JP2529888B2 - Heating element with thermistor characteristics - Google Patents

Description

TECHNICAL FIELD The present invention relates to a heating element having thermistor characteristics.

b. Prior Art FIG. 2 is an electric circuit for a heating element using a resistor according to the prior art.

The current flowing through the resistor R _L as the heating element in such a circuit, it is possible to control the resistance of the resistor R _L and the control resistor R _C coupled in series, the control resistor R _C itself There is a problem that heat is generated. Further, since a typical resistor material is a metal such as nichrome wire, it is necessary to form a small-sized heating element that does not have a large specific resistance (or resistivity) and can apply a high voltage. It is difficult.

There is a material having a high resistivity for the thick film resistor, but when this material is used, the heat resistant temperature is limited and it is not so preferable for use as a heating element.

There is a thermistor with a positive characteristic called PTC, and it has the ability to self-control to a high resistance above a certain temperature, but there is an upper limit to the temperature setting, and it is difficult to set it above 300 ° C. Also, in the case of PTC, a large inrush current may be a problem. Further, it is a high resistivity material and has a temperature coefficient of resistance of 0.
Or there are few positive ones in practice.

c. Problems to be Solved by the Invention Focusing on high-resistivity materials, there is a resistor made of a semiconductor-based material having a so-called thermistor characteristic mainly composed of a metal oxide.

When a voltage is applied to cause the resistor having the thermistor characteristic to self-heat, the Joule heat flowing at this time reduces the resistance of the resistor. As a result, the current increases, the Joule heat further increases, the resistance further decreases, and this repetition continues until destruction occurs, causing so-called thermal runaway.
In order to control this, a method of using a constant current power supply device or connecting a fixed resistor for current limiting in series to simply prevent thermal runaway has been adopted.

When a constant current power supply device is used to control a heating element having a thermistor characteristic in this way, it becomes expensive, current rise due to resistance slows the start-up at low temperature, or heat generation on the fixed resistance side cannot be ignored during heat generation. There's a problem.

d. Means for Solving the Problems The above problems are solved by a heating element made of a material having a so-called thermistor characteristic in which the temperature coefficient of resistance is negative, and a heating element made of a capacitor connected in series to this element. It was

e. In the case of an AC circuit in which a resistor is connected in series to a working capacitor, if the resistance value of the resistor is changed, the current that flows in this circuit is determined by the resistance value, the capacitance, voltage and frequency of the capacitor. At this time, for the region where the resistance value is relatively small,
This change in current becomes small and the so-called constant current characteristic appears.

Therefore, even if the heat generation of the heating element progresses and the resistance value decreases, the value of the flowing current is constant, so that thermal runaway does not occur. In addition, since the current is constant, the voltage across the heating element decreases due to the progress of heat generation and the decrease in resistance value, the amount of power applied to the heating element decreases, and heat generation is suppressed. That is, this prevents thermal runaway.

f. Example The constant current control capability of the capacitor is stably exhibited in the region where the heating element has a low resistance.

As an effective example of the present invention, a heating element having a resistance value that does not enter the constant current region at room temperature but that enters the constant current region when heat is generated is used. At this time, it becomes possible to apply most of the power supply voltage to the heating element side when the power is turned on, and the power given by the product of voltage and current shows the maximum when the power is turned on, although it depends on the temperature coefficient of resistance. It is possible to obtain the characteristic that the maximum electric power is exhibited after the power is turned on and before the heat is generated, and further, the electric power is gradually reduced due to the subsequent heat generation and the characteristic is directed to the self-control direction. Further, depending on the selection of conditions such as the resistance value, it is possible to set the maximum power at the saturation temperature.

Such characteristics are close to the self-controllability with PTC. However, the difference from PTC is that the control temperature of PTC is determined by the heating element itself, whereas in the case of a heating element with capacitor and thermistor characteristics, the equilibrium point can be selected by the capacitor capacity, resistance value and B constant, For example, the same heating element can be set below 100 ° C or above 200 ° C depending on the conditions. In addition, even if the heat resistance of the heating element does not matter even if it is 300 ° C or higher, which is the temperature range that could not be set by PTC, it can be set.

