Abstract:
An electronic device includes a supply circuit board having a switching power supply circuit for outputting DC voltages in conformity with a type of commercial power, and a control circuit, which is connected to the supply circuit board, for performing control in accordance with the type of the commercial power. The control circuit discriminates the type of commercial power based upon a voltage value that is the result of dividing a DC voltage by the supply circuit board, and changes over control of a fixing heater in dependence upon the type of commercial power discriminated. When the door of an image forming apparatus is opened, the supply circuit board halts the supply of DC to a high-voltage circuit, thereby suppressing the generation of high voltage, and the control circuit is notified of the fact that the door has been opened.

Description:
FIELD OF THE INVENTION 
   This invention relates to an electronic device that includes a control circuit for performing control that differs depending upon the power supply voltage of a commercial power supply, and to a method of controlling an image forming apparatus. 
   BACKGROUND OF THE INVENTION 
   In a conventional laser printer, power is supplied from a commercial power supply and the supplied power is applied to a power supply circuit built in the printer. From the power applied thereto, the power supply circuit generates DC voltages necessary for the laser printer, such as 3.3 V used as a low voltage for control circuits and the like, and 24 V used in motors, other drive systems and high-voltage circuits, etc. Commercial power differs in voltage and frequency depending upon the country or area. 
   Safety standards for the control devices of image printers such as laser printers have been defined in various countries. For example, according to the IEC-1000-3-3 standard in Europe, limit values on voltage fluctuation and flicker have been defined. Furthermore, according to the specifications of IEC-1000-3-3, the rated supply voltage and frequency of commercial power are 220 to 250 V and 50 Hz, respectively. 
   Ordinarily, in control of a fixing heater in a laser printer, the resistance value of a ceramic heater is optimized so as to satisfy the specifications and obtain appropriate electrophotographic fixation. In terms of controlling the temperature of the fixing heater, on/off duty and frequency are regulated to perform control so as to optimize voltage fluctuation and flicker value. 
   In North America, on the other hand, the rated voltage and frequency of commercial power are 120 V and 60 Hz, respectively. Consequently, it is necessary to control the resistance value and fixing temperature of the ceramic heater optimum for the commercial power supply. 
   In the prior art, therefore, the power supply circuit and fixing heater unit are each constructed differently depending upon the supply voltage of each country or area. In addition, the control board of the printer also performs heater control that differs depending upon the supply voltage, and therefore the control board is constructed differently according to the country or area. If printer control boards are thus prepared in accordance with the commercial power supply of each country or area, this will lead to an increase in types of printer control boards and have an adverse effect upon productivity. 
   SUMMARY OF THE INVENTION 
   Accordingly, a feature of the present invention is to provide an electronic device having a control circuit that is capable of supporting various commercial power supplies by identifying the type of commercial power supply connected, and a method of controlling an image forming apparatus. 
   In accordance with a preferred embodiment of the present invention, generation of high voltage is halted when the door of an electronic device or image forming apparatus is opened, and the fact that the door has been opened can be reported to a control circuit. 
   According to the present application, there is providing an electronic device comprising: 
   a first circuit board having a power supply circuit for outputting DC voltages in conformity with a type of commercial power; and 
   a second circuit board connected to the first circuit board and having a control circuit for performing control in accordance with the type of the commercial power; 
   wherein the first circuit board has generating means for generating an analog voltage, which corresponds to the type of the commercial power, based upon a prescribed DC voltage, and the control circuit changes over control in accordance with the analog voltage. 
   Other features and advantages of the present invention will be apparent from the following description taken in conjunction with the accompanying drawings, in which like reference characters designate the same or similar parts throughout the figures thereof. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention. 
       FIG. 1  is a block diagram illustrating the general structure of a laser printer according to a first embodiment of the present invention; 
       FIG. 2  is a flowchart illustrating door open/close detection processing in the printer according to the first embodiment; 
       FIG. 3  is a flowchart illustrating processing for discriminating the type of commercial power supply circuit in the printer according to the first embodiment; 
       FIG. 4  is a block diagram illustrating the general structure of a laser printer according to a second embodiment of the present invention; and 
       FIG. 5  is a flowchart illustrating processing for determining whether a door has been opened and for discriminating the type of commercial power supply circuit in the printer according to the second embodiment. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Preferred embodiments of the present invention will now be described in detail with reference to the accompanying drawings. 
   First Embodiment 
     FIG. 1  depicts a block diagram illustrating the general structure of a laser printer according to a first embodiment of the present invention. 
