JP4824007B2 - Current measuring device - Google Patents

Description

  The present invention relates to a current measuring apparatus that automatically switches a measurement range and measures current with a predetermined accuracy over a wide range.

  Various means for measuring current have already been proposed and put into practical use. A configuration is also known in which a measured current value is converted into a digital signal for display and control. In general, the measurement range is switched to measure a wide range of current values while maintaining a predetermined measurement accuracy. For example, as shown in FIG. 9, a resistor R is connected between a power supply unit 101 such as a charging / discharging device and a load unit 102 such as a capacitor or a battery, and the voltage across the resistor R due to the flowing current is converted into a conversion ratio. A voltage is converted by voltage converters 103, 104, and 105 of A, B, and C (A> B> C), input to the AD board 106, converted into a digital signal, and output as a measured digital current value. Are known.

  In this current measuring device, when the current flowing through the load unit 102 is IA <IB <IC, the measured current value is in a small range when the current IA, and the voltage at both ends of the resistor R becomes a small value. When the current IB is larger than the current IA, it is multiplied by A by the voltage converter 103. When the current IC is larger than the current IB, the voltage is multiplied by B. When the current IC is larger than the current IB, the voltage is multiplied by C. The digital value is converted by the board 106. Thereby, current measurement can be performed with substantially the same accuracy over the range of currents IA to IC.

  10 detects the current flowing from the power supply unit 111 to the load unit 112 such as a capacitor or a battery as the voltage across each of the resistors Ra and Rb connected in series, and the AD board 113 detects the current. When the resistor Ra is used for measuring a small current and the resistor Rb is used for measuring a large current, the resistance value has a relationship of Ra> Rb, and the diode Da, Db is connected in parallel. When the current flowing through the load unit 112 is always only in one direction, it is sufficient to connect only one diode having forward characteristics in the direction in which the current flows, and the current flows in the direction of the arrow shown in the figure. Therefore, it is sufficient to connect only the diode Da. When measuring a small current, the voltage at both ends of the resistor Ra is equal to or less than the forward voltage of the diodes Da and Db. When measuring a large current, the voltage at both ends of the resistor Ra becomes high, and the power loss due to the resistor Ra is large. However, at that time, when the voltage exceeds the threshold voltage in the forward direction of the diode, the current is shunted and flows to the diode, so that power loss at the time of measuring a large current can be reduced.

In addition, as a current detection means when a current of about several A and a current of several mA to several tens of mA flow depending on the operation mode, a resistor having a large resistance value and a resistor having a small resistance value are switched according to the operation mode, or several A In the case of an operation mode with a large current level, a resistor having a large resistance value and a resistance value having a large resistance value connected in series are short-circuited by a switch to switch between a large current operation mode and a small current operation mode. There has been proposed a means for performing current measurement while maintaining equal current detection accuracy (see, for example, Patent Document 1).
JP 2000-194456 A

  Although the current detection device shown in FIG. 9 of the conventional example can measure a wide range of current, there is a problem of cost increase by providing a plurality of voltage converters. The current detection device shown in FIG. 10 connects a diode in parallel with a resistor for measuring a large current, but suppresses the voltage at both ends of the resistor to a forward threshold voltage of the diode, for example, 0.6 V or less. Therefore, there is a problem that the loss when measuring a large current is relatively large. In Patent Document 1, the current measurement range is switched based on the operation mode switching control signal, and a configuration for automatically switching the measurement range is not shown.

  An object of the present invention is to solve the problems of the conventional example described above, and to automatically switch the measurement range according to the flowing current and reduce the power loss.

  A current measuring apparatus according to the present invention includes a plurality of resistors having different resistance values connected in series between a power supply unit and a load unit, and digitally converting voltages at both ends of these resistors and outputting them as measured digital current values A field effect transistor connected in parallel to a resistor for measuring a small current having a large resistance value compared to a resistor for measuring a large current, a voltage across both ends of the resistor having a large resistance value, and a reference Comparing with the voltage, a comparator that turns on the field effect transistor when the voltage across the resistor having a large resistance value exceeds the reference voltage is provided.

