Abstract:
A current sensor has a measurement unit for measuring the current flowing through an electrical wire using electromagnetic induction caused by the magnetic flux around the electrical wire, a wireless transmission unit for wirelessly transmitting measurement results, a power generation unit for generating power using the same electromagnetic induction caused by the magnetic flux around the electrical wire, and a battery that is charged by the power generation unit and supplies power to the measurement unit and the wireless transmission unit. A sudden change in the current flowing through the electrical wire causes measurement to be carried out. The timing at which transmission is carried out is controlled according to the size of the current flowing through the electrical wire. When the measurement unit is measuring, power generation by the power generation unit is stopped. Measurement results obtained during insufficient charging are stored and sent after charge is secured.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to a current sensor. 
       BACKGROUND ART 
       [0002]    For the purpose of providing a power measurement system and the like that need no special installation to be made by any skillful professionals, it has been proposed to provide a measurement apparatus and the like having a current sensor for detecting a waveform of current flowing through an electrical wire, in a noncontact manner, by means of electromagnetic inductive coupling and communication means (see Patent Literature 1 listed below). 
       CITATION LIST 
     Patent Literature 
       [0003]    Patent Literature 1: Re-publication of PCT International Publication No. 2009-099082 
       SUMMARY OF INVENTION 
     Technical Problem 
       [0004]    However, there are many problems to be further considered to achieve a more convenient current sensor. 
         [0005]    In view of the above, an object of the present invention is to propose a more convenient current sensor. 
       Solution to Problem 
       [0006]    To achieve the above object, according to one aspect of the present invention, there is provided a current sensor including a measurement unit that measures current flowing through a target electrical wire that is a target of measurement, a wireless transmission unit that wirelessly transmits a result of measurement performed by the measurement unit, a power generation unit that generates power by means of electromagnetic induction caused by magnetic flux around the target electrical wire, and a storage battery that is charged by the power generation unit and supplies power to the measurement unit and the wireless transmission unit. This makes it possible for the measurement to be performed with supply of power from the current flowing through the target. 
         [0007]    According to a specific feature of the present invention, the current sensor has a control unit that performs control such that the measurement unit performs measurement by means of change in the current flowing through a target electrical wire. This makes it possible for the measurement to be performed without missing any change in the current. According to more specific feature of the present invention, the control unit performs control such that the measurement unit performs measurement by means of a sudden change in the current flowing through the target electrical wire. According to further specific feature of the present invention, the control unit performs control such that the measurement unit performs measurement also at predetermined time intervals. 
         [0008]    According to another specific feature of the present invention, the current sensor includes a control unit that controls, by means of magnitude of the current flowing through the target electrical wire, timing for the wireless transmission unit to perform transmission. This makes it possible to receive supply of power for measurement from the storage battery in a manner balanced with charging by the power generation unit. 
         [0009]    According to another specific feature of the present invention, the current sensor has a control unit that makes the power generation unit stop power generation when the measurement unit performs measurement. This makes possible current measurement uninfluenced by the charging of the storage battery. According to another specific feature, the current sensor has a control unit that keeps the measurement unit from performing measurement when the control unit makes the power generation unit perform power generation. 
         [0010]    According to another specific feature of the present invention, the current sensor has a storage unit in which the result of measurement performed by the measurement unit is stored, and also has a control unit that stores and maintains the result of measurement in the storage unit and makes the transmission unit transmit the result of measurement stored in the storage unit at timing different from timing of measurement. This makes it possible to perform more elaborate measurement. According to a more specific feature, the control unit stores and maintains the result of measurement in the storage unit when supply of power to the transmission unit from the storage battery is insufficient, and the control unit makes the transmission unit transmit the result of measurement stored in the storage unit when sufficient amount of power is securely supplied from the storage battery to the transmission unit. 
