JP2006092932A - Heater current control circuit - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a heater current control circuit which securely suppresses the rush current even when the circumferential temperature varies, and a heater is repeatedly energized. <P>SOLUTION: When the heater current control means 15 to control on/off operation of the heater current based on the heater temperature turns on the heater current, one of switching means 10, 11, 12 is turned on based on the temperature of a thermistor 5, and the heater current is made to flow through one of different thermistor circuits. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a heater current control circuit, and more particularly to a heater current control circuit for a heating and fixing device used in an image processing apparatus such as a copying machine, a printer, and a facsimile machine.

  A block diagram of an example of a conventional heater current control circuit is shown in FIG.

  The heat fixing device includes a heater 21 and a temperature detector 22. A halogen lamp or the like is used as the heater. A switch element 26 for turning on / off the energization of the heater 21 is provided in series in the heat fixing device, and a thermistor 25 is provided in series between the switch element 26 and the heater 21 in series. Further, a temperature control circuit 28 for turning on / off the switch element 26 is provided. The thermistor 25 has a characteristic that the resistance value decreases as its temperature rises.

  Even if the resistance value of the heater 21 is small, the inrush current when the heater 21 electrically connected in series with the thermistor 25 is small is suppressed because the resistance value of the thermistor 25 itself is low when the temperature is low. It is done.

  Thereafter, the temperature of the thermistor 25 gradually increases due to the current flowing through the heater 21, so that the resistance value of the thermistor 25 decreases. However, due to self-heating of the heater 21, the resistance value of the heater 21 increases and the heater current gradually increases. The heater 21 approaches the rated current.

  Next, the temperature detector 22 detects the temperature of the heater 21 to operate the temperature control circuit 28, and the temperature control circuit 28 controls the on / off of the switch element 26 so that the heater 21 reaches a fixing possible temperature.

  However, in the conventional heater current control circuit described above, the inrush current to the heater can be suppressed when the heater is energized for the first time, but the inrush current cannot be suppressed when the energization is turned off after the heater is turned off.

  This is because current flows through the thermistor while the heater is energized, and the thermistor continues to generate heat. This is because if a current is applied again while the thermistor is not cooled, a large inrush current flows because the resistance value of the thermistor decreases.

  In order to solve this, as shown in FIG. 6, a first switch element 27 directly connected to the heater 21 and a second switch element 26 via a thermistor 25 are provided in parallel between the heater 21. An example in which a temperature control circuit 28 for turning on and off these switch elements 26 and 27 according to the temperature of the heater 21 and the measurement time of the timer 29 is known.

  In this example, when 20 seconds or more have elapsed after the first switch element 27 is turned off, the energization of the heater 21 is started by the second switch element 26 and the second switch element 26 is turned on. When the predetermined time (200 ms) has elapsed, the second switch element 26 is turned off, and the first switch element 27 is turned on for switching. Thereafter, when the temperature of the heater 21 (heat fixing device) reaches a predetermined temperature, the first switch element 27 is turned off. When only 20 seconds or less have passed since the first switch element 27 is turned off, the energization of the heater 21 is started from the first switch element 27, and the temperature of the heater 21 (heating fixing device). When the temperature reaches a predetermined temperature, the first switch element 27 is turned off (see Patent Document 1).

However, in this method, since the operation of disconnecting the thermistor 25 is performed only by the measurement time of the timer 29, current may flow directly to the heater 21 in a state where the heater 21 is not sufficiently warmed due to a change in ambient temperature. In such a case, there is a problem that the inrush current cannot be reliably controlled. Further, when the heater 21 is repeatedly turned on and off, the heater 21 may be turned on before the temperature of the thermistor 25 is sufficiently lowered. Even in such a case, the inrush current must be reliably controlled. There is a problem that can not be.
Japanese Patent Laid-Open No. 9-329988

  An object of the present invention is to realize a heater current control circuit that can accurately control an inrush current and reliably suppress the inrush current even when the heater is repeatedly energized even if the ambient temperature changes. It is.

