JP3784168B2 - Current measuring device - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a current measuring device that measures a current flowing through a load resistance by applying a constant voltage.
[0002]
[Prior art]
FIG. 5 shows a configuration of a main part of a conventional current measuring apparatus that measures a current flowing through a load resistance to be measured by applying a constant voltage. The current measuring device 9 includes five high-precision resistors R1 to R5 whose resistance values are different by 10 times, five mercury relays S1 to S5 respectively connected to these resistors, and an amplifier circuit 93. The resistors R1 to R5 are connected to an input terminal 91 to which a predetermined voltage is applied, and the mercury relays S1 to S5 are connected to an output terminal 92 connected to a load resistor (not shown).
[0003]
When any one of the mercury relays S1 to S5 is turned on, the input terminal 91 and the output terminal 92 are connected, current flows from the output terminal 92 to the load resistance, and the resistance connected to the conducted mercury relay includes the load resistance. A current equal to the current flowing through Amplifying circuit 93 amplifies the voltage between input terminal 91 and output terminal 92, and measures the amplified voltage appearing at terminal 93c with a voltage detection circuit (not shown), thereby measuring the current flowing through the load resistance.
[0004]
By switching the mercury relay to be conducted and switching the resistance through which a current is passed, the voltage appearing between the input terminal 91 and the output terminal 92 is changed, thereby switching the measurement range. This current measuring device 9 has a five-step measurement range from the minimum range R8UA to the maximum range R80MA, and switches the measurement range sequentially starting from the maximum range R80MA so that the voltage applied to the voltage detection circuit is within a predetermined range. It has.
[0005]
[Problems to be solved by the invention]
As described above, the conventional current measuring apparatus uses a mercury relay as a switch element for switching the measurement range. However, in the mercury relay, both the operation / recovery time, that is, the time from the start of the conduction operation to the complete conduction and the time from the start of the conduction operation to the complete non-conduction are as long as about 3 msec (milliseconds).
[0006]
For this reason, when the current flowing through the load resistance is measured by frequently switching the measurement range, a considerable time is required for the measurement. For example, when the load resistance to be measured in the minimum range R8UA is automatically set in the range starting from the maximum range R80MA in the current measuring device 9, it takes about 27 msec to set the appropriate range.
[0007]
In order to enable quick measurement, a switch element having a short operation / recovery time may be used instead of the mercury relay. For example, the operation / recovery time of the photo moss relay is about 0.5 msec and 0.2 msec, respectively. By using this, the measurement range can be quickly switched. However, the on-resistance of the mercury relay, that is, the resistance value during conduction is about 100 mΩ, whereas the on-resistance of the photo-moss relay is about 10Ω, which is 100 times higher. For this reason, in a large-range measurement in which a current is passed through a resistor having a small resistance value, an on-resistance equivalent to the resistance value is newly added, and the current cannot be measured correctly.
[0008]
The same applies to other switch elements having a short operation / recovery time, and these switch elements cannot be used as they are instead of mercury relays.
[0009]
The present invention has been made in view of the above problems, and provides a current measuring device capable of accurately measuring from a minute current flowing through a load resistance to a large current and capable of quickly switching a measurement range. The purpose is to do.
[0010]
[Means for Solving the Problems]
In order to achieve the above object, in the present invention, a first terminal, a second terminal, a plurality of high-precision resistors having one end connected to the first terminal and having different resistance values, and one end being a second terminal. A plurality of switch elements connected to the other terminal and the other end individually connected to the other end of the plurality of resistors, a first input terminal connected to the first terminal, and a second input terminal to the second terminal Having a connected amplifier circuit, connecting a load resistor to one of the first terminal and the second terminal, applying a predetermined voltage to the other, and causing any one of the plurality of switch elements to conduct; In a current measurement device including a current measurement circuit that measures a current flowing through a load resistor by amplifying and measuring a voltage between one terminal and a second terminal, the current measurement circuit includes a plurality of switch elements. As well as a photo MOS relay, and the resistance value of multiple resistors is A plurality of analog switches for connecting the second input terminal of the second input terminal and the second terminal and the amplifier circuit end the amplifier circuit of the switching element side of less than value respectively.
