Abstract:
A current sensor which constitutes an overload protection apparatus and senses a current supplied from a power source to a load is constituted by providing a magnetic sensor having the effect of magnetic impedance (MI), an AC supply means which impresses AC on this sensor, a bias current supply means which supplies a bias current to a bias coil, a peak sensing means which senses the peak or a change in impedance of the magnetic sensor as a change in voltage, and a switch which selects the output of the peak sensing means in accordance with each phase. A holding means which holds switch outputs one after another and an amplification means are provided in common to enable current sensing for each phase. Thus, a range of current sensing is enlarged to reduce power consumption and cost.

Description:
TECHNICAL FIELD OF THE INVENTION 
     The present invention relates to an over-current protection device for detecting a current flowing through a conductor and for shutting oft the current when the current exceeds a predetermined threshold value. More particularly, the present invention relates to an over-current protection device capable of controlling power supplied to a load such as an electric motor. 
     DESCRIPTION OF TECHNICAL BACKGROUND 
     An over-current protection device detects a current flowing through a load such as a three-phase motor via a contactor, and shuts off the current flowing to the motor when the current exceeds a safe threshold value. Conventionally, such a device is provided with a bi-metal switching element, and a part or all of the current to the motor flows through the bi-metal switching element. That is, the current flows though a switch consisting of the bi-metal element so that the bi-metal element is heated according to an intensity of the current. When the motor current exceeds a sate threshold value for a period of time longer than a predetermined time, the bi-metal element bends due to the heat to hold a switch contact in an open state, thereby shutting off the power to a control input terminal of a contactor. However, in the device using the bi-metal switch, it is difficult to adjust the current in the state that the switch is opened, so that the incorrectly adjusted condition tends to continue for a long time. 
     On the other hand, when an electric device is used instead of the bi-metal element, it is possible to electronically perform the function of the bi-metal switch. Accordingly, it is possible to improve reliability and easily adjust the device. However, the electronic device includes a complex circuit, and in order to properly detect a current to operate a contactor, it is necessary to provide a constant-voltage power supply and a large number of components. In addition, a current detection transformer has been used as a device for detecting current. Accordingly, it is difficult to obtain a wide range for detecting a current due to magnetic saturation of an iron core. It is possible to provide a magnetoresistive element as a device for detecting a current. However, it is necessary to provide an iron core due to a low sensitivity of the magnetoresistive element. Accordingly, similar to the current detection transformer, it is difficult to obtain a wide range for detecting a current. 
     To solve these problems, as a high sensitive magnetism detection element for replacing a Hall element and the magnetoresistive element, a magnetic impedance element using an amorphous wire has been disclosed (refer to Patent Document 1) Further, an amorphous magnetic thin film formed via a sputtering method has been used (refer to Patent Document 2). 
     When one of the magnetic impedance elements is used, it is possible to obtain high sensitivity in the magnetism detection characteristic. However, as shown in  FIG. 17 , impedance changes non-linearly relative to a magnetic field of the amorphous wire element, so that the magnetic impedance element has a non-linear output of the magnetism detection characteristic (refer to Patent Document 3). Accordingly, the linear output relative to the magnetic field is obtained from a difference in a variation of the magnetic impedance element obtained from a sum of the positive and negative magnetic fields generated by the AC bias magnetic field and the immeasurable external magnetic field, so that an AC bias magnetic field is applied to the magnetic impedance element (refer to Patent Document 4). 
     [Patent Document 1] 
     
         
         Japanese Patent Publication (Kokai) No. 06-281712 
         (page 4, FIG. 5 to FIG. 12)
 
[Patent Document 2]
 
         Japanese Patent Publication (Kokai) No. 08-075835 
         (pages 4 to 5, FIG. 1 to FIG. 6)
 
[Patent Document 3]
 
         Japanese Patent Publication (Kokai) No. 2000-055996 
         (page 3, FIG. 23)
 
[Patent Document 4]
 
         Japanese Patent Publication (Kokai) No. 09-127218 
         (pages 4 to 5, FIG. 3) 
       
    
     Incidentally, in principle, the magnetic impedance element generates a magnetic impedance effect. Accordingly, it is necessary to apply a high-frequency current of several mA and at least several MHz to the element, thereby increasing the power consumption and a size of the power-supply transformer, and making it difficult to downsize the device and reduce cost of the device. 