  Also, a current measuring device including a plurality of resistors having different resistance values connected in series between a power supply unit and a load unit, and an AD board that digitally converts voltages at both ends of these resistors and outputs them as measured digital current values. An absolute value that outputs an absolute value of a potential difference between a field effect transistor connected in parallel to a resistor for measuring a small current having a large resistance value compared to a resistor for measuring a large current and a resistor having a large resistance value The circuit includes a comparator that compares the output of the absolute value circuit with a reference voltage and turns on the field effect transistor when the output of the absolute value circuit exceeds the reference voltage.

  Also, a current measuring device including a plurality of resistors having different resistance values connected in series between a power supply unit and a load unit, and an AD board that digitally converts voltages at both ends of these resistors and outputs them as measured digital current values. Two series-connected field effect transistors connected in parallel to a resistor for measuring a small current having a large resistance value compared to a resistor for measuring a large current, each having a source commonly connected to a reference potential, and a resistor The absolute value circuit that outputs the absolute value of the potential difference between both ends of a resistor with a large value, the output of this absolute value circuit, and the reference voltage connected to the reference potential were compared, and the output of the absolute value circuit exceeded the reference voltage In some cases, two comparators that turn on two field-effect transistors connected in series are provided.

  Further, it is possible to provide a rectifying unit that rectifies and smoothes the voltages at both ends of a plurality of resistors having different resistance values connected in series between the power supply unit and the load unit and inputs them to the AD board. Further, the output of the comparator that turns on the field effect transistor can be input to the AD board as a measurement range switching signal.

  Also, a current measuring device including a plurality of resistors having different resistance values connected in series between a power supply unit and a load unit, and an AD board that digitally converts voltages at both ends of these resistors and outputs them as measured digital current values. A field effect transistor connected in parallel to a resistor for measuring a small current having a large resistance value compared to a resistor for measuring a large current, an operational amplifier for detecting a voltage at both ends of the resistor having a small resistance value, and this calculation Comparing the output signal of the amplifier with a reference voltage, and a comparator that turns on the field effect transistor when the output signal exceeds the reference voltage and turns off the field effect transistor when the output signal falls below the reference voltage. .

A plurality of resistors having different resistance values are connected in series between the power supply unit and the load unit, and a field effect transistor is connected in parallel with a resistor for measuring a small current with a large resistance value. When the voltage at both ends exceeds the reference voltage due to the flow of a large current , the field effect transistor is turned on, and the field effect transistor bypasses a resistor for measuring a small current with a large resistance value . Compared to the conventional example that bypasses a resistor for measuring a small current with a large resistance value by a diode, there is an advantage that the power loss due to the resistor at the time of measuring a large current can be greatly reduced. As a configuration in which two series-connected field effect transistors connected to a reference potential are connected in parallel to a resistor having a large resistance value for measuring a small current, whichever current direction is There is an advantage that it becomes possible to measure be in.

  The current measuring device of the present invention will be described with reference to FIG. 1. A plurality of resistors R1 and R2 having different resistance values connected in series between the power supply unit 1 and the load unit 2 and the resistors R1 and R2 And an AD board 3 that converts the voltage at both ends into a digital value and outputs it as a measured digital current value, and is connected in parallel to a small current measuring resistor R2 having a larger resistance value than the large current measuring resistor R1. The voltage at both ends of the transistor 4 and the resistor R2 having a large resistance value is compared with the reference voltage Vr1, and the field effect transistor 4 is turned on when the voltage at both ends of the resistor R2 having the large resistance value exceeds the reference voltage Vr1. And a comparator 5 for setting the state.

  FIG. 1 is an explanatory diagram of Embodiment 1 of the present invention, where 1 is a power supply unit such as an inverter or a charge / discharge device, 2 is a load unit such as a capacitor, battery, or motor, and 3 is a plurality of A / D conversion units. AD board including 4 is a field effect transistor, 5 is a comparator, Vr1 is a reference voltage, R1 is a resistor for measuring a large current, R2 is a resistor for measuring a small current, and the resistance values of the resistors R1 and R2 are R1 <R2 Select the relationship. A field effect transistor 4 is connected in parallel with the small current measuring resistor R2. When the load unit 2 is a capacitor or a battery and the power supply unit 1 is a charging device, a direct current in the direction of the arrow shown in the figure flows through resistors R1 and R2 connected in series. The voltages across these resistors R1 and R2 are input to the AD board 3 and converted into digital values.

