JPS6035274A - Current measuring circuit - Google Patents

Abstract

PURPOSE:To enables the measurement of fine current using a relatively long cable by connecting a detection resistor between the inversion input terminal and the output terminal of a differential amplifier, one end of an object to be measured to the inversion input terminal and the other end of the object to the output terminal through the power source. CONSTITUTION:A differential amplifier 17 with a high gain and non-saturable is used. The non-inversion input terminal is grounded and a detection resistor 13 is connected between the inversion input terminal and the output terminal thereof. One end of an object 11 to be measured is connected to the inversion input terminal while the other end of the object 11 is connected to the output terminal through the power source 12. With such an arrangement, with the feedback action through the detection resistor 13, the inversion input terminal has almost the ground potential. The current I1 flowing through the object 11 to be measured runs to the detection resistor 13 and the product RsI1 of the current I1 and the resistance value Rs of the detection resistor 13 is measured with a voltometer 18 to determine the current I1.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a current measuring circuit that measures a current flowing through an object to be measured by applying a voltage to the object.

<Prior art> As shown in FIG. 1, a voltage source 12 is connected to an object to be measured 11, the current generated by the voltage source 12 is passed through a detection resistor 13, and the voltage drop generated in the detection resistor 13 is collected by an amplifier. 14 and measuring the amplified output voltage with a voltmeter 15, the current flowing through the object to be measured 11 is measured.

If the object to be measured 11 is, for example, an insulation resistance, the current flowing through the object to be measured 11 will be minute, and in order to make this minute current a measurable voltage, the resistance value of the detection resistor 13 must be increased. This has no choice but to increase the input impedance of the amplifier 14. As the cable 16 connecting the detection resistor 13 and the amplifier 14 becomes longer, its stray capacitance with respect to the ground increases, and the time it takes for the output voltage, that is, the measured value, to stabilize becomes longer. For this reason, the cable 16 cannot be made long. On the other hand, it is also possible to measure small currents by converting them into voltages using parallel feedback amplifiers, but in this case a high-gain amplifier is required, and noise is also greatly amplified before being output. There were drawbacks that made it easy to accept. Furthermore, a high-resistance bridge circuit is sometimes used to measure high resistance, but this also has the drawback of being susceptible to stray capacitance due to its high resistance. As described above, in the past, the influence of stray capacitance was unavoidable, and it was not possible to connect a relatively long cable between the object to be measured and the measuring instrument.

<Summary of the Invention> An object of the present invention is to provide a current measurement circuit that allows a relatively long cable to be present between an object to be measured and a measuring device and is less susceptible to noise.

According to this invention, a detection resistor is connected between the inverting input terminal and the output terminal of a differential amplifier whose non-inverting input terminal is grounded, and one end of the object to be measured is connected to the inverting input terminal. Connect the other end of the object to be measured to the output terminal of the differential amplifier via a power supply.

<Embodiment> FIG. 2 shows an embodiment of the current measuring circuit according to the present invention,
A differential amplifier 17 that has a sufficiently high gain and does not cause saturation within the measurement range is used, the non-inverting input terminal of the differential amplifier 17 is grounded, and a detection resistor is connected between the inverting input terminal and the output terminal. 13 are connected. One end of the device under test 11 is connected to the inverting input terminal of the differential amplifier 17, and the other end of the device under test 11 is connected to the output terminal of the differential amplifier 17 via the power supply 12. A voltmeter 18 is connected to the output terminal of the differential amplifier 17.

With this configuration, the negative feedback effect through the detection resistor 13 causes the inverting input terminal of the amplifier 17 to be at the V ground potential. Since the input impedance of the differential amplifier 17 is sufficiently high, the current 11 flowing through the device under test 11 flows to the detection resistor 13, and the product R511 of this current 11 and the resistance value Rs of the detection resistor]3 is measured by the voltmeter 18. By measuring the stain flow at 11
can be used.