         [0011]    According to another feature of the present invention, there is provided a current sensor that has a measurement unit that measures current flowing through a target electrical wire that is a target of measurement, a power generation unit that generates power by means of electromagnetic induction caused by magnetic flux around the target electrical wire, and a storage battery that is charged by the power generation unit and supplies power to the measurement unit. Here, the measurement unit is used both for measuring current flowing through a target electrical wire and for measuring current with which the storage battery is charged by the power generation unit. This makes it possible to perform current measurement and to confirm a secured state of power supply for the measurement. 
         [0012]    According to another feature of the present invention, there is provided a current sensor having a measurement unit that measures current flowing through a target electrical wire by means of electromagnetic induction caused by magnetic flux around the target electrical wire, a power generation unit that generates power by means of electromagnetic induction caused by magnetic flux around the target electrical wire, and a storage battery that is charged by the power generation unit and supplies power to the measurement unit and the wireless transmission unit. Here, the measurement unit and the power generation unit share a common iron core. This makes it possible to measure current and secure power supply for the measurement with a simple configuration. 
       Advantageous Effects of Invention 
       [0013]    As has been discussed above, according to the present invention, there is provided a current sensor that is more convenient to use. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]      FIG. 1  is a system diagram illustrating an overall configuration of a first example of the present invention (Example 1); 
           [0015]      FIG. 2  is partly a conceptual diagram and partly a block diagram of a current sensor in Example 1 illustrated in  FIG. 1 ; 
           [0016]      FIG. 3  is a flow chart illustrating an operation of a control unit of the current sensor in Example 1; 
           [0017]      FIG. 4  is partly a conceptual diagram and partly a block diagram of a current sensor in a second example of the present invention (Example 2); 
           [0018]      FIG. 5  is a flow chart illustrating an operation of a control unit of the current sensor in Example 2 illustrated in  FIG. 4 ; 
           [0019]      FIG. 6  is a flow chart illustrating an operation of a control unit of a current sensor in a third example of the present invention (Example 3); 
           [0020]      FIG. 7  is a flow chart illustrating an operation of a control unit of a current sensor in a fourth example of the present invention (Example 4); 
           [0021]      FIG. 8  is partly a conceptual diagram and partly a block diagram of a current sensor in a fifth example of the present invention (Example 5); and 
           [0022]      FIGS. 9A, 9B, 9C, and 9D  are each a diagram illustrating timing of an operation of a control unit in Example 5 illustrated in  FIG. 8 . 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
     EXAMPLE 1 
       [0023]      FIG. 1  is a system diagram illustrating an overall configuration of Example 1 where a current sensor according to an embodiment of the present invention is used. Example 1 constitutes a smart meter system in a house. In the house, there exist a first household appliance (e.g., a lighting appliance)  2 , a second household appliance (e.g., a television set)  4 , a third household appliance (e.g., a refrigerator)  6 , etc. These household appliances are each connected to a commercial power supply  8  via a wall socket to receive power. Furthermore, there are also arranged a first current sensor  10 , a second current sensor  12 , and a third current sensor  14  corresponding to the first, second, and third household appliances, respectively. The first current sensor  10  detects density of magnetic flux around a cord through which power is supplied to the first household appliance  2 , and thereby, the first current sensor  10  measures magnitude of current consumed by the first household appliance  2 . The first current sensor  10  has short distance communication means, and transmits data of the measured current to a smart meter  20  by means of a radio wave  18 . This operation also applies to the second and third current sensors  13  and  14 . 
         [0024]      FIG. 2  is partly a conceptual diagram and partly a block diagram of a current sensor that is commonly used as the first current sensor  10 , the second current sensor  12 , and the third current sensor  14 . A cord  22  is connected to the first household appliance  10 , and around the cord  22 , there is disposed an iron core ring  24  having a shape corresponding to magnetic flux that is generated around the cord  22 . Around the iron core ring  24 , a coil  26  is wound (the coil  26  is wound around unillustrated part of the iron-core ring  24 , too, as indicated by an alternate dash and dot line  26   a ), and current extracted from an outgoing wire of the coil  26  charges a storage battery  32  in a power supply circuit  30  via a rectifier  28 . The power supply circuit  30  includes a voltage detection unit  33  for checking a voltage to which the storage battery  32  is charged. As has been hitherto described, the power supply unit of the current sensor is configured to generate power by means of the current flowing through the cord  22  and accumulates the thus generated power therein. 
