JPH11326400A - Overcurrent detection device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a compact and low-priced overcurrent detection device that can measure a load current and detect an overcurrent without a current sensor and a shunt resistance. SOLUTION: A drain-source voltage of a load drive FET 4 activating a load 3 is detected by an amplification circuit 5 and also a temperature of the FET 4 is measured by a temperature measuring circuit 8. Then, a current value flowing in the load 3 based upon the drain-source voltage and the temperature is calculated by a load current calculating means placed in a microcomputer 9 and an overcurrent is detected.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an overcurrent detecting device for detecting that an overcurrent flows in a load.

[0002]

2. Description of the Related Art FIG. 2 is a block diagram showing a configuration of a conventional overcurrent detecting device disclosed in, for example, Japanese Patent Application Laid-Open No. 7-7848, wherein 21 is an AC power supply, 22 is a breaker installed indoors, 23 , 24, and 25 are devices that are loads connected to the AC power supply 21, and 25 is an air conditioner having a control function for controlling its own input current. Also, 26
Is a current detection circuit 2 for detecting a current flowing through the breaker 22.
7, a microcomputer 28 for converting an input signal voltage from the current detection circuit 27 into a predetermined output voltage, and an output signal of the microcomputer 28
5 is a current detection device including a transmission circuit 29 for transmitting data to the current detection device 5.

A current detection circuit 27 includes a current sensor 30 for detecting a current (load current) of an indoor wiring and resistors R1, R2, R
3, R4, diodes D1 and D2, a capacitor C, and the like. An output voltage corresponding to a load current obtained from the middle point of the current sensor 30 and the resistor R1 is input to a start port of the microcomputer 28 via the diode D1. And the output voltage is divided by resistors R2 and R3.
An input is made to the A / D port of the microcomputer 28 via the resistor R4. Although the voltage Vcc (5 V) is always applied to the microcomputer 28, this voltage Vcc only holds a specific function inside the microcomputer 28, and other circuits are in the sleep mode (power supply is not performed). (Stopped state), and is activated when a voltage equal to or higher than a predetermined value is supplied to the activation port.

[0004] By the way, if the capacity of the breaker 22 is set to 20 amps and the current flowing through the indoor wiring reaches, for example, 18 amps, the overcurrent detecting device determines that the current is an overcurrent. , The current sensor 30 and the resistor Rl are selected such that a predetermined output voltage is obtained when the load current reaches 18 amps. When this output voltage is input to the start port of the microcomputer 28,
The microcomputer 28 starts up, converts the input signal voltage from the A / D port into a predetermined output signal, and
9 is output. The transmission circuit 29 is the microcomputer 2
8 is converted into, for example, an infrared signal and transmitted to the air conditioner 25. On the air conditioner 25 side,
Signal processing of the infrared signal is performed by the same method as that for processing the remote control signal for controlling the air conditioner 25, for example,
The input current of the air conditioner 25 is suppressed by, for example, reducing the rotation speed of a compressor (not shown). The transmission circuit 29
When the transmission to the indoor unit is difficult because the air conditioner and the indoor unit of the air conditioner 25 are not in the same room, the transmission from the transmission circuit 29 may be performed by radio waves or wired.
There is also a method of shunting a part of the load current to a shunt resistor for current measurement and detecting an overcurrent flowing in the load.

[0005]

As described above, in the conventional overcurrent detection device, since the load current is measured using the current sensor and the shunt resistor, it is difficult to reduce the size and cost of the device. There was a problem.

SUMMARY OF THE INVENTION The present invention has been made in view of the above-mentioned conventional problems, and is a small and inexpensive overcurrent capable of measuring a load current and detecting an overcurrent without using a current sensor or a shunt resistor. It is intended to provide a detection device.

[0007]

According to a first aspect of the present invention, there is provided an overcurrent detection apparatus comprising: a load driving FET for driving a load;
And means for detecting a drain-source voltage of the load driving FET, and means for calculating a value of a current flowing through the load based on the detected drain-source voltage, to detect an overcurrent. It is like that.

An overcurrent detecting device according to a second aspect of the present invention includes a temperature measuring circuit for measuring the temperature of the load driving FET, based on the detected drain-source voltage and the measured temperature. The current value flowing to the load is calculated.

An overcurrent detection device according to a third aspect of the present invention includes at least one or both of a means for stopping the driving of the load when an overcurrent is detected and a means for warning that there is an overcurrent.

