Circuit for measuring battery current

Abstract: A current measuring circuit for measuring discharging current and charging current of a battery (10), which is selectively connected to a load (20) and a charger (21). The current measuring circuit comprises a resistor (RA) connected in series with the battery having a sufficiently low resistance value as not to substantially restrain the output current from the battery and a differential amplifier (1b) for amplifying a potential difference between both ends of the resistor. By providing a switch (8, 9) for short-circuiting one input terminal and the other input terminal of the differential amplifier, an offset/drift voltage of the differential amplifier can be obtained. This offset/drift voltage is used for correcting the output voltage from the differential amplifier at the time of current measurement and, thereby, the discharging current and charging current of the battery can be measured accurately.

Description: The present invention relates to a current measuring circuit for measuring discharging current and charging current of a battery, which is divided into first and second parts and is selectively connectable to a load such as an electronic apparatus or the like and a battery charger. EP-A-0 216 662 describes a current measuring circuit for measuring discharging current and charging current of a battery comprising: means for permitting the battery to be selectively connected to a load or a charger; a measuring resistor for connection in series with said battery having a sufficiently low resistance value as not to substantially restrain the output current from said first battery; a first differential amplifier for amplifying a potential difference between both ends of said measuring resistor, said first differential amplifier comprising a first operational amplifier, a first resistor connected between one end of said measuring resistor and one input terminal of said first operational amplifier, a second resistor connected between the one input terminal and the output terminal of said first operational amplifier, and a third resistor connected between the other end of said measuring resistor and the other input terminal of said first operational amplifier; a fourth resistor connected between the other input terminal (+) of said first operational amplifier and ground; means for interconnecting the one input terminal (-) and the other input terminal (+) of said first operational amplifier via a variable resistor; a second differential amplifier for amplifying the potential difference between both ends of said measuring resistor, said second differential amplifier comprising a second operational amplifier, a further resistor connected between the other end of said measuring resistor and the one input terminal (+) of said second operational amplifier; and said first resistor being connected between the one end of said measuring resistor and the other input terminal (-) of said second operational amplifier and another further resistor being connected between the other input terminal (-) of said second operational amplifier and ground. Mobile terminals such as portable telephone sets are generally driven by a battery. Therefore, it is desirable to constantly measure the charge condition of the battery and to charge the battery when the residual capacity of the battery decreases. In such a battery as a nickel-cadmium battery, the voltage does not fall so much but the discharging current remarkably decreases when the residual capacity decreases. Accordingly, so-called residual capacity meters adapted to detect the discharging current of a battery to thereby measure the residual capacity of the battery have so far been in wide use. As a discharging current measuring circuit of the prior art, such a circuit, as disclosed for example in Japanese Laid-open Patent Publication No. 59-145485, in which a resistor is connected to the positive terminal of a battery and a voltage for detecting current is directly taken out from both ends of the resistor through an A/D converter has so far been used. With such a current measuring circuit, the current value I can be obtained by substituting the potential difference V between both ends of the resistor and the resistance value R in Ohm's law I = V/R. In the prior art current measuring circuit, the discharging current of the battery is restrained by the inserted resistor, and therefore, there have been problems with such prior art current measuring circuit that it is not suitable for measuring the discharging current of a battery supplying a large current and it is unable to measure a wide range of currents. There is also known such a type of current measuring circuit in which it is adapted such that the potential difference between both ends of a resistor for current detection is amplified by a differential amplifier before being output to an A/D converter. However, in correctly measuring the discharging current of a battery, an operational amplifier of an expensive low offset/low drift voltage type must be used in the differential amplifier. Hence, there has been a problem with this type that the current measuring circuit incurs an increase in cost. An object of the present invention is to provide a current measuring circuit capable of correctly measuring the battery discharging current