       FIG. 16  shows an example of a conventional detecting circuit using the magnetic impedance element. The circuit includes sheared oscillating means  31  and bias-current applying means  13   a   1 . A current of several mA and a bias current of about several tens of mA are constantly applied to magnetism detection elements  1   a ,  1   b , and  1   c , thereby increasing the power consumption in proportion to the number of the elements. Further, it is necessary to provide wave detection means  6   a   1 ,  6   b   1 , and  6   c   1 ; holding means  8   a   1 ,  8   b   1 , and  8   c   1 ; and amplifying means  11   a   1 ,  11   b   1 , and  11   c   1  in proportion to the number of the magnetism detection elements, thereby increasing la size of the circuit and cost of the parts. 
     In view of the problems, the present invention has been made, and an object of the present invention is to provide an over-current protection device with a compact and low cost configuration having a low-cost power-supply source in which a constant-voltage regulated power-supply is not necessary. Further, it is possible to obtain a wide range of the current detection. 
     SUMMARY OF THE INVENTION 
     To solve the problems described above, according to a first aspect of the present invention, an over-current protection device for shutting off power supply to a load when an over-loaded-current is generated includes a switching unit for switching a current supplied from a power-supply source to the load; a current detector for detecting the current supplied from the power-supply source to the load; and a controlling power-supply source for supplying power to each of the component elements. The current detector includes a magnetism detection element corresponding to a phase of the power-supply source and having a magnetic impedance effect; AC-current supply means for supplying an AC current to the magnetism detection element via oscillating means and a first switch corresponding to the magnetism detection element; bias-current supply means formed of a bias coil wound on the magnetism detection element, a third switch, and bias-current applying means for supplying a current to the bias coil via the third switch; wave detection means corresponding to the magnetism detection element for converting an individual impedance variation into a voltage and for passing a peak of the converted voltage the wave detection means; a second switch corresponding to the wave detection means for selecting an output of the wave detection means; holding means for holding the selected output of the wave detection means; and amplifying means for amplifying the voltage held by the holding means. It is possible to detect the current for each phase based on the selective operation of the first through third switches. 
     According to a second aspect of the present invention, an over-current protection device for shutting off power supply to a load when an overloaded current is generated includes a switching unit for switching a current supplied from a power-supply source to the load; a current detector for detecting the current supplied from the power-supply source to the load; and a controlling power-supply source for supplying power to each of the component elements. The current detector includes a magnetism detection element having a magnetic impedance effect and corresponding to a phase of the power-supply source; AC-current supply means for supplying an AC current to the magnetism detection element via oscillating means and a first switch corresponding to the magnetism detection element; bias-current supply means formed of a bias coil wound on the magnetism detection element, bias-current applying means, and dividing means connected to the oscillating means via a third switch for dividing a signal output from the oscillating means for feeding a current having a different polarity to the bias coil based on first and second timings; wave detection means corresponding to the magnetism detection element for converting an impedance variation into a voltage and for passing a peak of the voltage; a second switch corresponding to the wave detection means for selecting a signal output from the wave detection means; a first holding means for holding the selected signal output from the wave detection means; a pair of fourth switches for selecting the held voltage based on the first and second timings; a pair of second holding means for holding the selected two voltages; and amplifying means for amplifying a difference in the signals output from the pair of the second holding means. It is possible to detect the current for each phase based on the selective operation of the first through fourth switches. 
     In the first and second aspects of the present invention, when the first switch and the second switch corresponding to the magnetism detection element disposed in the phase of the power-supply source are selected, it is possible to select the third switch (according to a third aspect of the present invention). Alternatively, it is possible to operate the oscillating means synchronous with the third switch (according to a fourth aspect of the present invention). 
