JP2008010319A - Heater device and image forming device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To arrange that a switching element installed between an AC input and a heater is made to be surely switched at a high-precision timing between a state that electric power is supplied from the AC input to the heater and a state that supply is stopped. <P>SOLUTION: The heater device 600 installed in a copy machine 1 is equipped with a rectifying means in which AC input voltage is acquired and full-wave rectification is carried out, a zero cross signal forming means in which the zero cross signal of the AC input is formed based on the voltage that has been full-wave rectified by the rectifying means, a current detecting means by which the current flowing in the heater is detected, and a control means in which the zero cross point timing of the AC input is specified based on the zero cross signal that is formed by the zero cross signal forming means and on the amount of the current that is detected by the current detecting means, and which switches the state of the switching elements at the specified timing. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a heater device comprising a heater and a switching element that is provided between an AC input and the heater and switches between a state in which power is supplied to the heater from the AC input and a state in which supply is stopped. The present invention relates to an image forming apparatus including such a heater device configured as a fixing device.

  2. Description of the Related Art Conventionally, image forming apparatuses such as a printing apparatus, a copier, a fax machine, and a multifunction machine are used. In such an image forming apparatus, a fixing device that performs a fixing process on a toner image formed on a recording sheet is provided, and the fixing apparatus performs a fixing process by applying heat to the toner image from a heater.

  For the drive control of the heater, a switching element (for example, triac) for controlling the power supply to the heater is used, and the power supply to the heater from the AC input is switched between a supply state and a supply stop state by the switching element. Thereby, the amount of heat generated by the heater can be changed.

  Here, when switching is performed by the switching element, switching noise occurs. Further, an inrush current is generated when the switching element is switched to the on state to start supplying power to the heater. Here, when the inrush current increases, the noise that returns to the AC input side increases, so that a larger and more expensive choke coil is required to clear the regulation of the voltage between the noise terminals.

  Therefore, in the technique of Patent Document 1, the state of the switching element is switched at the timing (zero cross point) when the output from the AC input to the heater becomes 0 so that the switching noise and the inrush current can be reduced as much as possible. ing.

For this purpose, a diode bridge that accepts an alternating voltage from an AC input and performs full-wave rectification is provided. As a result, during the period in which the AC voltage is negative, the voltage is inverted between positive and negative, and a voltage that maintains the positive voltage at all times according to the absolute value of the output voltage of the AC input is obtained. By providing a comparator that compares the voltage obtained by inverting the negative part with the reference voltage, the output voltage of the AC input is high level only when the absolute value of the output voltage is smaller than the reference voltage, and low level during the other periods. A signal (zero cross signal) is generated. By switching the switching element based on this zero-cross signal, switching noise and inrush current can be prevented, and the regulation of the voltage between the noise terminals can be easily cleared to reduce the size and cost of the choke coil. Or trying to make it easier.
JP-A-11-327668

  However, in the circuit from the rectifier circuit to the zero cross signal generation circuit including the comparator or the like, there is a variation for each individual, and as a result, for example, the response speed at which the zero cross signal responds to a voltage change of the AC power supply varies. For example, the zero cross signal may become inaccurate due to the occurrence of. For this reason, with the technique of the conventional patent document 1, switching of a switching element was not able to be performed reliably and accurately.

  The present invention has been made in view of such a point, and an object of the present invention is to make it possible to reliably switch the state of a switching element at a precise timing.

The heater device according to claim 1,
A heater that generates heat when power is supplied from an AC input;
A switching element that is provided between the AC input and the heater and switches between a state in which power is supplied to the heater from the AC input and a state in which supply is stopped;
Rectifying means for acquiring the voltage of the AC input and performing full-wave rectification;
Zero-cross signal generating means for generating a zero-cross signal of the AC input based on the voltage that has been full-wave rectified by the rectifying means;
Current detecting means for detecting a current flowing through the heater;
The timing of the zero cross point of the AC input is specified based on the zero cross signal generated by the zero cross signal generation means and the amount of current detected by the current detection means, and at the specified timing, the switching element It is a heater apparatus provided with the control means which switches a state.