[0011]
When conducting a switch element connected to a resistor having a resistance value less than a predetermined value among the plurality of switch elements, only a switch connected to the end of the resistor among a plurality of analog switches is conducted. When an element connected to a resistor having a resistance value equal to or higher than a predetermined value is turned on, only one of the plurality of analog switches connected to the second terminal is turned on.
In addition, a noise removal circuit having an integration circuit is provided, and a predetermined voltage from which noise has been removed by the noise removal circuit is supplied to the current measurement circuit.
[0012]
The measurement range changes depending on which of the switch elements is turned on, but the measurement range can be switched very quickly by using a photoMOS relay with a very short operation / recovery time as the switch element. . When a current flows through a resistor having a resistance value less than a predetermined value, the resistor and the amplifier circuit are directly connected by an analog switch connected to the resistor, and the second terminal and the amplifier circuit are disconnected. At this time, the voltage between both ends of the resistor is applied to the amplifier circuit instead of the voltage between both ends of the resistor and the photo MOS relay connected in series, and the on-resistance of the photo MOS relay does not affect the measured value. .
[0013]
Moreover, since the input impedance of the amplifier circuit is so large that the resistance value of the analog switch can be ignored, the on-resistance of the analog switch does not make the measurement inaccurate. Therefore, even when measuring in a large range with a small resistance value, the current can be measured accurately.
[0014]
Also, when a current is passed through a resistor having a resistance value equal to or greater than a predetermined value, the second terminal and the amplifier circuit are connected, and measurement is performed based on the voltage between both ends of the resistor connected in series and the photoMOS relay. Become. Here, for example, if the predetermined value is about 10 kΩ or more, the on-resistance of the photo-moss relay is 1000 times or more, and the influence of the on-resistance on measurement is suppressed to 0.1% or less. Therefore, the current can be accurately measured even in a small range. By setting in this way, it is possible to avoid unnecessary provision of many analog switches.
[0015]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, an embodiment of a current measuring device of the present invention will be described with reference to the drawings. FIG. 1 shows a schematic configuration of the current measuring apparatus 1. The current measuring device 1 includes a noise removing circuit 10, a current measuring circuit 20, a voltage detecting circuit 30, and a feedback circuit 40.
[0016]
The noise removal circuit 10 includes an integration circuit 11, an inverting amplification circuit 12, and a buffer 13. The noise removal circuit 10 removes noise from a predetermined voltage of about 8V given through a resistor 10a from a constant voltage source (not shown), and obtains a voltage after noise removal. This is supplied to the current measurement circuit 20. The current measurement circuit 20 applies the voltage from the noise removal circuit 10 to the load resistance RL, which is the object of current measurement, via the two-core shielded wire 20a, thereby converting the current i flowing in itself into a voltage. Output. The current measurement circuit 20 will be described in detail later.
[0017]
The voltage detection circuit 30 includes an inverting amplifier circuit 31 and a voltmeter 32. The voltage output from the current measurement circuit 20 is amplified by the inverting amplifier circuit 31 at a predetermined magnification, and the amplified voltage is measured by the voltmeter 32. The voltage measured by the voltmeter 32 changes according to the current i flowing through the load resistor RL, but the voltage value does not directly represent the current value flowing through the load resistor RL, and the correspondence between the voltage value and the current value is It varies depending on the internal setting of the current measurement circuit 20. In order to adjust the reference level of the voltage applied from the current measurement circuit 20 to the voltage detection circuit 30, a variable resistor 30a is provided between the two circuits.
[0018]
The feedback circuit 40 includes a differential amplification circuit 41 having a magnification of 1. The current measurement circuit 20 inverts the difference between the voltage applied to the load resistor RL and the voltage decreased by flowing through the load resistor RL, and the variable resistor 10b. Is fed back to the noise removal circuit 10 via The input impedance of the differential amplifier circuit 41 is high, and no current flows from the current measurement circuit 20 to the feedback circuit 40. Therefore, the current i flowing through the load resistor RL is equal to the current i flowing through the current measuring circuit 20.