     In the first and second aspects of the present invention, the controlling power-supply source may include at least a pair of power-supply transformers having a primary coil and a secondary coil and connected to a current supply line from the controlling power-supply source to a load; a storage battery for storing the current at a secondary side; and a voltage adjuster (according to a fifth aspect of the present invention). Alternatively, the controlling power-supply source may include a power-supply transformer having at least a pair of primary coils and a secondary coil and connected to a current supply line between the power-supply source and the load; a storage battery for storing the current at a secondary side; and a voltage adjuster (according to a sixth aspect of the present invention). In the sixth aspect of the present invention, at least a pair of the primary coils and the secondary coil may be wound on a single iron core, and the primary coils may have different winding turns according to the phase (according to a seventh aspect of the present invention). In the seventh aspect of the present invention, a pair of the primary coils provided in the power-supply transformers may have a winding ratio of 1:2 (according to an eighth aspect of the present invention). 
     Further, in the first and second aspects of present invention, it is possible to integrate the magnetism detection element, a terminals for applying an AC current to the magnetism detection element, the bias coil, and a terminal for feeding a bias current to the bias coil with a resin molding process (according to a ninth aspect of the present invention). Alternatively, it is possible to integrate the magnetism detection element, a terminal for applying an AC current to the magnetism detection element, the bias coil, a terminal for feeding a bias current to the bias coil, and a circuit for outputting a signal proportional to the signal output from the magnetism detection element with a resin molding process (according to a tenth aspect of the present invention). Alternatively, it is possible to use a thin-film device as the magnetism detection element (according to an eleventh aspect of the present invention). 
     That is, in the present invention, the magnetism detection element having the magnetic impedance (MI) effect is used as the current detection means to prevent magnetic saturation caused by an iron core in a widely used conventional current detection transformer, thereby increasing a range of the current detection. Further, the controlling power-supply source does not need external power supply from a constant-voltage regulated power source. As a result, it is possible to provide an over-current protection device having wide applicability and is capable of decreasing the total cost. 
     When a multi-phase AC power-supply source is used, it is not necessary to provide a power-supply transformer for each phase, thereby providing the over-current protection device with a smaller number of parts and low cost. In this case, the oscillating means is a single unit instead of several oscillating means for each phase in the conventional method. It is possible to apply the AC current to the elements and devices disposed for each phase only when selected. Accordingly, it is possible to decrease power consumption. When the power is supplied to the bias coils only upon the detection, it is possible to further reduce the power consumption is further lowered by solely. When the oscillating means is operated only upon the detection, it is possible to further reduce power consumption. 
     Further, it is possible to provide only a single system of the holding means and the amplifying means, thereby further reducing power consumption and cost. The positive and negative bias magnetic fields are alternately applied to the magnetism detection element, and a difference in the detected voltages at the time of applying the bias magnetic field is determined. Accordingly, it is possible to improve linearity or the output. Further, a pulse is used to drive intermittently in place of the conventional AC biasing system, thereby further reducing power consumption. 
