JPS62266426A - Temperature detecting circuit - Google Patents

Abstract

PURPOSE:To enable the condition of the disconnection of a thermistor to be surely detected without reducing a temperature detecting accuracy by connecting a diode in the parallel circuit of a characteristic compensating resistor and the thermistor in a direction wherein a voltage is applied to the thermistor. CONSTITUTION:In a normal condition wherein a thermistor Rt is not disconnected, the thermistor Rt forming one arm of a resistance bridge circuit and a characteristic compensating resistor R4 are connected in parallel via a diode D. Thus, a resolution in a temperature detection in a relatively high temperature region can be increased. Further, since the backward diode D is connected between a thermistor disconnection detecting resistor R5 and the characteristic compensating resistor R4, a power supply voltage or an earth voltage is outputted via the thermistor disconnection detecting resistor R5 as a thermistor output voltage when the thermistor Rt is disconnected. Accordingly, since a greatly different voltage from the thermistor output voltage is outputted in the normal condition wherein the thermistor Rt is not disconnected, the condition of the disconnection of the thermistor Rt can be surely detected.

Description

[Detailed Description of the Invention] Technical Field> The present invention relates to a temperature detection circuit used in a device that detects the temperature of an object whose temperature is to be controlled and performs temperature control, and particularly relates to a temperature detection circuit that detects the temperature of an object whose temperature is to be controlled. Reliable detection of

<Summary of the Invention> The temperature detection circuit according to the present invention can be used, for example, in a copying machine.
This is used for the temperature control circuit of the fixing device provided therein. In particular, in order to prevent abnormal temperature control from being performed due to a thermistor used as a temperature detection element being disconnected, this device detects the disconnection of the thermistor, and accurately detects the temperature of the object. At the same time, disconnection of the thermistor can also be reliably detected.

<Prior Art and Its Disadvantages> Conventionally, in order to control the fixing temperature of a fixing device in, for example, a copying machine, a resistance bridge circuit using a thermistor Rt as a temperature detection element as shown in FIG. 4 has been used. Since the resistance value of the thermistor changes depending on the temperature, the potential at point a in the figure changes depending on the temperature. By comparing the potential at point a with the reference potential at point b using comparator A, it is possible to detect whether the temperature of the fixing device is higher or lower than the reference temperature.

The resistance value characteristic of a thermistor with respect to temperature is usually R=R
It is expressed as a exp (C/T). Here, C is a constant and T is an absolute temperature. Therefore, the potential at point a in FIG. 4 has a characteristic represented by ■ in FIG. 5. When such a temperature detection circuit is used as a temperature control circuit for a fusing device of a copying machine, it is necessary to sensitively detect the temperature near the fusing device (usually 150°C to 200°C). Therefore, very high detection accuracy (resolution) cannot be obtained.

In order to improve the temperature detection accuracy in a relatively high temperature region, a circuit is used in which the characteristic compensation resistor R4 shown by the broken line in FIG. 4 is connected in parallel to the thermistor Rt. In the case of a temperature detection circuit that has undergone characteristic compensation, the detection accuracy is high in a relatively high temperature range such as the fixing temperature, but the detection accuracy is decreased in a low temperature range, as shown by (2) in FIG.

By the way, in devices that heat objects to a relatively high temperature, such as the fusing device of a copying machine, a thermistor disconnection state is detected in order to prevent an abnormal rise in temperature that would occur if the thermistor were disconnected. There is a need to. The state where the thermistor is disconnected is when its resistance value is infinite, and corresponds to the characteristics of an extreme state at low temperature. Therefore, when the thermistor is disconnected, the potential at point a has a value of 0 as shown in FIG. Therefore, the potential at point a is ■0
It is possible to detect that the thermistor is disconnected by detecting that the thermistor is disconnected, but when the object to be temperature controlled has not yet warmed up, such as immediately after the power is turned on, the resistance of the thermistor is quite high, and the thermistor is disconnected. The output voltage is almost the same as VO. Furthermore, even if the target object is controlled at a predetermined temperature, there is a risk of false detection due to variations in circuit constants, noise, etc. due to the low detection accuracy in the low temperature range as mentioned above. .

<Object of the Invention> The object of the invention is to provide a temperature detection circuit that can reliably detect a thermistor disconnection state based on the thermistor output voltage without reducing the detection accuracy (resolution) of temperature detection in a relatively high temperature range. Our goal is to provide the following.

<Configuration and Effects of the Invention> In a temperature detection circuit connected in parallel to detect temperature from the output of the bridge circuit and to detect a disconnection state of the thermistor based on the thermistor output voltage, the characteristic compensation resistor A diode is inserted in a parallel circuit of the thermistor in a direction in which a voltage is applied to the thermistor, and a thermistor disconnection detection resistor is connected between the thermistor and the power source.

