JPH06276674A - Safety protective circuit - Google Patents

Abstract

PURPOSE:To protect a varistor from burning even when the varistor has deteriorated. CONSTITUTION:A detection circuit (b) detects the amount of an input current carried from a power input terminal (A) through a varistor 1 to a power-supply circuit unit (i), and the detected data is entered in a microprocessor (a). The microprocessor (a) calculates a gap between the detected data and a normal input current data as the varistor 1 is in a normal state. The microprocessor (a) estimates the varistor 1 to be deteriorated when the gap is larger than a given value, and generates a command to operate a switching circuit (c). Then, the input current is turned off by the switching circuit (c) so that the varistor l is protected from burning by heat.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a safety protection circuit using a varistor to protect electrical equipment from excessive voltage such as surge.

[0002]

2. Description of the Related Art FIG. 4 is a circuit diagram showing a part of a power source for controlling a microwave oven. When a surge or the like due to induced lightning enters the AC line of the microwave oven, the control electronic circuit is destroyed. Therefore, in order to protect this, a safety protection circuit consisting of the pattern fuse 2 and the varistor 1 is provided on the primary side of the transformer 3. Is provided. That is, when an excessive voltage such as a surge is applied to the AC line, an excessive current flows due to the characteristics of the varistor 1 and the pattern fuse 2 is blown to protect the control electronic circuit and the like.

[0003]

However, in the case of the above-mentioned conventional example, when the varistor 1 deteriorates due to the application of an excessive voltage, there is a problem in terms of safety measures. That is, the pattern fuse 2 is affected by the leakage current of the varistor 1.
However, if the varistor 1 is destroyed before the melting, there is a possibility that it will generate heat and ignite.

The present invention has been made in view of the above background, and an object thereof is to provide an improved safety protection circuit which does not ignite even if the varistor deteriorates.

[0005]

In order to achieve the above object, the safety protection circuit of the present invention is connected between the power source input terminal side of an electric device and the main body circuit side. A switch circuit that cuts off the input current flowing to the varistor according to the command, a detection circuit that detects the magnitude of the input current, and data of the magnitude of the input current when the varistor is normal are prepared in advance. Therefore, the error between the data and the detection data output from the detection circuit is obtained, and it is determined whether or not the varistor is deteriorated according to the size of the error. When the determination result indicates varistor deterioration, the switch circuit is And a determination circuit that issues a command to operate the.

[0006]

The magnitude of the input current flowing from the power supply input terminal to the body circuit and the varistor is detected by the detection circuit.
This detection data is guided to the determination circuit. The judgment circuit finds an error between the data of the magnitude of the input current and the detection data when the varistor is normal. When the obtained error is larger than a predetermined value, it is determined that the varistor is deteriorated,
It issues a command to operate the switch circuit. Then, the input current is cut off by the switch circuit, and as a result, there is no possibility that the varistor will generate heat and fire.

[0007]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the safety protection circuit of the present invention will be described below with reference to the drawings. 1 is a circuit diagram of a safety protection circuit installed in a microwave oven, FIG. 2 is a block diagram of the microwave oven, and FIG. 3 is a program flow chart of a microprocessor.

The safety protection circuit provided in the power source for controlling the microwave oven will be described by way of example. A in the figure
Is a power supply input terminal of the control power supply. The transformer 3 is connected to the power supply line B led from the power supply input terminal A. The varistor 1 is connected in parallel to the primary side of the transformer 3, and the pattern fuse 2, the relay RY, and the current transformer CT are connected in series. A power supply circuit section i is connected to the secondary side of the transformer 3. Relay RY
Is a component of the switch circuit c, and the current transformer CT is a component of the detection circuit b.

The power supply circuit section i is a circuit for generating a power supply voltage for energizing the microprocessor a for controlling the entire microwave oven, the display section d, etc., and is composed of a diode bridge, a smoothing capacitor and the like. This power supply circuit section i is a separate system from the main power supply for operating the magnetron and the heater (not shown). Therefore, the input current flowing from the power supply input terminal A to the power supply circuit section i via the power supply line B is substantially constant regardless of whether the magnetron is on or off, as long as the varistor 1 and the like are normal. Here, the data of the magnitude of the input current when the varistor 1 is normal is I
_Shall be represented as _IN .

The switch circuit c has a transistor Tr and a relay RY, and a transistor T for buffering a command (defined as a control signal α) from the microprocessor a.
The coil of the relay RY is applied via r to open the contact of the relay RY, and the input current is cut off in response to the command. A diode D2 for malfunction prevention is connected in parallel to the coil of the relay RY.