         [0025]    Next, a description will be given of a configuration for receiving supply of power from the power supply circuit  30  and detecting current to transmit the detection result to the smart meter  20 . Around the cord  22 , there is disposed an iron core ring  34  configured in the same manner as the one for the power supply unit, and a coil  36  is wound around the iron core ring  34 . An outgoing wire of the coil  36  is connected to a resistor  40  of a current detection unit  38 , and the current detection unit  38  detects current flowing through the cord  22  as a voltage appearing across the resistor  40 . Data of magnitude and variation of the current detected by the current detection unit  38  is processed by a processing unit  42 , and is then transmitted from a transmission unit  44  to the smart meter  20 . A control unit  46  controls the current detection performed by the current detection unit  38 , the detection-data processing performed by the processing unit  42 , and the transmission of the processed data performed by the transmission unit  44 . The current detection unit  38 , the processing unit  42 , the transmission unit  44 , and the control unit  46  receive power from the power supply circuit  30  (as indicated by bold arrows). 
         [0026]      FIG. 3  is a flow chart illustrating an operation of the control unit  46  of the current sensor in Example 1 illustrated in  FIG. 2 . When current is generated in the coil  26  based on current flowing through the cord  22  and the storage battery  32  is charged to a lowest level sufficient to start up the control unit  46 , the control unit  46  is started up and the flow of operation starts. Then in Step S 2 , it is checked whether or not the storage battery  32  has been charged to a predetermined voltage that is necessary for current detection and transmission of the result of the detection. When the storage battery  32  is found to have been charged to the predetermined voltage or higher, the flow proceeds to step S 4 , where current detection by the current detection unit  38  is started, and then the flow shifts to step S 6 . Here, in a case where the current detection has already been started, nothing is done in step S 4 , and the flow shifts directly to step S 6 . 
         [0027]    In Step S 6 , it is checked whether or not current measurement and transmission of the result of the current measurement immediately after the start of the current detection in step S 4  have been completed, and when not, the flow proceeds to step S 8 , where the current detection unit  38  and the processing unit  42  perform measurement and the transmission unit  44  performs transmission. Next, a counter for determining a detection interval is reset to start counting in step S 10 , and the flow reaches step S 12 . In this way, immediately after the current detection is started, measurement and transmission are each performed once first. On the other hand, when it is found in step S 6  that current has been measured and a result of the current measurement has been transmitted immediately after the start of the current detection, no further measurement or transmission is performed and the flow shifts to step S 12 . Thereafter, measurement and transmission are performed when a condition is satisfied as described later. 
         [0028]    Current measurement has been continuously performed by the current detection unit  38  and the processing unit  42  since the start of the current detection, and in step S 12 , it is checked whether or not a moving average value of the detected current is equal to or larger than a predetermined value. When the moving average value is found to be equal to or larger than the predetermined value, the flow proceeds to step S 14 , where a count-up value is set to a minimum (two seconds, for example), and the flow shifts to step S 16 . On the other hand, when the moving average value is found to be smaller than the predetermined value, the flow proceeds to step S 18 , where the count-up value is set to a maximum (10 seconds, for example), and the flow shifts to step S 16 . In this manner, when the moving average value of the current detected by the current detection unit  38  is large, then the current flowing through the coil  26  can also be regarded as large, and charging current of the storage battery  32  can also be regarded as large, and thus, the count-up value is reduced to increase the frequency of measurement and transmission, to thereby perform fine measurement and transmission. On the other hand, when the moving average value of the current detected by the current detection unit  38  is small, then the current flowing through the coil  26  can also be regarded as small and the charging current of the storage battery  32  can also be regarded as small, and thus, the count-up value is increased to reduce the frequency of the measurement and the transmission, to thereby reduce consumption of power from the storage battery  32 . Here, from step S 12  through step S 18  in the flow, the count-up value is changed stepwise between two large and small values, but the count-up value may be changed more finely between more than two levels of values, or may be changed substantially non-stepwise and continuously. 
