JPS61112969A - Current measuring circuit - Google Patents

Abstract

PURPOSE:To measure the resistance value of a resistor to be measured or the current flowed to said resistor at a high speed, by using two detection resistors before and behind the resistor to be measured and taking out detection voltages obtained from said resistors in inverse polarity to differentially add the same by a differential amplifier 53 while mutually negating noise. CONSTITUTION:A current is flowed to a resistor 12 to be measured from a power source 11 corresponding to the resistance value to be measured thereof and detection voltages corresponding to the flowed current are respectively generated in detection resistors 22, 24 to be amplified by amplifiers 27, 28. The output of the amplifier 27 is supplied to a differential amplifier 53 by an isolation amplifier 52 while the level based on the high potential side of the power source up to that time is converted to the level based on earthing. In this case, because the detection voltages obtained in the detection resistors 22, 24 are obtained in the output sides of the amplifiers 27, 28 as amplified outputs in mutually inverse polarity, the output of the differential amplifier 53 comes to the added one of the outputs of the amplifiers 27, 28 and corresponds to the current flowed to the resistor 12 to be measured to be indicated on an indicator 54.

Description

DETAILED DESCRIPTION OF THE INVENTION "Field of Industrial Application" The present invention relates to a current measuring circuit that measures the total current flowing through a resistor of a high resistor.

``Prior Art'' Conventionally, as a method for measuring particularly high resistance, such as insulation resistance, there is a voltage-current method in which a voltage is applied to the resistance to be measured and the current flowing at that time is measured, and a bridge circuit including the resistance to be measured is balanced. There is a zero position method that measures the resistance when For high-speed measurements, the voltage-current method was used. This is because the zero position method using a bridge will constitute a υ bridge due to the high resistance due to the nature of the bridge circuit.
This is because parasitic capacitance is inevitably generated due to high resistance, and even if the capacitance is small, the time constant becomes large because the resistor is high, making it difficult to perform measurements at high speed.

Both the voltage-current method and the zero-position method deal with minute currents and are therefore greatly affected by inductive noise. For this reason, the conventional insulation meter was constructed as shown in FIG. 3. That is, power is supplied from the power supply 11 to one end of the resistor 12 to be measured! E is applied, the bent end of the resistor to be measured 12 is grounded through the detection resistor 13, and the detected voltage obtained on the resistor to be measured 12 side of the small resistor 13 is amplified by the amplifier 14 and sent to the indicator 15. supply Based on the instruction from the indicator 15, the value of the current flowing through the resistor 12 to be measured is known, and the resistance (K) of the resistor 12 to be measured is known.
I know. In this case, a shield 17 is placed over the lead wire 16 between the resistance to be measured 12 and the detection resistor 13, and the shield 17 is
7 was grounded to prevent external induction noise from being applied to the lead wire 16. However, it is difficult to achieve complete shielding by the shield 17, and depending on the measurement system, it may not be possible to provide substantial shielding. For this reason, conventional measurement circuits are susceptible to noise.

Furthermore, to discuss the general problem of resistance under test, the insulation resistance of the cable that connects the measuring instrument and the resistance under test is about the same resistance value as the resistance under test, for example, the insulation resistance. This results in a parallel circuit between the resistance to be measured and the insulation resistance of the cable.

In addition, due to its structure, the resistor under test always has parasitic capacitance, and even if the parasitic capacitance is small, the time constant of charging and discharging for the parasitic capacitance is It will be fleeting and long. Further, a current (absorption current) due to a chemical change flows through the resistance to be measured, and this current exhibits a characteristic of gradually decreasing over time.

Due to these problems, according to the current-voltage method described earlier, the current flowing through the connected cable is measured in addition to the current flowing through the resistor being measured, and the leakage current of the cable is also measured, resulting in a measurement current that is less than the leakage current of the cable. It cannot be measured. In addition, the measurement time can be shortened by quickly charging the capacitance parasitic to the resistance being measured, but in general, in ammeters that measure minute currents, one resistor of the detection resistor for current detection is If the resistance value of the detection resistor is made small, the detected voltage (voltage drop across the detection resistor) will be small, and the resistance value of the detection resistor cannot be set to a small value due to the noise limit of the amplifier. Therefore, if the detection resistor serves as a charging resistance for the parasitic capacitance, high-speed measurement becomes difficult.

