JPH0562773A - Temperature detecting device for induction-heated cooking device - Google Patents

Abstract

PURPOSE:To detect the temperature of a pot with no time lag and dispense with constituting parts. CONSTITUTION:The input current I from an AC power source 7 is rectified and fed to an induction coil 5, and a pot 3 is heated by the induction coil 5 in an induction-heated cooking device l. A temperature detecting device has an input current change quantity detecting means 16, an A/D converting means 20, and a temperature judging/processing means 21. The input current 1 is changed by the temperature change of the pot 3. The input current I (primary current) is converted into the secondary current by a current transformer 15, thus the change quantity of the input current I is indicated as the change quantity of the secondary side voltage Vi. An input current change quantity detecting means 16 detects only the change portion of the input current I, i.e., the secondary side voltage Vi, and the voltage V0 is outputted. The A/D converting means 20 converts the output voltage V0 of the input current change quantity detecting means 16 into a digital value. The temperature judging/processing means 21 judges the temperature corresponding to the digital value converted by the A/D converting means 20.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a temperature detecting device used in induction heating cookers such as induction heating rice cookers and electromagnetic cookers.

[0002]

2. Description of the Related Art Conventionally, as a temperature detecting device for a pan of an induction heating cooker, a temperature detecting element such as a thermistor is brought into contact with the pan to directly detect the temperature, or the temperature detecting element is inserted through a space. Generally, the temperature is indirectly detected by placing it close to the pan.

[0003]

However, in the conventional temperature detecting device, since the temperature detecting element detects the heat transmitted from the pot by heat conduction or radiation, there is a time delay in the temperature detection, and it is difficult to add or subtract a slight amount of heat. Met. Especially in the case of directly detecting the temperature of the pan with the temperature detecting element, a complicated structure including a spring for biasing the temperature detecting element toward the pan and a supporting member for supporting the spring so that the temperature detecting element is pressed against the pan. Since a large movable mechanism is required, the number of parts is large and the assemblability is poor. Further, since the temperature detecting element projects from the cooking surface and interferes with the cleaning, there is a problem that it is difficult to care for the cooking device. An object of the present invention is to solve such a problem, and an object thereof is to provide a temperature detection device for an induction heating cooker that can detect the temperature of a pan without a time lag and that does not require any mechanical parts. is there.

[0004]

In an induction heating cooker, the electric power Ps supplied to a metal pot is generally represented by Ps = KSIc ² √ (fμρ ₀ (1 + αt)). Where K is the coupling coefficient between the induction coil and the pan,
S is the metal area, Ic is the induction coil current, f is the frequency,
μ is the magnetic permeability, ρ ₀ is the specific resistance of the metal at 0 ° C., α is the temperature coefficient, and t is the temperature rise. On the other hand, the input power Pi
Is expressed as Pi = IV, where V is the input voltage and I is the input current.

Since Pi = Ps and the input current I≈induction coil current Ic, I = V / KS√ (fμρ ₀ (1 + αt)). As a result, when V and f are constant, the input current I
Is a function of the temperature rise t, and it is clear that the input current I changes with the temperature rise t of the pan. However, α
= 1.5 to 6 × 10 ⁻³ , and the temperature rise t is 100.
Even if the temperature is ° C, the input current I changes only by about 10%, so that accurate temperature detection cannot be performed only by measuring the input current I.

Therefore, the present invention rectifies an input current from an AC power source and supplies the rectified current to an induction coil, and the induction coil is used to detect the temperature of a pan of an induction heating cooker. Input current change amount detection means for detecting only a changed portion of current, A / D conversion means for converting the change amount of input current detected by the input current change amount detection means into a digital value, and the A / D conversion And temperature determination processing means for determining the temperature corresponding to the digital value converted by the means.

[0007]

An embodiment of the present invention will be described below with reference to the drawings. FIG. 1 shows a circuit diagram of an induction heating rice cooker including a temperature detecting device according to the present invention. In the figure, a rice cooker 1 comprises a container body 2, a pot 3 housed in the container body 2, and a lid body 4 for covering the container body 2. An induction coil 5 is arranged in the container body 2. This induction coil 5
Is connected in series with the switching transistor 6 and is connected to a DC power supply which is obtained by rectifying the AC power supply 7 with the full-wave rectifier circuit 8 and smoothing it with the choke coil 9 and the smoothing capacitors 10 and 11.

