off a switch provided in a current path from a battery to a load, prevents overcurrent from flowing in the current path (see JP 2014-34291A, for example). 
       FIG. 1  is a circuit diagram illustrating a conventional power source device  8 . According to the conventional power source device  8  described in JP 2014-34291A, the positive terminal of a battery  80  is connected to one end of a load  81  via a switch  82 , whereas the negative terminal of the battery  80  and the other end of the load  81  are grounded. 
     A switch  83  is a PNP bipolar transistor, and a switch  84  is an NPN bipolar transistor. Regarding the switch  83 , the emitter is connected to the positive terminal of the battery  80 , the collector is connected to one end of the load  81  and one end of a resistor Ra, and the base is connected to one end of a resistor Rb. The other end of the resistor Ra is connected to the base of the switch  84 , one end of a switch  85 , and one end of a resistor Rc. The collector of the switch  84  is connected to the other end of the resistor Rb. The emitter of the switch  84 , the other end of the switch  85 , and the other end of the resistor Rc are grounded. 
     According to the conventional power source device  8 , the switches  82 ,  83 ,  84 , and  85  are off, on, on, and off, respectively, in the case where the load  81  is stopped. 
     Because the switch  82  is off, current flows from the positive terminal of the battery  80  to the switch  83  and the resistors Ra and Rc in order. The resistors Ra and Rc divide the output voltage of the battery  80 , and the divided voltage is applied to the base of the switch  84 . As a result, current flows from the base to the emitter of the switch  84  such that the switch  84  stays on. 
     In the case where the switch  84  is on, current flows from the positive terminal of the battery  80  to the emitter and the base of the switch  83  in order. Current flows from the base of the switch  83  to the resistor Rb and the switch  84  in order. As a result, the switch  83  also stays on. The battery  80  supplies dark current to the load  81  via the switch  83 . 
     In the case where the load  81  has shorted, a value Ia of the current flowing in the resistor Ra goes to zero amperes. The voltage difference between the base and the emitter of the switch  84  goes to zero volts as a result, and thus no current flows from the base to the emitter, which switches the switch  84  off. In the case where the switch  84  has been switched off, a value Ib of the current flowing in the resistor Rb, or in other words, the value of the current flowing from the emitter to the base of the switch  83 , goes to zero amperes. As a result, the switch  83  is also switched off. As a result, overcurrent is prevented from flowing from the battery  80  to the load  81  via the switch  83 . 
     Based on the above, the circuit constituted of the switches  83 ,  84 , and  85  and the resistors Ra, Rb, and Rc functions as an interrupting device that, by switching the switch  83  off, interrupts current flowing in the current path from the battery  80  to the load  81  so as to prevent overcurrent from flowing in the current path. 
     However, in the conventional power source device  8 , a constant current continues to flow to both the resistors Ra and Rb while the load  81  is stopped, regardless of the value of the current supplied to the load  81 . The resistors Ra, Rb, and Rc thus continue to consume power. There is thus a problem in that the circuit functioning as an interrupting device has high power consumption. 
     Having been achieved in light of such circumstances, an object of the present invention is to provide an interrupting device having low power consumption. 
     SUMMARY 
     An interrupting device according to the present invention is an interrupting device that, in the case where a current greater than or equal to a predetermined value flows in a current path, interrupts the current flowing in the current path. The device includes: a switch provided in the current path; a current mirror circuit that draws in a current from each of one end and another end of the switch and outputs a current obtained by combining the two drawn-in currents; and a switching circuit that switches the switch off in the case where a value of the current outputted from the current mirror circuit is greater than or equal to a current threshold. A value of each of the two currents drawn in by the current mirror circuit is higher the higher a value of the current flowing in the switch is. 
     According to the present invention, the switch is provided in the current path. The current mirror circuit draws in the current from each of the one end and the other end of the switch and outputs a current obtained by combining the two drawn-in currents. The value of each of the two currents drawn in by the current mirror circuit is higher the higher the value of the current flowing in the switch is. In the case where the current flowing in the current path is greater than or equal to the predetermined value, the value of the current outputted from the current mirror circuit becomes greater than or equal to the current threshold, and the switching circuit switches the switch off so as to interrupt the current flowing in the current path. 
     As a result, overcurrent is prevented from flowing in the current path. Furthermore, because the values of the two currents drawn in by the current mirror circuit are greater the greater the value of the current flowing in the switch is, the value of the current flowing through the current mirror circuit will be low in the case where the value of the current flowing in the current path is low. Accordingly, for example, in the case where the current path is a current path from a battery to a load, the load consumes almost no power when not operating, and thus the device also consumes almost no power. Thus the amount of power consumed is low. 
     In the interrupting device according to the present invention, the switching circuit is configured to keep the switch off after switching the switch off regardless of the value of the current outputted by the current mirror circuit, and the interrupting device further includes a canceling unit that cancels the off state the switch is kept in. 