         [0029]    In step S 16 , it is checked whether or not time has been counted up to the set count-up value to complete the counting up. When it is found that the counting up has not yet reached the count-up value (time has not yet been counted up to the count-up value), the flow proceeds to step S 20 , where it is checked whether or not instantaneous current has been caused to increase by a predetermined amount or more by a sudden increase of the current flowing through the cord  22 . When NO in step S 20 , the flow proceeds to step S 22 , where it is checked whether or not the instantaneous current has been caused to decrease by a predetermined amount or more by a sudden decrease of the current flowing through the cord  22 . When NO in step S 22 , the flow shifts to step S 24 . Here, in step S 2 , when it is found that the storage battery  32  has not yet been charged to the predetermined voltage that is necessary for current detection and transmission of the result of the detection, the flow proceeds to step S 26 , where the current detection is stopped to reduce power consumption, and then, the flow shifts directly to step S 24 . 
         [0030]    On the other hand, when completion of the counting up is detected in step S 16 , when increase of the instantaneous current by the predetermined amount or more is detected in step S 20 , or when decrease of the instantaneous current by the predetermined amount or more is detected in step S 22 , the flow returns to step S 8 , where measurement and transmission are performed. Thus, measurement and transmission are basically performed regularly at time intervals based on the set count-up value. Even out of the regular timing, measurement and transmission are immediately performed when increase or decrease of the instantaneous current by the predetermined amount or more has occurred. 
         [0031]    In Step S 24 , it is checked whether or not the storage battery  32  has been exhausted and the control unit  46  should be brought into a standby state. When NO in step S 24 , the flow returns to step S 2 , and steps from step S 2  through step S 26  are repeated until exhaustion of the storage battery  32  is detected in step S 24 . While the steps are repeatedly performed in this manner, each time it is detected in step S 16  that the counting up has reached the count-up value, the flow returns to step S 8 , and thereby, measurement and transmission are regularly performed. Furthermore, while the steps are repeatedly performed, the measurement and the transmission are extraordinarily performed to deal with changes in the instantaneous current. Here, since steps S 20  and S 22  are provided, it is possible to deal with peak-like current changes, where current suddenly increases and then suddenly decreases, and measure the behavior of the current, and transmit result of the measurement. On the other hand, when the storage battery is detected to have been exhausted in step S 24 , the flow is ended, and the control unit  46  enters the standby state. 
       EXAMPLE 2 
       [0032]      FIG. 4  is partly a conceptual diagram and partly a block diagram of a current sensor in Example 2 of the present invention. Example 2 has the same overall system configuration as Example 1, and the current sensor of Example 2 can be adopted as the first current sensor  10 , the second current sensor  12 , the third current sensor  14 , etc., illustrated in  FIG. 1 , and thus illustration and description of the entire system of Example 2 will be omitted. Furthermore, the current sensor in Example 2 illustrated in  FIG. 4  has many portions in common with the current sensor in Example 1 illustrated in  FIG. 2 , and thus the same portions as in Example 1 are denoted by the same reference signs, and descriptions thereof will be omitted unless necessary. 
         [0033]    The current sensor in Example 2 illustrated in  FIG. 4  is different from the current sensor in Example 1 illustrated in  FIG. 2  in that an iron core ring  52  is used for both charging and current measurement, that a Hall element  54  is adopted for current measurement, that a switch  56  is provided for avoiding influence of charging on current measurement, and that a storage unit  58  for storing a measured value therein is disposed in a control unit  60  for the purpose of separating timing of measurement from timing of transmission. 
