Patent Publication Number: US-6701096-B2

Title: Image-forming device having consumable component with internal fuse

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an image-forming device such as a printer, more specifically to the management of a consumable component in such a device. 
     2. Description of the Related Art 
     One example of an image-forming device in which the present invention can be practiced is the tandem color electrophotographic printer  1  shown in FIGS.  17  and  18 : FIG. 17 is a side sectional view; FIG. 18 is a schematic block diagram of the printing engine. 
     The printer in these drawings has a low-voltage power source  2 , a high-voltage power source  3 , and four printing mechanisms: a yellow (Y) printing mechanism  4 , a magenta (M) printing mechanism  5 , a cyan (C) printing mechanism  6 , and a black (K) printing mechanism  7 . The printing mechanisms include respective photosensitive drum units (ID units)  4   a - 7   a , light-emitting diode (LED) heads  4   b - 7   b,  discharge lamps  4   c - 7   c,  and transfer rollers  4   e - 7   e,  and are driven by respective motors  4   d - 7   d.    
     Printing media such as sheets of paper, not shown, are placed in a cassette tray  8 , and fed into the printer  1  by the rotation of a hopping roller  9 . An attraction roller  10  generates a static electric charge that holds the printing media to a transfer belt in a transfer belt unit  11 . Driven by the rotation of a transfer-belt driving roller  11   a,  the transfer belt carries the printing media past the printing mechanisms  4 - 7 , which perform printing processes that transfer yellow, magenta, cyan, and black toner images onto the printing media. The media next pass through a fuser  12 , which fuses the toner images onto them, and are finally delivered into a stacker  13 . The printing media may also be supplied manually, in which case they are fed into the printer  1  by a front roller  14 , but the subsequent printing operations are the same. 
     These printing operations are controlled by the engine controller  15  in FIG.  18 . The engine controller  15  controls the LED heads  4   b - 7   b  through a relay board  16 , and directly controls the discharge lamps  4   c - 7   c,  the above-mentioned motors (M)  4   d - 7   d,  a hopping motor  9   d  that drives the hopping roller  9 , a belt motor  11   d  that drives the transfer-belt driving roller  11   a , a heater motor  12   d  that drives a heating roller in the fuser  12 , a front motor  14   d  that drives the front roller  14 , and the power sources  2 ,  3 . The low-voltage power source  2  supplies power to a heat source such as a halogen lamp (not shown) in the fuser  12 . The high-voltage power source  3  supplies power to the ID units  4   a - 7   a  and the transfer belt unit  11 . The engine controller  15  is also connected to various sensors  17 , such as a sensor that senses the presence of printing media and a sensor that senses whether the printer&#39;s cover is open or closed. 
     In this printer  1 , the ID units  4   a - 7   a,  the transfer belt  11 , and the fuser  12  are consumable components that must be replaced at the end of their service lives. To tell the user when to replace the consumable components, the printer has counters that count the cumulative number of rotations made by rotating parts such as the photosensitive drums. When a counter reaches a predetermined value, the printer displays a service-life alarm indicating that the corresponding consumable component needs replacement. Notified by this alarm, the user can replace the consumable component at the appropriate time. 
     When the consumable component is replaced, it is also necessary to reset the counter. It is known art to reset the counter automatically by means of the structure shown in FIG.  19 . The consumable component  20 , which may be any one of the ID units  4   a - 7   a,  or the transfer belt  11  or fuser  12 , includes an internal fuse F 1 . The printer has a consumable-component sensing section  18  that senses whether the fuse F 1  is blown. If the fuse F 1  is not blown, the consumable-component sensing section  18  blows it and resets the counter. 
     The consumable-component sensing section  18  includes a transistor TR 1 , a resistor R 1 , and a central processing unit (CPU)  19 , the functions of which will be described below with reference to the flowchart in FIG.  20 . 