  In this configuration, power is supplied from the AC input and the heater generates heat, and a switching element is provided between the AC input and the heater. By switching the state of the switching element, the AC Electric power is supplied to the heater from the input, or the supply is stopped. On the other hand, the AC input voltage is acquired by the rectifying means and full-wave rectification is performed, and the zero-cross signal of the AC input is generated by the zero-cross signal generating means based on the voltage that is full-wave rectified by the rectifying means, Further, the current flowing through the heater is detected by the current detection means, and thereby the AC input is based on the zero cross signal generated by the zero cross signal generation means and the amount of current detected by the current detection means. The zero cross point timing is specified by the control means, and the state of the switching element is switched at that timing.

  Therefore, the timing of the zero cross point is specified accurately and accurately based on the zero cross signal generated by the zero cross signal generating means and the amount of current detected by the current detecting means, and the switching of the state of the switching element is performed. It can be carried out reliably at a precise timing.

  And by switching the state at a precise timing in this way, switching noise and inrush current can be suppressed, regulation of noise terminal voltage can be easily cleared, choke coil can be downsized and cost can be reduced Will be able to.

The heater device according to claim 2,
2. The heater device according to claim 1, wherein the control unit specifies the timing of the zero cross point of the AC input based on the timing at which the current flowing through the heater is detected as 0 by the current detection unit and the zero cross signal. is there.

  With this configuration, the control means specifies the timing of the zero cross point of the AC input based on the timing at which the current flowing through the heater is detected as 0 by the current detection means and the zero cross signal.

  In this way, the timing of the zero cross point of the AC input is reliably and accurately specified based on the timing at which the current flowing through the heater is detected as 0 by the current detection means and the zero cross signal, thereby ensuring high accuracy. The state of the switching element can be switched at the timing.

  In addition, even when the frequency and voltage of the AC input fluctuate for some reason, the pulse width of the zero cross signal generated by the zero cross signal generating means varies, whereas the current flowing through the heater by the current detecting means is 0. Therefore, the timing is reliably and accurately determined based on the zero-cross signal and the invariable timing at which the current flowing through the heater is detected as 0 by the current detecting means. Identified.

The heater device according to claim 3,
The control means includes
A time difference specifying means for specifying a time difference between a timing at which the zero cross signal rises and a timing at which a current 0 is detected by the current detection means;
3. The heater device according to claim 2, wherein the timing of the zero cross point is specified by compensating a variation time of the zero cross signal according to the time difference.

  With this configuration, the time difference between the timing when the zero cross signal rises and the timing when the current 0 is detected by the current detection means is specified by the time difference specifying means. The timing of the zero cross point is accurately identified by compensating for the variation time of the zero cross signal according to the time difference.

The invention according to claim 4
It is an image forming apparatus provided with the said heater apparatus in any one of Claims 1-3 comprised as a fixing device.

  If it is this structure, the said heater apparatus in any one of Claims 1-3 comprised as a fixing device can provide the image forming apparatus with the said effect.

  Note that when the heater starts to be driven, such as when the heater device or image forming apparatus is turned on, or returned from the energy saving mode (power save mode, sleep mode, etc.), the time difference described above is used. For some time, blurring occurs and it becomes unstable.

  Therefore, the above-mentioned control means is provided with storage means for storing the time difference specified by the time difference specifying means in the past, and the time difference stored in advance in the storage means is used when the heater starts to be driven. The timing of the zero cross point may be specified.

  The control unit may include a time difference history storage unit that stores a history of the time difference specified by the time difference specifying unit, and may specify the timing of the zero cross point based on the stored history.

  The time difference history storage means may store an average value of the time differences specified by the time difference specifying means.

  According to the present invention, the AC input zero crossing is reliably and accurately switched between the AC input and the heater, and the switching of the switching element between the AC input and the power supply state is stopped. This can be done at the point timing. In addition, since the state can be switched at a precise timing in this way, it becomes possible to suppress switching noise and inrush current, and the regulation of the voltage between the noise terminals can be easily cleared and choke The coil can be downsized and the cost can be reduced.