   As shown in  FIG. 1 , a commercial power supply  1  has a power-supply voltage and frequency that differ depending upon the country or area. A supply circuit board  12  generates DC voltages necessary for the laser printer by means of a switching power supply circuit  12   a  based upon supply voltage supplied from the commercial power supply  1 . Examples of the DC voltage are 3.3 V used for control circuits and small signals, and 24 V used in motors, other drive mechanisms and in a high-voltage circuit  12   e,  etc. The supply circuit board  12  further includes a circuit for controlling passage of current to a fixing heater  4   a.  Current is fed to or cut off from the fixing heater  4   a  by turning a triac  12   b  on and off, thereby making it possible to regulate the temperature of the ceramic heater  4   a  to a prescribed temperature. A relay  12   c  cuts off the supply of power to the ceramic heater  4   a  during printer standby or when a malfunction occurs in control of fixing temperature. A printer control board  13  has a CPU  13   a  such as a microprocessor for controlling an electrophotographic process necessary to achieve printing by the laser printer. The CPU  13   a  further controls the on/off operation of the triac  12   b  and the operation of the relay  12   c.  A fixing heater unit  4  has the ceramic heater  4   a  and a temperature fuse  4   b,  which is a safety circuit. The temperature fuse  4   b  opens the circuit by melting when the ceramic heater  4   a  attains an abnormally high temperature. A thermistor  4   c  detects the temperature of the ceramic heater  4   a.  Temperature information detected by the thermistor  4   c  is input to the CPU  13   a,  which proceeds to control the ceramic heater  4   a  to the prescribed temperature in accordance with this temperature information. 
   The 24 V from the switching power supply circuit  12   a  is input to the high-voltage circuit  12   e,  which generates the high voltage used in a well-known electrophotographic process. An interlock switch  12   d  cuts off the input voltage (24 V) to the high-voltage circuit  12   e  when a door on the body of the laser printer is opened by the user in order to unjam paper, for example, thereby interrupting the output from the high-voltage circuit  12   e.  Further, the voltage (24 V) on the output side of the interlock switch  12   d  is divided and input to the input port of the CPU  13   a  as a door-open signal “dopen”. As a result, if the dopen signal becomes approximately 0 V, the CPU  13   a  is capable of recognizing the fact that the door of the laser printer has been opened. 
   Furthermore, the DC supply voltage (24 V in this embodiment) output from the switching power supply circuit  12   a  is divided by resistors R 1  and R 2 , and the analog voltage resulting from this voltage division is input to an A/D input port of the CPU  13   a . The resistance values of the resistors R 1  and R 2  are each set to different values depending upon whether the supply circuit board  12  is a 120-V board or a 220-V board. Accordingly, by inputting the voltage value obtained by voltage division by the resistors R 1  and R 2  to the CPU  13   a , the CPU  13   a  can discriminate whether the supply circuit board  12  mounted within the laser printer is a 120-V board or a 220-V board. In other words, the CPU  13   a  can discriminate the type of commercial power supply  1  to which the laser printer has been connected. Optimum control of the temperature of the ceramic heater  4   a  for the purpose of satisfying the above-mentioned standards can be achieved in dependence upon the type of commercial power supply  1  discriminated. As a result, the printer control board  13  can be used as a common board suited to all commercial power supplies. This makes it unnecessary to change the board in accordance with each type of commercial power supply and leads to commonality of units. 
   In this embodiment, the A/D input port of the CPU  13   a  is used in discriminating the voltage value obtained by voltage division. This means that the commercial power supply is not limited to two types, i.e., 120V and 220V. For example, the commercial power supply may be 100 V/50 Hz or 100 V/60 Hz, as in Japan, or 200 V/50 Hz, as in Hong Kong. By changing the resistance ratio between the resistors R 1  and R 2  accordingly, the CPU  13   a  can discriminate the type of commercial power supply that corresponds to the supply circuit board  12 . As a result, temperature control of the fixing heater that conforms to each commercial power supply can be carried out and voltage fluctuation and flicker value can be minimized. 
     FIG. 2  is a flowchart of processing executed by the CPU  13   a  of the laser printer according to this embodiment in a case where the printer door (cover) has been opened. 
   First, at step S 1  in  FIG. 2 , the CPU  13   a  determines whether the dopen signal being applied to its input port is at the low level (about 0 V). As described above, this laser printer is such that when the door of the printer is opened, the supply of 24-V power from the switching power supply circuit  12   a  is cut off by the interlock switch  12   d,  as a result of which the signal level of the dopen signal becomes approximately 0 V (“YES” at step S 1 ). In this case, control proceeds to step S 2 , at which processing is executed to display a “DOOR OPEN” message on a control panel and halt the printing operation. If the dopen signal is at the high level (“NO” at step S 1 ), on the other hand, this means that the door is closed and, hence, the voltage of 24 V is being output normally. Control therefore proceeds to step S 3 , at which the usual processing is executed. 
     FIG. 3  is a flowchart illustrating processing executed by the CPU  13   a  of the laser printer according to this embodiment for discriminating the commercial power supply circuit supported by the supply circuit board  12 . 
   First, at step S 11  in  FIG. 3 , a digital value resulting from analog-to-digital conversion of the voltage value (obtained by voltage division) entering the A/D input port of the CPU  13   a  is input. Control then proceeds to step S 12 , at which the CPU  13   a  discriminates, based upon the digital value, the commercial power supply supported by the supply circuit board  12 . For example, the CPU  13   a  determines that: 
   (1) the commercial power supply is Japan-specific (100 V, 50 Hz) when the voltage value obtained by voltage division is 3 V (resistance ratio 7:1); 
   (2) the commercial power supply is Japan-specific (100 V, 60 Hz) when the voltage value obtained by voltage division is about 2.67 V (resistance ratio 8:1); 
   (3) the commercial power supply is Europe-specific when the voltage value obtained by voltage division is 2.4 V (resistance ratio 9:1); and 
   (4) the commercial power supply is USA-specific when the voltage value obtained by voltage division is about 2.1 V (resistance ratio 10:1). 