     When the printer&#39;s power is switched on or its cover is opened and then closed, to determine whether the consumable component  20  has been replaced, the CPU  19  reads (step S 201 ) and tests (step S 202 ) the input value at a one-bit digital input port IN, which is connected through fuse F 1  to ground and through resistor R 1  to a power supply (Vcc). If the input value is at the high logic level, indicating that fuse F 1  is already blown and the consumable component  20  is not new, the CPU  19  terminates the process in FIG.  20 . If the input value is at the low logic level, indicating that fuse F 1  is not blown and the consumable component  20  is new, the CPU  19  resets the counter that keeps track of the service life of the consumable component  20  (step S 203 ), and outputs a ‘0’ pulse from an output port OUT (step S 204 ), sending a current pulse through transistor TR 1  to blow fuse F 1 . To confirm that fuse F 1  has blown, the CPU  19  reads (step S 205 ) and tests (step S 206 ) the input value at the input port IN again. If the input value is at the high logic level, the process ends; if the input value is at the low logic level, steps S 204 , S 205 , and S 206  are repeated until the input value becomes high, or until a limit number of repetitions is reached. 
     Consumable components such as the ID units, transfer belt, and fuser have different specifications for different printers, and when they are replaced, the user may mistakenly install a consumable component of the wrong type. Since there are four ID units with different toner colors, the user may also install an ID unit of the wrong color. 
     When this happens, a conventional printer cannot recognize that the consumable component has been incorrectly replaced, and operates as if the replacement had been made correctly, creating various problems. One problem is that the user does not realize that the wrong consumable component has been installed until a defective printing result is obtained, at which point the user must replace the consumable component again, repeat the printing job, and either dispose of the consumable component that was mistakenly installed, or store it for later use. Another problem is that the mistakenly installed consumable component now has a blown fuse, so if it is later reinstalled and used, its counter will not be reset, and its service life will not be indicated correctly. 
     If consumable components with different specifications or colors have different external shapes, these problems can be avoided by a mechanical interlocking mechanism that prevents the installation of the wrong type of consumable component, but such mechanisms increase the manufacturing cost of the printer and the consumable component. 
     Instead of a fuse, the consumable component may have an internal memory circuit storing, for example, identification information and either a count value or a flag indicating whether the consumable component is new or not, but this memory circuit also increases the cost of the consumable component. 
     Another problem is that when a new consumable component is installed, its fuse may fail to blow. In this case, a conventional printer displays an alarm indicating that the consumable component is defective, and disables printing. The user must then replace the consumable component again, even though its functioning is not normally impaired by the fuse failure, and the failure may be due to a temporary condition that will disappear later. 
     A further problem is that the printer cannot distinguish between the state in which the consumable component is not installed, and the state in which the consumable component is installed but has a blown fuse. One conventional solution to this problem is shown in FIG.  21 . The consumable component  20  and sensing section  18  make electrical contact at three points  21 ,  22 ,  23 . In the consumable component  20 , contact point  22  is coupled directly to contact point  21 , and is coupled to contact point  23  through the fuse F 1 . The consumable-component sensing section  18  now includes a transistor TR 1 , resistors R 11 -R 16 , a CPU  19  with input ports IN 1  and IN 2 , and switching means (not shown) for making and breaking electrical contact at points  21  and  23 . In the consumable-component sensing section  18 , contact point  22  is coupled to the power supply (Vcc) through resistor R 11 , and contact point  23  is grounded. The functions of these elements will be explained with reference to the flowchart in FIG.  22 . 
     When the printer&#39;s power is switched on or its cover is opened, then closed, the CPU  19  commands the switching means to make electrical contact at point  21  (step S 211 ), then reads and tests the input value at input port IN 1 , which is connected through resistor R 15  to contact point  21  and through resistor R 16  to ground (step S 212 ). If the IN 1  input value is at the low logic level, indicating that the consumable component  20  is not installed, the CPU  19  displays an alarm indication on, for example, a display panel (step S 213 ), then terminates the procedure. 