         [0034]    Specifically, as in Example 1, in Example 2 as well, the iron core ring  52  having a shape corresponding to magnetic flux generated around one cord  22 . Around the iron core ring  52 , a coil  62  is wound. (The coil  62  is wound around unillustrated part of the iron-core ring  52 , too, as indicated by an alternate dash and dot line  62   a .) Also as in Example 1, current extracted from an outgoing wire of the coil  62  charges a storage battery  32  in a power supply circuit  30  via a rectifier  28 , but unlike in Example 1, the switch  56  is provided in a charging path, such that the switch  56  remains open while the current measurement is being performed to thereby prevent charging from having an influence on current measurement. 
         [0035]    Furthermore, in part of a magnetic circuit that the iron core ring  52  forms, the Hall element  54  is inserted such that the magnetic flux crosses the Hall element  54 . Here, the power supply circuit  30  supplies power to the Hall element as well. Magnetic flux density of the iron core ring  52  dependent on the current flowing through the cord  22  is converted into a voltage by the Hall element  54 . In this manner, the current flowing through the cord  22  is detected by a current detection unit  64  to which the Hall element is connected. The current detection by the current detection unit  64  is performed also during the charging of the storage battery  32 , and is used to measure a moving average current for setting the count-up value and to make a judgement on increase and decrease of the instantaneous current. However, in order to avoid influence of the combined use of the iron core ring  52  on measurement, the control unit  60  opens the switch  56  and suspends charging at the time of measurement. 
         [0036]    Furthermore, when the voltage of the storage battery  32  is insufficient to transmit a measured value, the control unit  60  controls such that only the measurement is performed and the measured value is stored in the storage unit  58 , and the measured value stored in the storage unit  58  is transmitted when the voltage of the storage battery  32  becomes sufficient for the transmission. For this purpose, the date and time of the measurement is simultaneously stored as a time stamp of the measured value to be stored. 
         [0037]      FIG. 5  is a flow chart illustrating an operation of the control unit  60  of the current sensor in Example 2 illustrated in  FIG. 4 . Furthermore, the flow chart of  FIG. 5  has many portions in common with the flow chart of Example 1 illustrated in  FIG. 3 , and thus the same steps as in the flow chart of Example 1 are denoted by the same step numbers, and descriptions thereof will be omitted unless necessary. Newly added steps in  FIG. 5  are indicated by bold letters. 
         [0038]    In the flow chart of Example 2 illustrated in  FIG. 5 , when it is found in step S 2  that the storage battery  32  has not yet been charged to the predetermined voltage that is necessary for current detection and transmission of the result of the detection, the flow proceeds to step S 28 , where the switch  56  is opened to turn power generation off. Then, the flow proceeds to step S 30 , where measurement by the current detection unit  64  is performed. At this time, since the voltage is insufficient, measurement by the transmission unit  44  is not performed, and the measured value is stored in the storage unit  58 . Then, the flow proceeds to step S 32 , where the switch  56  is closed to turn power generation on. With power generation turned on, the flow proceeds to step S 26 , where the current detection is stopped, and then, the flow shifts to step S 24 . 
         [0039]    In the flow chart of Example 2 illustrated in  FIG. 5 , when it is found in step S 2  that the storage battery  32  has not yet been charged to the predetermined voltage that is necessary for current detection and transmission of the result of the detection, the flow proceeds to step S 34 , where it is checked whether or not there is any measured value stored in step S 30 . When a measured value is found stored, the flow proceeds to step S 36 , where the stored value is read and transmitted, and then, the flow proceeds to step S 4 . On the other hand, when no measured value is found stored, the flow proceeds directly to step S 4 . 
