JPH01158365A - Method and apparatus for detecting small current - Google Patents

Abstract

PURPOSE:To enhance sensitivity and accuracy, by detecting the small current of the conductor passing through the center hole of a cylindrical iron core using a high permeability material whose magnetic histeresis curve has a square- shaped characteristic from the induced voltage of the detection coil wound around the iron core. CONSTITUTION:An iron core 1a composed of a high permeability material is formed into a closed magnetic path and the conductor 2a passing through the center hole of the iron core 1a is connected between the power supply of an apparatus to be detected and load. The high frequency exciting coil 7 connected to a high frequency power supply 10 through a control circuit 9 and the detection coil 12 connected to an output part 15 are wound around the iron core 1a through a control circuit 9. The exciting current of a high frequency exciting part 11 is set to the linear inclined part in the vicinity of the coercive force of the magnetic histeresis curve of the iron core 1a and, when a small magnetic field is applied by a conductor 2a, large change is imparted to a magnetic field and large voltage is induced in the coil 12. Therefore, a small current can be detected with high sensitivity and high accuracy.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to a small current detection method and device that are used in earth leakage circuit breakers and the like and utilize magnetic phenomena of an iron core.

[Conventional technology]

2. Description of the Related Art Zero-phase current transformers such as earth leakage circuit breakers are known as a main use of small current detection devices. FIG. 15 is a schematic diagram showing the main structure of a zero-phase current transformer in order to explain its operating principle. In FIG. 15, two conductors 2. It consists of a resistor 4 connected near both ends of a detection coil 3 wound around an iron core l,
Both ends of the conductor 2 are connected to an AC power source 5 and a load 6, respectively. Under normal conditions, the values of the reciprocating currents 1 , , I 1 flowing through the two conductors 2 and whose directions are indicated by arrows are the same, and since the directions are opposite, the iron core 1 is not magnetized and the detection coil 3 no voltage is induced in

When a leakage occurs on the load 6 side, the current! ,, The value of I g changes, the difference current magnetizes the iron core 1, the induced voltage generated at this time causes current to flow through the small resistor 4, and the voltage drop across the small resistor 4 can be taken out as a control signal. can.

Although FIG. 15 shows the case of single phase, in the case of three phases, the conductor 2 may be represented as three, and the principle is the same as that of single phase.

The detected current of this zero-phase current transformer is as small as 5 to 30 A, so even if the iron core is made of a high magnetic permeability material such as Fe-Ni alloy, such as Permalloy (trade name), the magnetization curve shown in Figure 16 will not change. Since the detected magnetic field is small, the magnetic flux obtained is also low, and the voltage induced in the detection coil 3 is very small. Therefore, it is preferable that such a zero-phase current transformer has higher sensitivity, and it can be made smaller depending on the equipment used. It is strongly (desired).

[Problem to be solved by the invention]

However, in order to increase the sensitivity of the zero-phase current transformer having the above structure and detect even lower currents, there are the following problems. To increase the output voltage (for example, increase the magnetic path cross-sectional area of iron core 1 in Fig. 15) or change the conductor 2 to
There is a method of increasing the magnetic field applied to the iron core 1 by winding the iron core 1 a large number of times.
Naturally, this leads to an increase in the dimensions of the device, and the reason why the number of turns of the conductor 2 around the iron core 1 is increased is because the current of the main circuit flows through this conductor 2, so the wire diameter is increased, and similarly, the wire diameter is increased. An increase in size is unavoidable, and therefore the method is usually to pass the conductor 2 through the center hole of a small iron core 1, increase the number of turns of the detection coil 3, and amplify the induced voltage using an electronic circuit. There is. However, even if the detection coil 3 is wound approximately 1,000 times using this method, it is very difficult to detect less than 5 A. Further, when the detection coil 3 is wound approximately 1000 times, the outer diameter of the coil portion including the iron core 1 becomes large even if a thin wire is used. The diameter of the ri copper wire used for the detection coil 3 may be small in principle, but since the strength of the wire itself is required for S wire work, it is impossible to use a wire that is too thin. , without increasing the number of windings of the detection coil 3, that is, without increasing the outer diameter of the detection coil portion including the iron core 1.
It is desirable that small currents of A or less can be detected.

An object of the present invention is to provide a small current detection device that can detect small currents with high precision and accuracy, and the device.

