JP2004120328A - Hysteresis comparator circuit and overheat prevention circuit - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To prevent malfunctions even when a pulse-like inductive noise due to electrostatic discharge or the like is mixed in the input of a comparator. <P>SOLUTION: Since positive feedback is provided with integration characteristics by providing not only a positive feedback resistor R4 but also an integration circuit 13 by a resistor R5 and a capacitor C in the positive feedback of the comparator 12, even when the pulse-like inductive noise due to the electrostatic discharge or the like is mixed in the input of the comparator 12, voltage fluctuation in the noninverted input terminal can be suppressed to be small and malfunctions by the pulse-like inductive noise are prevented. <P>COPYRIGHT: (C)2004,JPO

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a hysteresis comparator circuit and a circuit for preventing overheating of various heating elements such as a heater and an engine.
[0002]
[Prior art]
Generally, in this type of overheating prevention circuit, a temperature change of a target heating element is monitored by a temperature sensor such as a thermistor disposed in the vicinity thereof, and the temperature change monitored by the temperature sensor is regarded as a voltage change. By inputting to the inverting input terminal of the comparator and comparing with the reference voltage corresponding to the predetermined temperature input to the non-inverting input terminal, it is detected whether or not the temperature reaches the preset predetermined temperature, and exceeds the reference voltage. In such a high temperature state, an overheat is prevented by outputting an output for shutting off power to the heating element from the comparator (for example, see Patent Document 1).
[0003]
In addition, with regard to this type of comparator, when the input voltage of the comparator approaches the reference voltage, the output changes frequently due to noise being mixed into the input. There is known a comparator in which a positive feedback using a positive feedback resistor is applied to a comparator so as to have a hysteresis characteristic (for example, see Patent Document 2). In other words, the operation of switching the output of the comparator between the H level and the L level is not one threshold level, but two threshold levels having a predetermined level width (threshold level for switching to H level and L level). (Threshold level for switching).
[0004]
[Patent Document 1]
JP 05-257545 A [Patent Document 1]
JP-A-11-215811
[Problems to be solved by the invention]
According to Patent Documents 1 and 2, by providing a positive feedback resistor between a non-inverting input and an output of a comparator, an open / close control output to a switch element to be controlled with respect to a constant fluctuation of an input voltage to the comparator. The hysteresis input function that holds
[0006]
However, in the case of a configuration using a comparator that only has a hysteresis characteristic by applying a positive feedback using a positive feedback resistor, for example, pulse-like inductive noise caused by electrostatic discharge or the like is generated from a temperature sensor such as a thermistor. If mixed into the signal, an H-level output will be output from the comparator at that point, causing a hysteresis operation, and the output of the comparator may malfunction due to the pulse-like induction noise. is there.
[0007]
SUMMARY OF THE INVENTION It is an object of the present invention to prevent a malfunction from occurring even when pulse-like induction noise caused by electrostatic discharge or the like is mixed in an input of a comparator.
[0008]
[Means for Solving the Problems]
According to a first aspect of the present invention, there is provided a comparator which compares the magnitude of an input voltage input to an inverting input terminal with a reference voltage input to a non-inverting input terminal, and generates an output from an output terminal according to the comparison result. A positive feedback resistor connected between the output terminal of the comparator and the non-inverting input terminal, wherein a positive and negative input between the output terminal and the non-inverting input terminal of the comparator is provided. An integration circuit including a resistor and a capacitor connected in a feedback path is provided.
[0009]
Therefore, by providing not only a positive feedback resistor for the positive feedback of the comparator but also an integrating circuit including a resistor and a capacitor, pulse-like induction noise due to electrostatic discharge or the like may be mixed into the input of the comparator. In addition, the voltage fluctuation at the non-inverting input terminal can be suppressed small by the integration characteristic in the positive feedback, and the comparator output can be prevented from changing, so that malfunction due to pulse-like induction noise can be prevented.
[0010]
An overheat prevention circuit according to a second aspect of the present invention includes the hysteresis comparator circuit according to the first aspect, wherein an input voltage corresponding to a temperature of a heating element to be detected is input to the inverting input terminal of the comparator. The energizing operation for the heating element is controlled according to the output of the output terminal.
[0011]
Therefore, since the hysteresis comparator circuit according to the first aspect is provided, pulse-like induction noise caused by electrostatic discharge or the like may be mixed into an input input to the comparator according to the temperature of the heating element. Also, no malfunction occurs in the operation for preventing overheating.
[0012]
BEST MODE FOR CARRYING OUT THE INVENTION
An embodiment of the present invention will be described with reference to the drawings. The hysteresis comparator circuit of the present embodiment shows an application example incorporated in an overheat prevention circuit for a rewritable printer, for example.
[0013]
FIG. 1 is a side view showing a schematic configuration of the rewritable printer. In this rewritable printer, for example, a heater unit 5 for erasing print media images is provided on a transport path formed by print media transport rollers 1, 2, 3, 4, and a platen roller 6 and a thermal head for printing. 7 are provided. Here, the print medium 8 is formed of an insulating material together with the transport rollers 1 to 4. Further, a thermistor RTH, which is a temperature sensor for detecting a temperature for preventing overheating, is provided for the heater unit 5.
[0014]