and charging current with an economical structure not using expensive parts. In accordance with the present invention, there is provided a current measuring circuit for measuring discharging current and charging current of a battery divided into first and second parts and selectively connectable to a load or a charger; comprising: a measuring resistor for connection in series with said battery having a sufficiently low resistance value as not to substantially restrain the output current from said battery; a first differential amplifier for amplifying a potential difference between both ends of said measuring resistor, said first differential amplifier comprising a first operational amplifier, a first resistor connected between one end of said measuring resistor and one input terminal of said first operational amplifier, a second resistor connected between the one input terminal and the output terminal of said first operational amplifier, a third resistor connected between the other end of said measuring resistor and the other input terminal of said first operational amplifier, a fourth resistor connected between the other input terminal of said first operational amplifier and ground; the resistance value of said first resistor being equal to the resistance value of said third resistor and the resistance value of said second resistor being equal to the resistance value of said fourth resistor; means for interconnecting the one input terminal and the other input terminal of said first operational amplifier through the first and third resistors; calculating means for calculating the difference between an output voltage from said first operational amplifier when the one input terminal and the other input terminal of said first operational amplifier are interconnected and an output voltage from said first operational amplifier when the one input terminal of said first operational amplifier is connected with the one end of said measuring resistor and the other input terminal of said first operational amplifier is connected with the other end of said measuring resistor; and a second differential amplifier for amplifying the potential difference between both ends of said measuring resistor, said second differential amplifier comprising a second operational amplifier, a seventh resistor connected between the other end of said measuring resistor and one input terminal of said second operational amplifier, an eighth resistor connected between the one input terminal and the output terminal of said second operational amplifier, a ninth resistor connected between the one end of said measuring resistor and the other input terminal of said second operational amplifier, and a tenth resistor connected between the other input terminal of said second operational amplifier and ground, the resistance value of said seventh resistor being equal to the resistance value of said ninth resistor and the resistance value of said eighth resistor being equal to the resistance value of said tenth resistor, wherein the positive power-supply terminals of said first operational amplifier and of said second operational amplifier are connected with the positive voltage output terminal of the first battery part and the negative power-supply terminals of said first operational amplifier and of said second operational amplifier are grounded, wherein the negative voltage output terminal of the first battery part is connected to the one end of the measuring resistor and wherein the second battery part is connected in series with the measuring resistor at the other end thereof. By using the offset/drift voltage for correcting the output voltage from the first operational amplifier at the time of current measurement, it becomes possible to attain accurate measurement of the discharging current and charging current of the battery without using an expensive operational amplifier of a low offset/low drift type. Preferably, the current measuring circuit is provided with a series connection of a fifth resistor and a first switch connected in parallel with said second resistor, a series connection of a sixth resistor and a second switch connected in parallel with said fourth resistor, a series connection of an eleventh resistor and a third switch connected in parallel with said eighth resistor; and a series connection of a twelfth resistor and a fourth switch connected in parallel with said tenth resistor. Then, by closing the first and second switches, the gain of the first operational amplifier can be decreased and, hence, the amplification factor of the first differential amplifier can be made smaller. By switching the gains of the operational amplifier as described above, it becomes possible to achieve measurement of a wide range of currents from a small current to a large current. The above and other objects, features and advantages of the present invention and the manner of realizing them will become more apparent, and the invention itself will best be understood from a study of the following description and appended claims with reference to the attached drawings. In the drawings: FIG. 1 is an explanatory circuit diagram of a first current measuring circuit for measuring battery discharging/charging current; FIG. 2 is a circuit diagram of a second current measuring circuit for measuring battery discharging/ charging