In this way, the input terminal (inverting input terminal) of the differential amplifier 17
is always at the y ground potential, and the impedance seen from the cable 16 connecting the device under test 11 and the differential amplifier 17 to the amplifier 17 side is the resistance value Rs of the detection resistor 13.
is divided by the gain G1 of the amplifier 17, resulting in the value Rs/G1,
If G1 is sufficiently large, the time constant between this Rs /Q 1 and the stray capacitance of the cable 16 will become small, and therefore the length of the cable 16 will become long, so even if the stray capacitance of the cable 16 to ground is large. , Since the current required to charge this stray capacitance is extremely small and is charged in a short time, even if the resistance value Rs is significantly increased to measure a minute current, after applying a voltage to the object to be measured 11, Since the measurement output becomes stable immediately, measurement operations can be performed at high speed. Furthermore, even if noise is included in the power supply 12 or inductive noise such as commercial power supply voltage is input to the device under test 11, these noises will be absorbed by the negative feedback of the amplifier 17, which includes the power supply 12, the device under test 11, and the cable 16. It is inserted into the circuit, has a gain of 1 against noise (no amplification effect on noise), and can be measured without being affected by noise.

FIG. 3 shows an example of the present invention configured as a bridge circuit. In FIG. 3, parts corresponding to those in FIG. 2 are given the same reference numerals, and in addition to the configuration shown in FIG.
A series circuit of comparison resistors 19 and 21 is connected between both ends of the series circuit of comparison resistors 19 and 13. The connection point of the comparison resistor 19.21 is connected to the input side of the detection comparison amplifier 22, and the voltmeter 18 is connected to the output side of the amplifier 22.

In this configuration, the object to be measured 11 and the resistor 13°19.2
1 constitute each arm of the bridge circuit, and as in the case of FIG. 2, the inverting input terminal of the amplifier 17 is brought to the ground potential by the action of the differential amplifier 17 and the detection resistor 13. One input side of the detection comparison amplifier 22 is grounded, and therefore the detection comparison amplifier 22 is isometrically connected between a pair of diagonal points of the bridge circuit, and the unbalanced component of the bridge circuit is is amplified by the amplifier 22.

It will be easily understood that the same effects as in the case shown in FIG. 2 can be obtained in this case as well.

When measuring with the configurations shown in FIGS. 2 and 3, noise will be generated if the object to be measured 11 is replaced while the power is applied. Therefore, when replacing the object to be measured 11, it is preferable to configure the device so that the power supply can be controlled so as to stop applying the power. In addition, when creating the DC voltage source 12 from the output of the DC voltage source 12, when one end of the DC voltage source 12 fluctuates, the other end fluctuates in the same way, and the voltage between both ends of the DC voltage source 12 is always maintained at a constant value. There is a need. From these points of view, it is preferable to use the power source 12 as shown in FIG. 4, for example. That is, a circuit 25 is provided between both ends 23 and 24 of the power supply 12 to obtain a voltage proportional to the difference voltage V b -V a by inputting the terminal voltages Va and Vb. A summing amplifier 26 adds a voltage proportional to the differential voltage vb-va and an input voltage ``o'' corresponding to the desired voltage ``1'' between the terminals 23 and 24, and supplies the output to the terminal 24. In the circuit 25, for example, terminal 2
3 is connected to an inverting amplifier 27 having a gain of 1/n. That is, the terminal 23 is connected to the inverting input terminal of an operational amplifier 29 through a resistor 28 with a resistance value of nR, the non-inverting input terminal of the operational amplifier 29 is grounded, and a feedback terminal with a resistance value of R is connected between the output terminal and the inverting input terminal. A resistor 31 is connected. The output of this inverting amplifier 27 and the voltage at the terminal 24 have a gain of 1/n.
is supplied to the differential amplifier 32. That is, the output side of the inverting amplifier 27 is connected to the operational amplifier 34 through a resistor 33 having a resistance value R.
This non-inverting input terminal is connected to the terminal 24 through a resistor 35 having a resistance value nR, and a resistance value R is connected between the output terminal and the inverting input terminal of the operational amplifier 34.
A feedback resistor 36 is connected, and this inverting input terminal is connected to the terminal 23 through a resistor 37 having a resistance value nR. The output of this differential amplifier 32 is (V b −2V a )
/n.