     In addition, the magnetism detection element, AC-current input terminal thereof, bias coil, and current input terminal thereof are integrated with a resin molding process, thereby decreasing the magnetic resistance and bias current and reducing a size. Further, the magnetism detection element, AC-current input terminal thereof, bias coil, current input terminal thereof, and circuit for outputting a signal proportional to the signal output from the magnetism detection element are integrated, thereby improving the S/N (signal-to-noise) ratio. In particular, various corrective data are incorporated in the system to improve the function thereof, thereby obtaining excellent environmental resistance, high precision, and lower power consumption. The thin-film device is used as the magnetism detection element, thereby eliminating the adverse influence of variable output caused by a strain in the wire-type element and reducing power consumption with high precision. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic block diagram of an over-current protection device according to a first embodiment of the present invention; 
         FIG. 2  is a schematic view of a magnetism detection unit; 
         FIG. 3   a  is a schematic block diagram of a first embodiment of a magnetism detection unit; 
         FIG. 3   b  is a time chart showing an operation of each of switches shown in  FIG. 3   a;    
         FIG. 3   c  is an explanatory view of an operating timing of oscillating means; 
         FIG. 3   d  is a partial view showing a modified example of the magnetism detection unit shown in  FIG. 3   a;    
         FIG. 3   e  is a time chart showing an operation of the modified example shown in  FIG. 3   d;    
         FIG. 4  is a schematic block diagram showing individual components shown in  FIG. 3   a;    
       FIG. b is a schematic block diagram of a second embodiment of the magnetism detection unit; 
         FIG. 6  is an explanatory view showing positive and negative biases; 
         FIG. 7  is a schematic block diagram showing individual components shown in  FIG. 5 ; 
         FIG. 8  is a view showing a circuit of another embodiment of the oscillating means; 
         FIG. 9  is a schematic block diagram of an over-current protection device according to a second embodiment of the present invention; 
         FIG. 10  is a view showing a constitution of a transformer; 
         FIG. 11  is a perspective view showing a constitution of a magnetic sensor; 
         FIG. 12  is an explanatory view showing a process of producing the magnetic sensor shown in  FIG. 11 ; 
         FIG. 13  is an explanatory view showing an assembled state of the loaded magnetic sensor shown in  FIG. 11 ; 
         FIG. 14  is an explanatory view showing an example of a magnetic shielding; 
         FIG. 15  is a perspective view showing an example of the magnetism detection unit; 
         FIG. 16  is a schematic block diagram of a conventional magnetism detection unit; and 
         FIG. 17  is an explanatory view showing magnetic impedance characteristic of an amorphous wire. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Hereunder, the present invention will be described.  FIG. 1  is a schematic block diagram of an over-current protection device according to a first embodiment of the present invention. 
     In  FIG. 1 , power-supply lines R, S, and T connected to a three-phase AC power-supply source (not shown) are linked with a motor  30  via a three-phase contactor  20  and a pair of power-supply transformers  161  and  162 . A current detection unit  11  detects a current supplied via the power-supply lines R, S, and T for each phase. Even when one of the three phases incurs disconnection, it is required that the entire system be operated normally. Accordingly, the embodiment includes a pair of power-supply transformers  161  and  162 . However, the transformer may be disposed for each phase. The contactor  20  has three contacts  201 ,  202 , and  203 , and each of the contacts is directly connected to the motor  30 , or individually connected via primary coils of the power-supply transformers  161  and  162  through the power-supply lines R, S, and T. The three contacts are mechanically united with each other, so that an electromagnetic coil  204  connected to a control circuit  10  can drive the three contacts simultaneously. The above-mentioned current detection unit  11 , a pair of power-supply transformers  161  and  162 , and the control circuit  10  constitute an electronic overload relay unit  1 . A current regulator (gain adjuster)  100  amplifies a signal output from the current detection unit  11  in correspondence with a preset current value. The signal output from the current detection unit  11  is sent to an analog input of a microcomputer (mi-con)  102  via a half-wave rectifier  101 . 
     In the controlling power-supply source shown in  FIG. 1 , a first capacitor  180  is connected to the secondary coils of the power-supply transformers  161  and  162  via a pair of rectifying diodes  171  and  172 . Protecting diodes  174  and  175  are respectively linked between an anode and a ground of the circuit. The first capacitor  180  is linked between a positive input of a voltage adjuster  19  and the ground of the circuit. A second capacitor  181  is linked between a positive output and the ground of the circuit, so that the voltage adjuster  19  to output a constant voltage level VCC as a power-supply source of the electronic overload relay unit  1 . 
       FIG. 2  is a view showing a configuration of a current detection unit. Reference numeral  111  shown in  FIG. 2  designates a magnetic impedance element (MI element),  122  designates a wiring for supplying a current to each phase,  121  designates a substrate for fixing the wiring and the MI element  1 , and  110  designates a detection circuit. 
     The MI element  111  may include one formed of an amorphous wire disclosed in Japanese Patent Publication (Kokai) No. 06-281712, and a thin film device disclosed in Japanese Patent Publication (Kokai) No. 08-075835.  FIG. 2  shows the MI element corresponding to a single phase, and each phase has an identical constitution. 