current; FIG. 3 is a circuit diagram of a third current measuring circuit for measuring battery discharging/ charging current; FIG. 4 is a circuit diagram of a fourth current measuring circuit for measuring battery discharging/charging current; FIG. 5 is a circuit diagram of a fifth current measuring circuit for measuring battery discharging/ charging current; FIG. 6 is a circuit diagram of a sixth current measuring circuit for measuring battery discharging/ charging current; FIG. 7 is a circuit diagram of a seventh current measuring circuit for measuring battery discharging/ charging current; FIG. 8 is a circuit diagram of an eighth current measuring circuit for measuring battery discharging/ charging current; FIG. 9 is a circuit diagram of a ninth current measuring circuit for measuring battery discharging/ charging current; FIG. 10 is a circuit diagram of a tenth current measuring circuit for measuring battery discharging/charging current; FIG. 11 is a circuit diagram of an eleventh current measuring circuit for measuring battery discharging/ charging current; and FIG. 12 is a circuit diagram of a current measuring circuit for measuring battery discharging/charging current according to an embodiment of the present invention. Referring first to FIG. 1, a first current measuring circuit will be described. Reference numeral 10 denotes a rechargeable battery such as a nickel-cadmium battery. By connecting a switch 22 to the side of a load 20 such as a portable telephone set, the load 20 comes to be driven by the battery 10. When the switch 22 is connected to the side of a battery charger 21, the battery 10 is charged. On the positive electrode side of the battery 10, there is connected a resistor R A in series with the battery. The resistor R A has a sufficiently low resistance value as not to virtually restrain the output current from the battery. Preferably, the resistance value of the resistor R A is set below 0.1Ω . When the switch 22 is connected to the side of the load 20 and the load is driven by the battery 10, a current I 1 flows in the direction of the arrow Y1 (discharging direction). On the other hand, when the switch 22 is connected to the side of the charger 21, the current I 1 flows in the direction of the arrow Y2 (charging direction) and the battery 10 is thereby charged. As the current I 1 flowing out of the battery 10 or flowing into the battery 10 flows through the resistor R A , there is generated a very small potential difference V 3 between both ends of the resistor R A and this potential difference V 3 is applied to both inputs of a differential amplifier 1a. The differential amplifier 1a comprises an operational amplifier 2. There are connected a first resistor R 1 between one input terminal (the inverting input terminal '-' in FIG. 1) of the operational amplifier 2 and one end of the resistor R A , a second resistor R 2 between the inverting input terminal '-' and the output terminal of the operational amplifier 2, a third resistor R 3 between the other input terminal (the noninverting input terminal '+' in FIG. 1) of the operational amplifier 2 and the other end of the resistor R A , and a fourth resistor R 4 between the noninverting input terminal '+' of the operational amplifier 2 and ground 3. When the resistance values of the first to fourth resistors R 1 to R 4 are so set as to be R 1 = R 3 and R 2 = R 4 in the arrangement of the differential amplifier 1a as described above, the gain of the operational amplifier 2 comes to be determined by R 2 /R 1 and this becomes the amplification factor of the differential amplifier 1a. Accordingly, by setting the resistance values of the resistors R 1 to R 4 at will under the conditions of R 1 = R 3 and R 2 = R 4 , the amplification factor of the differential amplifier 1a can be set as desired. The differential amplifier 1a amplifies the voltage V 3 applied thereto by the factor R 2 /R 1 and outputs the amplified voltage from its output terminal 12 as a voltage V 0 . The output voltage V 0 becomes a positive or a negative voltage depending on the direction of the current with respect to the battery 10. The voltage V 0 becomes positive when the current flows out of the battery 10 in the discharging direction (in the direction of the arrow Y1) and becomes negative when the current flows in the charging direction (in the direction of the arrow Y2). Thus, by measuring the voltage V 0 with a measuring apparatus, which is not shown, the discharging current or charging current of the battery 10 can be measured. However, since the output voltage V 0 becomes positive or negative, it is required to apply a bias voltage to the A/D converter circuit in the measuring apparatus for determining the positive or negative polarity of the voltage V 0 . In the case of the circuit configuration shown in FIG. 1, operating power-supply voltages are applied to the positive power-supply terminal 13 and the negative power-supply terminal 14 of the operational amplifier 2. As the positive power-supply voltage, a positive voltage higher than the sum of the voltage of the battery 10 and the voltage of the potential difference V 3 is required, and as the negative power-supply voltage, a negative voltage lower than - V 3 is required. Accordingly, the positive power-supply terminal 13 is connected with a positive power supply, not shown, to