JP2001028288A - Electric heater - Google Patents

Abstract

PROBLEM TO BE SOLVED: To suitably detect temperature of a heating plate, suitably heat based on the detection result by installing an electric heater in the heating plate, changing the electrical resistance value of the heater according to the temperature change of the heating plate, and controlling supply power to the heater, based on change of the electric resistance value. SOLUTION: A heater 2 is installed on the bottom surface of a heating plate 1. In the heating plate 1, an enameled metal plate, a mica plate, heat resistant glass, or fine ceramics can be used. The heater 2 is formed by applying a paste- like heating element to the heating plate 1 and baking the heating element. The hat 2 generates heat with AC voltage supplied from an AC power source. The heater 2 detects the electrical resistance value change according to temperature change caused by DC voltage applied and functions as a temperature sensor. A microcomputer drive-controls a photocoupler and a transistor based on a pulse signal, and thereby detects temperature of the heating plate 1 and controls power supply to the heater 2.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electric heater such as a hot plate and an electric heater.

[0002]

2. Description of the Related Art Generally, in an electric heater, electric power supplied to a heater provided on a heating plate is controlled based on a temperature detected by a thermistor, and energization to the heater is prohibited by a bimetal.

[0003]

However, in the above-described conventional electric heater, a temperature detecting means such as a thermistor or a bimetal is provided at a specific location such as an edge of the heating plate, and can detect the temperature in the vicinity thereof. is there. Therefore, when the temperature changes partially, such as when the object to be cooked is placed on a part of the heating plate, the heating by the heater may be inappropriate. For example, even when the object to be cooked is placed at a specific location on the hot plate and the temperature of that portion drops, if the location is away from the thermistor, heating by the heater becomes insufficient. In addition, the temperature detecting means must be attached separately from the heater, which takes time and effort.

Accordingly, an object of the present invention is to provide an electric heater capable of performing appropriate heating based on an appropriate detection result of the temperature of a heating plate with a simple configuration.

[0005]

According to the present invention, as a means for solving the above-mentioned problems, an electric heater is provided on a heating plate, and an electric resistance value is changed according to a temperature change of the heating plate. And control means for controlling the power supplied to the heater based on a change in the electric resistance value.

With this configuration, even if the temperature of the heating plate partially changes, the electric resistance value of the heater changes based on the change. Then, the change in the electric resistance value is detected, and the power supplied to the heater, that is, the heating amount is adjusted based on the detection result.

In this case, the control means may control the energization based on a divided voltage between the heater and a reference resistor connected in series to the heater.

Further, there is provided switching means for applying an AC voltage to the heater when heating the heating plate and applying a DC voltage to the heater when taking in a divided voltage between the heater and the reference resistor. I just need.

A bidirectional thyristor can be used for the switching means.

It is preferable that the electric resistance value of the reference resistor is variable in that the divided voltage can be adjusted to correct the variation in the resistance value of the heater.

[0011]

Embodiments of the present invention will be described below with reference to the accompanying drawings.

FIG. 1 shows a heating plate 1 of a hot plate as an example of an electric heater. A heater 2 is provided on a lower surface of the heating plate 1. As the heating plate 1, one obtained by subjecting a metal to enamel treatment, a mica plate, heat-resistant glass, fine ceramics, or the like can be used. The heater 2 is obtained by applying a paste-like heating element to the heating plate 1 and firing the heating element. The heating element is formed in a single stroke over the entire area of the heating plate 1 in various forms such as a spiral shape, a zigzag shape, etc., in addition to the crank shape shown in FIG. For example, silver /
A palladium-based paste, a ruthenium oxide-based paste, a carbon-based paste, or the like that has a large change in electric resistance with a temperature change (a material having a temperature coefficient such as PTC characteristics) is used. That is, it is sufficient if the temperature of the heating plate 1 can be detected by a change in the electric resistance value of the heater 2. Note that the surface of the heating element is covered with an insulating layer (not shown) obtained by applying and firing a paste-like insulator.

The heater 2 generates heat when an AC voltage is applied from an AC power supply 3 by the electric circuit shown in FIG. Further, the heater 2 functions as a temperature sensor when a DC voltage is applied and a change in electric resistance value due to a temperature change is detected as described later.

A bidirectional thyristor 4 serving as a switching means is connected between the AC power supply 3 and the heater 2. The bidirectional thyristor 4 includes a photocoupler 5 input to a gate terminal.
On / off in response to a signal from the controller 2 to permit / disable the energization of the heater 2. Further, a variable resistor 6 as a reference resistor is connected to the heater 2 in series. The variable resistor 6 is provided to correct the variation in the electric resistance value at normal temperature due to the processing of the heater 2. A DC voltage can be applied to and cut off from the heater 2 and the variable resistor 6 by turning a transistor 7 on and off. The divided voltage between the heater 2 and the variable resistor 6 when a DC voltage is applied is input to a microcomputer (hereinafter abbreviated as a microcomputer 9) via an amplifier circuit 8. The microcomputer 9 turns on / off the photocoupler 5 or the transistor 7. A pulse signal is input from the zero-cross detection circuit 10 to the interrupt input port of the microcomputer 9 every time a zero-cross point of the AC power supply 3 is detected. The microcomputer 9 detects the temperature of the heating plate 1 and controls the energization of the heater 2 by controlling the drive of the photocoupler 5 and the transistor 7 based on the pulse signal.

Next, the operation of the hot plate will be described with reference to the flowchart of FIG.