         [0040]    Furthermore, in the flow chart of Example 2 illustrated in  FIG. 5 , when it is found in step S 6  that neither current measurement nor transmission immediately after the start of the current detection has not been done yet, the flow proceeds to step S 38 , where the switch  56  is opened to turn power generation off. With power generation turned off, the flow proceeds to step S 8 , where measurement by the current detection unit  64  and the processing unit  42  and transmission by the transmission unit  44  are performed. Then, the flow proceeds to step S 40 , where the switch  56  is closed to turn power generation on. With power generation turned on, the flow proceeds to step S 10 , where a counter is reset and started. The turning on/off of power generation before/after measurement and transmission as in the flow chart of  FIG. 5  is not limited to immediately after the start of current detection as described above; power generation is turned on/off in the same manner, in repetition of the flow, also when it is detected in step S 16  that the counting up has reached the count-up value, when increase of the instantaneous current by the predetermined amount or more is detected in step S 20 , and when decrease of instantaneous current by the predetermined amount or more is detected in step S 22 . That is, in these cases, too, the flow proceeds, via the turning off of power generation in step S 38 , to step S 8  where measurement and transmission is performed. 
       EXAMPLE 3 
       [0041]      FIG. 6  is a flow chart illustrating an operation of a control unit of a current sensor in Example 3 of a current sensor according to an embodiment of the present invention. The same hardware configuration as in Example 2 illustrated in  FIG. 4  is employed in Example 3. The flow chart of  FIG. 6  has many portions in common with the flow chart of Example 2 illustrated in  FIG. 4 , and thus the common portions are illustrated in a packaged-up manner, and the same portions as in the flow chart of Example 2 are denoted by the same step numbers, and descriptions thereof will be omitted unless necessary. Example 3 illustrated in  FIG. 6  is different from Example 2 illustrated in  FIG. 5  in that Example 3 is configured such that continuous measurement starts to be performed for a predetermined period of time after increase or decrease of instantaneous current by a predetermined amount. 
         [0042]    First, a description will be given on such steps in  FIG. 5  as are illustrated in packages in  FIG. 6 . Step S 52  in  FIG. 6  includes a flow that proceeds from step S 2 , via step S 34 , to reach step S 36  in  FIG. 5 , and a flow that proceeds from step S 2 , via steps S 28 , S 30 , S 32 , and S 26 , toward step S 24  in  FIG. 5 . Step S 54  in  FIG. 6  includes a flow that proceeds from step S 38 , via step S 8  and step S 40 , to reach step S 10  in  FIG. 5 . Step S 56  in  FIG. 6  includes a flow that proceeds from step S 12 , via step S 14  or step S 18 , toward step S 16  in  FIG. 5 . These steps are the same as in  FIG. 5 , and thus their descriptions will be omitted. 
         [0043]    In  FIG. 6 , when the flow reaches step S 57 , it is checked whether or not the instantaneous current has been caused to increase by the predetermined amount or more by a sudden increase of the current flowing through the cord  22 . When NO in step S 57 , the flow proceeds to step S 58 , where it is checked whether or not the instantaneous current has been caused to decrease by the predetermined amount or more by a sudden decrease of the current flowing through the cord  22 . When YES in step S 57  or step S 58 , the flow proceeds to step S 60 , where it is checked whether or not time that has elapsed since the previous continuous measurement is within a predetermined period of time. This is to avoid exhaustion of the storage battery  32  due to repetition of continuous measurement in a short period of time. When it is confirmed in step S 60  that the time elapsed since the previous continuous measurement is not within the predetermined period of time, the flow proceeds to step S 62 , where power generation is turned off 
         [0044]    Next, the flow proceeds to step S 64 , where measurement by the current detection unit  64  and the processing unit  42  and transmission by the transmission unit  44  are performed. Then the flow proceeds to step S 66 , where it is checked whether or not the measurement-target current is stable without large variation. When NO in step S 66 , the flow proceeds to step S 68 , where it is checked whether or not time for the continuous measurement is up (lapse of two minutes, for example). When YES in step S 68 , the flow proceeds to step S 70 , where the switch  56  is closed to turn power generation on. The flow proceeds to step S 70  to turn power generation on also when current stability is confirmed in step S 66 . These steps in the flow are provided for the purpose of avoiding exhaustion of the storage battery  32  that would result from idle continuation of the continuous measurement. On the other hand, when it is not detected in step S 68  that the time for the continuous measurement is up, the flow returns to step S 64 , and then the process from step S 64  through step S 68  are repeated such that continuous measurement and transmission are repeatedly performed until current stability is confirmed in step S 66  or it is detected in step S 68  that the time for the continuous measurement is up. Here, in the case where the flow proceeds to step S 70  to turn power generation on as described above, the flow subsequently proceeds to step S 24 , where the same operation is performed as in the flow of Example 2 illustrated in  FIG. 5 . 