With the above configuration, in a normal state where the thermistor is not disconnected, the thermistor and the characteristic compensation resistor constituting one side of the resistance bridge circuit are connected in parallel via the diode. Therefore, the resolution of temperature detection can be improved in a relatively high temperature region. Also,
Since a diode in the opposite direction is connected between the thermistor disconnection detection resistor and the characteristic compensation resistor, if the thermistor is disconnected, the thermistor output voltage will be connected to the power supply voltage or ground via the thermistor disconnection detection resistor. Voltage is output. Therefore, in a normal state where the thermistor is not disconnected, a voltage that is significantly different from the thermistor output voltage in the low temperature range is output, so that the disconnection state of the thermistor can be reliably detected.

<Example> FIG. 1 shows a temperature detection circuit according to an embodiment of the present invention.

In the figure, the thermistor Rt is connected in parallel with a characteristic compensation resistor R4 via a diode D. This parallel-connected circuit constitutes one side of the resistor bridge, and resistors R1,
R2 and R3 constitute the other three sides of the resistor bridge. Comparator At detects the potential difference between point a and point b, and outputs the detection signal to output terminal 0UTI.

Since the diode D is inserted in the direction in which the voltage is applied to the thermistor Rt via the resistor R1, the potential at point a is equal to the current 11 flowing through the thermistor and the current 12 flowing through the characteristic compensation resistor R4, as shown in the figure. Determined by Note that the resistor R5 is a thermistor disconnection detection resistor, but the resistor R1
, R4 and thermistor RL, the current i3 flowing from the thermistor disconnection detection resistor R5 to the thermistor Rt is much smaller than the current 11. Therefore, the influence of the thermistor disconnection detection resistor on the temperature detection characteristics can be ignored.

When thermistor Rt is disconnected, currents i1 and 13 stop flowing, and the potential at point C becomes the power supply voltage Vc. Comparator A2 compares the reference potential at point d obtained by resistors R6 and R7 connected to the power supply with the potential at point C, and outputs the detection signal to output terminal 0UT2. By setting the potential at point d between the potential generated at point C and the potential of power supply voltage VC in the low temperature region, it is possible to reliably detect a disconnection state of the thermistor.

Next, an example of specific circuit constants and their characteristics will be shown.

As a thermistor, it has the characteristics shown in Figure 2, for example, its resistance value is 1.5Ω at 170°C, and 1.5Ω at 190°C. The following values can be considered as circuit constants for increasing the resolution around 180"C using an OKΩ resistor.

Power supply voltage Vc: 20V R1: 5. IKΩ R2:5. IKΩ R3: 590Ω R4: 1.5Ω R55MΩ If the circuit constants are determined as above and the circuit shown in Figure 1 is configured, the potential at point a and The potential at point C has characteristics as shown in FIG. As shown in the figure, even in the extremely low temperature region around -20°C, the potential at point C only increases to about 4.2cm, which is the same as the power supply voltage (20V) generated at point C when the thermistor is disconnected. It's very different compared to Therefore, it is possible to clearly distinguish between the thermistor output voltage in a low temperature region and the thermistor output voltage in a disconnected state of the thermistor.

In the embodiment shown in FIG. 1, temperature is detected by detecting the potential at point a with reference to the potential at point b, but in a normal state where the thermistor is not disconnected, point a→ Since the potential difference between ÷→ and point C is only the forward voltage drop of the diode (approximately 0.7 V), temperature can also be detected by detecting the potential at point C.

However, it is also possible to configure a circuit by reversing this positional relationship and accordingly reversing the direction of the diodes. In this case, a disconnection state of the thermistor can be detected by detecting that the thermistor output voltage has become substantially equal to the ground voltage.

[Brief explanation of drawings]

Fig. 1 is a diagram showing a temperature detection circuit that is an embodiment of the present invention, Fig. 2 is a diagram showing the temperature characteristics of a specific thermistor, and Fig. 3 is a diagram showing the relationship between a specific temperature and the thermistor output voltage. FIG. 4 is a diagram showing a conventional temperature detection circuit, and FIG. 5 is a diagram for explaining the characteristics of the conventional temperature detection circuit. Rt - thermistor, R4 - resistance for characteristic compensation, R5 - resistance for thermistor disconnection detection, R1, R2, R3, R4, Rt - resistance bridge, D - diode gain.

Claims (1)

[Claims] (1) A resistance bridge circuit including a thermistor is provided, a resistance for characteristic compensation is connected in parallel to the thermistor, temperature is detected from the output of the bridge circuit, and a disconnection state of the thermistor is detected based on the thermistor output voltage. In the temperature detection circuit, a diode is inserted in a parallel circuit of the characteristic compensation resistor and the thermistor in a direction in which a voltage is applied to the thermistor, and a diode is inserted between the thermistor and the power source for detecting disconnection of the thermistor. A temperature detection circuit characterized by connecting a resistor.