     According to the present invention, no current flows in the switch in the case where the switching circuit has switched the switch off, and thus the value of the current outputted by the current mirror circuit drops. However, the switching circuit keeps the switch off regardless of the value of the current outputted by the current mirror circuit. In the case where, for example, a signal for canceling this off state has been inputted to the device, the off state of the switch, kept by the switching circuit, is canceled. 
     As described above, the switch is kept off, and thus overcurrent is prevented from continuing to flow in the current path. Furthermore, canceling the off state of the switch being kept makes it possible for current to flow in the current path again. 
     In the interrupting device according to the present invention, the switch is a transistor, and is configured to enter a non-conductive state in the case where a voltage value at a control terminal that takes a potential at a current input terminal as a reference is greater than or equal to a voltage threshold less than zero, and to enter a conductive state in the case where the voltage value at the control terminal is less than the voltage threshold. 
     In the present invention, the switch is a transistor, for example a P-channel Field Effect Transistor (FET). In the switch, in the case where the voltage value at a control terminal, such as a gate, that takes the potential at a current input terminal, such as a source, as a reference, is greater than or equal to a voltage threshold less than zero, the switch enters a non-conductive state and is switched off. Additionally, in the switch, in the case where the voltage value at the control terminal that takes the potential at the current input terminal as a reference is less than the voltage threshold, the switch enters a conductive state and is switched on. 
     Accordingly, by keeping the voltage between the current input terminal and the control terminal at zero volts or substantially zero volts, the switch can be kept on, and it is not necessary to keep the potential at the control terminal at a potential greater than the potential at the current input terminal using a charge pump circuit, for example. The amount of power consumed is therefore low. 
     The interrupting device according to the present invention further includes a resistance circuit, having at least one resistor, in which the current outputted by the current mirror circuit flows, and the switching circuit is configured to switch the switch off in the case where a voltage value at both ends of the resistance circuit is greater than or equal to a second voltage threshold. 
     According to the present invention, the current outputted by the current mirror circuit flows in the resistance circuit, which has at least one resistor. Accordingly, the voltage value between both ends of the resistance circuit is higher the higher the value of the current outputted by the current mirror circuit is. In the case where the current outputted by the current mirror circuit is greater than or equal to the current threshold, the voltage value between both ends of the resistance circuit becomes greater than or equal to the second voltage threshold, and the switching circuit switches the switch off. A configuration in which the current flowing in the current path is interrupted in the case where a current greater than or equal to the predetermined value flows in the current path can thus be realized easily. 
     In the interrupting device according to the present invention, the resistance circuit includes a first resistor, and a series circuit, constituted of a second resistor and a capacitor, connected in parallel to the first resistor. 
     According to the present invention, in the resistance circuit, a series circuit constituted of the second resistor and the capacitor is connected in parallel to the first resistor. In the case where no power is stored in the capacitor, the resistance value of the resistance circuit is substantially the resistance value of the parallel circuit constituted of the first resistor and the second resistor. As the power stored in the capacitor increases, the resistance value of the resistance circuit rises and approaches the resistance value of the first resistor. 
     A load to which a large current is temporarily supplied during operation can be considered as the load to which current is supplied via the current path. In this case, a low amount of power is stored in the capacitor, and the resistance value of the resistance circuit is low, at the point in time when the load operated. Accordingly, even in the case where the load has operated and a large current has temporarily flows in the current path, the voltage between both ends of the resistance circuit will not become greater than or equal to the voltage threshold, and the switch will not be switched off. 
     In the interrupting device according to the present invention, the switching circuit is configured to output an interruption signal indicating the interruption of the current flowing in the current path in the case where the switch has been switched off. 
     According to the present invention, in the case where the switch has been switched off, the interruption signal indicating the interruption of the current flowing in the current path is outputted from the switching circuit, and thus a notification that the current flowing in the current path has been interrupted, for example, can be made. 
     Advantageous Effects of Invention 
     According to the present invention, the value of a current outputted by a current mirror circuit is greater the greater the value of a current flowing in a switch is, and thus the amount of power consumed is low. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a circuit diagram illustrating a conventional power source device. 
         FIG. 2  is a block diagram illustrating the configuration of primary components of a power source device according to a first embodiment. 
         FIG. 3  is a circuit diagram illustrating an interrupting device. 
         FIG. 4  is a timing chart illustrating operations of the interrupting device. 
         FIG. 5  is a circuit diagram illustrating an interrupting device according to a second embodiment. 
         FIG. 6  is a timing chart illustrating effects of a resistance circuit. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     The present invention will be described in detail hereinafter on the basis of drawings illustrating embodiments thereof. 