  In order to solve the above-mentioned problems, a first aspect of the present invention provides a heater current control means for controlling on / off of a heater current based on a heater temperature, a thermistor connected in series to the heater, and the thermistor. Switch means for turning on and off the current flowing through the heater, and when the heater current control means controls the heater current to be turned on, a heater current control circuit for energizing the heater current through the switch means and the thermistor The thermistor is added as a first thermistor, a second thermistor connected in series to the heater, and the switch means is added as a first switch means, and the heater is connected via the second thermistor. The second switch means for turning on and off the current flowing through the first and the temperature of the first thermistor The first or second switch means based on the temperature of the first thermistor detected by the thermistor temperature detecting means when the heater current control means controls the heater current to be turned on. And selecting means for turning on any one of the above.

  According to a second aspect of the present invention, in the first aspect of the invention, the second thermistor and the second switch means are in series with the first thermistor and in parallel with the first switch means. It is connected.

  According to a third aspect of the present invention, in the second aspect of the present invention, a third thermistor connected in series to the heater in parallel with the first thermistor, and the third thermistor via the third thermistor. And a third switch means for turning on and off the current flowing through the heater, and the selection means detects the first temperature detected by the thermistor temperature detecting means when the heater current control means controls the heater current to be turned on. It is characterized by comprising means for turning on any one of the first to third switch means based on the temperature of one thermistor.

  According to a fourth aspect of the present invention, in the first aspect of the invention, the second thermistor and the second switch means are connected in parallel to the first thermistor and the first switch means. It is characterized by.

  A fifth aspect of the present invention is the invention according to any one of the first to fourth aspects, wherein each of the thermistors has a negative temperature characteristic in which a resistance value decreases as its temperature increases.

  As described above, the heater current control circuit of the present invention uses a plurality of thermistors, and selects one of the plurality of thermistor circuits from the temperature of the first thermistor separately from on / off of the heater current. Therefore, even if the ambient temperature changes, even if the heater is repeatedly turned on and off, the inrush current can be reliably suppressed.

  Hereinafter, the heater current control circuit of the present invention will be described in detail with reference to the drawings.

  FIG. 1 is a circuit block diagram of an embodiment of a heater current control circuit of the present invention. FIG. 2 is a flowchart for explaining the operation of this embodiment. FIG. 3 is a time chart showing waveforms of respective parts of the circuit according to this embodiment.

  The heater current control circuit shown in FIG. 1 controls, for example, the heater current of a heating and fixing device used in an image processing apparatus. The heater 1 is a heat source of the heating and fixing device, and the first detects the temperature of the heater 1. Temperature detector 2, AC power supply 3, second temperature detector 4 for detecting the temperature of first thermistor 5 described later, first thermistor 5 connected in series to heater 1, first thermistor 5, The second thermistor 6 connected in series, the third thermistor 7 connected in series to the heater 1 in parallel with the first thermistor 5, and the current flowing through the heater 1 via the first thermistor 5 are turned on and off. The second switch element (switch element 2) that turns on and off the current flowing through the heater 1 through the series circuit of the first switch element (switch element 1) 10, the first thermistor 5 and the second thermistor 6 1) The third switch element (switch element 3) 12 that turns on and off the current flowing through the heater 1 via the third thermistor 7 and the heater current that is turned on based on the heater temperature detected by the first temperature detector 2 are turned on. When the heater current control circuit 15 controls the heater current to be turned on, the first switch is controlled based on the temperature of the first thermistor 5 detected by the second temperature detector 4. The selector 16 is configured to turn on any one of the element 10, the second switch element 11, and the third switch element 12.

  The thermistors 5, 6 and 7 all have negative temperature characteristics in which the resistance value decreases as the temperature of the thermistors 5, 6 and 7 increases.

  The operation of the heater current control circuit of the present invention will be described with reference to FIGS.

