JPH0630579A - Current detecting circuit - Google Patents

Abstract

PURPOSE:To obtain a simple-structured current detecting circuit for both AC and DC. CONSTITUTION:A shunt resistor 10 for detection having a small resistance value is serially inserted into a circuit in which current to be detected flows. A voltage drop by the shunt resistor 10 is amplified by means of an operational amplifier 11 through protective diodes 12, 13 and then current which flows in the circuit to be detected is detected by means of the output of the amplifier 11. By this method, both AC and DC of the circuit to be detected can be detected with a simple-structured circuit without expensive circuit components.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a current detection circuit for detecting a current flowing through a load such as controlling a current flowing through a load such as a motor.

[0002]

2. Description of the Related Art Conventionally, an instrument current transformer (CT) 30 and a current detecting element 31 shown in FIGS. 5 and 6 are used for controlling a current to a motor or detecting an overcurrent. There is. CT30 of FIG. 5 is a kind of transformer, the primary winding 32 is connected in series to the load 33, and the secondary winding 34 constitutes a detection circuit. That is, the primary current can be immediately known by measuring the secondary current. Further, the current detection element 31 of FIG. 6 is a CT using the Hall effect using a magnetic material and a magnetic conversion element, and an output voltage proportional to the measured current is obtained.

[0003]

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention
Since the T30 and the current detection element 31 are relatively expensive, for example, if a circuit for detecting the current is formed by using them in the control circuit of the motor, the cost of the entire control circuit is considerably increased, and in the CT30 using the transformer, due to its structure. It also includes the problem that direct current cannot be detected.

The present invention has been made to solve the above-mentioned conventional problems, and an object of the present invention is to provide a low-cost current detection circuit for both AC and DC, which has a simple circuit configuration.

[0005]

In the current detection circuit according to the first aspect of the present invention, a shunt resistor for detection having a small resistance value is inserted in series with the circuit to be detected, and a voltage drop due to this shunt resistor is passed through a protection diode. It is amplified and output by an operational amplifier.

In the current detecting circuit according to the second aspect of the invention, the motor is connected to both output terminals of a single-phase inverter having two arms formed of semiconductor switching elements via a shunt resistor having a small resistance value. The voltage drop due to the shunt resistance is switched by a switch and amplified and output by an operational amplifier through a protection diode.

[0007]

In the first aspect of the invention, the current flowing through the shunt resistance of the circuit to be detected is input to the operational amplifier as a voltage drop due to the shunt resistance via the protection diode, and the operational amplifier functions as a differential amplifier and flows into the circuit to be detected. An output voltage proportional to the current will be obtained. Since the high voltage from the detected circuit is not applied to the operational amplifier by the protection diode, it is possible to detect the current of the detected circuit to which a high voltage is applied.

According to the second aspect of the present invention, by switching the switch according to the ignition of the semiconductor switching element of the single-phase inverter, the current can be detected by the operational amplifier using any one of the shunt resistors. The current detection period will be extended.

[0009]

EXAMPLES Example 1. FIG. 1 is a circuit diagram in which a current detection circuit according to an embodiment of the present invention is applied to a motor drive circuit, and FIGS. 2 and 3 are operation explanatory diagrams of the current detection circuit.

In the figure, reference numerals 1 and 2 denote upper-stage transistors to which diodes 3 and 4 are connected in antiparallel, respectively.
Reference numerals 5 and 6 are transistors in the lower stage, to which diodes 7 and 8 are connected in antiparallel, respectively, which form a semiconductor switch group consisting of two arms for outputting the winding current of the motor 9, and which have a high voltage, for example, a power supply Vdc of about 140V. Together with this, it constitutes a single-phase inverter. The load motor 9 is connected to the output terminal of the transistor 1 and the output terminal of the transistor 2 in the inverter. 10 is a motor 9 and a transistor 1
The shunt resistor for detection has a small resistance value of, for example, several tens of mΩ, which is inserted in series with the motor 9 between the output terminal and the output terminal. Reference numeral 11 is an operational amplifier, and the voltage across the shunt resistor 10 is input to the + input terminal and the − input terminal, respectively. 12, 13 are protection diodes respectively connected between both ends of the operational amplifier 11 and the shunt resistor 10, 14,
Reference numerals 15 and 16 are resistors connected to the input side of the operational amplifier 11, and 17 is a feedback resistor. Further, 18 is each protection diode 12, 13 with a resistor 19, 20 and a capacitor 2
It is a low voltage power source of about 5 V to 20 V, for example, which applies a voltage by 1 and 22.