supply such a positive voltage and the negative power-supply terminal 14 is connected with a negative power supply, not shown, to supply such a negative voltage. When the load 20 such as a portable telephone set or the like is driven by the battery 10, the current I 1 flows in the direction of the arrow Y1 and thereby a very small potential difference V 3 is produced between both ends of the resistor R A . The very small potential difference V 3 is multiplied in the differential amplifier 1a by the factor R 2 /R 1 and output from the output terminal 12 as the voltage V 0 . Then, the discharging current I 1 of the battery 10 can be measured by measuring the output voltage V 0 . As the battery 10 is used up, the discharging current I 1 decreases, and therefore, by observing the discharging current, the timing for charging the battery 10 can be determined. When the battery 10 is used up, the switch 22 is connected to the side of the charger 21 and the battery 10 is charged. When the battery 10 is sufficiently charged, the charging current I 1 decreases and, hence, the output voltage V 0 of the differential amplifier 1a lowers. Therefore, the timing at which the battery 10 is fully charged can be determined by observing the output voltage V 0 . Referring to FIG. 2, there is shown a second current measuring circuit. The current measuring circuit in this embodiment is composed of two sets of the differential amplifiers la shown in FIG. 1. In order to distinguish between the two differential amplifiers, a single prime (') is attached to reference numerals of one differential amplifier. The second current measuring circuit is arranged by interconnecting differential amplifiers 1a and 1a'. One end of the resistor R 1 of the differential amplifier la is connected with one end of the resistor R 3 ' of the differential amplifier 1a', while one end of the resistor R 3 is connected with one end of the resistor R 1 '. Further, the negative power-supply terminals 14 and 14' of the operational amplifiers 2 and 2' are connected with ground 3. In the described arrangement, when the discharging current I 1 of the battery 10 flows through the resistor R A , a very small potential difference V 3 is generated between both ends of the resistor R A and the voltage V 3 of the potential difference is applied between both the input terminals of the differential amplifiers 1a and 1a'. The voltage V 3 is amplified by the factor R 2 /R 1 in the differential amplifier 1a and a positive voltage V 01 is output from the output terminal 12. At this time, the output voltage V 02 of the differential amplifier 1a' becomes 0 volt. On the other hand, when the battery 10 is charged by the charger 21, the voltage V 3 of the potential difference due to the charging current I 1 is generated between both ends of the resistor R A , and the voltage V 3 of the potential difference is applied between both the input terminals of the differential amplifiers 1a and 1a'. At this time, the output voltage V 01 of the differential amplifier la becomes 0 volt, while in the differential amplifier 1a', the voltage V 3 of the potential difference is amplified by the factor R 2 '/R 1 ' and this voltage is output from the output terminal 12' as a positive voltage V 02 . Thus, no matter whether in discharging direction or in charging direction the current may be flowing, the output voltage can be measured as a positive voltage. Hence, it becomes unnecessary to distinguish between positive and negative voltages as in the above described first circuit. Therefore, by measuring the positive voltage V 01 or V 02 with a measuring apparatus, the discharging current or charging current of the battery 10 can be measured. Since measurement of only positive voltage is made in the present circuit, negative power supplies as the operating power supplies for the operational amplifiers 2 and 2' are not required. That is, the negative power-supply voltage can be 0 V. Therefore, the negative power-supply terminals 14 and 14' of the operational amplifiers 2 and 2' can be connected to ground 3 as described above. Thus, the negative power supply can be eliminated and, hence, the power supply circuit can be simplified. Referring to FIG. 3, there is shown a third current measuring circuit. In this third circuit, the battery 10 in the first and second circuits is divided into two batteries 15 and 16 and the resistor R A is connected between the batteries 15 and 16 in series with them. The positive power-supply terminal 13 is connected with the positive side of the battery 16. As a current I 2 A 2 flowing out of or into the batteries 15 and 16 flows through the resistor R A , a very small potential difference V 4 is generated between both ends of the resistor R A . The potential difference V 4 is amplified by the differential amplifier 1a to be output therefrom as a positive or negative voltage V 03 . As mentioned herein-above, the positive power-supply voltage as the operating power supply for the operational amplifier 2 is only required to be a voltage higher than the sum of the voltage of the battery 15 and the voltage of the potential difference V 4 . Therefore, as shown in FIG. 3, the positive power-supply terminal 13 can be connected to the positive side of the battery 16 to thereby utilize the battery voltage. Thus, in the present circuit, a separate