  Hereinafter, a copying machine 1 according to an embodiment of the present invention will be described with reference to the drawings.

  FIG. 1 is a diagram showing a copying machine 1 according to an embodiment of the present invention.

  The copying machine 1 includes an image reading unit 100 that reads an image of a document to be copied, a paper feeding unit 200 that includes a recording paper P, a conveyance path 200t of the recording paper P that extends from the paper feeding unit 200, and a conveyance path 200t. An image forming unit 300 that forms an image read by the image reading unit 100 described above on the recording paper P that is transported, and is provided downstream of the image forming unit 300 in the transport path 200t. A fixing device 400 that performs fixing processing of an image formed on the recording paper P, and a discharge unit 200d that discharges the recording paper P on which the image is fixed by the fixing device 400 are provided.

  The fixing device 400 heats the image formed on the recording paper P to fix the image on the recording paper P. For this heating, the fixing device 400 is provided with a heater 401 (FIG. 2) that generates heat when supplied with AC power.

  FIG. 2 is a diagram showing a heater device 600 configured in the copying machine 1. The heater 401 is a heater provided in the fixing device 400 (FIG. 1), and generates heat used for fixing processing. The copier includes a heater 401 and a fixing device 400 including the heater 401, a transformer 603, a rectifier circuit 604, a zero cross signal generation circuit 605, a heater drive circuit 601, a heater current detection circuit 602, and a control unit 606. 1 includes a heater device 600 that generates heat by the heater 401.

  The heater device 600 is merely an example of the “heater device” described in the claims, and the “heater device” described in the claims may be configured by a configuration different from the illustrated configuration. Needless to say, you can. The “fixing device” described in the claims corresponds to the entire fixing device 400 (FIG. 1) having the heater 401 and the fixing roller and the heater device 600 shown in FIG. 2 in this embodiment.

  The heater drive circuit 601 is provided between the heater 401 and the AC input 500 and drives the heater 401. The heater drive circuit 601 includes at least a switching element. Based on a control signal input from the control unit 606 to the heater drive circuit 601, the switching element is switched and the state is switched, whereby the power supply from the AC input 500 to the heater 401 is in the supply state and the supply stop state. Switch between and. A thyristor can be used as the switching element, and a bidirectional thyristor (triac) can be particularly preferably used.

  An AC input 500 is an input unit that inputs power from a commercial power source to the copier 1 via an outlet or the like.

  A heater current detection circuit 602 is also provided between the AC input 500 and the heater 401 as illustrated. The heater current detection circuit 602 detects an instantaneous current value flowing through the heater 401, that is, an average current value of a 50 Hz or 60 Hz alternating current flowing from the AC input 500 to the heater 401, and the like.

  The current value detected by the heater current detection circuit 602 is input to the control unit 606.

  On the other hand, a zero-cross signal of the output voltage of the AC input 500 that supplies power to the heater 401 in this way is generated by the transformer 603, the rectifier circuit 604, and the zero-cross signal generation circuit 605. The control unit 606 switches the state of the switching element of the heater drive circuit 601 based on the zero cross signal and the amount of current detected by the heater current detection circuit 602 described above.

  The transformer 603 acquires the voltage (100 V) output from the AC input 500 and steps down the voltage to an appropriate voltage (for example, 5 V) lower than 100 V.

  The rectifier circuit 604 receives the voltage stepped down by the transformer 603 and performs full-wave rectification.

  FIG. 3 is a diagram showing changes in current and voltage of each part of the heater device 600. In the uppermost part of FIG. 3, the voltage of the AC input 500 is shown. In the second stage, the voltage rectified by the rectifier circuit 604 is shown. The third stage shows a zero cross signal generated by a zero cross signal generation circuit 605 (described in detail later) from the output voltage of the rectifier circuit 604. The lowermost fourth stage shows the current that flows to the heater 401 that is supplied with power from the AC input 500 and is detected by the heater current detection circuit 602.

  As shown in the first and second graphs of FIG. 3, the voltage when the voltage becomes negative with respect to the voltage of the AC input 500 is inverted in the output voltage of the rectifier circuit 604, Positive voltage. In order to perform such voltage conversion, the rectifier circuit 604 is provided with, for example, a diode bridge configured by combining four rectifier diodes.