[Means to solve the problem]

In order to detect a small current flowing through a conductor passing through the center hole of a cylindrical core in a closed magnetic circuit, the present invention uses a high-frequency power source separate from the power source of the current to be detected, so that the positive magnetic field in the magnetic hysteresis curve of the core is almost saturated. point, the negative magnetic field is the differential 12 near the coercive force
1m ratio is set at each point, and the iron core is excited. When a small current of the current to be detected is added to this, the magnetic flux of the iron core changes greatly due to a slight change in the a magnetic field side, and even for small currents. Since a large induced voltage can be obtained, the device's configuration is as follows: A high-frequency excitation coil wound around an iron core is connected to a high-frequency power source via a control circuit that sets the positive and negative celadon magnetic fields generated in the iron core to a predetermined magnitude. It is connected to a detection coil wound around an iron core separately from the high frequency excitation section and the high frequency excitation coil, and detects the induced voltage due to the total magnetic flux in which the magnetic flux due to the high frequency Ill magnetic section and the magnetic flux due to the current to be detected in the conductor are superimposed. This is a small current detection device that also includes an output section and a circuit that separates the induced voltage of conductor current.

[Effect]

As described above, the small current detection method and device of the present invention set the excitation current (magnetic field) by the high-frequency excitation section to the slope of the straight line near the coercive force of the magnetic hysteresis curve of the iron core, that is, to the point where the differential permeability is large. Therefore, even when a small magnetic field due to the current to be detected in the conductor is applied, the magnetic flux shows a significant change, and a large voltage is induced in the detection coil.

Moreover, since the core is magnetized using a high-frequency current using an excitation coil, the time required for the magnetic flux to change is fast.

Therefore, in the method and device of the present invention, a magnetic field is constantly applied by a high-frequency excitation section, and when a magnetic field due to a current to be detected in a conductor is superimposed on this magnetic field, the induced voltage due to the total magnetic flux is detected, and the induced voltage due to the total magnetic flux is detected.

By removing the DC component, an output voltage with a correct detection current waveform can be extracted from the output end, and from the above, it is possible to detect smaller currents with high sensitivity and precision.

〔Example〕

The present invention will be explained below based on Examples.

FIG. 1 is a schematic diagram for explaining the main part configuration of a small current detection device according to the present invention, and the method of the present invention will also be described in the following figures. First, in FIG. 1, the iron core 1a is made of a material whose magnetic hysteresis curve exhibits rectangular characteristics and has a high i3 magnetic coefficient, and is formed into a closed magnetic path, for example, in the shape of a ring. A conductor 2a is connected to the power supply and load of the detected device through the center hole of the iron core 1a, but these are not shown. In addition, to detect zero-sequence current, there are two or three conductors 2a, and the difference current between each conductor is used, but the principle is the same as detecting a small current in a single wire, so here In the following description, the conductor 2a is represented by one conductor, and the current to be detected whose direction is indicated by an arrow is Io. Also, iron core 1a
The high-frequency excitation coil 7 wound around the thick part of the high-frequency excitation coil 7 is connected to the high-frequency power supply 10 via a control circuit 9 formed by, for example, two sets of a rectifier 8a, a resistor R11, a rectifier 8b, and a resistor R1 connected in series. A high-frequency excitation section 11 connected to the high-frequency excitation coil 7 is provided, and a detection coil 12 wound around the iron core 1a separately from the high-frequency excitation coil 7 is connected to a low-pass filter 13. with a rectifier 8c interposed therebetween. An output section 15 is formed in which the DC remover 14 is connected in this order.

The DC remover 14 can be easily constructed with a capacitor or an insulating transformer.As described above, the device of the present invention has a high frequency excitation section 11. which has an excitation coil 7 on the iron core 1a. It is equipped with two elements: an output section 15 having a detection coil 12, and in use, detects a small current 1o flowing through a conductor 2a of a device to be detected through a center hole of an iron core 1a.

Next, Figures 2 and 3 show the magnetic hysteresis curve of the iron core 1a, and Figure 2 shows that the magnetic hysteresis curve is simply the relationship between the magnetic field strength (H) and the magnetic flux density (B). Although this is a conceptual diagram showing the squareness, FIG. 3 is an enlarged version of FIG. 2 for convenience of explanation below. In the straight line section B, ~-B4 in Fig. 3, B becomes thicker due to a slight change in H (changes), and the induced voltage due to this change in magnetic flux increases the magnetic flux by φ1.
If time is t, it is proportional to dφ/dt, so the higher the frequency, the larger the magnetizing current becomes.The present invention is characterized by the effective use of these points, and its operation will be explained below as shown in Fig. 1. The device configuration, the magnetic hysteresis curve shown in FIG. 3, and other necessary drawings will be added to the description.