The detection circuit b is mainly composed of a current transformer CT which detects an input current. The output current of the current transformer CT is converted into a voltage by a resistor R1, and the converted AC voltage is converted into a diode D1 and a resistor. It is configured to be rectified by a rectifying circuit composed of R2 and a smoothing capacitor C, and the rectified DC voltage is output to the microprocessor a as a detection signal β indicating the magnitude of the input current.
A discharge resistor R3 is connected in parallel to both ends of the capacitor C.

At the input port of the microprocessor a,
As shown in FIG. 2, in addition to the detection circuit b, a sensor unit f for detecting the temperature and humidity in the cooking chamber and a key operation unit g for inputting the operation of the microwave oven are connected. Similarly, in the output port, in addition to the switch circuit c, a microwave / heater heating section e for driving a magnetron and a heater, a start of cooking,
A speaker h that emits a buzzer sound or voice to notify the end and procedure and a display unit d that displays time and information are connected.

The microprocessor a is provided with a program necessary for controlling these constituent parts. Here, a program related to the safety protection circuit will be described with reference to FIG. 3 (the program causes the microprocessor a to function as a determination circuit described in the claims).

When a power switch (not shown) is turned on and the power circuit section i or the like is operated, first, the detection signal β is input, the magnitude of the input current is recognized by this, and this is detected data as a predetermined register. (S1).

Then, the I _IN data previously recorded in a predetermined memory is read out, the error between the detected data and the I _IN data is obtained, and it is recognized whether or not the varistor current flowing through the varistor 1 is fluctuating. . If the error is within the predetermined range, it is determined that the varistor 1 is normal, while if it exceeds the predetermined range, it is determined that the varistor 1 has deteriorated and is abnormal. The threshold value that serves as a criterion for determining whether or not the varistor 1 is normal is determined in consideration of the variation amount of the input current and the characteristics of the varistor 1 (S2, S3).

When it is determined that the varistor 1 has deteriorated and is abnormal, a warning is displayed on the display section d so that it can be understood that the varistor 1 has deteriorated. A warning sound may be output to the speaker h (S4). After performing this warning display for a certain period of time, the control signal α is output (S5, S6).

When the control signal α is output, the switch circuit c operates to cut off the input current flowing from the power input terminal A to the varistor 1 and the power circuit section i. As a result, heat generation of the varistor 1 also stops, and the power supply circuit section i also stops operating. Naturally, since the operation of the microprocessor a is also stopped, the power supply to the microwave oven is also stopped.

Since the series of programs are sequentially processed by the timer function of the microprocessor a at predetermined intervals, even if the varistor 1 deteriorates due to some cause, the varistor 1 generates heat and ignites. The electric power to the microwave oven is automatically stopped before the dangerous state of being turned on.

The safety protection circuit according to the present invention is not limited to the application only to the microwave oven, but can be applied to any electric equipment as long as it is a circuit using a Batastar.

[0020]

As described above, the safety protection circuit according to the present invention is
Detects the magnitude of the input current flowing from the power input terminal to the main circuit and varistor, judges whether the varistor has deteriorated, and if it judges that it has deteriorated, it cuts off the input current. Therefore, even if the varistor deteriorates, it does not ignite, and there is no possibility that the varistor will generate heat. Therefore, it is of great significance in improving the safety of electric devices.

[Brief description of drawings]

FIG. 1 is a diagram for explaining an embodiment of a safety protection circuit of the present invention, which is a circuit diagram of a safety protection circuit equipped in a microwave oven.

FIG. 2 is a block diagram showing an overall configuration of a microwave oven.

FIG. 3 is a program flowchart of a microprocessor.

FIG. 4 is a diagram for explaining a conventional safety protection circuit, which is a circuit diagram showing a part of a control power supply for a microwave oven.

[Explanation of symbols]

 1 Varistor A Power input terminal i Power circuit part B Power line b Detection circuit c Switch circuit a Microprocessor

Claims (1)

[Claims] 1. A safety protection circuit for protecting an electric device from an excessive voltage such as a surge by using a varistor, which is connected between a power input terminal side and a main circuit side of the electric device and is connected from the power input terminal to the main circuit. And a switch circuit that cuts off the input current flowing to the varistor in response to a command, a detection circuit that detects the magnitude of the input current, and data of the magnitude of the input current when the varistor is normal. Therefore, the error between the data and the detection data output from the detection circuit is obtained, and it is determined whether or not the varistor is deteriorated according to the magnitude of the error. A safety protection circuit comprising: a determination circuit that issues a command to operate the circuit.