  The zero-cross signal generation circuit 605 obtains the output voltage of the rectifier circuit 604 that is always a positive voltage as shown in the second graph of FIG. A zero cross signal as shown in the stage is generated. For this purpose, the zero cross signal generation circuit 605 is provided with a comparator that compares the reference voltage Th (see FIG. 3) with the output voltage of the rectifier circuit 604 (the voltage in the second graph in FIG. 3). Based on the comparison result by the comparator, the signal is in the high state only during the period when the output of the rectifier circuit 604 is smaller than the reference voltage Th, and is in the low state during the other periods (third graph in FIG. 3). (Zero cross signal) is output.

  The zero cross signal generated by the zero cross signal generation circuit 605 in this way is input to the control unit 606 (see FIG. 2). The control unit 606 can roughly specify the zero cross point of the AC input 500 (the point at which the output of the AC input 500 instantaneously becomes 0) based on the zero cross signal.

  The control unit 606 switches the state of the switching element provided in the heater drive circuit 601 based on the zero cross signal input from the zero cross signal generation circuit 605 and the detection result signal input from the heater current detection circuit 602. The power timing is accurately identified as the zero cross point of the AC input 500.

  In the graph of the current flowing through the heater 401 shown in the lowermost stage of FIG. 3, the switching element is in the OFF state between the timings T1 and T2 even though the voltage of the AC input 500 shown in the uppermost stage increases. Therefore, the current flowing through the heater 401 is maintained at 0.

  On the other hand, the timing T2 shows the timing when the trigger signal is input from the control unit 606 to the gate of the switching element and the switching element is switched to the ON state. From this time, the bottom graph of FIG. As shown, current begins to flow from AC input 500 through this switching element to heater 401. Thereafter, the current flowing through the heater 401 changes according to the voltage change of the AC input 500. However, since the voltage of the AC input 500 becomes 0 at the timing T3, the current flowing through the heater 401 becomes 0 (see the graph at the bottom of FIG. 3), and then the current of the heater 401 becomes positive again at the timing T4. When it is going to become, it is suppressed that an electric current flows by the function of a switching element. In the lowermost graph of FIG. 3, after this timing T4, the switching element is switched to the ON state again at timing T5. From this time, the current is applied to the heater 401 according to the voltage change of the AC input 500. It is flowing.

  The current of the heater 401 changes in this way by switching the state of the switching element. Then, the heater 401 has a period in which a current is passed through the heater 401 as in timing T2 to T3 and a period in which current is prevented from flowing in as in timing T4 to T5 is included in the unit time. The total amount of current per unit time that flows through is determined. The controller 606 changes the total amount of current that flows to the heater 401 per unit time by changing the ratio of the time during which such current flows by switching the state of the switching element of the heater drive circuit 601, thereby generating heat from the heater 401. The amount can be varied. The control unit 606 thus maintains the temperature of the fixing roller provided in the fixing device 400 at an appropriate temperature by changing the amount of heat generated by the heater 401 per unit time.

  In FIG. 3, the switching element is turned on at timing T2 to start energization of the heater 401. However, if this switching is not performed, the current of the heater 401 continues to be 0 after that, thereby causing the timing T1. Thereafter, the power supply to the heater 401 is suppressed by the amount of electric power during the period up to the timing T3 when the 1/2 cycle of the AC input 500 (the cycle in which the zero cross signal is in the high state) elapses. Similarly, the ½ cycle from timing T4 is also completely suppressed in the same manner when the switching element is not turned on at timing T5.

  In this way, the control unit 606 completely suppresses the power supply in the period of ½ cycle without turning on the switching element, or immediately after the start of the period (timing timing T1 to T2 in FIG. 3). In T2), the switching element is turned on to suppress only for a short period, or to switch for a long period by switching immediately before the end as in timing T5. Thus, the total amount of current flowing through the heater 401 per unit time is changed, and consequently the amount of heat generated by the heater 401 per unit time is changed.