  When the operation of the heater current control circuit is started, first, the temperature of the heater 1 is detected using the first temperature detector 2 (step 1 in FIG. 2). The detected temperature is compared with the set temperature (step 2 in FIG. 2). If the detected temperature has reached the set temperature, the heater current on / off control circuit 15 sends a heater current off signal to the selector 16 to All of the first switch element 10, the second switch element 11, and the third switch element 12 are turned off to cut off the heater current (step 3 in FIG. 2).

  When the detected temperature is lower than the set temperature in step 2, the heater current on / off control circuit 15 sends a heater current on signal to the selector 16. In response to this, the selector 16 detects the temperature of the first thermistor 5 using the second temperature detector 4 (step 4 in FIG. 2). Then, the selector 16 selects any one of the first switch element 10, the second switch element 11, and the third switch element 12 based on the detected temperature, and supplies a current to the heater 1.

  At this time, if the temperature of the first thermistor 5 is sufficiently low, the selector 16 selects and turns on the first switch element 10 (step 5 in FIG. 2). Since the temperature of the first thermistor 5 is low, its resistance value is high, so that the inrush current does not increase.

  In this state, a current flows through the heater 1, and the temperature of the heater 1 is repeatedly detected using the first temperature detector 2 (step 8 in FIG. 2). When the temperature of the heater 1 eventually reaches the set temperature (step 9 in FIG. 2), the heater current on / off control circuit 15 sends a heater current off signal to the selector 16 to turn off all the switch elements 10, 11 and 12. The heater current is cut off (step 3 in FIG. 2), and the process returns to step 1 again.

  Since the heater current is cut off and the temperature of the heater 1 decreases and the detected temperature becomes lower than the set temperature in step 2, the heater current on / off control circuit 15 sends a heater current on signal to the selector 16. The selector 16 detects the temperature of the first thermistor 5 using the second temperature detector 4. At this time, if the temperature of the first thermistor 5 has risen to some extent due to the effect of energization or the like, the selector 16 selects and turns on the second switch element 11 (step 6 in FIG. 2). ). Then, the heater current is supplied through a series circuit of the first thermistor 5 and the second thermistor 6. Although the temperature of the first thermistor 5 is increased and the resistance value thereof is decreasing, the temperature of the second thermistor 6 inserted in series is low and the resistance value is high at this stage. The series circuit of the two thermistors 6 also has a high resistance value, so that the inrush current does not increase.

  If the heater current on / off control circuit 15 repeats the heater current on / off repeatedly, and the process proceeds to step 4, the temperature of the first thermistor 5 further rises and becomes high. 3 switch element 12 is selected and turned on (step 7 in FIG. 2). As a result, the heater current is supplied via the third thermistor 7. Prior to this stage, current flowed through the first thermistor 5 or the series circuit of the first thermistor 5 and the second thermistor 6, and no current flowed through the third thermistor 7. The temperature of the third thermistor 7 is low and its resistance value is high, so that the inrush current does not increase.

  Changes in the waveform and temperature of each part of the heater current control circuit during this period will be described with reference to FIG.

  FIG. 3A shows a signal waveform indicating ON / OFF of the heater current. If the temperature of the heater 1 does not reach the set value in step 2 of FIG. 2, the heater current on / off control circuit 15 sends a heater current on signal to the selector 16 to turn on one of the switch elements to turn on the heater current. Shed.

  At this time, the selector 16 selects one of the switch elements 10, 11 and 12 according to the temperature of the first thermistor 5 detected by the second temperature detector 4 and turns it on.

  When the temperature of the first thermistor 5 shown in FIG. 3E is low, the selector 16 selects and turns on the first switch element 10 in step 5 of FIG. FIG. 3B is a signal waveform indicating ON / OFF of the first switch element 10. As a result, a current flows through the heater 1. Since the temperature of the first thermistor 5 is low, its resistance is high, and the inrush current shown in FIG. When the first switch element 10 is turned on and a heater current flows through the first thermistor 5, the temperature of the first thermistor 5 gradually increases as shown in FIG.

