Abstract:
A heater activating apparatus includes a power source, a heater heated to which is applied to voltage from the power source, and a current regulating element. The voltage from the power source is applied to the heater through the current regulating element when the heater is initially to energized, and thereafter, the voltage is applied to the heater without passing through the current regulating element.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a heater activating apparatus for activating a heater such as a halogen heater and the like, and more particularly, it relates to a heater activating apparatus for activating a heater so that a heating element heated by the heater is maintained at a predetermined temperature. 
     2. Related Background Art 
     Recently, in an image forming apparatus such as an optical printer copying machine, a heat fixing device wherein a toner image is heated or heated and pressurized by a heating element such as a heat roller which surface is maintained at a predetermined temperature has widely been used. 
     An example of a heater activating circuit which is used with such heat fixing device and which provides a background of the present invention is shown in FIG. 7. 
     In FIG. 7, the reference numeral 1 denotes an AC voltage source; 2 denotes a switch; 3 denotes a halogen heater; 4 denotes a large electric power thermistor (referred to as &#34;power thermistor&#34; hereinafter); 5 denotes zero cross-controlled SSR (solid state relay); 6 denotes a thermistor for detecting a temperature of the heater; 7 denotes a comparator having an open collector output; 8 denotes a transistor; and 9 through 13 denote resistors. 
     Next, an operation of the above mentioned heater activating circuit will be explained. 
     When the switch 2 is turned ON, the voltage V 3  becomes a GND level (i.e., earthed), the transistor 8 goes to an OFF condition, and a light emitting element 5b of the SSR 5 also goes to an OFF condition. Since a triac 5a of the SSR 5 is in an OFF condition when the light emitting element 5b is turned OFF, the voltage is not applied to the heater. 
     On the other hand, when the switch 2 is turned OFF, the voltage V 1  is determined by the partial voltage of the resistor 9 and the temperature detecting thermistor 6. That is to say, when the temperature of the thermistor 6 is decreased the value of V 1  is reduced, and, when the temperature of the thermistor 6 is increased the value of V 1  is also increased. If the voltage V 1  is lower than a voltage V 2  of the other input terminal of the comparator 7, the output V 3  of the comparator 7 will be HIGH. In this case, since the transistor 8 is turned ON, the light emitting element 5b is also turned ON. Since the SSR 5 is zero cross-controlled, after the light emitting element 5b has been turned ON, the triac 5a is turned ON when the voltage at both ends thereof become zero or thereabout. And, the triac 5a is maintained in the ON condition until the voltage of the power source becomes zero or thereabout (i.e., the current in the heater becomes zero) after the light emitting element 5b has been turned OFF. When the temperature of the thermistor 6 increases to increase the voltage V 1  higher than the voltage V 2 , the comparator 7 is inverted to provide a LOW output. Consequently, the transistor 8 is turned OFF, thereby deenergizing the heater 3. Such condition is shown in FIG. 10. 
     In FIG. 10, A indicates a time point when the temperature adjustment is initiated by turning the switch 2 OFF in a condition that the temperatures of the heater 3, power thermistor 4 and the temperature detecting thermistor 6 are reduced to a room temperature; and B indicates a time point when the heater 3 is energized in a condition that the heater 3 is controlled substantially at the set temperature and the temperature of the temperature detecting thermistor 6 is substantially in the set value. The heater current in the time point A is shown in FIG. 8, and the heater current in the time point B is shown in FIG. 9. The power thermistor 4 is used to control the rush current to the heater. The power thermistor has a resistance of a few ohms when the temperature of the power thermistor itself is cooled to the room temperature. However, when the power thermistor is heated by the heater current, the resistance thereof is reduced below one ohm, thereby reducing the power consumption. 
     In the above-mentioned arrangement wherein the power thermistor 4 is connected to the heater 3 in series, since the current is always flowing in the power thermistor while the heater is being activated, the temperature of the power thermistor is considerably increased. Since the power thermistor has the thermal time constant larger than that of the heater, once the power thermistor has been heated up, it is not readily cooled. Accordingly, such arrangement provides the rush current controlling action when it is cooled to the room temperature (see FIG. 8). However, upon normal temperature adjustment, since the temperature of the heater is decreased to some extent but the temperature of the power thermistor itself is till high, the resistance thereof is small as mentioned above, with the result that the rush current control cannot be attained (see FIG. 9). 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a heater activating apparatus which can provide a rush current control to a heater for a long time. 