The magnetic field of the iron core 1a is normally Ha on the positive side of the third leap and -H3 on the negative side due to the high frequency current of the high frequency excitation section 11 shown in FIG.
It has been added up to. The waveform is as shown in Figure 4, where the high-frequency excitation current l is larger on the positive side than on the negative side, and the magnetic field H
a and H+ are set by appropriately selecting rectifiers 8a+8b and resistors R+ and R* of control circuit 9. Note that these resistors R+ and Rz are the current of the conductor 2a, that is, the current to be detected! This is also necessary to prevent fluctuations in the high frequency excitation coil l due to zero. If the resistor R++R8 is small, the high frequency excitation current i changes depending on the magnitude of the detected current 1°, so the impedance of the resistors R+ and Ri is preferably at least twice the impedance of the high frequency excitation coil 7.

At this time, the magnetic flux and magnetic field applied to the iron core 1a are B + * (Hm) and B + (-Ht)t Bm (Hs) in Fig. 3.
Draw a hysteresis curve A that changes in the direction of the arrow in the order of
A voltage is induced in the detection coil 12. When an AC sinusoidal current flows through the zero-order conductor 2a, the voltage waveform of which is shown in FIG. (middle), and the magnetic flux changes by ΔB in both positive and negative directions corresponding to ΔH1-ΔH, and the magnetic hysteresis curve is on the positive side Δ
H forms the loop of C shown by the dotted arrow in Figure 3, and the negative side -ΔB forms the loop of D shown by the dashed-dotted arrow.The induced voltage is small when ΔB is in the positive direction, and large when ΔB is in the negative direction. , the voltage waveform becomes a modulated wave as shown in FIG. Note that the horizontal axis in Figure 5 is (al (b
) (C) has a common time axis.

By removing the high frequency component from the waveform of FIG. 5(C) with the low-pass filter 13 of the output section 15 and the DC component with the DC remover 14,
The detected current waveform can be accurately reproduced and determined. Therefore, according to the present invention, as is clear from the above operating principle, it is possible to detect a small current having any waveform such as a distorted waveform or a rectangular wave.

The present invention utilizes the fact that the magnetic flux in the straight line between the second and third quadrants of the magnetic hysteresis curve of the iron core 1a, Bm- to -B9 in FIG. 3, changes greatly with a slight change in the magnetic field. , the effective use of this magnetic property of the iron core 1a is to set the magnetic field that is obtained by the high-frequency excitation described above and determines the reference hysteresis curve A to B9 within a range where ΔB does not deviate from the linear section when the detected current is small. It is best to set them as close as possible. In this way, the voltage constantly induced by high frequency excitation can be lowered, and the ratio between this induced voltage and the induced voltage due to the detection current 1o increases, increasing the detection accuracy. This is advantageous in that the loss of the iron core 1a is also low. Also, the magnetic field of the A loop, which is the reference, is −H
When the coercive force is set to c, that is, B-0 of the iron core 1a, the detectable current value at that time is the detection limit current in the present invention.

Although the waveform of the high-frequency excitation current l has been described as a sine wave as shown in Figure 4, other waveforms may be used. For example, you may add an offset to the high frequency power supply 10 and move the initial zero point shown by the dotted line to the solid line as shown in FIG. 6. If you do this as shown in FIG. 8b and resistance Rr,
The quantity of R* etc. used can be reduced.

In principle, the higher the frequency of the high-frequency excitation current i, the better, but in practice it must be determined by carefully considering the frequency of the current to be detected, its harmonic components, the required detection accuracy, the frequency characteristics of the iron core material, etc. This frequency is about 5 kHz when detecting a commercial frequency stream, and about 20 kHz when high-order frequency components are included. As is clear from Fig. 3 already mentioned, the magnetic properties of the iron core 1a are such that the hysteresis curve has good squareness with respect to the magnetization of high-frequency current, the absolute value of the core density is large, and the coercive force is small. It is necessary that there be.