positive power supply can be eliminated and, hence, the power supply circuit can be simplified. Referring to FIG. 4, there is shown a fourth current measuring circuit arranged by adding a differential amplifier 1a' to the current measuring circuit shown in FIG. 3. More specifically, one end of a resistor R 3 ' of the differential amplifier 1a' is connected to one end of the resistor R 1 of the differential amplifier la and one end of a resistor R 1 ' is connected to one end of the resistor R 3 , and a positive power-supply terminal 13' of an operational amplifier 2' is connected to the positive side of the battery 16 the same as the positive power-supply terminal 13 of the operational amplifier 2. In operation of the third current measuring circuit, as in the second circuit shown in FIG. 2, a positive output voltage V 04 is output from the output terminal 12 of the differential amplifier 1a when a discharging current flows out of the batteries 15 and 16, while a positive output voltage V 05 is output from the output terminal 12' of the differential amplifier 1a' when the batteries 15 and 16 are charged by the charger 21. Therefore, the current flowing either in the discharging direction or in the charging direction can be measured as a positive voltage. Thus, the negative power-supply voltages for the operational amplifiers 2 and 2' can be 0 V and, hence, the negative power-supply terminals 14 and 14' can be connected to ground to eliminate the negative power supply. Further, since voltages of the batteries 15 and 16 can be utilized as the positive power supplies for the operational amplifiers 2 and 2', no positive and negative power-supply apparatuses are required to be additionally provided for driving the operational amplifiers 2 and 2'. Referring to FIG. 5, there is shown a fifth current measuring circuit arranged by adding a fifth resistor R 5 and a sixth resistor R 6 to the differential amplifier la shown in FIG. 1. More specifically, the fifth resistor R 5 is connected in parallel with the second resistor R 2 through a first switch 4 and the sixth resistor R 6 is connected in parallel with the fourth resistor R 4 through a second switch 5. Further, the first and second switches 4 and 5 are adapted to be turned on/off responding to a first control signal 6 output from a control means, not shown. When the first and second switches 4 and 5 are turned on responding to the first control signal 6 as indicated by the respective broken lines, the resistance value determining the gain of the operational amplifier 2 becomes {R 2 ·R 5 /(R 2 + R 5 )}/R 1 , which is smaller than the resistance value determining the gain R 2 /R 1 in the previous state. Thus, the gain of the operational amplifier 2 can be decreased and, hence, the amplification factor of the differential amplifier 1b can be made smaller. Accordingly, even when the current flowing through the resistor R A is a large current, it becomes possible, by turning on the first and second switches 4 and 5, to output the potential difference generated between both ends of the resistor R A after amplifying the same by a smaller amplification factor adjusted according to the magnitude of the large current. Thus, it becomes possible to bring the output voltage V 0 of the differential amplifier 1b to such a suitable voltage value as can be measured by a measuring means. That is, currents widely ranging from a large current to a small current can be measured by switching the first and second switches. The above described advantage can be equally obtained by the sixth to eighth circuits shown in FIG. 6 to FIG. 8. The sixth circuit shown in FIG. 6 is proved, from its comparison with the second circuit shown in FIG. 2, to be using differential amplifiers 1b and 1b' in place of the differential amplifiers 1a and 1a' in the second circuit. The seventh circuit shown in FIG. 7 is proved, from its comparison with the third circuit shown in FIG. 3, to be using a differential amplifier 1b in place of the differential amplifier la in the third circuit. Further, the eighth circuit shown in FIG. 8 is proved, from its comparison with the fourth circuit shown in FIG. 4, to be using differential amplifiers 1b and 1b' in place of the differential amplifiers 1a and 1a' in the fourth circuit. Since the sixth to eighth circuits can easily change the amplification factors of the differential amplifiers, they can make measurement of a wide range of currents. Operations and other advantages of the sixth to eighth circuits are equal to those of the second to fourth circuits shown in FIG. 2 to FIG. 4. Referring now to FIG. 9, a ninth current measuring circuit will be described. This current measuring circuit is arranged by using a third switch 8 and a fourth switch 9 in addition to the components making up the fifth circuit. More specifically, there is provided the third switch 8 which, responding to a second control signal 7, either connects one input terminal of the differential amplifier 1b (one end of the first resistor R 1 in FIG. 9) and one end of the resistor R A or isolates that input terminal of-the differential amplifier 1b and that end of the resistor R A and, in addition, short-circuits that input terminal and the other input terminal of the differential amplifier 1b (one end of the third resistor R 3 in FIG. 9). Further, there is provided the