     Another object of the present invention is to provide a heater activating apparatus which can suppress the increase in temperature of a current controlling element. 
     A further object of the present invention is to provide a heater activating apparatus wherein the energization of a heater without a current passing through a controlling element and the energization of the heater with a current passing through the current controlling element can be selectively utilized. 
     Other objects of the present invention will be apparent from the following explanation. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a circuit showing a heater activating apparatus according to a first embodiment of the present invention; 
     FIG. 2 is a graph showing voltage waves in various parts of the circuit of FIG. 1; 
     FIG. 3 is a graph showing a rush current controlling action in a room temperature; 
     FIG. 4 is a graph showing a rush current controlling action in a normal temperature adjustment condition; 
     FIG. 5 is a circuit showing a heater activating apparatus according to a second embodiment of the present invention; 
     FIG. 6 is a graph showing voltage waves in various parts of the circuit of FIG. 5; 
     FIG. 7 is a circuit showing a heater activating apparatus relating to a background art for the present invention; 
     FIG. 8 is a graph showing a rush current controlling action in a room temperature; 
     FIG. 9 is a graph showing a rush current controlling action in a normal temperature adjustment condition; 
     FIG. 10 is a graph showing voltage waves in various parts of the circuit of FIG. 7; and 
     FIG. 11 is a sectional view of a heat fixing device using a heater activating apparatus according to the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention will now be explained in connection with preferred embodiments thereof with reference to the accompanying drawings. 
     FIG. 11 is a sectional view of a heat fixing device using a heater activating apparatus according to the present invention. 
     In FIG. 11, the reference numeral 61 denotes a heat roller including a halogen heater 3 therein; 62 denotes a pressure roller urged against the heat roller to form a nip therebetween; 64 denotes a separating claw for separating a recording medium or recording sheet; 65 denotes a web for cleaning a surface of the heat roller and for applying the separating agent to the surface of the heat roller; and 6 denotes a thermistor for detecting a surface temperature of the heat roller 61. On the basis of the detection output from the thermistor 6, by energizing or disenergizing the halogen heater 3, the surface of the heat roller 61 is maintained at a predetermined temperature. 
     The recording sheet on which an unfixed toner image is present is pinched by the nip between the heat roller 61 and the pressure roller 62 and is passed through the nip; meanwhile, the toner image is fixed to the recording sheet by heat and pressure. 
     Next, a heater activating portion for activating the halogen heater 3 will be explained. FIG. 1 shows a circuit of the heater activating portion. 
     In FIG. 1, the reference numeral 21 denotes a zero cross-controlled SSR; 22 denotes a one-shot timer; 23, 24 denote inverters; 25 denotes an OR gate; 26, 27 denote transistors; and 28-34 denote resistors. 
     FIG. 2 shows voltage waves illustrating an example of the changes in voltage of various parts of the circuit of FIG. 1, regarding the time elapsed. 
     Now, the operation of the various parts will be explained with reference to FIGS. 1 and 2. 
     When the temperature of the temperature detecting thermistor 6 is high and the voltage V 1  is higher than the voltage V 2 , the heater 3 remains in the OFF condition; however, if the temperature of the temperature detecting thermistor 6 is decreased and the voltage V 1  becomes lower than the voltage V 2 , the comparator 7 outputs the LOW level signal. In this case, the one-shot timer 22 outputs a LOW pulse having a width or duration t 2  starting from the edge of the building-up wave of the voltage V 4 . Since the voltage V 6  corresponds to the inversion of the voltage V 5  inverted in the inverter 23, the voltage V 6  has a HIGH pulse having a width t 2  starting from the edge of the building-up wave of the voltage V 4 . Since the voltage V 7  is obtained by the voltages V 4  and V 6  treated by the OR gate, the voltage V 7  goes to a LOW level at the edge of the building-up wave of the voltage V6, and returns to a HIGH level at the edge of the building-up wave of the voltage V 4 . The voltage V 8  becomes a LOW pulse having a width t 2  starting from the edge of the building-up wave of the voltage V 4 , similar to the voltage V 5 . When the voltage V 7  becomes the LOW level, the transistor 26 is turned OFF to energize a light emitting element 21b, thus turning a triac 21a ON. 