The purpose of this invention is to provide a current measurement circuit that can measure the current flowing through a high resistance at high speed even when a capacitance is loaded in parallel with the resistance, and is not easily affected by external noise such as induced hum. It is something to do.

"Means for solving the problem" According to the present invention, first and second detection resistors are inserted in series on the current input side and the current output side of the resistance to be measured, respectively,
Each detection voltage obtained at the first and second detection resistors is
, are amplified by the second amplifier, and the amplified outputs thereof are of opposite polarity. Buffer circuits each having a gain of 1 are connected to the measured resistance sides of the first and second detection resistors, and between the output sides of the first and second amplifiers and the output sides of the second and first buffer circuits. A first and second resistance divider circuit is connected to each of the first and second resistor divider circuits. The division ratios of these resistance divider circuits are the same. First and second feedback amplifiers are connected between each dividing point of the first and second resistance dividing circuits and the resistance-to-be-measured sides of the first and second detection resistors, respectively.

Because of this configuration, the resistance value of the resistor to be measured is apparently small, and if there is a large amount of parasitic capacitance, the first
, a correspondingly large current flows through the second detection resistor, and in response to this, the output of the amplifier is fed back through the feedback amplifier, causing a large current to flow through the resistance to be measured, and if there is a parasitic capacitance, the parasitic capacitance is rapidly charged. It will be done. Conversely, when moving to the next measurement and changing the measurement resistance, the voltage charged in the parasitic capacitance up to that point may be large, or
Alternatively, if the capacitance of the measuring resistor is charged for some reason and the voltage is high, a current flows in the opposite direction through the feedback amplifier and discharge occurs. In this way, even if a resistor to be measured has a high resistance value and a parasitic capacitance exists, the parasitic capacitance can be charged and discharged at high speed, and measurement can be performed at high speed.

In order to determine whether the resistance value of the resistor to be measured is greater than or equal to a predetermined value, or whether the current flowing through the resistor to be measured is less than or equal to a predetermined value, the voltage at the dividing point of the first and second resistance dividing circuits is determined. It is possible to determine whether the magnitude of the current flowing through the resistor to be measured is greater than or equal to a predetermined value based on whether the difference is greater than or equal to a predetermined value. When measuring the current flowing through the shorted resistor to be measured, the feedback circuits of the first and second feedback amplifiers are cut off when the current flowing through the resistor to be measured is in a steady state, and in this state, the first and second feedback amplifiers are cut off. Taking out the output of the second amplifier differentially,
The output can be measured with an indicator.

In this way, the measurement of whether the current value is higher than the standard or the absolute measurement of the magnitude of the current is performed by differentially extracting it.
Even if external force or induced noise is superimposed on the current flowing through the first and second detection resistors, this is in the same phase, so when the above-mentioned differential output is taken out, there is a risk that they will be canceled and affected by this noise. Not in.

"Embodiment" Next, an embodiment of the current measuring circuit according to the present invention will be described with reference to FIG. The resistor 12 to be measured is connected to one end of a detection resistor 22 through a lead wire 21, and the curved end of the detection resistor 22 is connected to one end of the measurement power supply 11, which is the positive side in this example. A curved end of the resistor to be measured 12 is connected to one end of a detection resistor 24 through a lead wire 23, and a curved end of the detection resistor 24 is connected to a curved end (negative side) of the power supply 11. Lead wire 21
A connection point 25 between the lead wire 23 and the detection resistor 22 is a current inflow side of the resistance to be measured 12, and a connection point 26 between the lead wire 23 and the detection resistor 24 is a current outflow side of the resistance to be measured 12.