  Thus, by performing switching control every 1/2 cycle of the AC input 500, the amount of heat generated by the heater 401 is changed.

  In this copying machine 1, the control unit 606 does not determine the timing for switching the state of the switching element based on only the zero cross signal shown in the third graph of FIG. By making a determination based on the current detection result by the heater current detection circuit 602 in which the current flowing through 401 is detected, the switching of the state of the switching element can be performed reliably and accurately.

  FIG. 4 is a diagram illustrating a state switching process of the switching element by the control unit 606.

  In FIG. 4, the graph of the third-stage zero-cross signal in FIG. 3 and the graph of the current of the fourth-stage heater 401 are taken up and shown in detail in an enlarged manner with respect to timings T1 to T2 and a certain period just before that. ing.

  FIG. 5 is a flowchart showing the flow of the switching process of the switching elements described in FIG.

  In step S1 of FIG. 5, the control unit 606 first determines whether or not to completely suppress the power supply for a period of ½ cycle without switching the switching element to the on state. The control unit 606 previously determines a ratio of a period for completely suppressing power supply per unit time, and performs this selection according to the determined period.

  When the switching element is turned on and the heater 401 is energized (step S1: YES), the control unit 606 executes the processing from step S2 onward to switch the switching element to the on state at a certain timing. Thus, current flows from the AC input 500 to the heater 401 through this switching element.

  In step S2, the control unit 606 detects timing Ta (FIG. 4) at which the zero cross signal switches from the low state to the high state.

  If the control unit 606 detects a timing Ta at which the zero cross signal rises from the low state to the high state (step S2: YES), the control unit 606 measures time from the timing Ta to the timing Tb in FIG. 4 to be described. To do. In step S3, the control unit 606 starts this time measurement.

  Here, in the vicinity of the timing Ta in FIG. 4, the current of the heater 401 shown in the lower part of FIG. 4 is negative. Timing Tb is a timing at which such a negative current rises to zero. The control unit 606 measures the time until the heater current detection circuit 602 detects that the current of the heater 401 becomes 0 at the timing Tb after the zero cross signal switches from the low state to the high state at the timing Ta. To do.

  In step S4, the control unit 606 determines whether or not the heater current detection circuit 602 has detected that the current of the heater 401 has become zero.

  If the heater current detection circuit 602 detects that the current of the heater 401 has become 0 (step S4: YES), in step S5, the control unit 606 sets the timing at the timing Tb in FIG. The timing is specified, and the time difference between this timing Tb and the timing Ta previously detected in step S2 is specified.

  In step S6, the control unit 606 suppresses power supply for a short time as described for the timings T1 to T2 in the description of FIG. 3, or relatively as in the case of the timings T4 to T5. Select whether to suppress power supply for a long time. The control unit 606 previously determines a ratio between a period for suppressing power supply for a short time and a period for suppressing the power supply for a long time according to the amount of heat to be supplied to the heater 401 per unit time. Accordingly, this selection in step S6 is performed.

  When the suppression is performed only for a short time (step S6: NO), the control unit 606 executes a process for switching the switching element to the ON state as follows.

  The control unit 606 measures the time until the time obtained by multiplying the calculated time difference by 0.3 passes (step S8a). That is, after the arrival of the timing Tb is detected in step S4, the process waits for the time until the time Tc in FIG. 4 arrives when the calculated time difference is multiplied by 0.3. And when it passes (step S8a: YES, timing Tc), the control part 606 switches the state of a switching element by this timing Tc by step S9, and is the timing in the graph of the current of the heater 401 of the lower stage of FIG. As indicated by Tc, a current starts to flow through the heater 401.

  In this way, a current as shown by a thick solid line in the lower graph of FIG. 4 flows to the heater 401, and energization of the heater 401 is started.

  Here, in this way, the control unit 606 does not refer to both the zero-cross signal and the detection result of the heater current detection circuit 602 (see steps S2 to S5), but refers to only the zero-cross signal for switching. A case where the state of the element is switched will be described for comparison.

  In this case, for example, the control unit 606 switches the switching element to the ON state at the falling timing Td of the zero cross signal in FIG. The current is a chain line.