  When the temperature of the heater 1 reaches the set value in step 9 of FIG. 2, the heater current on / off control circuit 15 turns off the heater current on signal sent to the selector 16 and turns all the switch elements 10, 11, 12 on. The heater current is cut off by turning off (step 3 in FIG. 2). Since the heater current flowing through the first thermistor 5 and the first switch element 10 is cut off, the temperature of the first thermistor 5 gradually decreases as shown in FIG.

  Returning to step 2 in FIG. 2 again, when the temperature of the heater 1 becomes lower than the set value, the heater current on / off control circuit 15 causes the selector 16 to select one of the switch elements and turn it on. If the temperature of the first thermistor 5 is lower than that of the time, but is still a moderate temperature (medium temperature), the selector 16 selects the second switch element 11 in step 6 of FIG. Let FIG. 3C shows a signal waveform indicating ON / OFF of the second switch element 11. As a result, a current flows through the heater 1. At this time, the resistance of the series circuit of the first thermistor 5 and the second thermistor 6 is high, and the inrush current shown in FIG. When the second switch element 11 is turned on and a heater current flows, the temperature of the first thermistor 5 gradually rises again as shown in FIG. Further, the temperature of the second thermistor 6 shown in FIG.

  Returning to step 2 in FIG. 2 again, when the temperature of the heater 1 becomes lower than the set value, the heater current on / off control circuit 15 causes the selector 16 to select one of the switch elements and turn it on. Assuming that the temperature of the first thermistor 5 is high, the selector 16 selects and turns on the third switch element 12 in step 7 of FIG. FIG. 3D is a signal waveform indicating ON / OFF of the third switch element 12. As a result, a current flows through the heater 1 via the third thermistor 7. At this time, the temperature of the third thermistor 7 is low because no current has flowed until then, its resistance value is high, and the inrush current shown in FIG. When the third switch element 12 is turned on and a heater current flows, the temperature of the third thermistor 7 rises as shown in FIG. On the other hand, the temperature of the first thermistor 5 and the second thermistor 6 gradually decreases as shown in FIGS. 3 (e) and 3 (f).

  Thus, in the heater current control circuit of the present invention, the thermistor circuit is selected according to the temperature of the first thermistor 5 and the resistance of the circuit through which the heater current flows is increased. The inrush current at the moment when the current flows can be reduced.

  FIG. 4 shows a circuit block diagram of another embodiment of the heater current control circuit of the present invention. This circuit is different from the circuit of FIG. 1 in that a fourth thermistor 8 (connected in series with the first thermistor 5) in parallel with the series circuit of the second thermistor 6 and the second switch element 11 is used. And a fourth switch element 13 (which turns on and off the current flowing through the heater 1 through the series circuit of the first thermistor 5 and the fourth thermistor 8). In FIG. 4, reference numerals 1 to 7, reference numerals 10 to 12, reference numerals 15, and 16 indicate the same elements as those shown in FIG. 1.

  In this embodiment, the selector 16 selects the thermistor circuit in four stages according to the temperature of the first thermistor 5 and causes the heater current to flow. That is, when the temperature of the first thermistor 5 is the lowest, the first switch element 10 is turned on. When the temperature of the first thermistor 5 is slightly higher than that, the second switch element 11 is turned on. When the temperature of one thermistor 5 is higher, the fourth switch element 13 is turned on, and when the temperature of the first thermistor 5 becomes the highest, the third switch element 12 is turned on.

  Thereby, by detecting the temperature of the first thermistor 5 more finely and selecting the switch elements 10, 11, 12, and 13, the inrush current can be further reduced and the specific thermistor can be used continuously. By reducing it, the burden on the thermistor can be reduced.

  The present invention is not limited to the configuration shown in FIGS. 1 and 4 and can be implemented.

  For example, the thermistor 7 and the switch 12 in FIG. 1 may be omitted, the thermistor 6 may be the second thermistor, and the switch 11 may be the second switch element.