     Further, when the voltage V 8  goes to the LOW level, the transistor 27 is turned OFF to energize a light emitting element 5b, thus turning a triac 5a ON. That is to say, during the turn-on time t 1 , since the triac 5a is turned ON for the initial time duration t 2 , the voltage from the power source 1 is applied to the heater 3 through the large electric power thermistor 4 having the resistance of negative temperature feature. After the time duration t 2 , since the triac 5a is turned OFF and the triac 21a turned ON, the power thermistor 4 and the triac 5a are short-circuited so as not to allow the to flow therethrough, with the result that all of the voltage of the power source 1 is applied to the heater 3. That is to say, the power thermistor is connected to the heater in series for a time duration required to prevent the rush current to the heater. 
     FIG. 3 shows the rush current control effected when the switch 2 is turned from ON to OFF in a condition that all of the elements are at the room temperature. During the initial time duration t 1 , since the power thermistor is connected to the heater 3 in series, the rush current to the heater is sufficiently suppressed. After the time duration t 1 , since the triac 21a is turned ON to short-circuit the power thermistor 4 and the triac 5a, the current is increased; however, in this point, since the heater 3 has already been heated considerably, the amount of the current increased by releasing the power thermistor 4 is a little. 
     As apparent from FIG. 4 showing the rush current control during the normal temperature adjustment, since the current does not flowed through the power thermistor 4 except as necessary, the increase in the temperature is suppressed to the minimum. Accordingly, also in the normal temperature adjustment condition, the resistance of the power thermister is still high to provide the rush current control action. 
     As mentioned above, by energizing the power thermistor for a predetermined time duration during the heater energization, it is possible to suppress the temperature increase in the power thermistor to the minimum so that the rush current control can be provided even in the normal temperature adjustment condition. 
     A heater activating apparatus according a second embodiment of the present invention is shown in FIG. 5. 
     In FIG. 5, the reference numeral 41 denotes a power resistor; 42, 43 denote zero cross-controlled SSRs; 44, 45 denote transistors; 46 denotes a CPU; and 47 through 50 denote resistors. 
     FIG. 6 shown voltage waves illustrating the changes in voltages of various parts of the apparatus of FIG. 5, regarding the time elapsed. Hereinafter, the second embodiment will be explained with reference to FIGS. 5 and 6. 
     In the above-mentioned first embodiment, while the arrangement wherein the power thermistor 4 and the triac 5a are short-circuited by the triac 21a was explained, in this second embodiment, only the current regulating power resistor 41 is short-circuited by a triac 43a. Thus, signals for activating triacs 42a, 43a will be as shown in FIG. 6. When the voltage V 1  is decreased below a predetermined voltage Vth, the CPU 46 changes the voltage V 9  and V 10  to LOW levels, respectively. The CPU controls the voltages in such a manner that the voltage V 9  is returned to a HIGH level when the time t 1  is elapsed after this voltage is changed to the LOW level, and the voltage V 10  is returned to a HIGH level when the voltage V 1  becomes higher than the voltage Vth. 
     Incidentally, in this embodiment, a power resistor is used in place of the power thermistor. When the power resistor is used for preventing the rush current, while, conventionally, the resistor having very high rated electric power was required, according to the present invention, since the power consumption of the power resistor is suppressed to the minimum, the resistor having the rated electric power considerably lower than a conventional one can be used, and the loss of the electric power can be reduced. Incidentally, in order to ensure the safety if the photo-triac 43a is damaged, the fuse resistor is used as the power resistor 41. 
     In this way, according to the present invention, it is possible to reduce the power consumption of the current regulating element, to suppress the increase in temperature of such element to the minimum, and to provide the effective rush current control. 
     As mentioned above, while the present invention has been explained with reference to the particular embodiments, the present invention is not limited to such embodiments, and various alterations and modifications can be adopted within the scope of the present invention.