  The resistance R2 for measuring the small current has a larger resistance value than the resistance R1 for measuring the large current. The field effect transistor 4 is connected in parallel with the resistance R2, and the voltage across the resistance R2 and the reference The voltage Vr1 is compared by the comparator 5, and when a current exceeding the small current measurement range flows, the voltage across the resistor R2 is set to exceed the reference voltage Vr1. Thereby, when a current exceeding the small current measurement range flows, the field effect transistor 4 is turned on by the output signal of the comparator 5. The field effect transistor 4 in the on state clamps the voltage across the resistor R2 to a value according to the reference voltage Vr1. The output signal of the comparator 5 is input to the AD board 3 as a measurement range switching signal. In the current measurement with a normal resistance, the voltage across the resistance is generally selected to be about 50 mV to 200 mV at maximum. For the resistors R1 and R2, the resistance values of the resistor R1 for measuring a large current and the resistor R2 for measuring a small current are selected based on the relationship between the maximum current value in the current measurement range and the maximum voltage selected in advance.

  Further, development of a field effect transistor whose voltage drop due to the on-resistance of the field effect transistor 4 is about several tens of mV to several hundred mV or less is being promoted. When the voltage across the resistor R2 exceeds the reference voltage Vr1, the field effect transistor 4 is turned on by the output signal of the comparator 5, and the voltage across the resistor R2 for small current measurement is clamped by the reference voltage Vr1. From the small current measurement range in which the output signal of the comparator 5 is input to the AD board 3 and the voltage across the resistor R2 is the measured digital current value, the voltage across the resistor R1 is the measured digital current value. Switch to the large current measurement range. The power loss due to the small current measurement resistor R2 in this large current measurement range is compared to the conventional example in which the field effect transistor 4 is turned on to bypass the current and the on resistance is small, so that it is bypassed by a diode. Can be greatly reduced.

  The AD board 3 also receives an A / D converter that inputs voltages across the resistors R1 and R2 and converts them into digital values, and a switching output means for switching and outputting the A / D converted measured digital current values. The switching output means is controlled by the output signal of the comparator 5, and the current measurement range is automatically switched. For example, the measurement digital converted by a plurality of A / D converters is shown. It is also possible to apply means for automatically selecting and outputting the current value by a subsequent circuit (not shown). Also, the measured digital current values by the resistors R1 and R2 converted by the A / D converter are compared, and when the difference between the measured digital current values by the resistors R1 and R2 due to the field effect transistor 4 being turned on exceeds a predetermined value. It is also possible to apply means for switching and outputting the measured digital current value by the resistor R2 to the measured digital current value by the resistor R1. Also, the measured digital current value can be input to a display device or the like (not shown) for digital display.

  As described above, when the direct current flowing through the load unit 2 is within the small current measurement range, the voltage across the resistor R2 is equal to or lower than the reference voltage Vr1, the field effect transistor 4 is in the off state, and the AD board 3 A measured digital current value obtained by converting the voltage at both ends of the resistor R2 into a digital value is output. When the current in the large current measurement range flows beyond this small current measurement range, the voltage at both ends of the resistor R2 exceeds the reference voltage Vr1. Therefore, the field effect transistor 4 is turned on by the comparator 5 and the voltage across the resistor R2 is clamped to the reference voltage Vr1. If this reference voltage Vr1 is, for example, 100 mV, the voltage across the resistor R2 for small current measurement is clamped to 100 mV even in the large current measurement range, and the AD board 3 converts the voltage across the resistor R1 to a digital value. The measured digital current value converted into can be output and the small current measurement range can be automatically switched to the large current measurement range. When the current flowing through the load unit 2 decreases and falls below the large current measurement range, the voltage across the resistor R2 falls below the reference voltage Vr1, the field effect transistor 5 is turned off, and current measurement using the first small current measurement range is performed. It becomes a state. In the large current measurement range, when the field effect transistor 4 is set to a complete ON state of a few dozen mΩ to reduce power loss and shift from the large current measurement range to the small current measurement range, It is also possible to add a control configuration for turning off the field effect transistor 4 by determining the range of the output measurement current value.