The voltages (falling electrons) detected by the detection resistors 22 and 24 are amplified by amplifiers 27 and 28, respectively, so that detection voltages with opposite polarities are obtained as their amplified outputs. That is, the connection point 25 side of the detection resistor 22 is connected to the inverting input side of the amplifier 27, and the curved end is connected to the non-inverting input side. - The connection point 26 side of the force sensing resistor 24 is connected to the inverting input side of the amplifier 28, and the curved end is connected to the non-inverting input side. The current passing through the detection resistors 22 and 24 based on the current flowing through the resistor 12 to be measured is as shown by the arrows. In the amplifier 27, the non-inverting input side is the positive side, and in the amplifier 28, the inverting input side is the positive side. , so the outputs of amplifiers 27 and 28 have opposite polarities. Note that the resistances of the detection resistors 22 and 24 should be equal to each other. Further, the negative side of the power supply 11 is grounded.

The inputs (111) of buffer circuits 31 and 32 each having a gain of 1 are connected to the connection points 25 and 26.The output side of the amplifier 27 is connected to the output I11 of the buffer circuit 32 through a resistance divider circuit 35 consisting of resistors 33 and 34. The output side of the amplifier 28 is connected to the output side of the buffer circuit 31 through a resistor divider circuit 38f consisting of resistors 36 and 37. The connection point of the resistors 33 and 34, that is, the resistor divider circuit 35 is connected to the connection point 25 through a feedback amplifier 41 and, if necessary, a feedback resistor 42 and a switch 43 in series.Furthermore, the dividing point 44 of the resistance divider circuit 38 is connected to the feedback amplifier 45, if necessary. feedback resistor 46,
It is connected to the connection point 26 through a series circuit of switches 47 .

Furthermore, in this embodiment, the leads 21,23 are each provided with a shield 48,49. The positive side of power supply 11 is connected to shield 48 through a unity gain buffer amplifier 51. The output side of the amplifier 27 and the output side of the buffer amplifier 51 are respectively connected to the non-inverting input side and the inverting input side of an isolation amplifier 52 for electrical isolation. The output side of isolation amplifier 52 is connected to the non-inverting input side of differential amplifier 53. Differential amplifier 5
30 inverting input side is connected to the output side of amplifier 28 and the output side is connected to indicator 54.

Further, the dividing point 39 of the resistance dividing circuit 35 is connected to the non-inverting input side of the isolation amplifier 55, the inverting input side of the isolation amplifier 55 is connected to the output side of the buffer circuit 31, and the output side of the differential amplifier 56 is connected to the non-inverting input side of the isolation amplifier 55. Connected to the non-inverting input side. The dividing point 44 of the resistor-split path 38 is connected to the non-inverting input side of the amplifier 57, and the output side of the buffer circuit 32 is connected to the inverting input side of the amplifier 57.
The output side of 7 is connected to the inverting input side of differential amplifier 56. The output of the differential amplifier 56 is supplied to a comparator 58, and the measurement result relative to the reference value is outputted to a terminal 59.

The common potential point of the Hiko amplifiers 27, 41, 51, 52, 55 and the buffer circuit 31 is on the positive side of the power supply 11, the high potential side of the power supply 11, and these high potential points 61 are connected to the shield 4.
Connected to 8. Further, the shield 49 is grounded.

In this configuration, a current flows from the power supply 11 to the resistance to be measured 12 according to the resistance to be measured, this current flows to the detection resistors 22, 24, and flows to the resistance to be measured 12 in these detection resistors 22, 24. Detection voltages corresponding to the currents are generated, and these detection voltages are each amplified 527.
．． The signal is amplified at 28. The isolation amplifier 52 converts the output of the amplifier 27 from a level based on the high potential side of the power supply 11 to a level based on the ground, and supplies the level to the differential amplifier 53 . In this case, each detection voltage obtained at the detection resistor 22.24 is applied to the amplifier 27.2.
8 are obtained as amplified outputs with opposite polarities, the output of the differential amplifier 53 is the sum of the outputs of the amplifiers 27 and 28, and the output of the amplifier 53 is the amplified output of the resistor 1 to be measured.
2, this is indicated by the indicator 54.

In FIG. 1, the detection voltage amplification part of the detection resistor 22 and the detection voltage amplification part of the detector 24 are similar circuits.For example, only the part that amplifies the detection voltage of the detection resistor 22 is taken out. If you look at it, it will look like the one shown in Figure 2. That is, the opposite side of the resistor 12 to be measured from the detection resistor 22 is grounded,
! 1: The amplifier 27 is grounded through the resistor divider circuit 35. The voltage of the telephone 11 is connected to the E1 resistor to be measured 12.
, the detection resistor 22, and the resistors fti of the resistors 33 and 34.
All RX.

