JPH11281499A - Temperature detecting circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reliably detect disconnection of a thermistor by holding end-to-end voltage of the thermistor as a sample in a temperature measuring mode, outputting sample hold voltage in a disconnection detecting mode, and comparing the end-to-end voltage of the thermistor with threshold value voltage. SOLUTION: A thermistor 20 having a negative temperature coefficient characteristic is connected to one pole of a DC power source, and a reference resistance 21 is connected to the other pole between it and the other end of the thermistor 20 so as to be switched. Next, end-to-end voltage of the thermistor 20 is sampled in the prescribed timing by a sample hold means 22 to be held up to the next sample time. Next, a comparing means 23 compares the end- to-end voltage of the thermistor 20 with predetermined threshold value voltage, and a latch means 24 latches output of the comparing means 23 in the prescribed timing. A command generating means 25 outputs a switching command and a sample command switched between a temperature measuring mode and a disconnection detecting mode in which a resistance value of a reference resistance 21 is a first/second resistance values.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a temperature detection circuit, and more particularly to a temperature detection circuit capable of maintaining a temperature detection accuracy of a thermistor and reliably detecting a disconnection of the thermistor.

[0002]

2. Description of the Related Art A technique for measuring a temperature using a thermistor whose resistance value largely changes in accordance with a change in temperature is well known. FIG. 1 shows a basic circuit for temperature measurement using a thermistor. As a thermistor 10, an NTC (Negative Tempe) whose resistance decreases as the temperature rises.
rature Coefficient) Thermistor is used.

One end of the thermistor 10 is connected to a DC power supply bus V _CC via a reference resistor 11, and the other end is grounded. Then, the potential at the connection point between the reference resistor 11 and the thermistor 10 is set to, for example, a sample / A / D converter /
The measurement by the holding means 12 makes it possible to detect a change in the resistance value of the thermistor 10, that is, a temperature.

When controlling the atmosphere of a relatively high temperature (for example, about 100 degrees Celsius) in controlling the amount of heat generated by the electric heater, the resistance of the thermistor 10 in the temperature range is several KΩ, so that the resistance of the reference resistor 11 is also small. Usually, it is set to several KΩ. In this case, when the thermistor 10 itself or the connection wiring to the thermistor 10 is disconnected, and when the ambient temperature is reduced to near 0 degrees Celsius, the resistance value at both ends of the thermistor 10 increases to the order of MΩ. It is difficult to distinguish between a state in which the ambient temperature has dropped to around 0 degrees Celsius.

For this reason, if the heating value control is operated in a state where the disconnection actually occurs, even if the temperature rises, the thermistor 1
The resistance value of 0 does not decrease, and heating by the electric heater may be continued. In order to solve this problem, it is common to provide a fail-safe function in which when the resistance value at both ends of the thermistor 10 exceeds a predetermined threshold value, the electric heater is assumed to be disconnected and the energization to the electric heater is stopped. .

[0006]

However, even when the ambient temperature drops to near zero degrees Celsius, the thermistor 10 can be used.
The resistance value at both ends increases to the order of MΩ, which causes a problem that the fail-safe function operates. If the resistance value of the reference resistor 11 is set to about 1 MΩ, it is possible to distinguish between a disconnection state and a state in which the ambient temperature is near zero degrees Celsius. In this case, the detection resolution of the temperature in the normal temperature range is reduced. As a result, the temperature control accuracy deteriorates.

SUMMARY OF THE INVENTION The present invention has been made in consideration of the above problems, and has as its object to provide a temperature detection circuit that can maintain the temperature detection accuracy and can reliably detect a disconnection of a thermistor.