     First Embodiment 
       FIG. 2  is a block diagram illustrating the configuration of primary components of a power source device  1  according to a first embodiment. The power source device  1  is preferably installed in a vehicle, and includes an interrupting device  10 , a battery  11 , a load  12 , and a notifying unit  13 . The interrupting device  10  is connected to a positive terminal of the battery  11 , one end of the load  12 , and the notifying unit  13 . A negative terminal of the battery  11  and the other end of the load  12  are grounded. 
     The load  12  is an electrical device installed in the vehicle, and power is supplied to the load  12  from the battery  11  via the interrupting device  10 . The load  12  consumes a set amount of power while operating, and thus a set current flows to the load  12 . When stopped, the load  12  consumes almost no power, and thus the value of current flowing in the load  12  is zero amperes or substantially zero amperes. 
     The interrupting device  10  is configured so that the battery  11  can normally supply power to the load  12 . In the case where current greater than or equal to a predetermined value flows in the current path from the battery  11  to the load  12 , the interrupting device  10  interrupts the current flowing in the current path. In the case where the interrupting device  10  has interrupted the current, the interrupting device  10  outputs, to the notifying unit  13 , an interruption signal indicating that the current flowing in the current path from the battery  11  to the load  12  has been interrupted. 
     The notifying unit  13  is inputted with the interruption signal from the interrupting device  10 . In the case where the notifying unit  13  has been inputted with the interruption signal from the interrupting device  10 , the notifying unit  13  makes a notification. The notifying unit  13  makes the notification by displaying, in a display unit (not illustrated), a message indicating that the current flowing in the current path from the battery  11  to the load  12  has been interrupted, for example. 
     A cancellation signal for canceling the interruption of the current flowing in the current path from the battery  11  to the load  12  is inputted to the interrupting device  10 . The cancellation signal is a binary signal having a high-level voltage and a low-level voltage. In the case where the cancellation signal is at the low-level voltage, the interrupting device  10  maintains the current interrupted or non-interrupted state of the current. In the case where the interrupting device  10  is in an interrupting state, and the cancellation signal switches from the low-level voltage to the high-level voltage, the interrupting device  10  cancels the interruption of the current. 
       FIG. 3  is a circuit diagram illustrating the interrupting device  10 . The interrupting device  10  includes a P-channel FET  20 , a current mirror circuit  21 , a resistance circuit  22 , an N-channel FET  23 , a switching circuit  24 , and a resistor R 1 . Each of the FETs  20  and  23  has a drain, a source, and a gate as terminals. The source of the FET  20  is connected to the positive terminal of the battery  11 , and the drain of the FET  20  is connected to one end of the load  12 . The source of the FET  20  is further connected to one end of the resistor R 1 . The drain of the FET  20  and the other end of the resistor R 1  are individually connected to the current mirror circuit  21 . 
     The current mirror circuit  21  is further connected to one end of the resistance circuit  22 . The one end of the resistance circuit  22  is further connected to the drain of the FET  23 . The other end of the resistance circuit  22  and the source of the FET  23  are grounded. 
     The switching circuit  24  is connected individually to the positive terminal of the battery  11 , the gate of the FET  20 , and the one end of the resistance circuit  22 . The gate of the FET  20  is also connected to the notifying unit  13 . 
     The FET  20  functions as a switch, and is provided in the current path from the battery  11  to the load  12 . In the FET  20 , current flows from the source to the drain. The FET  20  enters a non-conductive state and is switched off in the case where a voltage value at the gate, which takes the potential of the source as a reference, is greater than or equal to a predetermined voltage threshold, which is less than zero volts. Meanwhile, the FET  20  enters a conductive state and is switched on in the case where the voltage value at the gate, which takes the potential of the source as a reference, is less than the voltage threshold. The gate functions as a control terminal, and the source functions as a current input terminal. 
     In the case where the FET  20  is on, current can flow from the battery  11  to the load  12 , and thus the battery  11  can supply power to the load  12 . In the case where the FET  20  is off, no current flows between the source and the drain of the FET  20 , and the current flowing from the battery  11  to the load  12  is interrupted. 
     The current mirror circuit  21  includes two PNP bipolar transistors  30  and  31  and two NPN bipolar transistors  32  and  33 . Each of the bipolar transistors  30 ,  31 ,  32 , and  33  has an emitter, a collector and a base as terminals. The emitter of the bipolar transistor  30  is connected to the drain of the FET  20 , and the emitter of the bipolar transistor  31  is connected to the other end of the resistor R 1 . The base of the bipolar transistor  30  is connected to the collector of the bipolar transistor  30  and the base of the bipolar transistor  31 . 
     The collector of the bipolar transistor  30  is further connected to the collector of the bipolar transistor  32 . The collector of the bipolar transistor  31  is connected to the base of the bipolar transistor  32 , and the collector and the base of the bipolar transistor  33 . The emitter of each of the bipolar transistors  32  and  33  is connected to the one end of the resistance circuit  22 . 