     If the IN 1  input value is at the high logic level, indicating that the consumable component  20  is installed, the CPU  19  commands the switching means to break the electrical contact at point  21  and make electrical contact at point  23  (step S 214 ), then reads and tests the input value at input port IN 2 , which is connected through resistor R 13  to contact point  22  and through resistor R 14  to ground (step S 215 ). If the IN 2  input is at the high logic level, indicating that fuse F 1  is already blown, the CPU  19  terminates the procedure. If the IN 2  input value is at the low logic level, indicating that fuse F 1  is not blown, the CPU  19  resets the counter that keeps track of the service life of the consumable component  20  (step S 216 ), and outputs a ‘0’ pulse from output port OUT (step S 217 ), sending current through transistor TR 1  and resistor R 12  to blow fuse F 1 , then reads and tests the IN 2  input value again (step S 218 ). Steps S 217  and S 218  are repeated until the IN 2  input goes high, or until a limit number of repetitions is reached. 
     The conventional art shown in FIGS. 21 and 22, like that in FIGS. 19 and 20, has the drawback of being unable to distinguish between different types of consumable components. A further disadvantage is the need for a third electrical contact point  21 , and the need for switching means for making and breaking the electrical contacts at points  21  and  23 . The third contact point and switching means both take up extra space. The switching means also adds to the complexity of the printer and increases its cost. 
     The problems described above are not limited to electrophotographic printers, but can occur in other types of image-forming devices as well. 
     SUMMARY OF THE INVENTION 
     One object of the present invention is to provide an image-forming device with low-cost means for preventing the mistaken installation of an incorrect type of consumable component. 
     Another object of the invention is to enable a consumable component in an image-forming device to be used despite the temporary failure of a fuse to blow. 
     A further object is to enable a consumable component with a blown fuse to be distinguished from a consumable component that is not installed without the need for an extra electrical contact point. 
     A still further object of the invention is to provide a convenient means of monitoring the temperature inside the image-forming device. 
     The invented image-forming device has a replaceable consumable component with an internal fuse. When the consumable component is installed, the fuse is blown to indicate that the consumable component is no longer new. In addition, a counter in the image-forming device may be reset; thereafter, the counter measures the remaining service life of the consumable component by counting a predetermined repetitive operation that is executed when the consumable component is used. 
     According to a first aspect of the invention, the consumable component includes a resistor connected to, e.g., connected in series with the internal fuse. The resistance value of the resistor indicates the type of consumable component. Before blowing the fuse, the image-forming device determines the type of the consumable component. For instance, it determines whether the consumable component is of the correct type by measuring the resistance value of the resistor, and warns the user if the consumable component is of an incorrect type. 
     The image-forming device may also have means for short-circuiting the two ends of the fuse, so that the resistance value of the resistor can be measured even after the fuse has been blown. This feature is useful when the consumable component is temporarily removed, then reinstalled. 
     In an electrophotographic printer with replaceable photosensitive drum units having different toner colors, the resistance value may indicate the toner color. 
     The image-forming device may also have means for enabling the user to decide whether or not to reset the counter and blow the fuse when a consumable component of the correct type is installed. This enables the consumable component to be tested without blowing its fuse. 
     According to a second aspect of the invention, the image-forming device has a memory that stores fuse defect information. While attempting to blow the fuse in the consumable component, the image-forming device measures its resistance, first to decide whether the resistance is normal, then to determine whether the fuse has blown. If the fuse has normal resistance but fails to below within a predetermined time, the fuse defect information is checked. If this information does not indicate that the fuse had failed to blow in a previous attempt, then the counter is cleared and the consumable component is used for the time being, but its failure to blow is recorded in the memory, so that if the fuse again fails to blow on the next attempt, an alarm warning can be given. If the fuse is blown successfully on the next attempt, the indication of its failure to blow is cleared in the memory. 
     Before attempting to blow the fuse, the image-forming device may use a resistance measurement to determine whether the fuse is already blown, and if it is, clear the indication in the memory without resetting the counter. 