[0019]
The numerical values of +15 and −15 attached to the terminals of each circuit in FIG. 1 and the drawings described below represent the supplied drive voltage (V), and CGND is connected to the ground potential.
[0020]
The configuration of the current measurement circuit 20 is shown in FIG. The current measurement circuit 20 includes an input terminal 21 that receives a voltage supplied from the noise removal circuit 10, an output terminal 22 that applies the voltage to the load resistor RL, five resistors R1 to R5, and six photo MOS relays that are switch elements. K1 to K6 are provided.
[0021]
The resistors R1 to R5 have high accuracy within an error of 0.1%, and their resistance values differ by 10 times. Specifically, the resistance R1 is maximum at 100 kΩ, and the resistance R5 is minimum at 10Ω. One end of the resistors R1 to R5 is connected to the input terminal 21, and the other end is connected to one end of each of the photoMOS relays K1 to K5. The other ends of the photo MOS relays K <b> 1 to K <b> 5 are connected to the output terminal 22. The photoMOS relay K6 is connected between the resistor R5 and the output terminal 22 in parallel with the photoMOS relay K5. The reason why the two photoMOS relays K5 and K6 are provided in parallel between the resistor R5 and the output terminal 22 is to prevent the absolute maximum rating of the continuous load current from being exceeded.
[0022]
The resistors R1 to R4 and the photomoss relays K1 to K4 and the resistor R5 and the photomoss relays K5 and K6 connected in series form five current paths in parallel between the input terminal 21 and the output terminal 22. Although not shown, the current measuring device 1 includes a control unit that controls the entire device, and this control unit conducts only one photoMOS relay K1 to K5 at a time. The photo moss relays K5 and K6 are operated simultaneously. The maximum value of the current flowing between the input terminal 21 and the output terminal 22 is 8 μA to 80 mA depending on the operating current path. The five current paths correspond to the five measurement ranges R8UA, R80UA, R800UA, R8MA, R80MA of the current measuring apparatus 1.
[0023]
The current measurement circuit 20 also includes a measurement amplification circuit 23 and four analog switches K7 to K10. One input terminal 23a of the measurement amplifier circuit 23 is connected to the input terminal 21 via the resistor R6, and the other input terminal 23b is connected to one end of the resistor R7. The other end of the resistor R7 is connected to the ends of the resistors R5, R4, and R3 on the photoMOS relays K5, K4, and K3 via analog switches K7, K8, and K9, and connected to the output terminal 22 via the analog switch K10. It is connected.
[0024]
The measurement amplifier circuit 23 amplifies the voltage applied between the input terminals 23a and 23b by 10 times and outputs the amplified voltage to the voltage detection circuit 30 from the output terminal 23c. Two resistors R8 and R9 having an accuracy error within 0.1% are connected in parallel to the measurement amplifier circuit 23. The configuration of the measurement amplifier circuit 23 is shown in FIG. The measurement amplifier circuit 23 includes three operational amplifiers 51a, 51b, 52 and two overvoltage protection circuits 53a, 53b.
[0025]
The input terminal (+) of the operational amplifier 51a is connected to the input terminal 23a via the overvoltage protection circuit 53a. Similarly, the input terminal (+) of the operational amplifier 51b is connected to the input terminal 23b via the overvoltage protection circuit 53b. ing. The other input terminals (−) of the operational amplifiers 51a and 51b are connected by resistors R8 and R9. The output terminal of the operational amplifier 51a is connected to the input terminal (−) of the operational amplifier 52 through the resistor 54a, and is connected to its own input terminal (−) through the resistor 55a. The output terminal of the operational amplifier 51b is connected to the input terminal (+) of the operational amplifier 52 through the resistor 54b, and is connected to its own input terminal (−) through the resistor 55b.