Although the case where the detected current 1o is AC has been described so far, it is also possible to detect DC by changing the circuit of the output section 15. FIG. 7 shows the circuit configuration diagram of the main part, and the conductor 2a and the high frequency excitation section 11 are connected to the first
Since it is the same as the figure, the illustration is omitted, and the circuit configuration will be described with reference to FIG. rectifier 8c,
The low-pass filter 13a inputs the positive component to the e terminal of the differential amplifier 16, and a reference voltage having the same value as this voltage is input to the Φ terminal of the differential amplifier 16 by the DC power supply 17. The output of the differential amplifier 16 becomes O. When a positive current flows through the primary conductor 2a (not shown), the induced voltage in the detection coil 12 becomes small and the difference from the reference voltage becomes a negative value, but the differential amplifier 16 Since the positive and negative outputs of the outputs are reversed, a positive voltage proportional to the current to be detected is obtained at the differential amplifier 16. Even when the current to be detected is negative, a negative voltage proportional to the current can be obtained using the same principle. Thus, according to the present invention, it is possible to detect whether the current to be detected is only a direct current, a combination of a direct current and an alternating current, or only an alternating current. Note that when only direct current is detected, the low-pass filter 13a in FIG. 7 may be replaced with a smoothing circuit.

The basic matters regarding the configuration and operation of a small current detection device using the method of the present invention have been explained above, and next, a specific example of the present invention will be described with reference to FIGS. 1 and 3 again. Iron core 1a has a weight percentage of 82. OCo −2, ONl −4,5
A Co-based amorphous alloy ribbon having a composition of Fe-8,5SL-3,OB and subjected to a predetermined heat treatment was used to form a cylindrical wound core. This amorphous alloy has excellent magnetic properties and has very low magnetostriction, so the influence of stress on the magnetic properties is very small, and it is easy to handle, making it suitable for use as the iron core 1a.

The dimensions of the iron core 1a are outer diameter 20tm, inner diameter 15fl, and height.
(Width of thin strip) 2m. A copper wire with a diameter of 2fiφ was used as the conductor 2a. High frequency excitation coil 7 and detection coil 1
In each case, formal insulated copper wire with a diameter of 0.1 fiφ was wound 20 times and 40 times around the thick part of the iron core 1a. Since the high frequency current of the high frequency excitation coil 7 is small, it is sufficient to use such a thin wire. The high frequency excitation coil has a sine wave of 100kHz, and the magnetic field on the negative side is about 3 in the negative direction from the magnetic field -H4 corresponding to B in Figure 3.
It was set at a point moved by A/m. Note that when the detected current is a sine wave with an effective value of 30 mA, the absolute value of ΔH in FIG. 3 is approximately 2.4 A/m.

After the induced voltage of the detection coil 12 is reduced to only the positive component of the induced voltage when the magnetic flux changes in the negative direction by the rectifier 8c, it is passed through a low-pass filter 13 with a cut-off frequency of 1 kHz to remove high-frequency components, and then passed through a DC remover. 14, the direct current component was removed. Note that a capacitor was used as the DC remover 14. Although it is possible to double the output by using both the positive and negative sides of the induced voltage of the detection coil 12, only the positive side bone was used here.

Next, FIG. 8 shows the output waveform and the original waveform of the current to be detected when an AC 50H2 sinusoidal current effective value of 0.51A is detected according to the present invention as described above.
FIG. 9 shows the waveform when a rectangular wave current (maximum value 0.7 mA) was detected. The output waveforms in both Figures 8 and 9 are exactly the same as the original waveform of the current to be detected, and Figure 9 shows that the output waveforms are accurate even for such small currents and waveforms that have many harmonic components. It can be seen that it can be detected. 1st
Is the 0 figure intersecting? It is a diagram showing the relationship between the detected current value and the output NLb obtained when detecting a 30 Hz sine wave current. For comparison, FIG. 10 shows a conventional iron core 1, for example, using the trade name Permalloy for the iron core 1 in FIG.
The output characteristics when the dimensions of the body 2 are the same as those of the present invention described above and the number of windings of the detection coil 3 is 1000 are also shown. In FIG. 10, a solid characteristic line A represents the present invention, and a dotted characteristic line represents the conventional case. As can be seen from Figure 10, both 410 characteristic lines show very good linearity, but the present invention can obtain a higher output voltage, and even though the number of turns of the detection coil is 1/25, the output Since the output of the device according to the present invention is proportional to the number of turns of the detection coil, the output of the device according to the present invention is approximately 9 times as large as 10
If you set it to 00 times, you can increase the output by 25 times. However, in the present invention, even if the outer diameter of the iron core 1a is the same as the conventional one, the number of turns of the high-frequency excitation coil 7 and the detection coil 12 is small, and it is possible to downsize while still obtaining high output. Therefore, trying to obtain even higher output by increasing the number of turns in the coil section goes against the trend of miniaturization. Therefore, in such a case, practically, the number of turns of the detection coil 12 can be amplified with a simple electronic circuit. It is preferable to amplify the voltage to a desired level.