FIG. 1 is a circuit diagram of a heating element according to the present invention, and FIG. 3 is a heating element having a thermistor characteristic, R ₂₅ = 7KΩ, B constant = 1,500K, and a capacitor C having a capacitance C of 1 μF.
The time change of the resistance value R of the heating element, the current I flowing through the heating element, the voltage V across the heating element, the heating power P in the heating element, and the temperature T of the heating element is shown.

The current does not perform constant current operation in the high resistance region and gradually increases as the resistance decreases, but eventually reaches a constant value.

Also, the voltage across the heating element decreases as the resistance value decreases, and the electric power given as the product of this and the current forms a maximum point on the way until the temperature stabilizes, and then decreases and enters the stable region.

Such an operation is preferable for a heating element in order to accelerate the rise of temperature. Further, it is shown that when the resistance value at the time of stability changes due to the heat load on the heating element, the applied power also changes accordingly. That is, when the heat load increases and the temperature of the heating element decreases, the resistance value increases and P
= 1 ² power by R increases. Also, the heat load becomes smaller,
When the heating element temperature rises, the resistance value decreases, and the electric power also decreases.

Since the resistance change is not as steep as PTC, it does not reach the constant temperature control, but it has the characteristic of correcting the self-heating value in response to load fluctuations.

Fig. 4 shows the same heating element (R ₂₅ about 7KΩ, B constant 1,500K)
It was measured under the condition that the capacitance of the capacitor was half, 0.47 μF. The applied power was reduced to less than half, and the heat generation temperature decreased from about 200 ℃ to about 100 ℃. The position of the power peak has also moved, and the characteristics have changed overall.

Thus, even with the same heating element, it is possible to change the characteristics by changing the conditions of the control side capacitor.

It is also possible to form the resistor having the thermistor characteristics and the capacitor on the same substrate. At this time, it is preferable to determine the capacitance while considering the temperature change of the capacitance of the capacitor.

g. Effects of the Invention i) It is possible to prevent and control thermal runaway by controlling the current with a capacitor for a heating element having thermistor characteristics that was difficult to control conventionally and used as a heating element.

ii) Even if the same heating element is used, the set heating temperature can be changed by changing the capacitance of the capacitor.

iii) When the temperature of the environment decreases, the amount of heat generated increases, and the temperature change is reduced.

iv) It is possible to reduce the resistance value of the resistor having the thermistor characteristic, bring the capacitor into a controlled state in a constant current region, and use the change in the voltage across the resistor as information on the temperature change for control.

v) The temperature change of the heating element due to the change of the air volume corresponding to the heating element having the thermistor characteristic can be converted into the change of the voltage across the heating element to obtain the information of the air volume change.

[Brief description of drawings]

FIG. 1 is a circuit diagram of a heating element according to the present invention, FIG. 2 is a circuit diagram of a heating element using a resistor according to the prior art, FIGS. 3 and 4
The figure shows the resistance value R of the heating element in the heating element according to the present invention,
It is a graph which shows the time change of electric current I, both-ends voltage V, heating power P, and heating temperature T.

Claims (5)

(57) [Claims] 1. A heating element made of a material having a thermistor characteristic having a negative temperature coefficient of resistance, a capacitor, and an AC power supply for supplying power to a series circuit including the heating element and the capacitor. A heating element that has the thermistor characteristics. 2. The heating element having thermistor characteristics according to claim 1, wherein the impedance of the capacitor is larger than the impedance of the heating element. 3. A current flowing through a heating element having a thermistor characteristic having a negative temperature coefficient of resistance value is adjusted by adjusting the capacity of a capacitor connected in series to the heating element.
A method for controlling a heating element having a thermistor characteristic, characterized by controlling the amount of heat generated by the heating element when an alternating current flows. 4. The method of controlling a heating element having thermistor characteristics according to claim 3, wherein the impedance of the capacitor is larger than the impedance of the heating element. 5. A method of controlling a heating element having a thermistor characteristic according to claim 4, wherein the voltage across the heating element is used as a temperature information signal.