       EXAMPLE 4 
       [0045]      FIG. 7  is a flow chart illustrating an operation of a control unit of a current sensor in Example 4 of a current sensor according to an embodiment of the present invention. The same hardware configuration as in Example 2 illustrated in  FIG. 4  is employed in Example 4. The flow chart of  FIG. 7  has many portions in common with the flow chart in Example 2 illustrated in  FIG. 5 , and thus the same steps as in the flow chart of Example 1 are denoted by the same step numbers, and descriptions thereof will be omitted unless necessary. Here, the same way of illustrating a plurality of steps in packages as is adopted in the flow chart of Example 3 illustrated in  FIG. 6  is adopted also in part of  FIG. 7  with the same step numbers. Example 4 illustrated in  FIG. 7  is different from Example 2 illustrated in  FIG. 5  in that measurement and transmission are completely separated from each other in such a manner that measured values are stored and accumulated in a predetermined period of time and then the stored and accumulated values are transmitted in a batch at a predetermined transmission timing. Here, in  FIG. 7  illustrating Example 4, changed or newly added steps in comparison with  FIG. 5  illustrating Example 2 are indicated by bold letters. 
         [0046]    In the example illustrated in  FIG. 7 , after power generation is turned off in step S 38 , current measurement is performed by the current detection unit  64  and the processing unit  42  and the measured value is stored in the storage unit  58 . At this time, the date and time of the measurement is simultaneously stored as a time stamp. Transmission is not performed at this stage, and the flow shifts to step S 40 , where power generation is turned on. 
         [0047]    In the example illustrated in  FIG. 7 , when it is not detected in step S 22  that the instantaneous current has decreased by a predetermined amount or more, the flow shifts to step S 74 , where it is checked whether or not the predetermined transmission timing (for example, every one minute) has been reached. When the transmission timing is found to have been reached, the flow shifts to step S 76 , where it is checked whether or not the storage battery  32  has a voltage sufficient for transmission. When the voltage of the storage battery  32  is found to be sufficient, the flow proceeds to step S 78 , where the measured values stored in the storage unit  58  are transmitted in a batch, and then the flow proceeds to step S 24 . On the other hand, when it is not confirmed that the predetermined transmission timing has been reached, or, when it is not able to confirm that the voltage of the storage battery  32  is sufficient for transmission, the batch transmission of the stored measured values is postponed until the next opportunity and the flow shifts directly to step S 24 . The operation after the flow proceeds to step S 24  is the same as in the flow of Example 2 illustrated in  FIG. 5 . 
       EXAMPLE 5 
       [0048]      FIG. 8  is partly a conceptual diagram and partly a block diagram of a current sensor in Example 5 of the current sensor according to an/the embodiment of the present invention. Example 5 has the same overall system configuration as Example 1, and the current sensor of Example 5 can be adopted as the first current sensor  10 , the second current sensor  12 , the third current sensor  14 , etc. illustrated in  FIG. 1 , and thus illustration and description of the entire system configuration of Example 5 will be omitted. Furthermore, the current sensor in Example 5 has many portions in common with the current sensor in Example 1 illustrated in  FIG. 2 , and thus the same portions as in Example 1 are denoted by the same reference signs, and descriptions thereof will be omitted unless necessary. 