Next, an example of a comparison of dimensions after coil winding including the iron core between the device of the present invention and the conventional device is shown in Table 1. Diameter of 0 winding (0.1 m)
and core dimensions are common to both.

Table 1 As shown in Table 1, even if the device of the present invention uses an iron core with the same dimensions as the conventional one, the number of turns of both coils is small, so the SvA portion protruding from the outer periphery of the iron core is small, and the protruding portion after winding is small. The diameter is considerably smaller than before. Also, on the inner diameter side of the iron core, the area occupied by the windings is smaller in the present invention, and the inner diameter can be larger, so the detection sensitivity of the present invention is 1.
The center hole of the iron core can be made small to the extent that the conductor can be passed through, taking into consideration the winding diameter and the like.

In addition, in the present invention, based on the same principle as above, the
It is also possible to detect even smaller currents on the order of -A. FIG. 11 is a schematic diagram showing the main structure of the device, and parts common to those in FIG. 1 are denoted by the same reference numerals. 1st
The difference between Fig. 1 and Fig. 1 is that the conductor 2b passes through the center hole of the iron core 1b and forms an excitation coil 2c wound around the thick part of the iron core 1b; other aspects are the same as Fig. 1. It is. Furthermore, since the method of detecting the detected current is basically the same as that described above, the explanation thereof will be omitted.

Next, an example of the dimensions and output characteristics of the iron core 1b of the device shown in FIG. 11 will be described. The original dimensions of the iron core 1b are an outer diameter of 12 mm, an inner diameter of 10 fl, and a height of 2 fi, and after winding, including the winding portion, the outer diameter is 14 mm, the inner diameter is 7 mm, and the height is 4 mm.

For the winding portion, each of the excitation coil 2c, the high frequency excitation coil 7, and the detection coil 12 was wound 50 times on the thick part of the iron core 1b using formal insulated copper wire with a diameter of 0.1 fi.

It is wound 30 times and 200 times. With this device, the output waveform when AC 50 Hz sinusoidal current effective (!
66 μA) is shown in FIG. 13. Figures 12 and 13 correspond to Figures 8 and 9 mentioned above, and from Figures 12 and 13, the output waveform matches well with the original waveform of the current to be detected. It can be seen that even such a minute current can be detected accurately. 1st
Figure 4 corresponds to the above-mentioned Figure 10, and is a line showing the relationship between the detected current value and the output voltage obtained when detecting an AC 50 Hz sinusoidal current using the device in Figure 11. As shown in the figure, the output characteristics show very good linearity.

For example, even if a conventional device is used and the number of turns of each coil is the same and the core dimensions after winding are the same as the device shown in Fig. 11, the detectable current value is limited to about 100μ8. The effectiveness of the present invention, which is capable of detecting a minute current of several μA while maintaining a small size, is fully recognized.

As mentioned above, the small ring 1 detection device of the present invention has been described as using one conductor, but as mentioned above, it is important that the waveform match and good linearity exist between the current to be detected and the output voltage. Therefore, the device of the present invention can of course be fully applied to current detection equipment that uses differential current between two or three conductors, and can be used in a wide range of other ways as necessary. It is something.

〔Effect of the invention〕

Conventionally, small current configuration devices, such as zero-phase current transformers,
The magnetizing force of the iron core by the detection coil is weak (and the induced voltage is small, so it is still not sufficient to detect small currents with high sensitivity and accuracy, and the number of turns of the coil winding must be increased. While it has not been possible to reduce the outer diameter of the wire portion, according to the present invention, as explained in the embodiment, a high frequency excitation coil is used to set the magnetic field at a point where the differential permeability of the rectangular magnetic characteristics of the iron core is large. is installed separately from the detection coil, magnetizes the iron core, causes a large magnetic flux change with a small magnetic field change in a shorter time, and superimposes the induced voltage due to high frequency excitation on the voltage induced in the detection coil, and from this superimposed voltage. Since the output voltage waveform obtained by removing harmonic components and DC components is made to be the same as the detected current waveform, the detected current is not limited to AC sine wave current, but can also be detected with direct current or other similar waveforms. It is also possible to accurately detect even minute currents compared to conventional methods, and the number of turns of the excitation coil and Mk output coil can be reduced compared to conventional methods, so there is no increase in size due to winding. This makes it possible to downsize the iron core.