       FIG. 3   a  is a view showing a first embodiment of the current detection unit. Reference numerals  1   a ,  1   b , and  1   c  respectively designate an MI element formed in a wire shape or a thin-film. Reference numerals  2   a ,  2   b , and  2   c  respectively designate a bias coil for feeding a bias to the MI elements  1   a ,  1   b , and  1   c . Reference numeral  3  designates an oscillating circuit. Reference numerals  4   a ,  4   b , and  4   c  respectively designate a first switch. The first switches  4   a ,  4   b , and  4   c  individually switch a signal output from the oscillating circuit  3 , and then apply a high-frequency AC current to the MI elements  1   a ,  1   b , and  1   c . Reference numeral  13   a  designates second current-applying means for feeding a current to the bias coils  2   a ,  2   b , and  2   c , and a third switch  14   a  turns on and off the current. Reference numerals  6   a ,  6   b , and  6   c  respectively designate wave-detecting means for outputting a peak value of varied impedance of the MI elements  1   a ,  1   b , and  1   c  converted into varied voltages. Reference numerals  7   a ,  7   b , and  7   c  respectively designate a second switch for extracting a signal output from the wave detection means in correspondence with the selected MI element. Reference numeral  8   a  designates first holding means for holding the signal output from the wave detection means  6   a ,  6   b , and  6   c . Reference numeral  11   a  designates amplifying means for amplifying the signal output from the first holding means  8   a.    
     In response to the control signals A 1 , A 2 , B 1 , B 2 , C 1 , and C 2  for the first switches  4   a ,  4   b , and  4   c  and the second switches  7   a ,  7   b , and  7   c , the microcomputer  102  selects one of the MI elements  1   a ,  1   b , and  1   c , and further outputs a control signal El for delivering a bias current. 
     More particularly, in response to the control signals A 1  or A 2 , the MI element  1   a  is selected. Likewise, in response to the control signals B 1  or B 2 , the MI element  1   b  is selected. Likewise, in response to the control signals C 1  or C 2 , the MI element  1   c  is selected. In addition, in response to the control signal E 1 , a bias current is delivered to one of the MI elements  1   a ,  1   b , and  1   c . Accordingly, it is possible to apply the AC current and bias current to the MI elements consuming the majority of power solely for a period of time when the control signal is, thereby reducing the consumed power. For example, when the MI element  1   a  is driven, the first switch  4   a  and second switch  7   a  are turned on at substantially the same time, and the control signal E 1  is simultaneously output only once to turn on the third switch  14   a , thereby reducing power consumption. Further, the first holding means  8   a  and the amplifying means  11   a  can be constituted in a single system, thereby further facilitating power consumption and lower cost. 
     In terms of the timing after the MI element  1   a  is operated, the first switch  4   b  and second switch  7   b  are turned on at substantially the same time. The control signal E 1  is simultaneously output only once to turn the third switch  14   a  on, thereby turning on the MI element  1   b  to further turn on the first switch  4   c  and the second switch  7   c  at substantially the same time. Simultaneously, the control signal E 1  is output only once to turn on the third switch  14   a , thereby activating the MI element  1   c .  FIG. 3   b  is a timing chart showing the above sequential operations. 
     As shown in  FIG. 3   c , the oscillating means  3  is activated based on the timing of operating the third switch in response to the control signal E 1 . Accordingly, the oscillating means  3  can execute an oscillating operation only when a bias current is fed to the bias coils  2   a ,  2   b , and  2   c . As a result, as compared with a case that the oscillating means  3  continuously executes the oscillating operation, it is possible to further reduce power consumption. 