  FIG. 2 is an explanatory diagram of a second embodiment of the present invention. The same reference numerals as those in FIG. 1 denote the same parts, 4-1 and 4-2 are field effect transistors, 5-1 and 5-2 are comparators, and R2. -1, R2-2 are resistors for medium current measurement and small current measurement, and Vr1-1 and Vr1-2 are reference voltages. A resistor R1 for measuring a large current, a resistor R2-1 for measuring a medium current, and a resistor R2-2 for measuring a small current are connected in series, and a field effect in parallel with the resistor R2-1 for measuring a medium current. The transistor 4-1 is connected, the field effect transistor 4-2 is connected in parallel with the small current measuring resistor R2-2, and the field effect transistors 4-1 and 4 are output by the output signals of the comparators 5-1 and 5-2. -2 is controlled and the measured digital current value converted by the AD board 3 is switched and selected by the output signals of the comparators 5-1 and 5-2, and is output to the subsequent circuit configuration not shown.

  Further, the resistance values of the resistor R1 for measuring a large current, the resistor R2-1 for measuring a medium current, and the resistor R2-2 for measuring a small current are represented by R1 <(R2-1). ) <(R2-2). The reference voltages Vr1-1 and Vr1-2 for the comparators 5-1 and 5-2 are selected based on the resistance values of the resistors R2-1 and R2-2 and the current value for switching the measurement range. Depending on the case, the same voltage can be used. The AD board 3 includes A / D converters for large current measurement, medium current measurement, and small current measurement, and switching means for switching and outputting the respective converted output digital current values. Can be controlled by the output signals of the comparators 5-1 and 5-2 to switch between the large current measurement range, the medium current measurement range, and the small current measurement range.

  A current in the direction of the arrow flows from the power supply unit 1 to the load unit 2, and voltages at both ends of the resistors R1, R2-1, and R2-2 are input to the AD board 3, and at that time, the field effect transistors 5-1 and 5-2 In the case of the small current measurement range in the off state, the measured digital current value of the small current measurement range obtained by converting the voltage across the resistor R2-2 for small current measurement into a digital value is selectively output from the AD board 3. When a current exceeding the current in the small current measurement range flows, the voltage across the resistor R2-2 exceeds the reference voltage Vr1-2, and the field effect transistor 5-2 is turned on by the output signal of the comparator 5-2. . As a result, the voltage across the resistor R2-2 is clamped by the reference voltage Vr1-2, and the AD board 3 uses the output signal of the comparator 5-2 as a medium current measurement range to cause the resistor R2-1. A / D conversion is performed on the voltage at both ends, and the output is selected and output as a measured digital current value in the medium current measurement range.

  When the current flowing through the load unit 2 further increases and the voltage across the resistor R2-1 exceeds the reference voltage Vr1-1, the field effect transistor 5-1 is turned on by the output signal of the comparator 5-1. As a result, the voltage across the resistor R2-1 is clamped by the reference voltage Vr1-1, and the AD board 3 uses the output signals from the comparator 5-1 and the comparator 5-2 as a large current measurement range. The voltage at both ends of the resistor R1 is A / D converted and selectively output as a measured digital current value in the large current measurement range. In this state, the voltage across the resistor R2-2 for small current measurement is clamped by the reference voltage Vr1-2, and the voltage across the resistor R2-1 for medium current measurement is the reference voltage Vr1. It becomes the state clamped by -1. In addition, the case where the measurement range is automatically switched between the three measurement ranges of the small current measurement range, the medium current measurement range, and the large current measurement range is shown, but the measurement range can be switched in more stages. It can be set as the structure to do. In that case, a plurality of resistors each having a different resistance value are connected in series, for example, a field effect transistor is connected in parallel to each of a plurality of resistors other than the resistor corresponding to the large current measurement range, and the voltages at both ends of each resistor are connected. And the respective reference voltages are compared by a comparator, and a control configuration in which a field effect transistor corresponding to a current measurement range equal to or lower than the current current measurement range is turned on and a resistor having a larger resistance value is set in a bypass state. Is also possible.