     In both of the bipolar transistors  30  and  31 , current flows from the emitter to the collector. In both of the bipolar transistors  30  and  31 , a resistance value between the emitter and the collector increases as the voltage at the base, which takes the potential at the emitter as a reference, increases. The resistance value between the emitter and the collector decreases as the voltage at the base, which takes the potential at the emitter as a reference, decreases. 
     The bipolar transistors  30  and  31  have the same or substantially the same characteristics. Accordingly, for both of the bipolar transistors  30  and  31 , the resistance value between the emitter and the collector, which corresponds to the voltage at the base that takes the potential at the emitter as a reference, is the same or substantially the same. 
     Meanwhile, in the bipolar transistors  32  and  33 , current flows from the collector to the emitter. In each of the bipolar transistors  32  and  33 , a resistance value between the collector and the emitter decreases as the voltage at the base, which takes the potential at the emitter as a reference, increases, and increases as the voltage at the base, which takes the potential at the emitter as a reference, decreases. 
     The bipolar transistors  32  and  33  also have the same or substantially the same characteristics. Accordingly, for both of the bipolar transistors  32  and  33 , the resistance value between the emitter and the collector, which corresponds to the voltage at the base that takes the potential at the emitter as a reference, is the same or substantially the same. 
     The current mirror circuit  21  configured as described above draws in current from both the source side and the drain side of the FET  20 . The values of the two currents drawn in by the current mirror circuit  21  are the same or substantially the same. The current mirror circuit  21  adjusts a value Ic of each of the two drawn-in currents such that the potentials at the emitters of the bipolar transistors  30  and  31  are the same or substantially the same. 
     A resistance value between the source and the drain of the FET  20 , the value of current flowing from the source to the drain of the FET  20 , and the resistance value of the resistor R 1  in the case where the FET  20  is on are represented by “ron”, “Id”, and “r 1 ”, respectively. 
     The potentials at the emitters of the bipolar transistors  30  and  31  being the same or substantially the same is equivalent to a voltage value between the source and the drain of the FET  20  and a voltage value between both ends of the resistor R 1  being the same or substantially the same. Accordingly, in the case where the FET  20  is on, ron×Id is the same or substantially the same as r 1 ×Ic. To rephrase, the current value Ic matches or substantially matches (ron×Id)/r 1 . 
     Accordingly, the current value Ic is proportional to the current value Id, being lower the lower the current value Id is, and higher the higher the current value Id is. In the case where the current value Id is zero amperes or substantially zero amperes, the current value Ic is also zero amperes or substantially zero amperes. 
     The current mirror circuit  21  outputs a current obtained by combining the two currents drawn in from the source side and the drain side of the FET  20 . In the case where the FET  20  is on, the value of the current outputted by the current mirror circuit  21  matches or substantially matches 2×Ic, or in other words, (2×ron×Id)/r 1 . 
     The FET  23  also functions as a switch, in the same manner as the FET  20 . In the FET  23 , current flows from the drain to the source. The FET  23  enters a conductive state and is switched on in the case where the voltage at the gate, which takes a ground potential as a reference, is greater than or equal to a set voltage. Meanwhile, the FET  23  enters a non-conductive state and is switched off in the case where the voltage at the gate, which takes the ground potential as a reference, is less than the set voltage. 
     In the case where the cancellation signal is at the high-level voltage, the voltage at the gate of the FET  23 , which takes the ground potential as a reference, becomes greater than or equal to the set voltage, and the FET  23  is switched on. In the case where the cancellation signal is at the low-level voltage, the voltage at the gate of the FET  23 , which takes the ground potential as a reference, becomes less than the set voltage, and the FET  23  is switched off. 
     The resistance circuit  22  includes a resistor R 2 . One end of the resistor R 2  serves as the one end of the resistance circuit  22 , and is connected to the emitters of the bipolar transistors  32  and  33  in the current mirror circuit  21 . The other end of the resistor R 2  serves as the other end of the resistance circuit  22 , and is grounded. Current outputted from the current mirror circuit  21  flows in the resistor R 2  of the resistance circuit  22  in the case where the FET  23  and a FET  40  (mentioned later) are off. 
     A resistance value of the resistor R 2  is represented by “r 2 ”. In the case where both the FETs  23  and  40  are off, a voltage value V 1  between both ends of the resistance circuit  22  matches or substantially matches (2×ron×r 2 ×Id)/r 1  (=2×r 2 ×Ic). The higher the value of the current outputted by the current mirror circuit  21  (2×Ic) is, the higher the voltage value V 1  is. 