  As described above, when switching the state of the switching element at the falling timing Td of the zero cross signal in FIG. 4, the variation in the zero cross signal directly affects the switching timing.

  On the other hand, in the case of the copying machine 1 described above, the state is switched to the timing Tc specified by measuring the time starting from the timing Ta at which the zero cross signal rises. Although the influence of variation does not become zero, the timing Tc changes according to the timing Tb when the current of the heater 401 becomes zero, and the timing Tc based on the detection result of the heater current detection circuit 602 is changed. The change compensates for the effects of variations in the zero cross signal, thereby bringing the timing Tc closer to the zero cross point of the correct AC input 500 output level.

  In this way, the switching of the switching element state is reliably performed at a precise timing, so that switching noise and inrush current can be suppressed, the regulation of the voltage between the noise terminals can be easily cleared, the choke coil can be downsized, and the cost can be reduced. Can be lowered.

  On the other hand, in step S6 in the flowchart of FIG. 5, when the control unit 606 has selected to suppress power supply for a long time (step S6: YES), the timing T4 in FIG. 3 (at the timing Tb in FIG. 4). 4) (step S4, S5), the control unit 606 waits until the timing T5 in FIG. 4 at which the zero cross signal rises next (step S7a: NO). If the timing T5 arrives (Step S7a: YES) When the time obtained by further multiplying the above time difference by 0.7 has elapsed from this timing T5 (Step S8b: YES), the control unit 606 executes the switching of the switching element (Step S9). .

  At this time, as in the case described above, the state switching timing is changed in accordance with the timing when the current of the heater 401 becomes 0, so that the influence due to the variation in the zero cross signal is compensated, and the timing is surely accurate. By switching the state of the switching element, switching noise and inrush current can be suppressed, or the regulation of the voltage between the noise terminals can be easily cleared, and the choke coil can be downsized and the cost can be reduced. It is like that.

  In steps S8b to S9, the state switching is executed at the timing when 0.7 times the time difference has elapsed, but the timing that does not reach the time difference multiplied by 0.3 for one time difference. The state is switched at a timing away from the timing at which the heater 401 current is detected by 0 and the heater current detection circuit 602 by a time obtained by multiplying the time difference by 0.3. It is the same as the above-mentioned case as what was aimed at.

  In the above description, the case where the AC input 500 outputs a positive voltage and thus the current is controlled by switching the state of the switching element in the period in which the positive current flows through the heater 401 has been described as an example. The same applies to the period in which the input 500 outputs a negative voltage and therefore a negative current flows through the heater 401.

  Next, a modified example will be described.

  (A) In the above description of the embodiment, a copying machine has been described as an example of an image forming apparatus. However, the same applies to image forming apparatuses of other forms such as a fax machine, a facsimile machine, and a multifunction machine.

  Further, the heater device 600 provided in the image forming apparatus has been described as an example. However, the present invention is not limited to such a case, and the case where a heater device similar to the heater device 600 is provided in a device that is not an image forming apparatus is also described. The invention is applicable.

  Further, although the heater 401 of the heater device 600 has been described as being a heater provided in the fixing device 400, the present invention is not limited to this case.

  (B) In the description of the embodiment described above, the rising timing of the zero cross signal (timing Ta in FIG. 4) and the timing at which the heater current detection circuit 602 detects the current of the heater 401 as 0 (timing Tb in FIG. 4). However, the present invention is not limited to this case, and the current of the heater 401 is detected as 0 by the zero cross signal and the heater current detection circuit 602. The state of the switching element may be switched at another appropriate timing specified based on the timing of the switching. Furthermore, it is not necessary to specify the switching timing based on the timing at which the current is detected as 0. For example, at the switching timing specified based on the timing at which the current amount becomes maximum or minimum, the state of the switching element May be switched.

  (C) The heater current detection circuit 602 may be operated only during a period in which the zero cross signal is in the high state and stopped during the low state in response to the zero cross signal. Alternatively, the control unit 606 may operate the heater current detection circuit 602 only while the zero-cross signal is in a high state.