  FIG. 3 is an explanatory diagram of Embodiment 3 of the present invention. The same reference numerals as those in FIG. 1 denote the same names, and Vr1a denotes a reference voltage. In the third embodiment, the direction of current flow is opposite to the case shown in FIG. 1 as indicated by an arrow. For example, in the case where discharge control is performed using the load unit 2 as a battery and the power source unit 1 as a charge / discharge control unit. Equivalent to. Therefore, the comparator 5 compares the voltage across the resistor R2 with the reference voltage Vr1a, but the polarity of the reference voltage Vr1a is also reversed because the current direction is opposite to that shown in FIG. ing. As in the case shown in FIG. 1, when the voltage at both ends of the small current measuring resistor R2 exceeds the reference voltage Vr1a, the field effect and the transistor 4 are turned on by the output signal of the comparator 5, and both ends of the resistor R2 are turned on. The voltage is clamped by the reference voltage Vr1a, and the AD board 3 A / D converts the voltage across the resistor R1 and the voltage across the resistor R2 into a measured digital current value. The voltage at both ends of the resistor R1 is output as a measured digital current value in the large current measurement range by the output signal of the comparator 5 that turns ON. That is, it is possible to automatically switch from the small current measurement range to the large current measurement range, and to reduce power loss due to the small current measurement resistor R2. In the second embodiment shown in FIG. 2 as well, when the current direction is opposite, the polarity of the reference voltage with respect to the comparator can be reversed to cope with large, medium and small current measurement ranges.

  FIG. 4 is an explanatory diagram of Embodiment 4 of the present invention. The same reference numerals as those in FIG. 1 denote the same parts, D is a parasitic diode of the field effect transistor 4, 6 is a differential amplifier, and 7 is an absolute value circuit. Show. The fourth embodiment enables current measurement with the same configuration when the current flows in either direction as compared to the configuration in which the current directions in FIGS. 1 and 3 are opposite. For example, The power supply unit 1 can be applied to a charge / discharge control circuit, and the load unit 2 can be applied to current measurement when the current direction is reversed due to charging / discharging of a battery or a capacitor. In addition, the differential amplifier 6 and the absolute value circuit 7 are configured such that the polarity of the voltage across the resistor R2 for measuring a small current is inverted corresponding to the current direction. Since the output polarity of the differential amplifier 6 is inverted according to the current direction, the absolute value circuit 7 inputs the voltage value of one polarity not related to the current direction to the comparator 5 and compares it with the reference voltage Vr1. The reference voltage Vr1 in this case is such that the negative side of the reference voltage Vr1 is connected to the reference potential and the positive side of the reference voltage Vr1 is input to the comparator 5 so as not to relate to the current direction. The potential between the output terminals of the power supply unit 1 can be ½ of the voltage.

  The parasitic diode D of the field effect transistor 4 has a forward threshold voltage of about 0.6 V, like the diode of the individual component, and the voltage across the resistor R2 due to the current in the small current measurement range is as described above. Thus, since the maximum is about 200 mV, the bypass function by the parasitic diode D does not occur, and when a current exceeding the small current measurement range flows, the field effect transistor 4 is turned on, and the small current flowing through the resistor R2 Current exceeding the current measurement range can be bypassed. Therefore, the power loss of the resistor R2 due to the current in the large current measurement range can be reduced. The current measuring device according to the fourth embodiment has an advantage that the current between the power source unit 1 and the load unit 2 can be measured in any direction.

  FIG. 5 is an explanatory diagram of Embodiment 5 of the present invention. The same reference numerals as those in FIGS. 1 and 4 denote the same parts, 4a and 4b denote field effect transistors, and Da and Db denote parasitic diodes. The fifth embodiment shows a configuration in which current measurement is possible regardless of the direction of current as in the fourth embodiment, and two field effects are provided at both ends of the small current measuring resistor R2. The transistors 4a and 4b are connected in parallel, for example, with the sources connected in common. The common connection point of the sources is connected to the reference potential connecting the negative side of the reference voltage Vr1. Similarly to the case shown in FIG. 4, the differential amplifier 6 and the absolute value circuit 7 obtain the potential difference between both ends of the resistor R2 by the differential amplifier 6, convert it to one polarity by the absolute value circuit 7, and 5 is compared with the reference voltage Vr1. When the reference voltage Vr1 is exceeded, a current exceeding the small current measurement range flows, the field effect transistors 4a and 4b are turned on, and the voltage across the resistor R2 is set to the reference voltage Vr1. Clamp to the value corresponding to. Also in the fifth embodiment, the voltage across the resistor R2 for measuring a small current having a large resistance value is clamped at the reference voltage Vr1 with the field effect transistors 4a and 4b turned on in the large current measuring range. The power loss due to the resistor R2 can be reduced.