fourth switch 9, which similarly responding to the second control signal 7, either connects the other input terminal of the differential amplifier 1b and the other end of the resistor R A or isolates them. With the third and fourth switches 8 and 9 provided as described above, if both the input terminals of the operational amplifier 2 are short-circuited, an offset/drift voltage V 0 ' is output from the operational amplifier 2. In measurement of the discharging current and charging current of the battery 10, by correcting the voltage V 0 output from the differential amplifier 1b using the offset/drift voltage V 0 ', the current measurement can be performed more accurately. At the time of the measurement, the third switch 8 and the fourth switch 9 are switched to the sides indicated by the broken lines. Thereby, the resistor R A is isolated from the resistor R 1 and the resistor R 3 . In addition, the inverting input terminal (- ) and the non-inverting input terminal (+ ) are short-circuited (connected together) through the resistors R 1 and R 3 and, hence, the potential difference between both the input terminals becomes zero. The offset/drift voltage V 0 ' output from the operational amplifier 2 when both its input terminals are short-circuited is measured by and stored in a processing unit through an A/D converter circuit, which is not shown. Then, the third and fourth switches 8 and 9 are switched to the sides indicated by the solid lines and the output voltage V 0 of the operational amplifier 2 at this time is measured by the processing unit through the A/D converter circuit, and the difference between the thus measured voltage V 0 and the earlier stored voltage V 0 ' is obtained. This difference provides a proper current value after correction. By correcting the output voltage of the operational amplifier at the time of current measurement using the offset/drift voltage, a highly accurate current measuring circuit can be provided economically, without using an expensive operational amplifier of a low offset/low drift voltage type. Further, such a merit is obtained that power consumption in the current measuring circuit can be reduced by turning off the fourth switch 9 thereby isolating the other input terminal of the differential amplifier 1b from the other end of the resistor R A and cutting off the useless current flowing through the resistors R 3 and R 4 when the current measurement is not performed. FIG. 11 shows an eleventh circuit having the above-described advantages. This circuit has the arrangement of the seventh circuit shown in FIG. 7 with a third switch 8 and a fourth switch 9 added thereto. Also in this circuit, an offset/drift voltage V 03' is similarly measured and stored. Then, the output voltage V 03 of the differential amplifier 1b is measured and the difference between this measured voltage V 03 and the earlier stored voltage V 03 , is obtained. Thereby, highly accurate measurement of the discharging current or charging current can be achieved. Further, by turning off the fourth switch 9 when the current I 2 is not measured, power consumption in the current measuring circuit can be reduced. The above described two advantages obtained in the ninth and eleventh circuits are similarly obtained by the respective circuits shown in FIG. 10 and FIG. 12. The circuit shown in FIG. 10 has the arrangement of the sixth circuit shown in FIG. 6 with a third switch 8 and a fourth switch 9 added thereto. In Fig. 10, the third switch 8 and the fourth switch 9 are switched in response to a second control signal 7 to the side indicated by the broken lines, and an offset/drift voltage V 01 ' or V 02 ' then output from the differential amplifier 1b or 1b' is measured by and stored in a processing unit through an A/D converter circuit, not shown. Then, the third and fourth switches 8 and 9 are switched to the side indicated by the solid lines. The output voltage V 01 or V 02 at this time is measured by the processing unit through the A/D converter circuit and the difference between the thus measured voltage V 01 or V 02 and the earlier stored voltage V 01 ' or V 02 ' is obtained. Thus, highly accurate measurement of the discharging current or charging current of the battery 10 can be achieved and, further, power consumption in the current measuring circuit when no current is measured can be reduced by turning off the fourth switch 9. FIG. 12 shows an embodiment of the present invention. This embodiment has the arrangement of the eighth circuit shown in FIG. 8 with a third switch 8 and a fourth switch 9 added thereto. Also in this embodiment, an offset/drift voltage V 04 ' or V 05 ' is similarly measured and stored. Then output voltage V 04 or V 05 of the differential amplifier 1b or 1b' is measured and the difference between this measured output voltage V 04 or V 05 and the earlier stored voltage V 04 ' or V 05 ' is obtained. Thereby, highly accurate measurement of the discharging current or charging current can be achieved. Further, power consumption in the current measuring circuit when no current is measured can be reduced by turning off the fourth switch 9. Accordingly, the described embodiment is able to provide a current measuring circuit capable of measuring battery discharging current and charging current over a wide range from a small current to a large current.