[0010]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to the drawings. FIG. 1 is a circuit block diagram showing an embodiment of the present invention, wherein 1 is an overcurrent detection device, 2 is a power supply, and 3 is a load. The overcurrent detection device 1 includes a field effect transistor (hereinafter, referred to as an FET) 4 for driving a load 3, an amplifying circuit 5 for detecting a voltage between a drain and a source of the FET 4 and amplifying the voltage, and a resistor 6.
A temperature measurement circuit 8 composed of
The microcomputer 9 outputs a drive signal to the ET 4 and controls the FET 4. The microcomputer 9 receives the FE input from the amplification circuit 5.
Current flowing to the load based on the drain-source voltage of T4 and the temperature input from the temperature measurement circuit (load current)
And a ROM 11 that is an internal storage device that stores temperature characteristic data of the on-resistance between the drain and source of the FET 4 and a determination value for determining whether or not the load current is an overcurrent. It is determined whether the load current is an overcurrent. Also, in the figure, reference numeral 12 denotes an overcurrent warning lamp for warning a user of an overcurrent;
Is a transistor for driving the overcurrent warning lamp 12, 14
Is a power supply for the overcurrent warning lamp.

Next, the operation of the overcurrent detecting device 1 having the above configuration will be described. In order to drive the load 3, the microcomputer 9 sends a drive signal (duty 10
0%), the FET 4 is turned on and the load 3
Load current flows through the At this time, the drain of the FET 4
A voltage corresponding to the load current is output between the sources. This voltage is the product of the on-resistance between the drain and source of the FET 4 and the load current, and since the voltage value is several tens to several hundreds mV, after being amplified in the amplifier circuit 5, the load current of the microcomputer 9 is calculated. Input to the means 10. On the other hand, the on-resistance between the drain and the source of the FET 4 is almost constant, but changes with temperature.
Is stored in the ROM 11 in advance, and the FE measured by the temperature measurement circuit 8 including the resistor 6 and the thermistor 7 is stored.
The temperature of T4 is input to the load current calculation means 10 of the microcomputer 9. The load current calculation means 10 reads the FE at the temperature from the ROM 11 based on the temperature data.
After reading the drain-source ON resistance of T4, the load current is calculated from the drain-source voltage of the FET 4 input from the amplifier circuit 5 and the drain-source ON resistance. The microcomputer 9 determines whether the load current is an overcurrent by comparing the calculated load current with a determination value stored in the ROM 11. Further, if it is determined that an overcurrent has occurred, the microcomputer 9 controls the FET 4 to stop the driving of the load 3 by stopping the FET 4, and turns on the transistor 13 to turn on the overcurrent warning lamp 12. To warn the user of overcurrent. The power supply 14 of the overcurrent warning lamp 12 may share the power supply 2 of the load. Further, a light emitting diode, a buzzer, or the like can be used instead of the overcurrent warning lamp 12.

[0012]

As described above, according to the first aspect of the present invention, means for detecting the drain-source voltage of a load driving FET for driving a load, and the detected drain-source voltage Means for calculating the value of the current flowing to the load based on the above is provided so as to detect overcurrent, so that a small and inexpensive overcurrent detection device can be obtained without using a current sensor or a shunt resistor. it can.

According to the second aspect of the present invention, there is provided a temperature measuring circuit for measuring the temperature of the load driving FET, and the current value flowing through the load is determined based on the drain-source voltage and the temperature of the FET. Since the calculation is performed, the load current can be more accurately obtained, and the detection accuracy of the overcurrent can be improved.

Further, according to the third aspect of the present invention, when the overcurrent is detected, the driving of the load is stopped or a warning that the overcurrent occurs is issued, so that the safety can be improved.

[Brief description of the drawings]

FIG. 1 is a circuit block diagram showing an embodiment of the present invention.

FIG. 2 is a circuit block diagram showing a conventional overcurrent detection device.

[Explanation of symbols]

1 Overcurrent detection device, 2 power supply, 3 load, 4 FE
T, 5 amplifying circuit, 6 resistor, 7 thermistor, 8 temperature measuring circuit, 9 microcomputer, 10 load current calculating means, 11 ROM, 12 overcurrent warning lamp,
13 Transistor, 14 Power supply (for warning lamp)

Claims (3)

[Claims] 1. A load driving FET connected to a load for driving the load, and a drain of the load driving FET.
Means for detecting a source-to-source voltage and means for calculating a value of a current flowing to the load based on the detected drain-source voltage, so as to detect an overcurrent. Current detector. 2. The overcurrent detecting device according to claim 1, further comprising a temperature measuring circuit for measuring a temperature of said load driving FET. 3. The overcurrent detecting device according to claim 1, further comprising at least one or both of a means for stopping the driving of the load and a means for warning of an overcurrent when an overcurrent is detected. .