The output terminal of the operational amplifier 34 is connected to the inverting input terminal of the summing amplifier 26 through a resistor 38 having a resistance value R, and this inverting input terminal is connected to the terminal 24 through a resistor 39 having a resistance value I]R. It is connected to the control input terminal 42 through a resistor 41 with a value of R/2, and is connected to the amplifier 26.
The output terminal of is connected to terminal 24. The non-inverting input terminal of summing amplifier 26 is grounded. The gain of the amplifier 26 is A1
Letting the input voltage of the control input terminal 42 be Vo, respectively,
The following equation holds between the input and output of the summing amplifier 26.

The output vb becomes ■. A ) ) When set to n, V b −V
a = n Vo = V 1, and terminal 23.2
The voltage between terminals 23 and 24 (■1) is n times the control input voltage (■0), and if the control input voltage VO is set to zero, the voltage between terminals 23 and 24 becomes v1-0, and the terminals 42 are connected to Vo and O. By controlling the voltage between the terminals 23 and 24, it is possible to turn the voltage between the terminals 23 and 24 OFF. Moreover, since feedback control is performed so that the sum of the output of the circuit 25 and the input of the terminal 42 becomes zero, the voltage between the terminals 23 and 24 is always held at nVo. Note that n is an attenuation coefficient for preventing saturation of the differential amplifier 32. Terminal 23 is connected to resistor 13.2 in FIG.
1, and the terminal 24 is connected to the connection point between the object to be measured 11 and the resistor 19. As the circuit 25, (V
Other types may be used as long as they provide an output proportional to b-Va).

In FIG. 3, for example, the ON/OFF control of the power supply 12 is @
When controlled as shown in FIG. 5A, the detection comparison amplifier 22 generates an output depending on the resistance value of the object to be measured 11, for example, when the power supply 12 is turned on, as shown in FIG. 5B. In FIG. 5B, the output of the bridge circuit indicates whether it is greater than or smaller than the threshold value Vt. However, the object to be measured 1
If the shielding for 1 is insufficient, commercial power supply power may be induced, and large noise may be input as shown in FIG. 5C, for example. In this case, the output of the bridge circuit is the sum of B and C in Fig. 5, as shown in Fig. 5, and when this is compared with the threshold value Vt, as shown in Fig. 5E, it is correct. If pl is smaller than the threshold value Vt, it may be judged to be larger than the threshold value, and vice versa, making it impossible to obtain a correct judgment output. In such a case, for example, the method shown in FIG. 6 may be used.

In FIG. 6, parts corresponding to those in FIG. 3 are given the same reference numerals. A switch 43 is inserted in series with the power supply 12,
A switch 44 is connected in parallel with the power supply 12 and the switch 43, and the switches 43 and 44 are controlled to be turned on and off in reverse by a control signal from the terminal 42. The output of the bridge circuit when the voltage of the power supply 12 is not applied to the object to be measured 11 is stored in the memory circuit 45. That is, the connection point between the resistors 19 and 21 is connected to one end of the capacitor 48 through the buffer circuit 46 and further through the switch 47, the other end of the capacitor 48 is grounded, and the connection point between the switch 47 and the capacitor 48 is connected to the input of the buffer circuit 49. connected to the side. Power supply 1
The comparator 51 compares the output of the bridge circuit when voltage No. 2 is applied to the object under test 11 and the value stored in the storage circuit 45 . That is, the connection point of resistor 19.21 is connected to the non-inverting input terminal of comparator 51, and the output terminal of buffer circuit 49 is connected to the inverting input terminal of comparator 510. switch 4
7 is controlled by the control signal at the terminal 42, and the switch 44
When the switch 47 is off, the switch 47 is also turned off.