  Further, in FIG. 1, the thermistor 6 and the switch 11 may be omitted, the thermistor 7 may be a second thermistor, and the switch 12 may be a second switch element.

  That is, the heater 1, the first temperature detector 2, the AC power source 3, the second temperature detector 4, the first thermistor 5, the first switch element 10, and the heater current control as shown in FIGS. In the configuration including the circuit 15 and the selector 16 (other configurations are not included), as the second thermistor, a thermistor connected in parallel to the first thermistor 5 or connected in series is provided. As the second switch element, a switch element for turning on and off the current flowing through the heater 1 via the second thermistor (or the series circuit of the first thermistor and the second thermistor 5) is added. When the heater current control circuit 15 controls the heater current to be turned on, the selector 16 controls the first switch element based on the temperature of the first thermistor 5 detected by the second temperature detector 4. 0 or any one may be configured to perform the selection control to turn on of the second switch element.

In the heater current control circuit of the present invention, a plurality of thermistor circuits are selected from the temperature of the first thermistor and current is passed through the heater via different thermistor circuits, so that the resistance value of the thermistor circuit is kept large. Even if the ambient temperature changes, the inrush current can be reliably suppressed even if the heater is turned on / off repeatedly and energized.
Therefore, it can be used effectively in heating and fixing devices of image forming apparatuses such as dry copying machines, laser printers, facsimiles, etc., and can be used in these fields because it can reduce safety and power consumption. High nature.

The circuit block diagram of one embodiment of the heater current control circuit of the present invention. The flowchart explaining operation | movement of the heater current control circuit shown in FIG. The time chart which shows the waveform of each part of the heater current control circuit shown in FIG. The circuit block diagram of other embodiment of the heater current control circuit of this invention. The block diagram of an example of the conventional heater current control circuit. The block diagram of the other example of the conventional heater current control circuit.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Heater 2 1st temperature detector 3 AC power supply 4 2nd temperature detector 5 1st thermistor 6 2nd thermistor 7 3rd thermistor 8 4th thermistor 10 1st switch element 11 2nd switch Element 12 Third switch element 13 Fourth switch element 15 Heater current on / off control circuit 16 Selector 21 Heater 22 Temperature detector 23 AC power supply 25 Thermistor 26 Switch element 27 Switch element 28 Temperature control circuit 29 Timer

Claims (5)

Heater current control means for controlling the heater current on and off based on the temperature of the heater, a thermistor connected in series to the heater, and a switch means for turning on and off the current flowing through the heater via the thermistor. A heater current control circuit for energizing the heater current through the switch means and the thermistor when the heater current control means controls the heater current to be on,
A second thermistor, wherein the thermistor is added as a first thermistor and connected in series to the heater;
The switch means is added as a first switch means, and a second switch means for turning on and off the current flowing through the heater via the second thermistor;
Thermistor temperature detecting means for detecting the temperature of the first thermistor;
When the heater current control means controls the heater current to be turned on, one of the first and second switch means is controlled based on the temperature of the first thermistor detected by the thermistor temperature detection means. A heater current control circuit comprising: a selection means for turning on. The heater current control circuit according to claim 1, wherein the second thermistor and the second switch means are connected in series to the first thermistor and in parallel to the first switch means. A third thermistor connected in series with the heater in parallel with the first thermistor;
And third switch means for turning on and off the current flowing through the heater via the third thermistor,
The selection means includes
When the heater current control means controls the heater current to be on, any one of the first to third switch means is selected based on the temperature of the first thermistor detected by the thermistor temperature detection means. The heater current control circuit according to claim 2, comprising a means for turning on. The heater current control circuit according to claim 1, wherein the second thermistor and the second switch means are connected in parallel to the first thermistor and the first switch means. 5. The heater current control circuit according to claim 1, wherein each of the thermistors has a negative temperature characteristic in which a resistance value decreases as its own temperature increases.

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