     If the replaceable consumable component is a photosensitive drum unit having a photosensitive drum making contact with a transfer roller through which current is supplied to charge the surface of the photosensitive drum, before measuring the resistance of the fuse, the image-forming device may measure the output voltage of the power source that supplies the current, to confirm that the photosensitive drum unit is properly installed, so that an uninstalled photosensitive drum unit will not be misinterpreted as an installed photosensitive drum unit with a blown fuse. If the photosensitive drum unit is not installed, the indication in the memory is not cleared and the counter is not reset. 
     According to a third aspect of the invention, the consumable component includes a resistor connected in parallel with the internal fuse between two points at which the consumable component makes electrical contact with the image-forming device. The electrical resistance between these two points then indicates whether or not the consumable component is installed, and if it is installed, whether or not its internal fuse is blown. The resistance value may also indicate whether the consumable component is of the correct type. The resistor may be a thermistor with a positive temperature coefficient, in which case the resistance value can be monitored to monitor the temperature inside the image-forming device. 
     The invention also provides a consumable component such as a photosensitive drum unit or a toner cartridge having a resistor coupled in parallel with an internal fuse. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the attached drawings: 
     FIG. 1 is a circuit diagram showing relevant parts of a consumable component and its sensing section in a first embodiment of the invented image-forming device; 
     FIG. 2 is a flowchart describing the operation of the first embodiment; 
     FIG. 3 is a circuit diagram showing relevant parts of a consumable component and its sensing section in a second embodiment of the invented image-forming device; 
     FIG. 4 is a flowchart describing the operation of the second embodiment; 
     FIG. 5 is a graph illustrating the recognition of consumable components in a third embodiment of the invented image-forming device; 
     FIG. 6 is a flowchart describing the operation of a fourth embodiment of the invented image-forming device; 
     FIG. 7 is a circuit diagram showing relevant parts of a consumable component and its sensing section in a fifth embodiment of the invented image-forming device; 
     FIGS. 8 and 9 are a flowchart describing the operation of the fifth embodiment; 
     FIG. 10 is a circuit diagram showing relevant parts of a consumable component and its sensing section in a sixth embodiment of the invented image-forming device; 
     FIGS. 11 and 12 are a flowchart describing the operation of the sixth embodiment; 
     FIG. 13 is a circuit diagram showing relevant parts of a consumable component and its sensing section in a seventh embodiment of the invented image-forming device; 
     FIG. 14 is a flowchart describing the operation of the seventh embodiment; 
     FIG. 15 is a circuit diagram showing relevant parts of a consumable component and its sensing section in an eighth embodiment of the invented image-forming device; 
     FIG. 16 is a flowchart describing the operation of the eighth embodiment; 
     FIG. 17 is a sectional view showing the structure of a color electrophotographic printer; 
     FIG. 18 is a block diagram of the printing engine of the printer in FIG. 17; 
     FIG. 19 is a circuit diagram showing the structure of a conventional consumable-component sensing section in a printer; 
     FIG. 20 is a flowchart describing the operation of the consumable-component sensing section in FIG. 19; 
     FIG. 21 is a partial circuit diagram showing the structure of another conventional consumable-component sensing section; and 
     FIG. 22 is a flowchart describing the operation of the consumable-component sensing section in FIG.  21 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Embodiments of the invention will now be described with reference to the attached drawings. All of the embodiments are electrophotographic printers with consumable components having internal fuses, and with consumable-component sensing sections that blow these internal fuses and reset counters to measure the service lives of the consumable components. 
     FIG. 1 schematically illustrates a consumable component  27  and its consumable-component sensing section  28  in a printer according to a first embodiment of the invention. The consumable-component sensing section  28  has a pnp bipolar transistor TR 1  and a resistor R 1  connected in parallel between a point P and, for example, a five-volt (5-V) power supply Vcc. The consumable component  27  has a resistor R 2  and fuse F 1 , which are connected in series between point P and ground through a pair of points  31 ,  32  (shown as lines in the drawing) at which the consumable component  27  makes electrical contact with the consumable-component sensing section  28 . Resistor R 2  has a prescribed resistance value that differs depending on the type and specifications of the consumable component  27 , but is low enough to permit fuse F 1  to blow. The combined series resistance of resistors R 2  and R 1  is high enough to prevent fuse F 1  from blowing. 