When R3+Ra+Rb and the gain of the amplifier 27 are G, the voltage EX obtained at the resistor 12 to be measured is and the voltage ES obtained at the resistor 34 is. Therefore, if the voltage Ex obtained at the connection point 25 and the voltage E5 obtained at the division point 39 are equal, no current will flow through the feedback amplifier 41. The resistance value Rx of the resistor to be measured 12 is smaller than Rx in this equilibrium state, and the voltage Ex at the connection point 25 is lower than the voltage at the dividing point 39.
When the voltage E5 becomes smaller than the voltage E5, current is supplied to the resistor 12 through the feedback amplifier 41. Conversely, when the measured resistance value Rx becomes larger than EX in the balanced state, EX becomes larger than R5, and current flows backward through the feedback amplifier 41 to the division point 39 side. That is, in this circuit, the dividing point 39 and the connecting point 25 constitute a pair of diagonals of the bridge, forming a kind of bridge circuit, and when the condition that EX and ES increase equally is removed, the bridge becomes unbalanced. A current flows through the feedback amplifier 41, and the magnitude of this unbalance can be detected from the voltage at the dividing point 39. In other words, if the voltage E at the dividing point 39 is greater than the voltage 1ilx at the connection point 25, the resistance to be measured 1
The resistance value Rx of 2 is smaller than the value determined by the balance condition of the bridge. Therefore, the measured resistance value RX% depends on whether the reference voltage EES is high or low. ll Measured current (
It is possible to measure the magnitude of the current (current flowing through the resistance to be measured 12).

In the example of FIG. 1, the voltage ES at the dividing point 39 and the connection point 2
5 and the voltage EX (output of the buffer circuit 31) is amplified by the isolation amplifier 55, and its reference potential point is lowered to the ground potential and supplied to the differential amplifier 56. Similarly to TFC, the voltage difference between the division point 44 and the connection point 26 is amplified by the amplifier 57 and supplied to the differential amplifier 56. In this case the gains of amplifiers 55, 57 are made equal;
Moreover, the amplified outputs have opposite polarities. The absolute directness of the potential difference between the connection point 25 and the division point 39 and the absolute directness of the potential difference between the connection point 26 and the division point 44 are equal but have opposite polarities, so these potential differences are added at the output side of the differential amplifier 56. The output voltage is outputted as a signal having a larger magnitude relationship with respect to the grid reference, and this output is compared with, for example, a ground potential in a comparator 58. Resistance value Rx of resistor to be measured 12
is smaller than the reference value and the current value is larger than the reference current value, the output of the comparator 58 is high level υ, and if the resistance value Rx is larger than the reference value and the current is smaller than the reference value, the output of the comparator 58 is The output is at Lee's level. In this way, it is possible to determine the magnitude relative to the reference value.

"Effects of the Invention" As mentioned above, in the explanation of FIG.
If the current flowing through the resistor 12 is larger than the reference value, the current is supplied to the resistor 12 through the feedback amplifier 41. Therefore, if a relatively large parasitic capacitance exists in the resistor to be measured 12, the resistor to be measured RX will appear small, and the current flowing through the trench detection resistor 22 will exert a large force. A current is supplied to the resistance to be measured 12 through the feedback amplifier 41, and the parasitic capacitance is rapidly charged.

For example, when the capacitance of the lead wires 21 and 23 is charged with a high voltage, and the resistance to be measured 12 is connected, the potential at the connection point 25 becomes higher than the potential at the dividing point 39, and the resistance to be measured becomes higher. 12, lead wire 21, etc., flows back through the feedback amplifier 41 and rapidly absorbs the charge. Similarly, in the detection resistor 241u11, a current flows through the feedback amplifier 45 due to the potential relationship between the connection point 26 and the dividing point 4'4, and the parasitic capacitance of the resistor 12 to be measured is rapidly charged and discharged. Therefore, measurements can be made that much faster.