A capacitor 12 forming an LC resonance circuit together with the induction coil 5 is connected in parallel to the induction coil 5, and a diode 13 is connected between the collector and the emitter of the transistor 6. Transistor 6
A transistor drive pulse control circuit 14 is connected to the base of the circuit, and this circuit 14 has a collector-emitter voltage V
A drive pulse is output to the base of the transistor 6 based on ce.

On the other hand, a current transformer 15 is provided on the power supply line of the AC power source 7, and an input current change amount detection circuit 16 is connected to the secondary side of the current transformer 15. As shown in FIG. 2, the input current change amount detection circuit 16 includes an input terminal 1
A variable resistor VR is connected between 7a and 17b, and
A first detection circuit 18a in which a diode D ₁ , a Zener diode ZD ₁ and a smoothing capacitor C ₁ are connected in series is connected, and a diode D ₂ , a Zener diode ZD ₂ and a first detection circuit 18a are connected in parallel with the first detection circuit 18a. The second detection circuit 18b in which the smoothing capacitor C ₂ is connected in series is connected.

The diode D of the first detection circuit 18a₁as well as
Zener diode ZD₁Are the second detection circuits 18
diode D of b₂, Zener diode ZD₂To the opposite polarity
It is connected. Each smoothing capacitor C₁, C₂Both poles
Between each of the resistance R₁, R_Two Connected to each flat
Smoothing capacitor C₁, C₂On the Zener diode connection side of
The output terminals 19a and 19b are respectively connected. So
The output terminal 19 of the input current change amount detection circuit 16
As shown in FIG. 1, the A and D converters 20a and 19b are
A temperature determination processing circuit 21 incorporating therein is connected.

The A / D converter 20 converts the analog voltage between the output terminals 19a and 19b of the input current change amount detection circuit 16 into a digital value. In the temperature determination processing circuit 21, the temperature change curve of the pot 3 with respect to the change of the input current (hereinafter, input current-
It is called a temperature change curve. ) Is set and stored.
Then, the temperature determination processing circuit 21 outputs the input current −
According to the temperature change curve, the temperature corresponding to the digital value is determined from the digital value converted by the A / D converter 20, and if the temperature is a predetermined cooking temperature, the transistor drive pulse control circuit 14 is determined. It outputs a stop signal.

In the induction heating rice cooker having the above structure,
When a drive pulse is applied from the transistor drive pulse control circuit 14 to the base of the transistor 6 to turn on the transistor 6 and a current is passed through the induction coil 5 for a predetermined time and then the transistor 6 is turned off, a high frequency current is generated in the induction coil 5. As a result of an eddy current being generated in the conductive pot 3 placed in the alternating magnetic field generated by this high-frequency current, the pot 3 heats up and rice is cooked.

The transistor drive pulse control circuit 14 is
In order to prevent the high frequency current flowing through the induction coil 5 from being attenuated with time, the transistor 6 is turned on at a certain point and then turned off after a certain period of time to maintain a high high frequency current. The timing for turning on the transistor 6 is the voltage V between the collector and the emitter of the transistor 6.
This is a time point when ce becomes a minimum and power consumption during switching is minimized.

During the cooking of rice by such induction heating, the temperature of the pan 3 at the time of boiling, the time of cooking after the water disappears, etc. is detected by the temperature detecting device according to the present invention. Hereinafter, the temperature detection operation at the time of cooking will be described. The input current I (primary current) from the AC power supply 7 is transformed into a secondary current I ′ by the current transformer 15. Therefore, the change amount of the input current I depends on the secondary side voltage Vi of the current transformer 15.
Can be considered as the amount of change in. This secondary voltage Vi is
Input terminals 17a and 17b of the input current change amount detection circuit 16
Entered in. The upper part of FIG. 3 shows the waveform of the secondary voltage Vi.

