JPH0720614Y2 - Current detection circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial Application Field> The present invention relates to a current detection circuit, and more particularly to a current detection circuit capable of setting a threshold for current detection without increasing the cost of parts.

<Prior Art> Conventionally, there are various types of current detection circuits. For example, in a circuit that detects a drive current of a motor, a shunt resistor is connected in series with the motor to detect the current. There is something. In this shunt resistance method, since the drive current of the motor can be detected linearly by utilizing the linearity of the resistance characteristic, the current can be detected over a wide range from a relatively small current. it can.

However, most of the current needs to flow through the shunt resistor, and it is necessary not only to use a large shunt resistor, but also to eliminate the spark noise generated in the motor commutator in parallel with the shunt resistor. There is a problem that the cost of parts will rise because it is necessary to connect a capacitor for rectification.

Also, there is a device in which a diode is connected in series with a motor and the presence or absence of a drive current for the motor is detected by utilizing the forward characteristic of the diode. In this diode system, it is effective for detecting the presence or absence of current, and
Since the change in forward voltage is small even if the current increases,
Compared with the shunt resistance method, the capacitor for removing the spark noise described above is not required, and thus there is a feature that the cost of parts can be reduced.

Incidentally, a transistor bridge circuit is used for the circuit for driving the motor in the forward / reverse direction, and the failure of each transistor is further diagnosed by the same applicant as Japanese Patent Application No. 61-291.
941. In this failure diagnosis circuit, for example, when one of the transistors is turned on while the motor is stopped and a corresponding other transistor is in a short-circuit failure state, a current flows through the circuit, so the current is detected. By doing so, the failure is diagnosed.

However, by turning on the transistor, the base current of the transistor will flow to the current detection circuit even if it is normal. Therefore, in order to detect the motor drive current and diagnose the short-circuit failure state of the transistor, it is necessary to set the threshold value for discriminating the two above the base current and below the motor drive current. There is.

<Problems to be Solved by the Invention> However, in the diode method, there is a problem that it is difficult to set the threshold because the voltage difference with respect to the current difference is small due to the diode characteristics. Further, in the shunt resistance method, the voltage difference with respect to the current difference can be made sufficiently large, but there is a problem that the cost of the parts rises as described above.

In view of such problems of the conventional technology, a main object of the present invention is to provide a current detection circuit capable of setting a current detection threshold value without increasing the cost of parts.

<Means for Solving Problems> According to the present invention, the purpose is to detect a current flowing through the transistor bridge circuit in order to detect a short circuit failure of a transistor in the transistor bridge circuit for forward and reverse rotation of the motor. A current detection circuit for performing, in the forward direction, a diode element connected in series to the transistor bridge circuit, a resistance element connected in parallel with the diode element, and a voltage change across both elements. And a comparator for detecting, wherein the failure detection threshold of the comparator is set in a small current region of the diode element in the combined characteristic of the both elements. It is achieved by

<Operation> In this way, the difference between the minute base current value flowing at the time of transistor diagnosis and the current value at the time of failure that will flow at the time of short-circuit failure of the transistor is determined by the combined characteristics of both the diode and the resistor. It is possible to increase the value by utilizing a region having a remarkable resistance characteristic which is a small current region of the diode, and it is possible to easily set a threshold value for discriminating the two.

<Embodiment> Hereinafter, the present invention will be described in detail with reference to the accompanying drawings.

FIG. 1 is a block diagram showing a motor drive circuit to which the present invention is applied. In this circuit, four transistors Q1 ...
A publicly known transistor bridge circuit is configured by Q4.

As shown in FIG. 1, a pair of transistors Q1 and Q2 are connected in parallel to each other at a power supply terminal 1 as a power supply terminal of a motor drive circuit.
The collector of Q1 is connected to terminal 3 of motor 2 and the collector of transistor Q2 is connected to terminal 4 of motor 2. The collector of the transistor Q3 is connected to the terminal 3 of the motor 2, and the collector of the transistor Q4 is connected to the other terminal 4. In addition, both transistors Q3 and Q4
Has its emitter grounded through the diode D1 in the forward direction.

The bases of the transistors Q1 to Q4 are connected to the transistor driving terminals 6 to 9 of the control circuit 5 having a CPU for controlling the motor driving circuit. The motor 2 is driven in the forward and reverse directions by the signal output from the terminals 6 to 9 of the control circuit 5. For example, by turning on the transistors Q1 and Q4 and turning off the transistors Q2 and Q3,
Of the motor 2 is turned forward, and conversely, the transistors Q2 and Q3 are turned on and the transistors Q1 and Q4 are turned off.
Will be reversed.