       FIG. 3   d  shows a modified example of the device shown in  FIG. 3   a . The holding means  8   a  is provided in common with the wave detection means  6   a ,  6   b , and  6   c  shown in  FIG. 3   a . As shown in  FIG. 3   d , the holding means  8   c ,  8   d , and  8   e  is individually provided in the wave detection means  6   a ,  6   b , and  6   c , respectively. As in the case shown in  FIG. 3   a , the first switch and the third switch are turned on at substantially the same time. However, with the above arrangement, it is possible to select the operating timing. Incidentally, the second switches  7   a ,  7   b , and  7   c  are integrated.  FIG. 3   e  is a timing chart of the integrated unit. 
       FIG. 4  is a view showing an embodiment of the device shown in  FIG. 3   a . The oscillating means  3  may include a quartz oscillator or transistor, and in this embodiment, the oscillating means  3  is formed of a CMOS gate as an example. The wave detection means  6   a ,  6   b , and  6   c  may be formed of analog switches, and in this embodiment, the wave detection means  6   a ,  6   b , and  6   c  are formed of diodes. The first holding means  8   a  is formed of a resistor and capacitor. The amplifying means  11   a  may be formed of a transistor, and in the embodiment, the amplifying means  11   a  is formed of an operational amplifier as an example. The first, second, and third switches are respectively formed of relays or analog switches. For the purpose of lowering the current fed to the MI elements  1   a ,  1   b , and  1   c , current limit resistors  5   a ,  5   b , and  5   c  are provided. However, the current limit resistors may be omitted. 
     The diagrams shown in  FIG. 3   a  and  FIG. 4  can detect magnetism with a simplified constitution. However, a variation in impedance relative to a magnetic field of the amorphous wire elements exhibits non-linearity as shown in  FIG. 17 , so that the output precision is not satisfactory. 
       FIG. 5  is a view showing a second embodiment with an improvement in the non-linear characteristic. As compared with that shown in  FIG. 3   a , the positive and negative bias magnetic fields are alternately applied to the MI elements  1   a ,  1   b , and  1   c , so that a difference in detected voltages upon the application of the individual bias magnetic fields is obtained, thereby improving the output linearity. 
     Reference numeral  12  designates frequency-dividing means for dividing a frequency of the signal output from the oscillating means  3 . The dividing means  12  outputs a signal containing a frequency lower than that of the AC current fed to the MI elements  1   a ,  1   b , and  1   c . Reference numeral  13   b  designates second current-applying means for alternately applying positive and negative bias magnetic fields in response to the positive and negative output timings delivered from the frequency-dividing means  12 . The second current-applying means  13   b  applies the output signal of the oscillating means  3  divided by the frequency-dividing means  12  via the third switch  14   b  to the bias coils  2   a ,  2   b , and  2   c . Further, the device is provided with first holding means  8   b  for holding a voltage corresponding to a variation in impedance caused by the positive and negative bias magnetic fields of the MI elements  1   a ,  1   b , and  1   c;  a pair of second holding means  10   a  and  10   b  for holding the voltage output from the first holding means  8   b  based on the positive and negative timings; a pair of fourth switches  9   a  and  9   b  operated by the timings D 1  and  2 ; and differential amplifying means  11   b  for differentially amplifying the voltage output from the second holding means  10   a  and  10   b.    
       FIG. 6  is an explanatory view showing an operation of the positive and negative biases. Note that the operating characteristic of the sensor (i.e. the MI elements) relative to the magnetic field shown in  FIG. 6  is that in a conventional magnetic impedance element. 
       FIGS. 6(   a ) and ( b ) are explanatory views of the operating characteristic when the bias magnetic field is added while the external magnetic field remains zero.  FIG. 6(   a ) schematically shows the operating characteristic when the bias magnetic field having an equal intensity of the positive and negative magnetic fields is added to a magnetic impedance element under the condition that no appreciable external magnetic field out of the measurable range is present.  FIG. 6(   a ) also shows a portion representing a variation in impedance relative to a variation in the intensity of the external magnetic field, and variations caused by the intensity of the bias magnetic field added to the magnetic impedance element and the adding duration. 