As for the secondary voltage Vi, the positive half-wave of the alternating current is rectified by the diode D ₁ of the first detection circuit 18a, and only the portion above the Zener voltage Vz is extracted by the Zener diode ZD ₁ . 3 The waveform becomes as shown by the two-dot chain line in the lower stage. Further, the rectified secondary voltage is smoothed by the smoothing capacitor C ₁ . In addition,
Due to the presence of the resistor R ₁ , the electric charge charged in the capacitor C ₁ is discharged through the resistor R ₁ , so that an inclined voltage waveform is obtained as shown in the lower part of FIG. Similarly, the AC negative half-wave of the secondary voltage Vi is rectified and smoothed by the second extraction circuit 18b.

Now, it is clear from the above equation that the input current I decreases as the temperature of the rice cooker 3 rises.
Due to the change of the input current I, the voltage waveform of the secondary side voltage Vi of the current transformer 15 changes to a waveform having a low peak portion when the temperature changes from low temperature to high temperature, as shown in the upper part of FIG. And
Only the changing portion of the secondary side voltage Vi is extracted by the input current change amount detecting circuit 16 and is output between both electrodes of the capacitors C ₁ and C ₂ . Therefore, although the change of the input current I due to this temperature change is about 10% as described above, the changed portion of the secondary side voltage Vi caused by the change of the input current I is extracted by the input current change amount detection circuit 16. .. In this way, the voltages V ₁ and V ₂ extracted by the input current change amount detection circuit 16 are superimposed and output.
It is output as Vo from 19b. This change in the output voltage V ₀ changes the input current I, that is, the temperature change t of the pan 3.
Corresponding to.

Next, the output voltage Vo of the input current change amount detection circuit 16 is converted into a digital value by the A / D converter 20 of the temperature judgment processing circuit 21. The temperature judgment processing circuit 21 judges the temperature corresponding to the digital value of the output voltage V ₀ according to the input current-temperature change curve, and if this temperature is the rising temperature, the stop signal is sent to the transistor drive pulse control circuit 14. Is output. As a result, the transistor drive pulse control circuit 14 turns off the transistor 6, the heating of the pan 3 is stopped, and the rice cooking is completed. By using a differential amplifier using an operational amplifier or the like as the input current change amount detection circuit 16, a voltage corresponding to the Zener voltage Vz is subtracted from the secondary side voltage of the current transformer 15,
The same purpose can be achieved by increasing the voltage of Vz or more to an appropriate magnitude.

[0018]

As is apparent from the above description, according to the present invention, the temperature of the pot is detected by the change of the input current without a time lag, so that it is possible to perform delicate cooking with heat. Further, since a temperature detecting element or the like for directly detecting the temperature of the pot is not used, the heating surface has no unevenness, does not disturb the cleaning, and is easy to maintain. Furthermore, since there is no spring for pressing the temperature detecting element against the pan and a support mechanism for supporting the spring as in the conventional case, the structure is simple and the assembly is easy. ..

[Brief description of drawings]

FIG. 1 is a circuit diagram of an induction heating rice cooker including a temperature detecting device according to the present invention.

FIG. 2 is a circuit diagram of an input current change amount detection circuit of FIG.

FIG. 3 is a voltage waveform diagram of an input voltage and an output voltage of the input current change amount detection circuit of FIG.

[Explanation of symbols]

1 ... Induction heating rice cooker, 3 ... Pot, 5 ... Induction coil, 7 ... AC power supply, 16 ... Input current change amount detection circuit, 20 ... A / D
Converter, 21 ... Temperature determination processing circuit.

Claims (1)

[Claims] 1. A temperature detecting device for rectifying an input current from an AC power source and supplying the rectified current to an induction coil, and detecting the temperature of the pan of an induction heating cooker in which the induction coil heats the pan. Input current change amount detecting means for detecting only the input current change amount, A / D converting means for converting the change amount of the input current detected by the input current changing amount detecting means into a digital value, and the A / D converting means for converting And a temperature determination processing means for determining the temperature corresponding to the digital value.