  (D) When the heater 401 starts to generate heat, such as when the copier 1 is turned on or restored from an energy saving mode (sleep mode, power save mode, etc.), the above time difference varies for a while. There is also blurring. Therefore, assuming that the time difference specified in step S5 in FIG. 5 has been stored in the past, the control unit 606 reuses the time difference specified and stored in the past when the heater 401 starts to generate heat. It may be a thing. In this case, the process of measuring the time difference in steps S3 to S5 is not performed, and the time difference stored in advance can be used in the process corresponding to step S8a or step S8b.

  Here, the time difference stored in the control unit 606 may be an average value of measurement results over several times.

  (E) The heater current detection circuit 602 may be as follows. That is, 0 V and 5 V are output to the control unit 606 depending on whether current is flowing in one direction or in the opposite direction. Using the heater current detection circuit 602, the control unit 606 determines whether the current flowing through the heater 401 is positive or negative, and consequently the current of the heater 401 changes from positive to negative, or changes from negative to positive. Thus, the timing at which the current value exceeds 0 is detected.

  The heater current detection circuit 602 may be the same as the zero cross signal generation circuit 605, or may generate a zero cross signal of the current flowing through the heater 401. In this case, the reference current corresponding to the above-described reference voltage Th (see FIG. 3) is preferably as small as possible.

  (F) In the above description, the amount of power supplied to the heater 401 is switched every half of the period of the AC input 500 to control the amount of heat generated by the heater 401. This switching is completely Between the three patterns of when the supplied power is 0 (step S1: NO in FIG. 5), when the small power is supplied (step S6: YES), when the large power is supplied (step S6: NO) It is supposed to be switched at.

  Thus, switching does not need to be performed in three patterns. For example, the supply power may be controlled by switching between two patterns among the three patterns described above.

  (G) A variety of switching elements can be used. For example, a so-called GTO (Gate Turn Off) thyristor is used, and only switching from the off state to the on state is controlled as described above. Instead of this, the switching timing from the off state to the on state may also be controlled. Also in this case, switching noise can be avoided when switching from the off state to the on state by accurately bringing the switching timing close to the zero cross point of the AC input 500.

1 is a diagram showing a copying machine according to an embodiment of the present invention. It is a figure which shows the heater apparatus comprised in the copying machine which concerns on embodiment. It is a figure which shows the change of the electric current of each part of a heater apparatus, and a voltage. It is explanatory drawing which shows the state switching process of the switching element by a control part. It is a flowchart which shows the flow of the state switching process of the switching element by a control part.

Explanation of symbols

P recording paper 1 copier 100 image reading unit 200 paper feeding unit 200d discharge unit 200t transport path 300 image forming unit 400 fixing device 401 heater 500 AC input 600 heater device 601 heater drive circuit 602 heater current detection circuit 603 transformer 604 rectifier circuit 605 Zero cross signal generation circuit 606 control unit

Claims (4)

A heater that generates heat when power is supplied from an AC input;
A switching element that is provided between the AC input and the heater and switches between a state in which power is supplied to the heater from the AC input and a state in which supply is stopped;
Rectifying means for acquiring the voltage of the AC input and performing full-wave rectification;
Zero-cross signal generating means for generating a zero-cross signal of the AC input based on the voltage that has been full-wave rectified by the rectifying means;
Current detecting means for detecting a current flowing through the heater;
The timing of the zero cross point of the AC input is specified based on the zero cross signal generated by the zero cross signal generation means and the amount of current detected by the current detection means, and at the specified timing, the switching element A heater device comprising control means for switching states.   2. The heater device according to claim 1, wherein the control unit specifies the timing of the zero cross point of the AC input based on a timing at which the current flowing through the heater is detected as 0 by the current detection unit and the zero cross signal. The control means includes
A time difference specifying means for specifying a time difference between a timing at which the zero cross signal rises and a timing at which a current 0 is detected by the current detection means;
The heater device according to claim 2, wherein the timing of the zero cross point is specified by compensating a variation time of the zero cross signal according to the time difference.   An image forming apparatus comprising the heater device according to claim 1 configured as a fixing device.

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