[0008]

FIG. 2 is a diagram showing a basic configuration of a temperature detecting circuit according to the present invention, wherein a thermistor 20 having one end connected to one pole of a DC power supply and having a negative temperature coefficient characteristic; One end is connected to the other pole of the DC power supply, and is connected between the other end of the thermistor 20 and the resistance between the first resistance and a second resistance larger than the first resistance. And a sample / hold means 22 for sampling the voltage across the thermistor 20 at a predetermined timing and holding the voltage until the next sampling,
Comparing means 23 for comparing the voltage between both ends of the thermistor 20 with a predetermined threshold voltage, latch means 24 for latching the output of the comparing means 23 at a predetermined timing, and a resistance value of the reference resistor 21 being a first resistance value , And a switching command for switching between the disconnection detection mode in which the resistance value of the reference resistor 21 is the second resistance value, a sample command to the sample / hold means 24 during the temperature measurement mode, and a disconnection. Command generation means 27 for outputting a latch command to the latch means 26 during the detection mode period.

In the temperature detection circuit according to the present invention, the voltage across the thermistor is sampled and held in the temperature measurement mode, the sampled and held voltage is output in the disconnection detection mode, and the voltage across the thermistor and the threshold voltage are compared. The disconnection of the thermistor is detected by comparison.

[0010]

FIG. 3 is a basic configuration diagram of a magnetic printing apparatus to which a temperature detecting circuit according to the present invention is applied. The magnetic drum 1 is rotated at a high speed in the direction of arrow 11 and the magnetic head 2 is rotated.
Is moved along the rotation axis of the magnetic drum 1, the magnetic head 3 performs main scanning in the circumferential direction on the magnetic drum 1 and performs sub-scanning in the rotation axis direction. Therefore, when a recording signal is supplied to the magnetic head in accordance with the main scanning and the sub-scanning, a magnetic latent image is recorded on the magnetic drum 1.

After the recording is completed, the magnetic head 2 is moved to the standby position, and the magnetic drum 1 is rotated at a low speed to develop the magnetic latent image by the developing device 3, so that the magnetic toner is attracted onto the magnetic drum 1. When the recording paper 4 is transported between the transfer roller 5 to which a high voltage is applied and the magnetic drum, the magnetic toner on the magnetic drum 1 is transferred to the recording paper 4 by static electricity. Then, the magnetic toner image transferred to the recording paper 4 is fixed by fixing rollers 61 and 62.

Excess magnetic toner on the magnetic drum 1 is removed from the magnetic drum 1 by cleaner devices 71 and 72. By repeating the above-described developing, feeding, transferring, and fixing processes, it is possible to make a plurality of copies at high speed. When recording a new magnetic latent image, the latent image on the magnetic drum is erased by the erasing head 8 in advance.

The developing unit 3 includes a magnetic toner 31, a sleeve 32 rotating in the direction of the arrow, a developing magnet 33 fixedly arranged in the sleeve 32, a regulating plate 35 for regulating the height of the magnetic toner 31, and a housing for accommodating the above. 36. Note that N ₁ , S ₁ , N ₂ , and S ₂ described in the developing magnet 33 represent the polarity of the developing magnet 33.

FIG. 4 is a detailed view around the fixing roller.
The recording paper 4 is conveyed by the conveyance belt 20, guided between the heating roller 61 and the pressure roller 62 by the paper passing guides 72 and 74, and the magnetic toner is fixed on the recording paper 4. The recording paper 4 on which the fixing is completed is peeled off by the peeling claw 28, and the paper passing guides 76 and 78 and the paper sensor 80
And is discharged by the paper discharge roller 30 via the The release material application device 84 is disposed in contact with the heating roller 61, and applies the release material to the heating roller 61 in order to promote the separation of the recording paper 4 from the heating roller 61.

The heating roller 61 is provided with a heater 23 along the rotation axis of the heating roller 61 in order to reliably perform fixing. In order to control the surface temperature of the heating roller 61 to be constant, a temperature sensor 82 is provided in contact with the heating roller 61. In this embodiment, the temperature sensor 82 is attached to the paper passing guide 72, but at a position where the position is fixed relative to the heating roller 61 such as a roller side plate (not shown) of the heating unit 22. Therefore, the recording paper 4 can be attached to another position that does not hinder the movement.