[0026]
The output terminal of the operational amplifier 52 is the output terminal 23 c of the measurement amplifier circuit 23. The output terminal 23c is connected to the input terminal (−) of the operational amplifier 52 via a resistor 56a for feedback. The input terminal (+) of the operational amplifier 52 is connected to the ground potential via the resistor 56b. With such a configuration, the measurement amplifier circuit 23 can amplify a given voltage with high accuracy and has a high input impedance of about 10 ¹⁰ Ω.
[0027]
The above-described control unit conducts only one analog switch K7 to K10 at a time. Specifically, the analog switch K7 is turned on when the photoMOS relays K5 and K6 are turned on, the analog switch K8 is turned on when the photomoss relay K4 is turned on, and the analog switch K9 is turned on when the photomoss relay K3 is turned on. Let Further, the analog switch K10 is turned on when the photoMOS relay K1 or K2 is turned on.
[0028]
Therefore, when the measurement range R8UA or R80UA is selected, the voltage between the input terminal 21 and the output terminal 22 is given to the measurement amplifier circuit 23, and when the measurement range R800UA, R8MA or R80MA is selected, The voltage between both ends of the resistor R3, R4 or R5 is given, respectively.
[0029]
The on-resistance of the photo MOS relay is about 10Ω, which corresponds to 1%, 10%, and 100% of the resistance values of the resistors R3, R4, and R5. However, when measuring the current flowing through the load resistor RL in the measurement ranges R800UA, R8MA, R80MA where the current flows through the photoMOS relays K3, K4, K5, K6, the voltage across the resistors R3, R4, R5 is directly measured. As a result, their on-resistance does not affect the measurement.
[0030]
Moreover, since the on-resistance of the analog switch is about 175Ω and is negligibly small compared to the input impedance of the measurement amplifier circuit 23, the analog switches K7, K8, which connect the resistors R3, R4, R5 and the measurement amplifier circuit 23, The on-resistance of K9 does not affect the measurement. Therefore, the accuracy of medium current or large current measured in the measurement ranges R800UA, R8MA, R80MA is high.
[0031]
When measuring in the measurement ranges R8UA and R80UA, the on-resistance of the photoMOS relays K1 and K2 is affected. However, the on-resistance of the photo-moss relay is only 0.01% and 0.1% of the resistance values of the resistors R1 and R2, which is less than the accuracy error of the high-precision resistor used in the current measuring device 1, The effect is negligible. Therefore, the precision of minute or small currents measured in the measurement ranges R8UA and R80UA is high.
[0032]
The current measuring device 1 has a range automatic setting function. When the range is automatically set, the control unit sets the measurement range to the maximum range so that the voltage output from the measurement amplification circuit 23 falls within the maximum voltage that can be measured by the voltage detection circuit 30 and within 5% of the maximum voltage. Start with R80MA and switch sequentially. The current measuring device 1 can quickly switch the measurement range by using a photo moss relay with an extremely short operation / recovery time of 0.5 / 0.2 msec. The operation / recovery time of the analog switch is further shortened to 1 / 0.5 μsec and does not affect the measurement at all.
[0033]
FIG. 4 shows the time required for measurement in the automatic range setting when the current flowing through the load resistance RL is 4 to 80 μA, in comparison with a conventional apparatus using a mercury relay. (A) is the time required for the current measuring device 1, and (b) is the time required for the current measuring device 9 of FIG. In both cases, the four measurement ranges from R80MA to R80UA will be switched. However, there is a difference of 2.5 msec in one operation time and 2.8 msec in one recovery time, so 21.2 msec until the measurement is completed. The difference arises. That is, the current measuring device 1 completes the measurement in about 12% of the conventional time.