     The consumable-component sensing section  28  includes a CPU  29  such as a microcontroller that receives the voltage level of point P at an analog input port having an analog-to-digital (A/D) conversion function. By means of this function, the CPU  29  internally converts the voltage level at point P to, for example, an eight-bit digital value. The CPU  28  also has a one-bit digital output port (OUT) that controls transistor TR 1 , ‘0’ output switching transistor TR 1  on and ‘1’ output switching it off. 
     Transistor TR 1  includes internal resistors through which its base electrode is coupled to the output port OUT, and to its emitter electrode. The emitter electrode is connected to the power supply Vcc, and the collector electrode is connected to point P. Transistor TR 1  is normally kept in the off state (OUT=‘1’). 
     The operation of the first embodiment will be described below with reference to the flowchart in FIG.  2 . 
     When the printer&#39;s power is switched on or its cover (not visible) is opened, then closed, the CPU  29  reads the A/D input value, representing the voltage level at point P, (step S 1 ). If the fuse F 1  is blown, this voltage level is substantially Vcc, the A/D input value is correspondingly high, and the subsequent steps in FIG. 2 are skipped. 
     If the A/D input value is not high enough to indicate a blown fuse, the CPU  29  next decides whether the A/D input value is equal to a prescribed value (step S 2 ). The prescribed value is equivalent to the power-supply voltage Vcc divided by the resistances of resistors R 1  and R 2 , provided resistor R 2  has the prescribed resistance value. If the value read from the A/D input port differs from the prescribed value, indicating that a consumable component  27  of an incorrect type is installed, the user is informed that the consumable component  27  is out of specification by a display on a control panel (not shown), or by an audible alarm or the like (step S 3 ). 
     If the A/D input value is substantially equal to the prescribed-value, indicating that the consumable component  27  is of the correct type and its fuse F 1  is not yet blown, a counter that measures the service life of the consumable component  27  is reset (step S 4 ). If, for example, the consumable component  27  is a photosensitive drum unit, its service life can be measured by counting rotations of the photosensitive drum. The counter, also referred to as a consumable component counter, may be a hardware counter, or a software counter that maintains a count value in, for example, an internal non-volatile memory in the CPU  29 . 
     After resetting the consumable component counter, to blow the fuse F 1 , the CPU  29  sends a ‘0’ pulse out from the output port OUT (step S 5 ), switching on transistor TR 1  for a certain interval. After this interval, to confirm that the fuse F 1  has blown, the CPU  29  reads the voltage of point P from the A/D input port (step S 6 ), and compares the A/D-converted value of the voltage with a predetermined value such as, for example, ‘F0’ (step S 7 ). ‘F0’ is a hexadecimal value near the top of the eight-bit A/D conversion scale. 
     If the A/D-converted value is equal to or greater than ‘F0’, indicating that the fuse F 1  has blown, the process in FIG. 2 is terminated; if the value is less than ‘F0’, indicating that the fuse F 1  has not yet blown, steps S 5 , S 6 , and S 7  are repeated until the fuse F 1  blows, or until a limit number of repetitions is reached. If fuse F 1  does not blow within the limit number of repetitions, a fuse alarm is indicated, although this is not indicated in the drawing. 
     Sensing the voltage at point P before blowing the fuse F 1  is equivalent to measuring the resistance value of resistor R 2 . Since this resistance value differs according to the type and specifications of the consumable component  27 , before blowing the fuse F 1 , the printer can determine whether the consumable component  27  is of the correct type. Problems caused by the installation of an incorrect type of consumable component  27 , such as defective printing results and the blowing of the fuse in the incorrectly installed consumable component, can therefore be avoided. 