     The switching circuit  24  includes a P-channel FET  40 , an N-channel FET  41 , a diode D 1 , and resistors R 3 , R 4 , R 5 , and R 6 . Each of the FETs  40  and  41  has a drain, a source, and a gate as terminals. The source of the FET  40  and one end of the resistor R 3  are connected to the positive terminal of the battery  11 . The other end of the resistor R 3  is connected to the gate of the FET  40  and one end of the resistor R 4 . The other end of the resistor R 4  is connected to the drain of the FET  41 , and the source of the FET  41  is grounded. 
     The gate of the FET  41  is connected to the drain of the FET  23  and the one end of the resistor R 2 . The drain of the FET  40  is connected to the notifying unit  13 , the gate of the FET  20 , the anode of the diode D 1 , and one end of the resistor R 5 . The other end of the resistor R 5  is grounded. The cathode of the diode D 1  is connected to one end of the resistor R 6 , and the other end of the resistor R 6  is connected to the gate of the FET  41 . 
     The FET  40  also functions as a switch, and in the FET  40 , current flows from the source to the drain. The FET  40  enters a non-conductive state and is switched off in the case where a voltage value at the gate, which takes the potential of the source as a reference, or in other words, a value obtained by subtracting an output voltage value Vb of the battery  11  from a voltage value V 2  at the gate that takes the ground potential as a reference, is greater than or equal to a voltage threshold (Vth 2 −Vb), which is less than zero volts. Meanwhile, the FET  40  enters a conductive state and is switched on in the case where the voltage value at the gate, which takes the potential of the source as a reference, or in other words, the value obtained by subtracting the output voltage value Vb from the voltage value V 2 , is less than the voltage threshold (Vth 2 −Vb). 
     To rephrase, the FET  40  is switched off in the case where the voltage value V 2  is greater than or equal to the voltage threshold Vth 2 , and is switched on in the case where the voltage value V 2  is less than the voltage threshold Vth 2 . The voltage threshold Vth 2  is a voltage value less than the output voltage value Vb of the battery  11 . 
     The FET  41  also functions as a switch, and in the FET  41 , current flows from the drain to the source. The voltage value V 1  is a voltage value between both ends of the resistance circuit  22 , and is also a voltage value between the gate and the source of the FET  41 . The FET  41  enters a conductive state and is switched on in the case where the voltage value V 1  is greater than or equal to a voltage threshold Vth 1 . The FET  41  enters a non-conductive state and is switched off in the case where the voltage value V 1  is less than the voltage threshold Vth 1 . 
     The FET  41  is off in the case where the voltage value V 1  is less than the voltage threshold Vth 1 . In the case where the FET  41  is off, no current flows in the resistors R 3  and R 4 , and the voltage value V 2  matches or substantially matches the output voltage value Vb of the battery  11 . At this time, the voltage value V 2  is greater than or equal to the voltage threshold Vth 2 , and the FET  40  is off. In the case where the FET  40  is off, no current flows in the resistor R 5 , and thus a voltage value V 3  at the drain of the FET  40 , which takes the ground potential as a reference, is zero volts or substantially zero volts. 
     In the FET  20 , a voltage value at the gate, which takes the source potential as a reference, can be expressed as (V 3 −Vb). In the case where the aforementioned predetermined voltage threshold is expressed as (Vth 3 −Vb), the FET  20  is switched off in the case where the voltage value V 3  is greater than or equal to the voltage threshold Vth 3 , and the FET  20  is switched on in the case where the voltage value V 3  is less than the voltage threshold Vth 3 . The voltage threshold Vth 3  is a voltage value less than the output voltage value Vb of the battery  11 . 
     In the case where the voltage value V 3  is zero volts or substantially zero volts, the voltage value V 3  is less than the voltage threshold Vth 3  and the FET  20  is therefore on. 
     In the case where the voltage value V 1  has become greater than or equal to the voltage threshold Vth 1 , the FET  41  is switched on, and current flows from the positive terminal of the battery  11  to the resistors R 3  and R 4  and the FET  41  in order. At this time, the resistors R 3  and R 4  divide the output voltage of the battery  11 , and the divided voltage is applied to the gate of the FET  40 . At this time, the voltage value V 2  becomes less than the voltage threshold Vth 2 , and the FET  40  is switched on. In the case where the FET  40  has been switched on, current flows from the positive terminal of the battery  11  to the FET  40  and the resistor R 5  in order, and the voltage value V 3  matches or substantially matches the output voltage value Vb of the battery  11 . 
     In the case where the voltage value V 3  matches or substantially matches the output voltage value Vb, the voltage value V 3  becomes greater than or equal to the voltage threshold Vth 3 , and the FET  20  is switched off. The current flowing in the current path from the battery  11  to the load  12  is interrupted as a result. 