     The impedance characteristic does not show a smooth curve in an area in which the intensity of the external magnetic field remains zero. In general, the impedance characteristic becomes unstable at a point where the polarity of the magnetic field changes. The blank circles shown on the impedance-characteristic curves designate the impedance values acquired from the values of the maximum positive/negative bias magnetic fields generated by the bias magnetic field that periodically oscillates the positive and negative magnetic fields with a rectangular waveform. Based on the relationship between the values and the high-frequency current available for the driving applied to the magnetic impedance element, an output voltage can be obtained. The difference in output voltages between the two points is detected. 
     As a result, in a case of no appreciable external magnetic field at outside of the measurable range, the output voltages at two points are identical, i.e. no difference, so that the output becomes zero after the differential amplifying operation as shown in  FIG. 6(   b ). 
     On the other hand,  FIGS. 6(   c ) and ( d ) are views showing an operation of applying the bias when a measurable external magnetic field exists. 
       FIG. 6(   c ) presents a schematic chart showing the characteristic in a case that positive magnetic field of ΔH is detected as the external magnetic field outside the measurable range. Blank circles shown on the curves for designating the impedance characteristic respectively represent the impedance values obtained from the maximum values of the positive and negative magnetic fields of bias. The blank circles shift to the closed circles due to the influence of the external magnetic field ΔH. Relative to the closed circles at the positive side and the negative side of the oscillating bias magnetic field, the polarity of the voltage is defined by the direction that a voltage value corresponding to the closed circles at the negative side changes the closed circles at the positive side. 
     Accordingly, the difference in the output voltages (differential output) becomes ΔV of the positive voltage. When an external magnetic field at outside of the measurable range ΔH is detected, as shown in  FIG. 6(   d ), the output after the differential amplifying operation is obtained as A×ΔV, in which A is an amplifying rate of the differential amplifier. 
     As described above, instead of the conventional AC bias driving, by intermittently driving the magnetic impedance elements with pulses as shown in  FIGS. 6(   a ) and  6 ( c ), it is possible to further reduce power consumption as compared with the conventional method of driving continuously. 
       FIG. 7  is a view showing an example of the device shown in  FIG. 5 . In contrast with that shown in  FIG. 4 , the circuit shown in  FIG. 7  corresponds to that provided with second holding means  10   a  and  10   b , differential amplifying means  11   b , and frequency-dividing means  12 . In this example, the second holding means  10   a  and  10   b  are formed of capacitors. The differential amplifying means  11   b  is formed of a differential amplifier of an operational amplifier. The frequency-dividing means  12  is formed of a flip-flop. 
     In place of the oscillating means  3  shown in  FIG. 4  and  FIG. 7 , as shown in  FIG. 8 , one ( 3   a   1 ) capable of oscillating only when the control signal E 1  remains at a High level may be used, so that the third switch  14   b  and the oscillating means  3   a   1  are turned on via the signal E 1  only when one of the groups including the first switches  4   a ,  4   b , and  4   c , and the second switches  7   a ,  7   b , and  7   c  corresponding to each power-source phase is turned on, thereby further reducing power consumption. 
     The above description refers to a case in which the three-phase AC power-supply source is used. In a case of a single phase, only the single phase is considered to employ the invention. 
       FIG. 9  is a schematic block diagram of an over-current protection device according to a second embodiment of the present invention. 
     The circuit shown in  FIG. 1  requires the power-supply transformers  161  and  162  corresponding to at least two phases. On the other hand, the circuit shown in  FIG. 9  includes a single core  145  to replace the primary coils  140  and  150  provided for each phase so as to receive power from the secondary coil  146 , thereby eliminating one of the two cores. Concretely, as shown in  FIG. 10 , the core  145  is formed of a toroidal core  145   a . A winding ratio between the primary coils  140  and  150  is selected to be, for example, 1:2, so that a proper current level is fed from the secondary coil  146 . The winding turns of the primary coils differ among individual phases, because if the winding turns are identical, it is not possible to detect a vacant phase. 