  FIG. 6 is an explanatory diagram of Embodiment 6 of the present invention, where 1 is an AC power supply unit, 2 is an AC load unit such as an induction motor operating with AC, and 3 is an AD including a plurality of A / D conversion units. Board, 4 is a field effect transistor, 5 is a comparator, D is a parasitic diode of the field effect transistor, Vr1 is a reference voltage, R1 is a resistor for measuring a large current, R2 is a resistor for measuring a small current, and 8 and 9 are full waves The rectification | straightening part which rectifies and smoothes is shown. Similarly to the above-described embodiments, the resistance values of the resistors R1 and R2 are selected so that R1 <R2. Further, the negative side of the rectifying unit 9 is set as a reference potential, the positive side is input to the positive terminal of the comparator 5, and the negative side of the reference voltage Vr <b> 1 is connected to the negative terminal of the comparator 5.

  The current flowing from the power supply unit 1 to the load unit 2 flows in the same way as the above-described bidirectional DC current, but the current direction is reversed according to the AC frequency. Accordingly, the voltage across the resistors R1 and R2 is also an AC voltage. The rectifiers 8 and 9 rectify and smooth the AC voltage at both ends of the resistors R1 and R2 to obtain a DC voltage, which is input to the AD board 3 and A / D converted to obtain a measured digital current value. The source and drain of the field effect transistor 4 are connected to both ends of the small current measuring resistor R2, and the output signal of the comparator 5 is input to the gate. The + side output voltage of the rectifying unit 9 is input to the + terminal of the comparator 5, and the − side of the rectifying unit 9 is connected to the reference potential. Further, the connection configuration is such that the reference voltage vr1 is applied between the negative terminal of the comparator 5 and the reference potential.

  The alternating current flowing from the power supply unit 1 to the load unit 2 flows to the series-connected resistors R1 and R2, and the AC voltage at both ends of the resistors R1 and R2 is converted into direct current by the rectifying units 8 and 9, and the rectified output of the rectifying unit 9 When the voltage exceeds the reference voltage Vr1, the field effect transistor 4 is turned on by the output signal of the comparator 5, and the voltage across the resistor R2 is clamped to the reference voltage Vr1. Thereby, there is an advantage that the power consumption by the resistor R2 for measuring a small current having a large resistance value can be reduced in the large current measuring range.

  FIG. 7 is an explanatory diagram of Embodiment 7 of the present invention. The same reference numerals as those in FIGS. 1 and 3 denote the same parts, and 10 denotes an operational amplifier. In addition, since the structure which outputs resistance value R1, R2 from which resistance value differs, and AD board 3 as a digital value is the same as that of Example 1 shown in FIG. 1, the overlapping description is abbreviate | omitted. In the seventh embodiment, the operational amplifier 10 detects the voltage across the resistor R1 having a small resistance value, and the comparator 5 compares it with the reference voltage Vr1. When a current in which the output signal of the operational amplifier 10 exceeds the reference voltage Vr1 flows through the resistor R1, power loss due to the resistor R1 having a large resistance value increases. At that time, the field effect transistor 4 is turned on by the comparator 5; Resistor R2 is bypassed to enter a large current measurement state. Thereby, the power loss at the time of measuring a large current can be reduced. When the current flowing through the resistor R1 decreases and the output signal of the operational amplifier 10 falls below the reference voltage Vr1, the voltage applied from the comparator 5 to the gate of the field effect transistor 4 becomes zero, and the field effect transistor 4 is turned off. Return to the state. That is, the state returns to the small current measurement state. Therefore, in the large current measurement state, the resistor R2 having a large resistance value for small current measurement is bypassed by the ON state of the field effect transistor 4 to reduce power loss, and in the small current measurement state, the field effect transistor 4 is turned off. As described above, measurement can be performed with high measurement accuracy using the resistor R2 having a large resistance value. If the current direction is opposite to the arrow direction shown in the figure, the absolute value circuit is connected to the input terminal of the operational amplifier 10 or the output terminal of the operational amplifier 10 so that the comparator 5 Therefore, the reference voltage Vr1 can be compared.