In the motor drive circuit having the above-mentioned structure, the transistors 1, 5, 2 and 6 forming the two arms operate so that two of the four transistors are turned on in order to pass a current to each phase. . When each of the transistors 1, 5, 2 and 6 is sequentially turned on in a predetermined firing order, either positive or negative voltage is applied to the armature winding of the motor 9 to drive the motor 9.
The current flowing in the motor 9 is input as a voltage drop to the operational amplifier 11 through the protection diodes 12 and 13 by the shunt resistor 10 having a small resistance value connected in series to the motor 9, is amplified by the operational amplifier 11, and has a proportional magnitude. Is used as information for controlling the current to the motor 9 and detecting overcurrent.

However, in the embodiment shown in FIG. 1, as shown in FIG. 2, when the transistor 1 is turned on and the diode 3 in antiparallel with the transistor 1 is turned on, the shunt resistor 1 is turned on.
As shown in FIG. 2, the potentials at both ends A and B of 0 are + Vdc >> V
Since it becomes c, the protection diodes 12 and 13 are in the OFF state, so that the current is not detected to protect the operational amplifier 11 against a high voltage. The current is detected when the transistor 5 is turned on as shown in FIG. 3 and when the diode 7 in antiparallel with the transistor 5 is turned on. At this time, the potentials of both ends A and B of the shunt resistor 10 are 0 << Vc. And the protection diodes 12 and 13 are turned on, and the operational amplifier 1
1 functions as a differential amplifier, a voltage proportional to the current is output, and the current is detected. The shunt resistor 10 and the operational amplifier 11 can detect an electric current in either AC or DC because of the structure thereof, and it is not necessary to use an expensive CT or a current detecting element, and the circuit structure is simple. Therefore, it can be widely used as part of a low-cost control circuit.

Example 2. FIG. 4 is a circuit diagram in which a current detection circuit according to another embodiment of the present invention is applied to a motor drive circuit. This embodiment eliminates the time when the current is not detected in the first embodiment, and the basic configuration for detecting the current is the same as that of the first embodiment. That is, as shown in FIG. 4, the motor 9 is connected to both output terminals of the inverter arm through the shunt resistor 10 having a small resistance value, and the voltage drop due to these shunt resistors 10 is switched to the switches 23, 2.
4, 25, 26 to switch protective diodes 12, 1
It is adapted to be amplified and output by the operational amplifier 11 via 3. Switches 25 and 26 are transistor 1
Is opened at the time of switching, and closed at the time of switching of the transistor 5. The switches 23 and 24 are closed when the transistor 5 is switching, and are opened when the transistor 1 is switching.

In this embodiment as well, the detection of the current flowing in the motor 9 is carried out as described in the first embodiment, but the switches 23, 24, 2 are switched in accordance with the ignition of the transistors 1, 5.
By switching between 5 and 26, the current can be detected by the operational amplifier 11 using any one of the shunt resistors 10, the current detection period is extended, and the detection delay can be prevented.

[0015]

As is apparent from the above description of the embodiments, according to the first invention, the current flowing through the shunt resistance of the circuit to be detected is amplified by the operational amplifier as a voltage drop due to the shunt resistance, and the circuit to be detected is amplified. An output voltage proportional to the current flowing in the circuit is obtained, and it is possible to detect the alternating current of the detected circuit with a simple configuration without expensive circuit parts.

According to the second aspect of the invention, by switching the switch in accordance with the ignition of the semiconductor switching element of the single-phase inverter, the current can be detected by the operational amplifier using any one of the shunt resistors. In particular, the current detection period can be extended, detection delay can be avoided, and the cost of the control circuit including the motor drive circuit can be reduced.

[Brief description of drawings]

FIG. 1 is a circuit diagram shown together with a motor drive circuit showing an embodiment of a first invention.

FIG. 2 is an operation explanatory diagram relating to current detection in FIG.

FIG. 3 is an operation explanatory diagram relating to current detection in FIG. 1.

FIG. 4 is a circuit diagram shown together with a motor drive circuit showing an embodiment of a second invention.

FIG. 5 is an explanatory diagram of a current detection circuit using a current transformer for an instrument as a conventional example.

FIG. 6 is a configuration diagram showing a current detection element as a conventional example.

[Explanation of symbols]

 1 Transistor 2 Transistor 5 Transistor 6 Transistor 9 Motor 10 Shunt Resistor 11 Operational Amplifier 12 Protective Diode 13 Protective Diode 23 Switch 24 Switch 25 Switch 26 Switch

Claims (2)

[Claims] 1. A detection shunt resistor having a small resistance value is inserted in series with a circuit to be detected in which a current to be detected flows, and a voltage drop due to this shunt resistor is amplified by an operational amplifier via a protection diode, and the output of the operational amplifier is output. The current detection circuit is configured to detect the current flowing through the circuit to be detected. 2. A motor is connected via a shunt resistor having a small resistance value to both output terminals of a single-phase inverter having two arms formed of semiconductor switching elements, and a voltage drop due to these shunt resistors is switched by a switch. Amplifies with an operational amplifier via a protection diode,
A current detection circuit characterized in that the current flowing through the motor is detected by the output of the operational amplifier.