JPH09113545A - Electric current measuring device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an electric current measuring device which can measure a dynamic electric current of a wide range at high speed without switching over it by a switch. SOLUTION: This device detects an electric current flowing to a capacitor 12 by impressing DC voltage on the capacitor 12. A resistance 13 for measuring is connected in series to the capacitor 12, and a amplifier 15 for measuring detects an electric current flowing in the measuring resistance by an electric potential difference on both ends of the measuring resistance. A logarithmic amplifier 14 is connected in series to the resistance for measuring. Outputs of the amplifier for measuring and the logarithmic amplifier are inputted to an operation processing circuit 17 through A/D converter 16 and 18. The operation processing circuit 17 finds an electric current value flowing to the capacitor 12 by selecting either of outputs of the amplifier for measuring and the logarithmic amplifier 14 with a prescribed threshold as a boundary.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a current measuring device, and more particularly to a current measuring device suitable for measuring a leak current flowing through a capacitor.

[0002]

2. Description of the Related Art Generally, in order to judge the quality of a capacitor, a DC voltage for measurement is applied to the capacitor, and the leakage current of the capacitor after being sufficiently charged is measured. Naturally, good products have low leakage current.

FIG. 1 shows an example of a conventional leakage current measuring device for a capacitor. This measuring apparatus comprises a constant voltage power supply 2 for applying a constant DC voltage to a capacitor 1, a measuring resistor 3 connected in series with the capacitor 1, an OP amplifier 4 having a negative input terminal connected to the resistor 3, and an OP amplifier. A feedback resistor 5 connected between the negative input terminal and the output terminal of the amplifier 4, an A / D converter 6 for A / D converting the output of the OP amplifier, and an A / D converter 6
And a CPU 7 that calculates a leakage current based on the output of. Since the leakage current of the capacitor 1 is variously different, a plurality of feedback resistors 5 are connected in parallel and the gains are switched by switching these resistors 5 with a switch 8 to select a measurement current band.

[0004]

However, in the case of the above measuring device, when the leakage current value changes over a wide range,
It is necessary to switch the range by the switch 8 during measurement. However, it is impossible to switch the switch 8 if the leakage current changes within a few milliseconds.
There is a problem that a dynamic current value that changes at high speed cannot be measured.

Therefore, an object of the present invention is to provide a current measuring device capable of measuring a dynamic current changing in a wide range at high speed without switching by a switch.

[0006]

To achieve the above object, the present invention provides a current measuring device for detecting a current flowing through an object to be measured by applying a DC voltage to the object to be measured. A measuring resistor connected in series, a measuring amplifier that measures the potential difference across the measuring resistor, a logarithmic amplifier that is connected in series with the measuring resistor and that converts the current flowing through the measuring resistor into a voltage logarithmically, and a measuring amplifier. A / D converter for A / D converting the signal output from the amplifier and the logarithmic amplifier, and the output of the A / D converter are input, and any one of the outputs of the measurement amplifier and the logarithmic amplifier across a predetermined threshold value. And an arithmetic processing circuit for determining the value of the current flowing through the DUT by selecting either.

The power supply current flows through an object to be measured (for example, a capacitor) and a measuring resistor, and a current that changes with characteristics according to the time constant of CR flows through the measuring resistor. At this time,
Since a relatively large current flows at first, the arithmetic processing circuit measures this current with the measuring amplifier. On the other hand, when the current value decreases, measurement cannot be performed with the measurement amplifier, so the arithmetic processing circuit switches from the measurement amplifier to the logarithmic amplifier at a predetermined threshold, and the leakage current is detected using the output of the logarithmic amplifier. measure. By doing so, it is not necessary to switch the switch as in the conventional case, and dynamic current measurement that changes in a wide range is possible. Since the arithmetic processing circuit is switched instantaneously by software, high-speed current measurement is possible.

[0008]

2 shows an example of a current measuring device according to the present invention. The DC measurement power supply 10 is connected to a capacitor 12, which is an object to be measured, via a switch 11. A resistor 13 is connected in series on the downstream side of the capacitor 12, and a logarithmic amplifier 14 is connected in series on the downstream side of the resistor 13. Both ends of the resistor 13 are connected to the positive and negative input ends of the measurement amplifier 15.

The output terminal of the logarithmic amplifier 14 is an A / D converter 1.
6 is connected to the arithmetic processing circuit (CPU) 17, and the output end of the measurement amplifier 15 is also connected to the arithmetic processing circuit (CPU) 17 via the A / D converter 18.

The CPU 17 calculates the leakage current by selecting one of the outputs of the amplifiers 14 and 15 depending on whether the leakage current of the capacitor 12 is larger than a predetermined threshold value I ₀ (for example, 1 mmA). That is, in the region where the leakage current of the capacitor 12 is larger than the threshold value I ₀ , the output V _{1 of the} measurement amplifier 15
Is used to obtain the leakage current, and in the region where the leakage current is smaller than the threshold value I ₀ , the leakage current is obtained using the output V ₂ of the logarithmic amplifier 14. The leak current threshold I ₀ is determined by the resistor 13.