     The consumable component  27  need not be a photosensitive drum unit, but may be, for example, a fuser, a belt unit, or a toner cartridge. 
     FIG. 3 schematically illustrates a consumable component  26  and its consumable-component sensing section  38  in a printer according to a second embodiment of the invention. The second embodiment differs from the first embodiment in that the consumable-component sensing section  38  has an npn bipolar transistor TR 2 , controlled through an output port OUT 2  of the CPU  39 , that can short-circuit the two ends of the fuse F 1  in the consumable component  26 . The emitter of transistor TR 2  is connected through contact point  32  to one end of fuse F 1 ; its collector is coupled through a third contact point  33  to the other end of fuse F 1 . The output port that controls transistor TR 1  is now denoted OUT 1 . 
     The operation of the second embodiment when the user removes the consumable component  26  to correct a paper jam, for example, then reinstalls the same consumable component  26  will be described below with reference to the flowchart in FIG.  4 . 
     The CPU  39  reads the voltage at point P from the A/D input port (step S 11 ). To decide whether the fuse F 1  has blown or not, the CPU  39  compares the read value with ‘F0’ (step S 12 ). If the value is equal to or greater than ‘F0’, indicating that the fuse F 1  is blown, the process proceeds to step S 13  to determine whether the consumable component  26  is of the correct type or not. 
     In step S 13 , output port OUT 2  is set for ‘1’ output, turning on transistor TR 2  and thereby short-circuiting the two ends of the fuse F 1 . Then the voltage at point P is read from the A/D input port again (step S 14 ) and compared with the prescribed value, that is, with Vcc divided by R 1  and R 2  (step S 15 ). If the voltage at point P has the prescribed value, the correct consumable component  26  is assumed to have been reinstalled, and the process ends. 
     If the value read from the A/D input port is lower than ‘F0’ in step S 12 , or if the value read from the A/D input port differs from the prescribed value in step S 15 , the wrong consumable component  26  is assumed to have been reinstalled, and the user is informed that the reinstalled consumable component is out of specification by an alarm display, an audible alarm, or the like (step S 16 ). 
     In the second embodiment, the fuse F 1  can be bypassed to measure the resistance of resistor R 2 , so even when a consumable component is temporarily removed and then reinstalled, the printer can check whether the reinstalled consumable component  26  is of the correct type. 
     In a variation of the second embodiment, if the A/D input value is less than ‘F0’ in step S 12 , indicating that the fuse F 1  is not blown yet, the CPU  39  proceeds with steps S 2  to S 7  in FIG.  2 . 
     In another variation of the second embodiment, transistor TR 2  is first switched on to measure the resistance of resistor R 2 , then switched off to determine whether the fuse F 1  is blown or not. 
     Next, a third embodiment will be described. The third embodiment concerns the recognition of the ID units  4   a - 7   a  in the electrophotographic printer in FIG. 17 by the consumable-component sensing section  28  or  38  in the first or second embodiment. A separate consumable-component sensing section is provided for each of the four ID units  4   a - 7   a.  In the following description, the resistances R 2   Y , R 2   M , R 2   C , R 2   K  of the resistors R 2  connected in series with the internal fuses in the yellow, magenta, cyan, and black ID units  4   a - 7   a  are related so that R 2   Y &gt;R 2   M &gt;R 2   C &gt;R 2   K . The resistances R 1   Y , R 1   M , R 1   C , and R 1   K  of resistor R 1  in the corresponding consumable-component sensing sections may all be identical. 
     In FIG. 5 the voltage value of point P is indicated on the vertical axis, and the eight-bit digital value to which this voltage is converted in the CPU is indicated on the horizontal axis. These values are nominally in a different range for each of the four colors yellow (Y), magenta (M), cyan (C), and black (K). An adequate recognition margin can be obtained by setting the resistance values of R 1  and R 2  to generate voltage values of, for example, 3.5-4.0 V in the Y-ID unit  4   a,  3.0-3.5 V in the M-ID unit  5   a,  2.5-3.0 V in the C-ID unit  6   a,  and 2.0-2.5 V in the K-ID unit  7   a.  In hexadecimal notation, the corresponding ranges of the A/D-converted values are BF-CC, 99-BF, 7F-99, and 66-7F. 