Also, the emitters and diodes of transistors Q3 and Q4
The part between D1 and D1 is connected to the current detection terminal 10 of the control circuit 5 via the resistor R1 and the comparator 11. The reference voltage divided by the resistors R3 and R4 is input to the comparator 11. Further, the diode D1 and the resistor R1 are grounded via the resistor R2, and thus the current detection circuit 12 in which the diode D1 and the resistor R2 are connected in parallel to each other is configured. Therefore, the current flowing through the motor drive circuit is input to the current detection terminal 10 of the control circuit 5 via the comparator 11 as the forward voltage of the diode D1 of the current detection circuit 12, and the output change of the comparator 11 changes the motor. The presence or absence of a drive current can be detected.

By the way, in the motor drive circuit according to the present embodiment, the failure diagnosis of each of the transistors Q1 to Q4 can be performed. For example, by outputting an ON signal to the transistor Q4 when the motor 2 is stopped. , Transistor Q1
It is possible to detect the short-circuited state of Q2. That is,
When the transistor Q1 or Q2 has a short circuit failure, a current flows in the current detection circuit 12, and the voltage in the substantially constant voltage region of the diode D1 is applied to the current detection terminal 10 via the comparator 11 as the voltage at the time of current detection. Therefore, the failure state can be detected.

Next, the voltage-current characteristics of the current detection circuit 12 will be shown below with reference to FIG. In Fig. 2, the horizontal axis represents voltage,
The current is plotted on the vertical axis, and the characteristic of only the diode D1 and the characteristic of only the resistor R2 are shown by the two-dot chain line. Therefore, the current detection circuit 12 according to the present embodiment has a characteristic obtained by combining the characteristic of the diode D1 and the characteristic of the resistor R2, as shown by the solid line in FIG.

As described above, at the time of failure diagnosis, a minute current (for example, 100 mA) due to the base current of the transistor is detected by the current detection circuit 12.
Flow to. By the way, with only the diode D1, the voltage corresponding to the base current becomes the voltage value indicated by Vt in FIG. 2, and the current when the motor 2 is unloaded (for example, 400 m
When A) flows, it becomes the voltage value indicated by Vm,
Since the difference (Vm-Vt) due to both is in the large current region where the slope of the characteristic curve of the diode D1 is large in the figure, it is small, so it is necessary to distinguish between Vt and Vm. It is difficult to set the threshold.

However, in the present invention, since the characteristic shown by the solid line in FIG.
When the base current of the transistor flows, it is V1 in the figure, and when the current when there is no load in the motor 2, it becomes V2. Therefore, the difference between them (V2-V1) is the figure of the diode D1. In the small current region where the slope of the characteristic curve is small, the characteristic of the resistor R2 becomes remarkable and is linear, so it is sufficiently large, so the threshold for discriminating between them is between V1 and V2. It is easy to set the reference voltage of the comparator 11 as, for example, Vs in FIG. Therefore, the threshold value for current detection can be easily set in the region of minute current, and even when a minute current such as the base current of the transistor flows at the time of failure diagnosis, the current when the motor 2 is unloaded can be discriminated. can do.

Further, in the present embodiment, since the drive current of the motor 2 is detected in the large current region of the diode characteristic, the motor 2 is not affected by the spark noise generated in the commutator of the motor 2. The drive current of can be detected.

<Effect of Device> As described above, according to the present invention, the current detection circuit is composed of the diode and the resistor connected in parallel with each other, so that it becomes remarkable in the small current region of the diode in the combined characteristic of both elements. Since it has a linear characteristic due to the resistance characteristic, it is possible to increase the difference between the minute base current value that flows when diagnosing a transistor and the fault current value that will flow when a short circuit occurs in the transistor. The detection threshold can be easily set. Also, by selecting resistors with different resistance values, the slope of the region of the resistance characteristic can be changed arbitrarily,
It can be easily applied to various circuits. Furthermore, since no voltage higher than the forward voltage of the diode is applied to the resistors connected in parallel, it is not necessary to increase the capacitance of the resistors, and the effect of the resistors is extremely large. is there.

[Brief description of drawings]

FIG. 1 is a block diagram showing a motor drive circuit to which the present invention is applied. FIG. 2 is a diagram showing a voltage-current characteristic of the current detection circuit according to the present invention. 1 ... Power supply terminal, 2 ... Motor 3, 4 ... Terminal, 5 ... Control circuit 6-9 ... Terminal, 10 ... Current detection terminal 11 ... Comparator, 12 ... Current detection circuit

Front page continuation (72) Inventor Kazuhiko Tachikawa 1-2681 Hirosawa-cho, Kiryu-shi Gunma Mitsuba Electric Mfg. Co., Ltd. (56) References JP-A-60-93963 (JP, A) Actually developed 59-21770 (JP, U)

Claims (1)

[Scope of utility model registration request] 1. A current detection circuit for detecting a current flowing through the transistor bridge circuit for detecting a short circuit failure of a transistor of the transistor bridge circuit for rotating the motor forward and backward, the serial circuit being connected to the transistor bridge circuit. And a diode element connected in the forward direction, a resistance element connected in parallel with the diode element, and a comparator for detecting a voltage change across the both elements, and the failure detection of the comparator. A current detection circuit, wherein a threshold value is set in a small current region of the diode element in a combined characteristic of the both elements.