  FIG. 8 is an explanatory diagram of an eighth embodiment of the present invention, where 1 is a power supply unit, 2 is a load unit, 3 is an AD board, 4-1 and 4-2 are field effect transistors, and 5-1 and 5-2. Is a comparator, R1 is a resistor for measuring a large current, R2-1 and R2-2 are resistors for measuring a medium current and a small current, Vr1-1 and Vr1-2 are reference voltages, and 11 is an operational amplifier. The operational amplifier 11 detects the voltage across the resistor R1 for measuring a large current, inputs it to the comparators 5-1 and 5-2, and compares it with the reference voltages Vr1-1 and Vr1-2. In the measurement range, the field effect transistors 5-1 and 5-2 are kept off. When a current exceeding the small current measurement range flows and the medium current measurement range is set, the output signal of the operational amplifier 11 exceeds the reference voltage Vr1-2, so that the field effect transistor 4-2 is turned on by the comparator 5-2. The state and the current flowing through the resistor R2-2 having a large resistance value are bypassed. When a current exceeding the medium current measurement range flows, the output signal of the operational amplifier 11 exceeds the reference voltage Vr1-1. Therefore, the field effect transistor 4-1 is also turned on by the comparator 5-1, and the resistance having the maximum resistance value is set. It is possible to automatically shift to a configuration in which the field effect transistors 4-2 and 4-1 are turned on for the R2-2 and the medium resistance R2-1, and current measurement is performed by the resistance R1 having the minimum resistance value. . In this embodiment, similarly to the above-described embodiment, when the current direction is opposite to the direction of the arrow shown in the figure, the absolute value circuit is connected to the input terminal of the operational amplifier 11 or to the output terminal of the operational amplifier 11. Thus, regardless of the direction of the current, the comparators 5-1 and 5-2 can be configured to be comparable with the reference voltages Vr1-1 and Vr1-2.

It is explanatory drawing of Example 1 of this invention. It is explanatory drawing of Example 2 of this invention. It is explanatory drawing of Example 3 of this invention. It is explanatory drawing of Example 4 of this invention. It is explanatory drawing of Example 5 of this invention. It is explanatory drawing of Example 6 of this invention. It is explanatory drawing of Example 7 of this invention. It is explanatory drawing of Example 8 of this invention. It is explanatory drawing of a prior art example. It is explanatory drawing of a prior art example.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Power supply part 2 Load part 3 AD board 4,4-1,4-2 Field effect transistor 5,5-1,5-2 Comparator R1, R2, R2-1, R2-2 Resistor Vr1, Vr1-1, Vr1 -2 Reference voltage 10,11 Operational amplifier

Claims (3)

In a current measuring device including a plurality of resistors having different resistance values connected in series between a power supply unit and a load unit, and an AD board that digitally converts voltages at both ends of the resistors and outputs them as measured digital current values ,
Two series-connected field effect transistors connected in parallel to a resistor for measuring a small current having a large resistance value compared to a resistor for measuring a large current , each having a source commonly connected to a reference potential ;
An absolute value circuit that outputs an absolute value of a potential difference between both ends of the resistor for measuring a small current with a large resistance value ;
A comparator that compares the output of the absolute value circuit with a reference voltage connected to the reference potential, and turns on the two field-effect transistors connected in series when the output of the absolute value circuit exceeds the reference voltage And a current measuring device. 2. The rectifying unit according to claim 1, further comprising: a rectifying unit that rectifies and smoothes voltages at both ends of resistors having different resistance values connected in series between the power supply unit and the load unit and inputs the same to the AD board. Current measuring device. The current measuring apparatus according to claim 1, further comprising: a configuration in which an output signal of the comparator that turns on the field effect transistor is input to the AD board as a measurement range switching signal .

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