     In the case where the FET  20  has been switched off, the current value Id becomes zero amperes or substantially zero amperes, the current value Ic becomes zero amperes or substantially zero amperes, and the value of the current outputted by the current mirror circuit  21  also becomes zero amperes or substantially zero amperes. Additionally, in the case where the FET  20  is switched off, current flows from the positive terminal of the battery  11  to the FET  40 , the diode D 1 , and the resistors R 6  and R 2  in order, and the voltage value V 1  stays at a voltage value greater than or equal to the voltage threshold Vth 1 . Accordingly, even in the case where the FET  20  has been switched off and the value of the current outputted by the current mirror circuit  21  has become zero amperes or substantially zero amperes, the FETs  40  and  41  remain on, and the FET  20  remains off. 
     As described above, in the case where the voltage value V 1  has become greater than or equal to the voltage threshold Vth 1  while the FET  20  is on, the switching circuit  24  switches the FET  20  from on to off. To rephrase, in the case where the value of the current outputted by the current mirror circuit  21  has become greater than or equal to a current threshold (Vth 1 /r 2 ) while the FET  20  is on, the switching circuit  24  switches the FET  20  from on to off. The voltage threshold Vth 1  corresponds to a second voltage threshold. 
     A value Ie of the current flowing from the battery  11  to the load  12  is expressed as (Id−Ic), and matches or substantially matches ((r 1 −ron)×Id)/r 1 . Accordingly, the current value Id matches or substantially matches (r 1 ×Ie)/(r 1 −ron). In the case where the FET  20  is on, the voltage value V 1  matches or substantially matches (2×ron×r 2 ×Id)/r 1 , as described earlier. Therefore, the voltage value V 1  matches or substantially matches (2×ron×r 2 ×Ie)/(r 1 −ron). Additionally, the current value Ie matches or substantially matches ((r 1 −ron)×V 1 )/(2×ron×r 2 ). 
     The current value Ie in the case where the voltage value V 1  is at the voltage threshold Vth 1 , or in other words, a current threshold Ieth at which the current flowing in the current path from the battery  11  to the load  12  is interrupted, matches or substantially matches ((r 1 −ron)×Vth 1 )/(2×ron×r 2 ). This current threshold Ieth is the aforementioned predetermined value. 
     Additionally, the current value Id in the case where the voltage value V 1  is at the voltage threshold Vth 1 , or in other words, a current threshold Idth at which the current flowing in the current path from the battery  11  to the load  12  is interrupted, matches or substantially matches (r 1 ×Vth 1 )/(2×ron×r 2 ). 
     In the case where the voltage value V 1  is greater than or equal to the voltage threshold Vth 1  and the FET  20  remains off, when the cancellation signal switches from the low-level voltage to the high-level voltage, the FET  23  switches from off to on and the voltage value V 1  becomes less than the voltage threshold Vth 1 . As a result, the FETs  41  and  40  switch off in that order, the FET  20  switches from off to on, and the off state of the FET  20  is canceled. 
     As described above, after switching the FET  20  from on to off, the switching circuit  24  keeps the FET  20  off regardless of the value of the current outputted by the current mirror circuit  21 . The FET  23  cancels the off state of the FET  20  established by the switching circuit  24 . Thus the FET  23  functions as a canceling unit. 
       FIG. 4  is a timing chart illustrating operations of the interrupting device  10 .  FIG. 4  illustrates transitions of the current values Ie and Ic, the voltage values V 1 , V 2 , and V 3 , the cancellation signal, and the on/off switching of the FET  20 . In  FIG. 4 , the high-level voltage is represented by “H” and the low-level voltage is represented by “L”. 
     With the interrupting device  10 , the cancellation signal normally stays at the low-level voltage in the case where the current value Ie is less than the current threshold Ieth. In the case where the current value Ie is less than the current threshold Ieth, the voltage value V 1  is less than the voltage threshold Vth 1 , and thus the FET  41  is off. As a result, the voltage value V 2  matches or substantially matches the output voltage value Vb of the battery  11 , and is thus greater than or equal to the voltage threshold Vth 2 . Because the voltage value V 2  is greater than or equal to the voltage threshold Vth 2 , the FET  40  is off, and the voltage value V 3  is zero volts or substantially zero volts. Accordingly, the voltage value V 3  is less than the voltage threshold Vth 3 , and thus the FET  20  stays on. 
     The FET  20  stays on while the current value Ie is less than the current threshold Ieth. The FET  20  therefore stays on while the load  12  is stopped and the current value Ie is zero amperes or substantially zero amperes as well. Furthermore, in the case where the load  12  is operating and the current value Ie exceeds zero amperes as well, the FET  20  stays on while the current value Ie is less than the current threshold Ieth. While the FET  20  is on, the voltage value V 3  is zero volts or substantially zero volts, and the switching circuit  24  outputs zero volts or substantially zero volts to the notifying unit  13 . 