         [0049]    The current sensor in Example 5 illustrated in  FIG. 8  is different from the current sensor in Example 1 illustrated in  FIG. 1  in that an iron core ring  72  and a coil  74  wound around the iron core ring  72  (which is wound around unillustrated part of the iron-core ring  72 , too, as indicated by an alternate dash and dot line  74   a ) are used both for charging and current measurement, and also in that time division between measurement time and charging time is achieved by using a switch  78  that is controlled by a control unit  76  to switch the connection destination of an outgoing wire of the coil  74 . 
         [0050]      FIG. 9A  to  FIG. 9D  are each a diagram illustrating timing of an operation of the control unit  76  in Example 5 illustrated in  FIG. 8 .  FIG. 9A  illustrates a case where the remaining capacity of the storage battery  32  is small and charging current caused by the current flowing through the cord  22  is also small, and the relative magnitude of the duty of the measurement time t 1  with respect to the duty of the charging time t 2  is smaller than in  FIGS. 9B, 9C, and 9D . That is, in  FIG. 9A , the width and the frequency of the measurement time tl are both smaller than in  FIGS. 9B, 9C, and 9D . 
         [0051]    In contrast to  FIG. 9A ,  FIG. 9B  illustrates a case where there is a little margin in the remaining capacity of the storage battery  32  and the magnitude of the charging current, and the width of the measurement time t 1  remains the same as in the case of  FIG. 9A , but the frequency is twice as large.  FIG. 9C  illustrates a case where there is a larger margin in the remaining capacity of the storage battery  32  and the magnitude of the charging current than in  FIG. 9B , and the frequency of the measurement time t 1  remains the same as in the case of  FIG. 9B , but the width of the measurement time t 1  is larger than in the case of  FIG. 9B . Thus, it is also possible to perform continuous measurement within the measurement time t 1 .  FIG. 9D  illustrates a case where there is the largest margin in the remaining capacity of the storage battery  32  and the magnitude of the charging current, and the width of the measurement time t 1  is larger than that of the charging time t 2 . The control of these cases is performed based on a judgment made by the control unit  76 , and the judgment is made based on the voltage of the voltage detection unit  33  and the magnitude of current measured in current measurement. 
         [0052]    The various features dealt with in the descriptions of the examples of the present invention are not necessarily unique to the respective examples, and as along as it is possible to make use of the advantages of the features of the examples, the features can be utilized by being appropriately replaced or combined with each other. For example, the current sensor of Example 1 illustrated in  FIG. 2  may be such a Hall element as in Example 2. Furthermore, in the continuous measurement in Example 3 illustrated in  FIG. 6 , measurement and transmission may be completely separated from each other as in Example 4 illustrated in  FIG. 7  such that only the measurement is continuously performed and the transmission is performed in a batch manner at each predetermined timing. 
         [0053]    Moreover, in Example 5 illustrated in  FIG. 8 , measurement and transmission may further be separated from each other within the measurement time t 1  such that the transmission is performed in a batch manner as in Example 4 illustrated in  FIG. 7 . In this case, in the state where the width and the frequency of the measurement time t 1  are made minimum as in  FIG. 9A , a larger number of cases may be assumed including a case where only the measurement is performed without performing the transmission when the charging current is even smaller. 
       INDUSTRIAL APPLICABILITY 
       [0054]    The present invention is applicable to a current sensor. 
       LIST OF REFERENCE SIGNS 
       [0000]    
       
           22  target electrical wire 
           34 ,  36 ,  54 ,  72 ,  74  measurement unit 
           44  radio communication unit 
           24 ,  26 ,  52 ,  62  power generation unit 
           32  storage battery 
           46 ,  60 ,  76  control section 
           58  storage unit 
           52 ,  72  common iron core