The gain of the amplifier 27 is ff1G, and the gain of the feedback amplifier 41 is 0.
1. If the resistance of the feedback resistor 40 is flijkRf,
This charging resistance value total detection resistor 22 is connected to the charging resistor.
For example, the resistance value of 'T-! By doing so, charging and discharging the parasitic capacitance of the resistance to be measured 12 through the feedback amplifiers 41 and 45 is more efficient than when charging and discharging the parasitic capacitance of the resistor under test 12 through the detection resistors 22 and 24. can also be done rapidly.

After the currents for charging and discharging are stabilized, the switch 43.47 is turned off, and when the output of the amplifier 53 determines that the value of the current flowing through the resistor 12 to be measured has been reached, the switch 43.47 is manually turned off. , or the stabilization is detected, the switches 43 and 47 are automatically turned off, and the resistance r of the resistor to be measured 12 or the current flowing through it is measured from the output of the differential amplifier 53 at that time.

Two detection resistors 22, 24'ff: are used, and the detection voltages obtained from these are taken out with opposite polarities and added differentially in the differential amplifier 53, or in the case of comparative measurement, .26 and the dividing point 39.44 are differentially added by a differential amplifier 56. Since the noises induced in the lead wires 21 and 23 are in the same phase, these noises appear in the same phase at the output side of the amplifiers 27 and 28, so in each differential amplifier 53 and 56, these noises are canceled by each other and the difference is generated. dynamic amplifier 53. ．． No noise appears on the output side of 56.

Toe 9 Not affected by external induced noise.

Furthermore, in this example, the output of the buffer amplifier 51 is shielded 4.
B to maintain the potential of the shield 48 on the high voltage side of the power supply 11, and the shield 49 is grounded, so the shield 48
Even if there is a leakage current flowing from the shield 49 to the shield 49, this leakage current is supplied from the buffer amplifier 51 and does not flow to the resistor 12 to be measured. Also shield 48
is the high potential of the power supply 11 (1111), and since the voltage drop in the detection resistor 22 is small, the charging and discharging of the capacitance between the shield 48 and the lead wire 21 is very small and can be almost ignored. Similarly, the shield 49 is grounded, and the charging and discharging between it and the lead wire 23 is very small.In this case, each of the shields 48 and 49 is a double shield, and for example, on the shield 48 side, the inner side When the outer shield is connected to the output side of the buffer circuit 31 and the outer shield is connected to the output side of the buffer amplifier 51, the influence of charging and discharging between the shield and the lead wire is also eliminated.

The buffer circuit 32 is provided to prevent current from inputting or outputting to the connection point 26 when the reference potential point of the resistance dividing circuit 350 is connected to the connection point 26. A buffer circuit 31 is provided for the same reason.

Note that when the isolation amplifier 52 is provided with a gain, the output of the amplifier 28 that is input to the differential amplifier 53 is similarly amplified by an Ike amplifier to match the gain. The part that measures the magnitude of current or resistance with respect to this reference, resistance to be measured 1
Either the current value flowing through 2 or the resistance [RX't'' measurement portion may be omitted.

[Brief explanation of the drawing]

FIG. 1 is a connection diagram showing an example of a current measuring circuit according to the present invention, FIG. 2 is a diagram showing a part of the circuit to explain its operation, and FIG. 3 is a diagram showing a conventional current measuring circuit. be. 12: Resistance to be measured, 22, 24: Detection resistor, 27,
28: Amplifier, 31, 32: Buffer circuit, 35.3
8: Resistor divider circuit, 41, 45: Feedback amplifier, 53, 5
6: Differential amplifier, 52° 55: Isolation amplifier, 58: Comparator.

Claims (1)

[Claims] (1) first and second detection resistors inserted in series on the current inflow side and current output side of the resistor to be measured, and voltage drops across these first and second detection resistors, respectively, are supplied; first and second amplifiers that output output voltages of opposite polarity; first and second buffer circuits respectively connected to the resistance to be measured side of the first and second detection resistors; , connected between the output side of the second amplifier and the output sides of the second and first buffer circuits, and having the same division ratio.
, a second resistance divider circuit, and first and second feedbacks connected between each division point of the first and second resistance divider circuits and the resistance under test side of the first and second detection resistors, respectively. A current measurement circuit comprising an amplifier.