     The current value Ic matches or substantially matches (ron×Ie)/(r 1 −ron), and is lower the lower the current value Ie is, and higher the higher the current value Ie is. Furthermore, in the case where the current value Ie is zero amperes or substantially zero amperes, the current value Ic is also zero amperes or substantially zero amperes. Accordingly, when the load  12  has stopped operating and almost no power is being consumed, the interrupting device  10  also consumes almost no power, and thus the interrupting device  10  has low power consumption. 
     As described earlier, the voltage value V 1  matches or substantially matches (2×ron×r 2 ×Ie)/(r 1 −ron), and is thus lower the lower the current value Ie is, and is higher the higher the current value Ie is. 
     Additionally, the FET  20  is a P-channel FET, and thus the FET  20  can be kept on by keeping the voltage between the source and the gate at zero volts or substantially zero volts. As such, it is not necessary to keep the potential at the gate at a higher potential than the potential at the source using a charge pump circuit, for example. The interrupting device  10  has an even lower power consumption as a result. 
     In the case where the current value Ie has risen due to an abnormality, the current value Ic and the voltage value V 1  also rise. In the case where the current value Ie has become greater than or equal to the current threshold Ieth, the voltage value V 1  becomes greater than or equal to the voltage threshold Vth 1 , and the FET  41  is switched on. In the case with FET  41  has been switched on, the voltage value V 2  drops to a voltage value less than the voltage threshold Vth 2 , and the FET  40  is switched on. As a result, the voltage value V 3  matches or substantially matches the output voltage value Vb of the battery  11 , and is thus greater than or equal to the voltage threshold Vth 3 . The FET  20  is thus switched off. In the case where the FET  20  has switched off, the current flowing in the current path from the battery  11  to the load  12  is interrupted, and the operation of the load  12  is stopped. 
     In the case where the FET  20  has switched off, the switching circuit  24  outputs, to the notifying unit  13 , a voltage having a value greater than or equal to the voltage threshold Vth 3  as the interruption signal indicating the interruption of the current flowing in the current path from the battery  11  to the load  12 . In the case where the notifying unit  13  has been inputted with the interruption signal, the notifying unit  13  makes a notification as described earlier. A notification indicating that the current flowing in the current path from the battery  11  to the load  12  has been interrupted can be made as a result. 
     In the case where the FET  40  has switched from off to on, current flows from the positive terminal of the battery  11  to the FET  40 , the diode D 1 , and the resistors R 6  and R 2  in order, and the voltage value V 1  stays greater than or equal to the voltage threshold Vth 1 . In the case where the FET  40  has switched from off to on, the current value Ie drops to a current value less than the current threshold Ieth, and the value of the current outputted by the current mirror circuit  21  drops. However, the switching circuit  24  keeps the FET  20  off regardless of the value of the current outputted by the current mirror circuit  21 . As a result, the interrupting device  10  can prevent overcurrent from continuing to flow in the current path. 
     In the case where the cancellation signal has switched from the low-level voltage to the high-level voltage while the FET  20  is off, the voltage value V 1  drops to zero volts or substantially zero volts, which is lower than the voltage threshold Vth 1 , and the FET  41  is switched off. In the case where the FET  41  has been switched off, the voltage value V 2  returns to greater than or equal to the voltage threshold Vth 2 , and the FET  40  is switched off. As a result, the voltage value V 3  returns to less than the voltage threshold Vth 3 , and the FET  20  is once again switched on. 
     In the case where the load  12  operates again after the cancellation signal has switched from the high-level voltage to the low-level voltage, current flows in the current path from the battery  11  to the load  12 , and the current value Ie again becomes a current value greater than or equal to zero amperes. 
     As described thus far, by canceling the state in which the FET  20  stays off, current can again flow in the current path from the battery  11  to the load  12 . 
     According to the interrupting device  10  configured as described thus far, a configuration in which, in the case where current greater than or equal to the current threshold Ieth flows in the current path from the battery  11  to the load  12 , the current flowing in the current path is interrupted, is easily realized using the voltage value V 1  between both ends of the resistance circuit  22 . 
     Second Embodiment 
       FIG. 5  is a circuit diagram illustrating an interrupting device  10  according to a second embodiment. The interrupting device  10  according to the second embodiment differs from the interrupting device  10  according to the first embodiment in terms of the configuration of the resistance circuit  22 . 
     A load to which a large current is temporarily supplied during operation can be considered as the load  12 . The interrupting device  10  according to the second embodiment is a device in which the FET  20  is unlikely to erroneously turn off even in the case where a large current flows in the current path from the battery  11  to the load  12  temporarily while the load  12  is operating. 
     Hereinafter, points of the second embodiment that are different from the first embodiment will be described. Configurations aside from those described hereinafter are the same as in the first embodiment, and will thus be given the same reference numerals, and detailed descriptions thereof will be omitted. 