[0034]
Here, the resistors R1 to R5 are connected to the input terminal 21 of the current measuring circuit 20, and the photo MOS relays K1 to K6 are connected to the output terminal 22. However, the arrangement order is reversed, and the photo MOS relay is connected. K1 to K6 may be connected to the input terminal 21 and resistors R1 to R5 may be connected to the output terminal 22. In this case, the resistor R7 is directly connected to the output terminal 22, the resistor R6 is connected to the input terminal 21 via the analog switch K10, and the resistor R6 is connected to the analog switches K7, K8, and K9. There is no difference in control action.
[0035]
In addition, only three of the five resistors R1 to R5 having a small resistance value are connected to the measurement amplifier circuit 23 via the analog switches K7 to K9. However, all the resistors R1 to R5 are connected via the analog switch. You may make it connect to the measurement amplification circuit 23. FIG. At this time, the analog switch K10 that connects the output terminal 22 and the measurement amplifier circuit 23 becomes unnecessary. However, when the on-resistance of the photoMOS relay is smaller than the accuracy error of the resistance, measuring only the voltage across the resistor does not contribute to the improvement of the measurement accuracy. Therefore, it is desirable to determine which resistor is connected to the measurement amplifier circuit 23 through the analog switch in consideration of the resistance value and the accuracy error.
[0036]
【The invention's effect】
When using the current measuring device of the present invention, the time required for switching the range is extremely short, so that the measurement efficiency is greatly improved. In addition, there is almost no error when measuring in any range, and a minute current to a large current can be accurately measured.
[Brief description of the drawings]
FIG. 1 is a diagram showing a schematic configuration of a current measuring device according to an embodiment of the present invention.
FIG. 2 is a diagram showing a configuration of a current measuring circuit of the current measuring device.
FIG. 3 is a diagram showing a configuration of a measurement amplification circuit of the current measurement circuit.
FIG. 4 is a diagram showing a comparative example of measurement times of the current measuring device and a conventional current measuring device.
FIG. 5 is a diagram showing a configuration of a main part of a conventional current measuring device.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Current measurement apparatus 10 Noise removal circuit 11 Integration circuit 12 Inversion amplification circuit 13 Buffer 20 Current measurement circuit 20a Two-core shielded wire 21 Input terminal 22 Output terminal 23 Measurement amplification circuit R1-R5 High precision resistance K1-K6 Photo-moss relay K7- K10 analog switch 30 voltage detection circuit 31 inverting amplifier circuit 32 voltmeter 40 feedback circuit 41 differential amplifier circuits 51a, 51b, 52 operational amplifiers 53a, 53b overvoltage protection circuit

Claims (1)

A first terminal;
A second terminal;
A plurality of high-precision resistors having one end connected to the first terminal and having different resistance values;
A plurality of switch elements having one end connected to the second terminal and the other end individually connected to the other ends of the plurality of resistors;
A first input terminal connected to the first terminal and a second input terminal connected to the second terminal; and a load on one of the first terminal and the second terminal. Connecting a resistor and applying a predetermined voltage to the other to make one of the plurality of switch elements conductive to amplify and measure the voltage between the first terminal and the second terminal In the current measuring device having a current measuring circuit for measuring the current flowing through the load resistance,
A noise removal circuit having an integration circuit is provided, and a predetermined voltage from which noise has been removed by the noise removal circuit is supplied to the current measurement circuit,
In addition to a predetermined voltage, the noise elimination circuit receives a voltage corresponding to a difference between a voltage applied to the load resistor by the current measuring circuit; and a voltage reduced by flowing through the load resistor. And
The current measurement circuit includes a photo moss relay as the plurality of switch elements, the switch element side end of the plurality of resistors having a resistance value less than a predetermined value, and a second input terminal of the amplifier circuit, And a plurality of analog switches that respectively connect the second terminal and the second input terminal of the amplifier circuit, and a resistance value of the plurality of switch elements is connected to a resistance that is less than the predetermined value. Of the plurality of analog switches that are connected to the ends of the resistors, and one of the plurality of switch elements that is connected to a resistor having a resistance value greater than or equal to the predetermined value is conducted. A current measuring device for conducting only the one connected to the second terminal among the plurality of analog switches.

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