A paper sensor 80 is arranged downstream of the peeling claw 28 and detects the movement of the recording paper 4 between the paper passing guides 76 and 78. In the present embodiment, a sensor for mechanically detecting the movement of the recording paper 4 is used, but a transmission type or reflection type optical sensor may be used. The temperature sensor 82 employs a thermistor, and its output signal is guided to the temperature detection circuit 25 according to the present invention. The output of the temperature detection circuit 25 is guided to a control unit 26, which controls the solid state switch SSR21.
To control the electric power supplied to the heater 23 inserted into the center of the heating roller 61 via the.

FIG. 5 is a circuit diagram of a temperature detecting circuit according to the present invention used in the magnetic printing apparatus. One end of the thermistor S has a switching element Q and a first reference resistor R.
_It is connected to the DC bus V _CC via _one series connection, and the other end is grounded. Furthermore, the second reference resistance R ₂ is connected in parallel with the reference resistor R ₁ of the switching element Q and the first.

Here, the first reference resistor R ₁ is a thermistor S
Is a reference resistance used for temperature measurement by using a relatively low resistance value of several KΩ to several tens KΩ. On the other hand, the second reference resistor R ₂ is a reference resistor used for detecting disconnection of the thermistor S, and has a relatively high resistance value of 1 MΩ or more. The connection points between the first reference resistor R ₁ and the second reference resistor R ₂ and the thermistor S are connected to the non-inverting input terminal of the first operational amplifier A ₁ . Since the first operational amplifier A ₁ is that a so-called voltage follower, the output is directly fed back to the inverting input terminal.

The output of the first operational amplifier A ₁ branches into two, one of which is connected via a sampler SW to the second operational amplifier A _2.
Is connected to the non-inverting input terminal. The non-inverting input terminal is grounded via the halt capacitor C. Further, since the second operational amplifier A ₂ also constitutes a voltage follower, the output proportional to the temperature detected by the thermistor S is directly fed back to the inverting input terminal.

The other output of the first operational amplifier A ₁ is connected to a non-inverting input terminal of a _third operational amplifier A ₃ constituting a comparator. A reference voltage source V _ref is connected to a third inverting input terminal of the operational amplifier A _3. The output of the third operational amplifier A ₃ is a third resistor R ₃ , a fourth resistor R ₄ ,
It is connected to a data input terminal of a monostable flip-flop FF via a _first diode D ₁ and a second diode D ₂ .

Note that the third resistor R ₃ , the fourth resistor R ₄ ,
The first diode D ₁ and the second diode D ₂ are for matching the output level of the third operational amplifier A _{3 with} the input level of the monostable flip-flop FF.
The switching element Q, the sampler SW, and the monostable flip-flop FF are controlled by a control signal output from the control signal generator G.

FIG. 6 is a timing chart of the temperature detection circuit according to the present invention, in which the first control signal MODE output from the control signal generator G and the first operational amplifier A _{1 in} order from the top.
The non-inverting input terminal voltage V _1, the second control signal SMPL output from the control signal generator G, the second operational amplifier A ₂ of
Non-inverting input terminal voltage V ₂ , monostable flip-flop F
F represents the data input terminal voltage V _{4 of the} F, the third control signal LATCH output from the control signal generator G, and the output voltage ALM of the monostable flip-flop FF.

The first control signal MODE is a pulse that alternates between high and low at predetermined intervals. When MODE becomes high and the switching element Q is turned off, the thermistor S and the second resistor R ₂ are connected in series. The connected disconnection detection mode is set. When the switching element Q is turned in the opposite MODE becomes low, the thermistor S and the temperature measuring mode in which the first resistor R ₁ are connected in series. Note that the first resistor R ₁
Since a resistance value << second resistance value of the resistor R _2, a temperature measuring mode the effect of the second resistor R ₂ is negligible.

When the first control signal MODE output from the control signal generator G is low, that is, after the temperature is in the temperature measurement mode and the first operational amplifier V ₁ is stabilized, the control signal generator G is turned on. 2 is output. This SM
When PL is high, the sampler SW is closed, and the output of the _first operational amplifier A1, that is, the voltage across the thermistor S, is held in the capacitor C.