As shown in FIG. 2, the hysteresis comparator circuit 11 of the present embodiment for the overheating prevention circuit of the heater unit 5 has an OP amplifier configuration having an inverting input terminal indicated by − and a non-inverting input terminal indicated by +. Is mainly configured. The inverting input terminal of the comparator 12 is connected to the midpoint between the current limiting resistor R1 connected between the + 5V power supply and the ground and the thermistor RTH. The non-inverting input terminal of the comparator 12 is connected to the connection midpoint between the reference voltage generating resistors R2 and R3 connected between the + 5V power supply and ground. Further, a positive feedback resistor R4 is connected in a positive feedback path between the output terminal Output of the comparator 12 and the non-inverting input terminal, and an integrating circuit 13 including a resistor R5 and a capacitor C is connected. Note that a pull-up resistor R6 is connected between the output terminal Output of the comparator 12 and the + 5V power supply.
[0015]
In such a configuration, the resistance value of the thermistor RTH disposed near the heater unit 5 decreases as the temperature of the heater increases, and the input voltage (thermistor output voltage) to the inverting input terminal of the comparator 12 decreases (FIG. 3 (a)). With respect to such a change in the input voltage (thermistor output voltage), whether the overheat prevention circuit (the output of the output terminal Output of the comparator 12) is at H level or L level is as shown in FIG. Has two levels.
[0016]
Under such circumstances, the input voltage corresponding to the heater temperature detected by the thermistor RTH reaches a reference voltage (overheat prevention circuit H level) corresponding to a predetermined temperature preset by the resistors R2, R3, and R4. In this case, the overheat prevention circuit (the output of the output terminal Output of the comparator 12) becomes H level.
This H-level signal is used as a heater power supply cutoff signal, and the power supply to the heater is cut off, thereby preventing the heater unit 5 from overheating. In this state, the output of the output terminal Output of the comparator 12 is positively fed back via the positive feedback resistor R4, so that the input voltage corresponding to the temperature of the heater detected by the thermistor RTH is previously set by the resistors R2, R3, R4. The operation is continued until the voltage drops to the reference voltage (overheat prevention circuit L level) corresponding to the set predetermined temperature (see FIG. 3B), and the hysteresis characteristic is shown. As a result, the overheat prevention circuit (the output of the output terminal Output of the comparator 12) is maintained at the H level as shown in FIG. 3C, and the overheat prevention circuit H level to the overheat prevention circuit L level are maintained. An overheating prevention operation that is not affected by a change in the input voltage can be performed.
[0017]
Here, as shown in FIG. 3A, consider a case where pulse-like induction noise is generated in the thermistor RTH due to electrostatic discharge or the like. For example, in the case of the rewritable printer shown in FIG. 1, all of the print media 8 come into contact with the transporting rollers 1 to 4 having an insulating property and receive a frictional force, so that static electricity is charged. When this approaches a conductive part such as the housing of the printer, discharge from the print medium 8 to the conductive part occurs. The electromagnetic field generated thereby mixes into a wire connecting the thermistor RTH and the overheat prevention circuit as pulse-like induction noise.
[0018]
This pulse-like induction noise may exceed (below) the overheat prevention circuit H level, for example, as shown in the example of the drawing when viewed from the output voltage of the thermistor RTH. However, considering the voltage at the non-inverting input terminal of the comparator 12, the positive feedback is provided through the integrating circuit 13 including the resistor R5 and the capacitor C. Is suppressed to a pulse state with small voltage fluctuation. Therefore, when viewed as the overheat prevention circuit (the output of the output terminal Output of the comparator 12), as shown in FIG. 3C, the circuit is maintained at the L level without malfunction due to pulse-like induction noise. Is done. Thereafter, when the original overheat is detected, a normal hysteresis operation is performed as described above.
[0019]
Although the present embodiment has been described as an example of application to an overheat prevention circuit for the heater unit 5 of the rewritable printer, the present invention is not limited to this, and can be similarly applied to overheat prevention circuits for various heating elements.
[0020]
【The invention's effect】
According to the hysteresis comparator circuit according to the first aspect of the present invention, not only a positive feedback resistor but also an integrating circuit including a resistor and a capacitor is provided for the positive feedback of the comparator, so that the pulse input caused by electrostatic discharge or the like is applied to the input of the comparator. Even if inductive noise is mixed in, the voltage fluctuation at the non-inverting input terminal can be suppressed small by the integration characteristic in the positive feedback, and the output of the comparator can be prevented from changing. Erroneous operation can be prevented.
[0021]
According to the overheat prevention circuit according to the second aspect of the present invention, since the hysteresis comparator circuit according to the first aspect is provided, the input to the comparator according to the temperature of the heating element has a pulse shape caused by electrostatic discharge or the like. Erroneous operation does not occur in the operation for preventing overheating even when the induction noise is mixed.
[Brief description of the drawings]
FIG. 1 is a schematic side view illustrating a configuration example of a rewritable printer to which an overheat prevention circuit according to an embodiment of the present invention is applied.
FIG. 2 is a schematic circuit diagram of an overheat prevention circuit including a hysteresis comparator circuit according to one embodiment of the present invention.
FIG. 3 is a waveform chart for explaining the operation.
[Explanation of symbols]
11 Hysteresis comparator circuit 12 Comparator 13 Integrator circuit R4 Positive feedback resistor R5 Resistor C Capacitor

Claims (2)

A comparator that compares the magnitude of the input voltage input to the inverting input terminal with the reference voltage input to the non-inverting input terminal, and generates an output according to the comparison result from the output terminal; and the output terminal of the comparator. A positive feedback resistor connected between the non-inverting input terminal, and a hysteresis comparator circuit,
A hysteresis comparator circuit comprising an integrating circuit including a resistor and a capacitor connected in a positive feedback path between the output terminal of the comparator and the non-inverting input terminal. 2. A hysteresis comparator circuit according to claim 1, wherein an input voltage corresponding to a temperature of a heating element to be detected is input to the inverting input terminal of the comparator, and the heating element is output according to an output of the output terminal of the comparator. An overheat prevention circuit, which controls an energizing operation of the circuit.

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