For these reasons, the device according to the present invention is useful as a small current detection device that is compact, has high sensitivity, and performs high-speed functions when applied to not only zero-phase current transformers but also other current detection devices. .

[Brief explanation of the drawing]

FIG. 1 is a schematic diagram showing the main part configuration of the device according to the present invention,
Figure 2 is a conceptual diagram showing the shape of the magnetic hysteresis curve of the iron core material, Figure 3 is an enlarged view of Figure 2, Figure 4 is a waveform diagram of high-frequency excitation current, and Figure 5 is detection when high-frequency excitation is performed. Waveform of induced voltage in the coil (a). Furthermore, when the current to be detected flows, the magnetic field waveform (bl) and the induced voltage waveform (C), Figure 6 is a diagram of the high frequency excitation current waveform that is different from Figure 4, and Figure 7 is a diagram of the output section that is different from Figure 1. A circuit configuration diagram, Fig. 8 is a waveform diagram of the output voltage and detected AC sine wave current by the device of the present invention, Fig. 9 is a waveform diagram of the output voltage and detected AC rectangular current, and Fig. 10 is the same output voltage. and the detected AC sine wave current value, FIG. 11 is a schematic diagram showing the main part configuration of the device according to the present invention, different from FIG. 1, and FIG. 12 is the output from the device of FIG. 11. A waveform diagram of the voltage and the detected AC sine wave current, FIG. 13 is a waveform diagram of the output voltage and the detected AC triangular current, and FIG. 14 is a relationship diagram between the output voltage and the detected AC sine wave current value. FIG. 15 is a schematic diagram showing the main part configuration for explaining the operation of a conventional zero-phase current transformer, and FIG. 16 is a conceptual diagram showing the shape of the magnetization curve of the iron core material in FIG. 15. 1, la, lb: iron core, 2.2a, 2b: conductor, 2C: excitation coil, 3.12; detection coil, 7: high frequency excitation coil, 8a, 8b+8c: rectifier, 9:
Control circuit, lO: high frequency power supply, 11: high frequency excitation section, 1
3, 13a: Low-pass filter, 14: DC remover, 15: Output section, 16: Temperature difference 1/-4 Figure 4 Time Figure 6 z1 Figure 8 Figure 9 Figure 10 Figure 11 Figure 12 Figure 13 Figure 14 Figure 15 Figure 16

Claims (1)

[Scope of Claims] 1) Strength of magnetic field generated in the core by a small current flowing through a conductor passing through the center hole of a cylindrical core in which a closed magnetic path is formed using a high permeability material whose magnetic hysteresis curve exhibits rectangular characteristics. 1. A method of detecting the small current from the induced voltage of a detection coil wound around the iron core by changing the current, the method comprising: changing the current of the small current; i) The iron core is excited at high frequency up to the point where the maximum magnetic field on the positive side is almost saturated in the magnetic hysteresis curve and the maximum magnetic field on the negative side is a straight line passing through the point of coercive force, and ii) The iron core is excited in this excited state. iii) greatly changing the magnetic flux density of the iron core by applying a magnetic field due to a small current of the conductor to and slightly moving the maximum magnetic field on the negative side of the high-frequency excitation on the straight line part of the hysteresis curve; iii) the high-frequency excitation After detecting the induced voltage of the total magnetic flux of the iron core in which the magnetic flux generated by the magnetic flux and the magnetic flux generated by the small current of the conductor are superimposed, the induced voltage of the small current of the conductor is separated from this induced voltage. 2) The strength of the magnetic field generated in the core is changed by a small current flowing through a conductor passing through the center hole of a cylindrical core in which a closed magnetic path is formed using a high magnetic permeability material whose magnetic hysteresis curve exhibits square characteristics. A device for detecting the small current from the induced voltage of a detection coil wound around the iron core, the device comprising: i) passing through the center hole of the iron core and winding a high frequency excitation coil around the thick part of the iron core to generate both positive and negative magnetic fields in the iron core; ii) A high frequency excitation section connected to a high frequency power source via a control circuit that sets the high frequency excitation coil to a predetermined size; ii) connected to a detection coil wound around the thick part of the iron core through the center hole of the iron core, separate from the high frequency excitation coil; and an output section having a circuit that separates the induced voltage component of the small current in the conductor after detecting the total magnetic flux in which the magnetic flux generated by the high frequency excitation section and the magnetic flux generated by the small current in the conductor are superimposed. A small current detection device featuring: 3) The small current detection device according to claim 2), wherein the conductor passing through the center hole of the cylindrical core forms an excitation coil wound around the thick part of the core.