While the first control signal MODE output from the control signal generator G is high, that is, in the disconnection detection mode, the sampler SW is opened and the voltage held in the capacitor C is measured by the thermistor S. Temperature signal T
Output as EMP. Further, during the disconnection detection mode, the third control signal LATC is output from the control signal generator G.
H is output. The LATCH is led to the clock terminal of the monostable flip-flop FF, the monostable flip-flop FF latches data input terminal voltage V ₄ when the LATCH becomes high.

The voltage of the reference voltage source V _ref is set to a slightly lower voltage than the voltage of the DC bus V _CC, the operational amplifier A ₁ be a disconnection detection mode first when the thermistor S is normal output voltages V ₁ is the reference voltage source V _ref
It is as follows. Therefore, as long as the thermistor S is normal, the output of the _third operational amplifier A3 constituting the comparator is maintained at a voltage near the negative power supply voltage, and the output ALM of the monostable flip-flop FF is maintained at logic "0". You.

When a disconnection occurs in the thermistor S,
First operational amplifier A₁Output voltage V ₁Is the reference voltage source V
_refThus, the third operational amplifier A_ThreeThe output of the positive power supply
Invert to a voltage near the voltage. Therefore, thermistor S is turned off.
After a line is generated, the mode shifts to the disconnection detection mode and LATCH
Is output, the output A of the monostable flip-flop FF is output.
LM becomes logic "1" and thermistor S is disconnected.
Is output.

Since the electric charge held in the capacitor C leaks during the hold period, when sampling is performed in the next temperature measurement mode, the second operational amplifier A ₂
A phenomenon occurs in which the output of the device rapidly rises by the droop voltage.
However, this phenomenon can be reduced to a negligible level by appropriately setting the switching interval and the circuit constant between the temperature measurement mode and the disconnection detection mode.

FIG. 7 shows a modification of the configuration around the monostable flip-flop FF. The circuit of FIG. 2 is such that the output ALM is also reset when the disconnection is not detected after the thermistor S is detected. Which solves
Once the disconnection of the thermistor S is detected, the output ALM continues to be output even if the disconnection is no longer detected. That is, the output AL which is the output of the monostable flip-flop FF
M is fed back to the inverting input terminal of the AND gate AND. A non-inverting input terminal of the AND gate AND has a third control signal LATCH output from the control signal generator G.
Connect. Then, the monostable flip-flop FF is controlled by the output of the AND gate AND.

According to this configuration, the monostable flip-flop FF is controlled by the third control signal LATCH as long as the output ALM of the monostable flip-flop FF is logic "0", that is, unless the disconnection of the thermistor S is detected. You. Then, once the disconnection of the thermistor S is detected,
Output ALM of monostable flip-flop FF is logic "1"
, The third control signal LATCH is blocked by the AND gate AND, so that even if the disconnection of the thermistor S is no longer detected, the monostable flip-flop F
The output ALM of F keeps logic "1".

According to this embodiment, unstable operation can be reliably prevented when disconnection such as poor contact of the connector to which the thermistor S is connected and return from disconnection are repeatedly detected. While performing sample / hold means by configured sample / hold circuit by a sampler SW and a capacitor C and a second operational amplifier A ₂ in the above embodiment, the first output of the operational amplifier A ₁ directly A The A / D converter is operated by the second control signal SMPL output from the control signal generator G, and the A / D converter itself is used as a sample / hold means, thereby forming a sample / hold circuit. It can be omitted.

Further, in the above embodiment, an FET is used as the switching element Q. However, other switching elements such as an analog switch or a transistor can be used. Also, if a track / hold circuit is used instead of the sample / hold circuit of the above embodiment, the pulse waveform of the first control signal MODE is set so that the temperature measurement mode period is longer and the disconnection detection mode period is shorter. Thus, the temperature hold time for disconnection detection can be shortened.