FIG. 3 shows a change in current flowing through the capacitor 12 after the switch 11 is turned on, and FIG.
2 shows changes in the output voltages V ₁ and V ₂ of the amplifiers 14 and 15 when a current as shown in FIG. 3 flows. Outputs V ₁ and V ₂ are A
A / D converters 16 and 18 convert into digital signals, and the CPU
At 17, the following calculation is performed. The formula for obtaining the current I ₁ from the output V ₁ of the measurement amplifier 15 is as follows.

[0012]

(Equation 1) On the other hand, the formula for obtaining the current I ₂ from the output V ₂ of the logarithmic amplifier 14 is as follows.

[0013]

(2) I ₂ = B ^−CV2 (2) In the above equation, A, B, and C are constants, and R is the resistance value of the resistor 13. The respective values of A, B and C are set so that the current values I ₁ and I _{2 at the} threshold value I ₀ are the same.

Next, an example of a concrete arithmetic processing operation of the CPU 17 will be described with reference to FIG. First, the A / D converter 1
The outputs V ₁ and V ₂ of the amplifiers 14 and 15 are input from 6 and 18 (step S ₁ ). Next, the output V ₁ is compared with the voltage V ₀ corresponding to the threshold value I ₀ (step S ₂ ). This voltage V ₀ is given by the following equation.

[0015]

When V ₀ = I ₀ · R / A V ₁ ≧ V ₀ , I ₁ is calculated from equation (1) (step S ₃ ), and this I ₁ is determined as the leakage current value (step S ₃ ). ₄ ). On the other hand, when V ₁ <V ₀ , I ₂ is calculated from the equation (2) (step S ₅ ) and this I ₂ is determined as the leakage current value (step S ₆ ).

As described above, since the large leak current is measured by the measuring amplifier 15 and the small leak current is measured by the logarithmic amplifier 14, the leak current of the capacitor 12 can be accurately measured even if it changes in a wide range. be able to. Moreover, the CPU 17 uses the software to amplify the amplifiers 14 and 15
Since one of the outputs is selected and the leakage current is calculated from the selected output, it is not necessary to switch the switch as in the prior art, and high-speed current measurement is possible.

In the above-mentioned embodiment, the example in which the present measuring device is applied to the measurement of the leakage current of the capacitor is shown, but it can also be applied to the current measurement of other electronic parts or electronic equipment.

[0018]

As is apparent from the above description, according to the present invention, a large leakage current is measured by a measuring amplifier, and a small leakage current is measured by a logarithmic amplifier. Even if it changes over time, it can be measured accurately. Moreover, since the arithmetic processing circuit instantaneously selects the outputs of both amplifiers by software, it is possible to measure at high speed even a current value that changes in a few milliseconds.

[Brief description of the drawings]

FIG. 1 is a circuit diagram of an example of a conventional capacitor leakage current measuring device.

FIG. 2 is a circuit diagram of an example of a current measuring device according to the present invention.

FIG. 3 is a diagram showing changes in leakage current of a capacitor.

FIG. 4 is a diagram showing changes in outputs of a measurement amplifier and a logarithmic amplifier.

[Explanation of symbols]

 10 DC Measuring Power Supply 12 Capacitor 13 Resistance 14 Logarithmic Amplifier 15 Measuring Amplifier 16, 18 A / D Converter 17 CPU

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[Procedure amendment]

[Submission date] February 9, 1996

[Procedure amendment 1]

[Document name to be amended] Statement

[Correction target item name] Brief description of drawings

[Correction method] Change

[Correction contents]

[Brief description of the drawings]

FIG. 1 is a circuit diagram of an example of a conventional capacitor leakage current measuring device.

FIG. 2 is a circuit diagram of an example of a current measuring device according to the present invention.

FIG. 3 is a diagram showing changes in leakage current of a capacitor.

FIG. 4 is a diagram showing changes in outputs of a measurement amplifier and a logarithmic amplifier.

FIG. 5 is a flowchart showing the operation of the current measuring device according to the present invention.
FIG.

[Explanation of symbols] 10 DC power supply 12 Capacitor 13 Resistor 14 Logarithmic amplifier 15 Measurement amplifier 16, 18 A / D converter 17 CPU

Claims (2)

[Claims] 1. A current measuring device for detecting a current flowing through an object to be measured by applying a DC voltage to the object to be measured. A measurement amplifier that measures the potential difference, a logarithmic amplifier that is connected in series with the measurement resistor and that performs logarithmic conversion of the current that flows through the measurement resistor, and a signal that is output from the measurement amplifier and the logarithmic amplifier
The A / D converter for A / D conversion and the output of the A / D converter are input, and either the output of the measurement amplifier or the output of the logarithmic amplifier is selected with a predetermined threshold value as a boundary, so that the DUT can be measured. A current measuring device, comprising: an arithmetic processing circuit for obtaining a flowing current value. 2. The current measuring device according to claim 1, wherein the object to be measured is a capacitor, and the leak current flowing through the capacitor is measured.