When a power switch (not shown) is turned on, first, zero-cross detection is performed based on a pulse signal from the zero-cross detection circuit 10 (step S1). Zero-cross detection is performed to remove the influence of noise superimposed on the AC voltage from the AC power supply 3. Then, the photocoupler 5 is turned on to drive the bidirectional thyristor 4,
An AC voltage is applied to the heater 2 (Step S2). Thereby, the heater 2 generates heat, and the heating by the heating plate 1 is started.

When a predetermined time has passed since the heating by the heater 2 (step S3), the photocoupler 5 is turned off,
The heating by the heater 2 is interrupted. Then, the transistor 7 is turned on, and a DC voltage is applied to the heater 2 and the variable resistor 6 (Step S4). At this time, a divided voltage between the heater 2 and the variable resistor 6 is input via the amplifier circuit 8. Therefore, the temperature of the heating plate 1 is detected based on the divided voltage (step S5). That is, since the electric resistance value of the heater 2 changes according to the difference in the temperature of the heating plate 1, the temperature of the heating plate 1 is estimated by detecting the change in the electric resistance value as the change in the divided voltage. In this case, since the heater 2 is formed over almost the entire area of the heating plate 1, the electric resistance value of the heater 2 changes even if the temperature changes partially. Therefore, even if the temperature changes in any part due to the placement of the object to be cooked, etc., the detection does not fail. The temperature detection is performed every 8 seconds at a low temperature when the temperature has not risen so much after the heating by the heater 2 is started, and then every 5 seconds when the temperature rises to a predetermined temperature. During this time, the heating by the heater 2 is interrupted, but the required time is as short as about 0.4 seconds, so that the heating state is not adversely affected.

Here, the detected temperature is compared with the set temperature (step S6). If the detected temperature exceeds the set temperature, heating by the heater 2 and temperature detection are continued, and the detected temperature becomes lower than the set temperature. For example, the photo coupler 5 is turned off, and the power supply to the heater 2 is stopped (step S
7). Thus, the temperature of the heating plate 1 is adjusted to the set temperature.

FIG. 4 shows how the temperature of the heating plate 1 is actually controlled when the temperature is detected by the heater 2 according to the present invention and when the temperature is detected by the bimetal according to the conventional example. 2 shows a graph comparing. In the case of controlling the temperature by the method according to the present invention, the actual temperature change of the heating plate 1 was detected at two points, that is, a center part O and a position A 50 mm away from the center part. In FIG. 4, the solid line indicates the actual temperature change of the heating plate 1 at the position O, and the dashed line indicates the actual temperature change of the heating plate 1 at the position A. In this method, when the food is placed at the position A (indicated by the horizontal axis a), thereafter,
The temperature difference between the position O and the position A opened up to about 12 ° C. (FIG. 4
It is indicated by t1 in the middle. ). On the other hand, when the temperature is controlled by the method according to the conventional example, similarly, at the position B where the bimetal is provided and the position C which is 50 mm away therefrom, the actual heating plate 1
Temperature change was detected. In FIG. 4, the two-dot chain line indicates the actual temperature change of the heating plate 1 at the position B, and the dotted line indicates the actual temperature change at the position C. In this method, when the food was placed at the position C (indicated by the horizontal axis b), the temperature difference between the position B and the position C opened by about 20 ° C. at the maximum (indicated by t2 in FIG. 4). As is clear from this graph, it was confirmed that appropriate temperature control can be performed by employing the method according to the present invention.

In the above embodiment, one heater 2 is provided over the entire area of the heating plate 1. However, the heater 2 may be divided into a plurality of areas, and the heater 2 may be provided for each area. As a result, a temperature change can be detected for each region, and it is possible to perform the appropriate temperature control by increasing the detection accuracy.

In the above embodiment, the case where the structure according to the present invention is applied to a hot plate has been described. However, other electric heaters, such as electric heaters, which need to detect a partial temperature change, are also applicable. Can be adopted.

[0022]

As is clear from the above description, according to the electric heater according to the present invention, the electric heater changes the electric resistance according to the difference in the temperature of the heating plate. No matter where the partial temperature change occurs, it can be regarded as a change in the electric resistance value of the heater, and appropriate temperature control can be performed. Further, since a temperature detecting means such as a thermistor is not required, not only the mounting operation is unnecessary but also the configuration can be simplified.

[Brief description of the drawings]

FIG. 1 is a bottom view of a hot plate of a hot plate according to the present embodiment.

FIG. 2 is an electric circuit diagram connected to the heater of FIG.

FIG. 3 is a flowchart illustrating control by the microcomputer of FIG. 2;

FIG. 4 is a graph showing a comparison result between a conventional example and this embodiment.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Heating plate 2 ... Heater 3 ... AC power supply 4 ... Bidirectional thyristor 5 ... Photocoupler 6 ... Variable resistance 7 ... Transistor 8 ... Amplification circuit 9 ... Microcomputer 10 ... Zero cross detection circuit

Claims (5)

[Claims] 1. A heater provided on a heating plate for changing an electric resistance value according to a temperature change of the heating plate, and a control means for controlling electric power supplied to the heater based on the change in the electric resistance value. An electric heater comprising: 2. The electric heater according to claim 1, wherein the control unit controls the energization based on a divided voltage between the heater and a reference resistor connected in series to the heater. 3. A switching means for applying an AC voltage to the heater when heating the heating plate, and applying a DC voltage to the heater when taking in a divided voltage between the heater and a reference resistor. The electric heater according to claim 2, characterized in that: 4. The electric heater according to claim 3, wherein said switching means is a bidirectional thyristor. 5. The electric heater according to claim 2, wherein an electric resistance value of the reference resistance is variable.