In the above-described embodiment, the first reference resistor R ₁ and the second reference resistor R ₂ are connected in parallel in the temperature measurement mode. However, the second reference resistor may be disconnected. Good.

[0034]

According to the temperature detecting circuit of the present invention, accurate temperature measurement by the thermistor can be performed in the temperature measuring mode, and the disconnection of the thermistor itself and its peripheral wiring can be reliably detected in the disconnection detecting mode. It becomes possible.

[Brief description of the drawings]

FIG. 1 is a basic circuit for temperature measurement using a thermistor.

FIG. 2 is a basic configuration diagram of a temperature detection circuit according to the present invention.

FIG. 3 is a basic configuration diagram of a magnetic printing apparatus.

FIG. 4 is a detailed view around a fixing roller.

FIG. 5 is a circuit diagram of a temperature detection circuit according to the present invention.

FIG. 6 is a timing chart of the temperature detection circuit according to the present invention.

FIG. 7 is a modified example of a configuration around a monostable flip-flop.

[Explanation of symbols]

S: Thermistor Q: Switching element R ₁ : First reference resistor R ₂ : Second reference resistor A ₁ : First operational amplifier A ₂ : Second operational amplifier A ₃ ... Third operational amplifier SW: Sampler C: capacitor _Vref : reference voltage FF: monostable flip-flop G: control signal generator

Claims (3)

[Claims] 1. A thermistor having a negative temperature coefficient, one end of which is connected to one pole of a DC power supply; and a thermistor connected between one other pole of the DC power supply and another end of the thermistor; The resistance value is set to the first resistance value and the first resistance value.
A reference resistance switchable between a second resistance value larger than the first resistance value, a sample / hold means for sampling a voltage between both ends of the thermistor at a predetermined timing, and holding the voltage until the next sampling time; Comparing means for comparing the voltage with a predetermined threshold voltage; latch means for latching the output of the comparing means at a predetermined timing; and a temperature measurement mode in which the resistance value of the reference resistor is a first resistance value. And a switching command for switching between a disconnection detection mode as a second resistance value, a sample command to the sample / hold means during the temperature measurement mode, and a latch command to the latch means during the disconnection detection mode. A temperature detection circuit comprising: a command generation unit that outputs the command. 2. A thermistor having a negative temperature coefficient characteristic in which one terminal is grounded; a switching element disposed between a positive electrode of a DC power supply and another terminal of the thermistor; A second reference resistor having a resistance value greater than a resistance value of the first reference resistor disposed in parallel with the series connection; and a voltage detection unit for detecting a voltage across the thermistor; Sample / hold means for sampling the output voltage of the voltage detection means at a predetermined timing and holding until the next sampling time; comparison means for comparing the output voltage of the voltage detection means with a predetermined threshold voltage; Latch means for latching the output of the comparison means at a predetermined timing; temperature measurement mode in which the switching element is on; and switching off of the switching element. Switching command to switch between a state in which disconnection detection mode, a sample command to said sample / hold means in the temperature measuring mode period,
And a command generation means for outputting a latch command to the latch means during a disconnection detection mode period. 3. A thermistor having one end connected to one pole of a DC power supply and having a negative temperature coefficient characteristic; a first reference resistor having one end connected to the other pole of the DC power supply; A second reference resistor having one end connected to the other pole of the power supply and having a resistance value larger than the resistance value of the first reference resistor; and the other end of the thermistor connected to the other end of the first reference resistor. A switching unit selectively connected to one end or the other end of the second reference resistor; a sample / hold unit that samples a voltage between both ends of the thermistor at a predetermined timing and holds the voltage until the next sampling time; Comparing means for comparing the voltage between both ends of the thermistor with a predetermined threshold voltage; latch means for latching an output of the comparing means at a predetermined timing; A switching command for switching between a temperature measurement mode in which a first reference resistor is connected and a disconnection detection mode in which the thermistor and the second reference resistor are connected; and the sample / hold during the temperature measurement mode And a command generation means for outputting a latch command to the latch means during a disconnection detection mode.