Claims: A current measuring circuit for measuring discharging current and charging current of a battery (15,16) divided into first (16) and second (15) parts and selectively connectable to a load (20) or a charger (21); comprising: a measuring resistor (R A ) for connection in series with said battery (15,16) having a sufficiently low resistance value as not to substantially restrain the output current from said battery (15,16); and a first differential amplifier (1b) for amplifying a potential difference between both ends of said measuring resistor (R A ), said first differential amplifier comprising a first operational amplifier (2), a first resistor (R 1 ) connected between one end of said measuring resistor (R A ) and one input terminal (-) of said first operational amplifier (2), a second resistor (R 2 ) connected between the one input terminal (-) and the output terminal of said first operational amplifier (2), a third resistor (R 3 ) connected between the other end of said measuring resistor (R A ) and the other input terminal (+) of said first operational amplifier (2), a fourth resistor (R 4 ) connected between the other input terminal (+) of said first operational amplifier (2) and ground; the resistance value of said first resistor (R 1 ) being equal to the resistance value of said third resistor (R 3 ) and the resistance value of said second resistor (R 2 ) being equal to the resistance value of said fourth resistor (R 4 ); means (8) for interconnecting the one input terminal and the other input terminal of said first operational amplifier (2) through the first and third resistors (R 1 ,R 3 ); calculating means for calculating the difference between an output voltage from said first operational amplifier (2) when the one input terminal and the other input terminal of said first operational amplifier (2) are interconnected and an output voltage from said first operational amplifier when the one input terminal of said first operational amplifier is connected with the one end of said measuring resistor (R A ) and the other input terminal of said first operational amplifier is connected with the other end of said measuring resistor (R A ); and a second differential amplifier (1b') for amplifying the potential difference between both ends of said measuring resistor (R A ), said second differential amplifier comprising a second operational amplifier (2'), a seventh resistor (R 1' ) connected between the other end of said measuring resistor and one input terminal (-) of said second operational amplifier (2'), an eighth resistor (R 2' ) connected between the one input terminal (-) and the output terminal of said second operational amplifier (2'), a ninth resistor (R 3' ) connected between the one end of said measuring resistor (R A ) and the other input terminal (+) of said second operational amplifier (2'), and a tenth resistor (R 4' ) connected between the other input terminal (+) of said second operational amplifier (2') and ground, the resistance value of said seventh resistor (R 1' ) being equal to the resistance value of said ninth resistor (R 3' ) and the resistance value of said eighth resistor (R 2' ) being equal to the resistance value of said tenth resistor (R 4' ), wherein the positive power-supply terminals of said first operational amplifier (2) and of said second operational amplifier (2') are connected with the positive voltage output terminal of the first battery part (16) and the negative power-supply terminals of said first operational amplifier (2) and of said second operational amplifier (2') are grounded, wherein the negative voltage output terminal of the first battery part (16) is connected to the one end of the measuring resistor (R A ) and wherein the second battery part (15) is connected in series with the measuring resistor (R A ) at the other end thereof. A current measuring circuit according to claim 1, further comprising a series connection of a fifth resistor (R 5 ) and a first switch (4) connected in parallel with said second resistor (R 2 ); a series connection of a sixth resistor (R 6 ) and a second switch (5) connected in parallel with said fourth resistor (R 4 ); a series connection of an eleventh resistor (R 5' ) and a third switch (4') connected in parallel with said eighth resistor (R 2' ); and a series connection of a twelfth resistor (R 6' ) and a fourth switch (5') connected in parallel with said tenth resistor (R 4' ). A current measuring circuit according to Claim 1 or 2, wherein the resistance value of said measuring resistor (R A ) is below 0.1Ω.

Inventors: GOTO SEIJI, MURAYAMA YUKIO, TSUNODA KAZUAKI, GOTO, SEIJI, C/O FUJITSU KYUSHU DIGITAL, MURAYAMA, YUKIO, C/O FUJITSU LIMITED, TSUNODA, KAZUAKI, C/O FUJITSU KYUSHU DIGITAL
Applicants: FUJITSU LTD, FUJITSU LIMITED
Application year: 1992
Main classification: G01R31
IPC codes: G01R19, G01R15, G01R31