     The resistance circuit  22  according to the second embodiment includes a capacitor C 1  and a resistor R 7  in addition to the resistor R 2 . One end of the resistor R 7  is connected to the current mirror circuit 21-side end of the resistor R 2 , and the other end of the resistor R 7  is connected to one end of the capacitor C 1 . The other end of the capacitor C 1  is grounded. In this manner, a series circuit constituted of the capacitor C 1  and the resistor R 7  is connected in parallel to the resistor R 2 . The resistors R 2  and R 7  function as a first resistor and a second resistor, respectively. 
     In the case where the power stored in the capacitor C 1  is zero watts, the resistance value of the resistance circuit  22  is substantially the resistance value of a parallel circuit formed by the resistor R 2  and the resistor R 7  connected in parallel. This resistance value is lower than the resistance value of the resistor R 2 . The resistance value of the resistance circuit  22  rises as the power stored in the capacitor C 1  increases. An upper limit value of the resistance value of the resistance circuit  22  is the resistance value of the resistor R 2 . 
     The resistance value of the resistance circuit  22  is represented by “rt”. Current outputted by the current mirror circuit  21  flows in the resistance circuit  22 . In the case where the value of the current outputted by the current mirror circuit  21  is greater than or equal to a current threshold (Vth 1 /rt), the FET  41  is switched on, and the switching circuit  24  switches the FET  20  off. Accordingly, in the case were the resistance value rt of the resistance circuit  22  is low, the FET  20  will not turn off even if a large current is outputted from the current mirror circuit  21 . In other words, the current threshold Ieth is greater the lower the resistance value rt of the resistance circuit  22  is, and is lower the greater the resistance value rt is. 
     In the case where the value of the current outputted by the current mirror circuit  21  is less than the current threshold (Vth 1 /rt), the FET  41  is switched off, and the switching circuit  24  switches the FET  20  on. 
       FIG. 6  is a timing chart illustrating effects of the resistance circuit  22 . In  FIG. 6 , transitions of the current value Ie are indicated by a bold line, whereas transitions of the current threshold Ieth are indicated by a narrow line. In the case where the current value Ie is zero amperes or substantially zero amperes and the current mirror circuit  21  is not outputting current, the capacitor C 1  discharges and thus little power is stored in the capacitor C 1 . The current threshold Ieth is therefore high. 
     In the case where the load  12  operates and current begins to flow from the battery  11  to the load  12 , the current mirror circuit  21  also begins to output current, and power is stored in the capacitor C 1 . As the power stored in the capacitor C 1  rises, the current threshold Ieth drops. 
     The current threshold Ieth is sufficiently high during the period when the load  12  operates and a large current is temporarily supplied, and thus the voltage value V 1  does not become greater than or equal to the voltage threshold Vth 1 , and the FET  20  remains on. Thus even in the case where the load  12  operates and a large current temporarily flows in the current path from the battery  11  to the load  12 , the voltage value V 1  will not become greater than or equal to the voltage threshold Vth 1 , and the FET  20  will not be switched off. 
     The interrupting device  10  according to the second embodiment has the same configuration as the interrupting device  10  according to the first embodiment, with the exception of the series circuit constituted of the capacitor C 1  and the resistor R 7  being connected to the resistor R 2 . Thus the interrupting device  10  according to the second embodiment achieves the same effects as the interrupting device  10  according to the first embodiment. 
     In the first and second embodiments, the values of the two currents drawn in by the current mirror circuit  21  from the source and the drain of the FET  20  need not be the same or substantially the same. The current mirror circuit  21  may draw in a current from the source of the FET  20  having a value that is a predetermined multiple of the value of the current drawn in from the drain of the FET  20 . Additionally, the current mirror circuit  21  is not limited to a circuit constituted using the bipolar transistors  30 ,  31 ,  32 , and  33 . The current mirror circuit  21  may be any circuit that draws in two currents, having values that are greater the greater the current value Id is, from the source and the drain, respectively, of the FET  20 , and that outputs a current obtained by combining the two drawn-in currents. 
     Additionally, a PNP bipolar transistor may be used instead of the FET  20 . Furthermore, in the switching circuit  24 , a PNP bipolar transistor may be used instead of the FET  40 , and an NPN bipolar transistor may be used instead of the FET  41 . 
     It is sufficient that the FET  23  function as a switch, and thus the FET  23  is not limited to an N-channel FET. The FET  23  may be a P-channel FET. Furthermore, a bipolar transistor may be used instead of the FET  23 . The first and second embodiments disclosed here are intended to be in all ways exemplary and in no ways limiting. The scope of the present invention is defined not by the foregoing descriptions but by the scope of the claims, and is intended to include all changes equivalent in meaning to and falling within the scope of the claims.