     In the controlling power-supply source, a first capacitor  180  is linked with the secondary coil  146  via a rectifying diode  176 . A protective diode  177  is connected between the anode of the rectifying diode  176  and the ground of the circuit. The first capacitor  180  is connected between the positive input terminal of the voltage adjuster  19  and the ground of the circuit. The second capacitor  181  is connected between the positive output terminal of the voltage adjuster  19  and the ground of the circuit. The voltage adjuster  19  outputs a constant voltage level VCC. 
     Other components shown in  FIG. 9  are identical to those shown in  FIG. 1 , and descriptions thereof are omitted. 
     Referring to  FIG. 11 , a concrete constitution of the magnetic sensor as described above is described below. 
     In  FIG. 11 , reference numeral  111  designates a magnetism detection element formed of a thin-film, and reference numeral  115  designates a resinous bobbin formed on an outside surface of the magnetism detection element  111  with an insert-molding process. Reference numeral  116  designates a coil for applying a bias magnetic field to the magnetism detection element  111 , reference numeral  117  designates a coil for applying a negative feedback magnetic field to the magnetism detection element  111 , and reference numeral  118  designates a resin case for protecting the magnetism detection element  111  and the coils  116  and  117  from environmental hazards formed with an insert-molding process. Reference numeral  114  designates terminals for applying a high-frequency current to both ends of the magnetism detection element  111 , and for applying a current to the coils  116  and  117 . The entire constitution of the magnetic sensor is designated by reference numeral  120 . In the constitution shown in  FIG. 11 , the coil is provided for applying the negative-feedback magnetic field to the magnetism detection element  111 . However, the coil may be omitted. 
       FIG. 12  is a view showing a process of assembling a magnetic sensor unit. Initially, as shown in (2), a magnetism detection element  111  is bonded between a pair of terminals on the lead frame  119  shown in (1). The bonding method includes a soldering process, an adhesive process, and bonding. Next, as shown in (3), a bobbin  115  is integrally molded with the lead frame  119  with the magnetism detection element  111 . Next, as shown in (4), after the lead frame  119  is cut off, a bias coil  116  and a negative feedback coil  117  are wound. Next, as shown in (5), a case  118  is molded directly above the coil unit. Next, as shown in (6), terminals  114  are folded to complete the assembly work. 
     It is possible to form the thin-film magnetism detection element into a substantially 1 mm square shape. Accordingly, it is possible to form the magnetic sensor  120  into a substantially 5 mm square shape, thereby decreasing the magnetic resistance between the magnetic detection element  111  and the coils  116  and  117 . 
       FIG. 13  shows an example of the magnetic sensor in a mounted state, wherein  FIG. 13(   a ) is a perspective view thereof and  FIG. 13(   b ) is a plan view thereof. 
     As shown in  FIG. 13  ( a ), the magnetic sensor  120  is mounted on a substrate  121  having a wiring  122  for connecting a current  200 . Due to the arrangement of the magnetic sensor  120  relative to a magnetic flux generated by the current  200  as indicated by hidden line in  FIG. 13(   b ), the output sensitivity of the magnetic sensor  120  is determined. Thus, by considering the arrangement of the magnetic sensor  120 , it is possible to adjust the output sensitivity of the magnetic sensor  120 . 
       FIG. 14  shows an example of a structure of a magnetic shield. A magnetic shield  123  is added to the one shown in  FIG. 11 . Although the shield has an oval shape here, it is desirable to adjust the shape in correspondence with the magnitude of the current  200 . Reference numeral  121  designates a substrate, and reference numeral  122  designates a wiring. 
       FIG. 15  is a concrete example of the magnetism detection unit. A detection circuit  110  is incorporated (integrated) into the magnetic sensor unit shown in  FIG. 11 . With this arrangement, it is possible to enhance the S/N ratio of the sensor signal. By internally storing various types of corrective data used for automatic calibration as described in  FIG. 6  for each magnetic sensor element, the precision can be further improved. 
     INDUSTRIAL APPLICABILITY 
     The present invention is applicable to the above-described over-current protection device, and is also applicable to general current detection devices for detecting the magnitude of the current flowing through a conductor, or general breakers for breaking the current when the magnitude of the detected current exceeds a pre-determined threshold value.