In this way, before the voltage is applied to the object to be measured 11, that is, the switch 43 is off, the switches 4, 4, 47
When the switch 47 is turned on, the noise obtained at the output of the bridge circuit is charged in the capacitor 48, and when a voltage is applied to the object under test 11, the switch 47 is turned off, and the noise is stored in the capacitor 48 just before that. The measured noise value and the measured output of the bridge circuit are compared by a comparator 51, and the noises superimposed on the measured output are canceled by each other, and a correct judgment output is obtained.

The stored value of the memory circuit 45 becomes as shown in FIG. 5F for the noise shown in FIG. 5C, and the output of the comparator 51 at this time is
The judgment result when there is no noise (5th
The same result as in Figure B) is obtained.

When the resistance values of the resistors 19.21 are Ra and Rb, and the resistance value of the object to be measured 11 is Rx, the output of the bridge circuit (voltage at the connection point of the resistors 19.21) Vd is as follows. ■1=vb-va Inductive noise current 1 at the connection point between the object to be measured 11 and the resistor 13
If n=Fsinωt is input, the noise voltage Vn
is V n, = F'5ln (IJ t-Rs (2). When determining the measured output using the ground potential as a threshold, Vd + Vn = 0 (31) is the determination condition. This equation (3) When expressed in terms of Rx, it becomes as follows.The phase of the noise when a voltage is applied to the object under test 11 is to, and the phase of the noise when the noise is stored in the memory circuit 45 immediately before that is tl, and these phases t.

, tx, the noise VnO value is Vn O+Vn'', the judgment condition in FIG. By bringing to closer to each other, syl sin ωt 1-sin ωt o I <<
It can be set to 1. The noise reduction rate at this time is: Since l5lnωtl is 1 at most, = I ωt
1-sinωt o I (becomes 81.ω-2π×5
If the interval between 0, tl and to is 1ms, the maximum value of the noise reduction rate is = Sm2πX50X10 :r-0,3
The minimum value of pl that is 1 is the value that satisfies S stone ωt 1 = sin ωtO, and K = 0. Therefore, if the measurement voltage is applied for 1 ms, 501
4z noise can be reduced to a maximum of 0.31 times or less.

Note that the switches 43 and 44 in FIG. 6 may be omitted, and the power source 12 shown in FIG. 4 may be used, and the switch 47 may be turned off when the voltage VO is applied to the terminal 42. Also, the idea shown in Fig. 6 is not only applicable to bridge circuit measurements, but also in general, when the power supply voltage is applied, the measurement output is obtained at the output terminal, and when the power supply voltage is not applied, the measurement output is obtained at the output terminal. This can be applied to measuring instruments that do not produce a constant output, but in which noise is output to the output terminal regardless of whether or not a power supply voltage is applied.

<Effects> As described above, according to the present invention, minute currents can be measured even with a relatively long cable, and can be measured at high speed, allowing accurate measurement without being affected by noise. can do.

[Brief explanation of drawings]

FIG. 1 is a circuit diagram showing a conventional current measurement circuit, FIG. 2 is a connection diagram showing an example of a current measurement circuit according to the present invention, and FIG. 3 is a connection diagram showing an example of applying the present invention to a bridge circuit.
FIG. 4 is a contact diagram showing an example of the power supply 12, FIG. 5 is a waveform diagram showing the influence of noise, and FIG. 6 is a connection diagram showing an example of the present invention in which the influence of noise is removed. 11: Object to be measured, 12: Power supply, 13: Detection resistor, 1
7: Differential amplifier. Patent Applicant Toa Denpa Kogyo Co., Ltd. Agent Takuo Kusano 1 Figure 172 Figure Left 3 Figure - Shore 4 Figure 25 Figure 5

Claims (1)

[Claims] (il) The non-inverting input terminal of the difference amplifier is grounded, a detection resistor is connected between the inverting input terminal and the output terminal, one end of the object to be measured is connected to the inverting input terminal of the differential amplifier, and the The other end of the object to be measured is a current measuring circuit connected to the output terminal of the differential amplifier through a power supply.