   It should be noted that the CPU  13   a  discriminates the corresponding types of commercial power supply based upon respective ones of the digital values corresponding to the above-cited voltage values. 
   When the type of commercial power supply is thus determined, control proceeds to step S 13 , at which the method of controlling the fixing heater  4   a,  for example, is decided in accordance with the type of commercial power supply determined. 
   Second Embodiment 
     FIG. 4  is a block diagram illustrating the general structure of a laser printer according to a second embodiment of the present invention. Components identical with those of  FIG. 1  are designated by like reference characters and need not be described again. 
   In the first embodiment above, the dopen signal indicating the door of the printer has been opened and the voltage value that is the result of dividing the supply voltage of 24 V by the resistances R 1  and R 2  are input separately to the CPU  13   a.  In the second embodiment, however, effects similar to those of the first embodiment are obtained by adopting a single common signal for both a voltage value, which is the result of dividing, by the resistances R 1  and R 2 , the 24 V for the high voltage interrupted by the interlock switch  12   d,  and the above-mentioned dopen signal. 
   More specifically, 0 V is input to the A/D input port of the CPU  13   a  when the door is open, and the analog voltage that results from voltage division by the resistances R 1 , R 2  is input to the A/D input port when the door is closed. Naturally, when the door is open, the temperature of the fixing heater  4   a  is not controlled and therefore it is possible to discriminate, by this single signal, when the door is open, when the door is closed and the type of commercial power supply voltage. In addition, since the same signal is used in detection of whether the door is open or closed and in discrimination of the type of commercial power supply voltage, the number of signal lines can be reduced and the cost of the apparatus can be reduced. 
   The processing executed in this case is as illustrated in the flowchart of  FIG. 5 . 
   First, at step S 21  in  FIG. 5 , a digital value resulting from analog-to-digital conversion of the voltage value (obtained by voltage division) entering the A/D input port of the CPU  13   a  is input. Control then proceeds to step S 22 , at which the CPU  13   a  determines whether the digital value is 0 or substantially 0, namely whether the door is open. If the door is open (“YES” at-step S 22 ), control proceeds to step S 23 , where processing similar to that of step S 2  above is executed in response opening of the printer door (cover). 
   On the other hand, if the digital value is not  0  or substantially 0 (“NO” at step S 22 ), control proceeds to step S 24 . Here the CPU  13   a  discriminates, in a manner similar to that of step S 12  in  FIG. 3  based upon the digital value entered at step S 21 , the commercial power supply supported by the supply circuit board  12 . When the type of commercial power supply is thus determined, control proceeds to step S 25 , at which the method of controlling the fixing heater  4   a,  for example, is decided in a manner similar to that of step S 13  in FIG.  3  in accordance with the type of commercial power supply determined. 
   The present invention can be applied to a system constituted by a plurality of devices (e.g., a host computer, interface, reader, printer, etc.) or to an apparatus comprising a single device (e.g., a copier or facsimile machine, etc.). 
   Further, it almost goes without saying that the object of the invention is attained also by supplying a storage medium (or recording medium) on which the program codes of the software for performing the functions of the foregoing embodiments to a system or an apparatus have been recorded, reading the program codes with a computer (e.g., a CPU or MPU) of the system or apparatus from the storage medium, and then executing the program codes. In this case, the program codes read from the storage medium themselves implement the novel functions of the embodiments, and the program codes per se and storage medium storing the program codes constitute the invention. Further, besides the case where the aforesaid functions according to the embodiments are implemented by executing the program codes read by a computer, it almost goes without saying that the present invention covers a case where an operating system or the like running on the computer performs a part of or the entire process based upon the designation of program codes and implements the functions according to the embodiments. 
   The present invention further covers a case where, after the program codes read from the storage medium are written in a function expansion card inserted into a computer or in a memory provided in a function expansion unit connected to the computer, a CPU or the like contained in the function expansion card or function expansion unit performs a part of or the entire process based upon the designation of program codes and implements the function of the above embodiments. 
   Though the above embodiments have been described independently, specific features and functionality thereof may be combined. 
   Thus, in accordance with the embodiments as described above, a power supply circuit board outputs an analog voltage signal that conforms to the type of each commercial power supply, and this signal is input to an analog input port of a CPU on a control board, whereby the CPU is capable of recognizing the type of commercial power supply to which a printer has been connected. As a result, arrangements corresponding to commercial power supplies of a plurality of types can be constructed by a printer control board of a single type. 
   Further, by using this analog voltage signal conjointly as a signal for sensing whether a door is open or closed, the number of signal lines can be reduced, thereby making it possible to reduce the cost of the apparatus. 
   The present invention is not limited to the above embodiments and various changes and modifications can be made within the spirit and scope of the present invention. Therefore, to apprise the public of the scope of the present invention, the following claims are made.