     In the third embodiment, when an ID unit is replaced, the printer can automatically determine the color of the newly installed ID unit, and warn the user if the color is incorrect. In a tandem color electrophotographic printer, for example, the user can be specifically informed as to the position in which an ID unit of the wrong color has been installed, so that the problem can be corrected without further mistakes. 
     Next, a fourth embodiment will be described. The fourth embodiment allows the user to decide whether to blow the fuse or not when a consumable component is replaced. This feature may be added to a printer according to the first, second, or third embodiment. The operation according to the fourth embodiment will be described below with reference to the flowchart in FIG.  6 . 
     After the replacement of a consumable component, the CPU reads the voltage of point P between resistors R 1  and R 2  from the analog input port (step S 21 ). If the input value is high enough to indicate a blown fuse, the subsequent steps are skipped. Otherwise, the input value is compared with a prescribed value (step S 22 ). If the value differs from the prescribed value, indicating that a consumable component of an incorrect type has been installed, the user is informed that the consumable component is out of specification by a control-panel display, an audible alarm, or the like (step S 29 ). If the value is equal to the prescribed value, indicating that a new consumable component of the correct type has been installed, a query is displayed on the control panel, asking whether to reset the consumable component counter or not (step S 23 ), and the user&#39;s response to this query is determined (step S 24 ). 
     If the user does not want to reset the consumable component counter, he pushes a button that operates a switch SW 2  in the printer, and the process ends. To reset the counter, the user pushes another button, operating a switch SW 1 , and the consumable component counter is reset (step S 25 ). Switches SW 1  and SW 2  may be operated by ‘Yes’ and ‘No’ buttons on the printer&#39;s control panel. 
     After the reset, to blow the fuse F 1 , the CPU sends a ‘0’ pulse out from output port OUT in FIG. 1 or OUT 1  in FIG. 3 (step S 26 ), switching on transistor TR 1  for a certain interval. Then the CPU reads the voltage of point P from the analog input port again (step S 27 ), and compares the A/D-converted value of the voltage with hexadecimal ‘F0’ (step S 28 ) to confirm that the fuse F 1  has blown. If the value now read is equal to or greater than ‘F 0 ’, indicating that the fuse F 1  has blown, the process ends; if the value is lower than ‘F 0 ’, indicating that the fuse F 1  has not blown, the process returns to step S 26 . 
     When a consumable component is manufactured, if its fuse is blown in the final functional inspection, the fuse must be replaced before shipment. In the fourth embodiment described above, the fuse need not be blown in this type of inspection, so the time, cost, and labor of replacement of the fuse can be saved. Since the user who purchases the consumable component can also select whether to blow the fuse or not, the user can install the consumable component temporarily and perform a trial printing without blowing the fuse, in order to pre-check the component for defects. 
     Next, a fifth embodiment of the invention will be described. FIG. 7 is a block diagram showing the structure of a consumable-component sensing section  100  that manages a photosensitive drum unit (ID unit)  24  in an electrophotographic printer according to the fifth embodiment. 
     The printer in FIG. 7 is controlled by a CPU  101 , and has an electrically erasable programmable read-only memory (EEPROM)  102  storing fuse defect information, described below. The CPU  101  has an analog input port (A/D) and an output port (OUT) as in the preceding embodiments, but the output port is connected to the base of an npn bipolar transistor TR 3 . The emitter of transistor TR 3  is connected to ground. The collector of transistor TR 3  is connected to the base of pnp bipolar transistor TR 1 , which is similar to transistor TR 1  in the preceding embodiments. 
     As in the preceding embodiments, transistor TR 1  and a resistor R 1  are coupled in parallel between a power supply Vcc and a point P. Differing from the preceding embodiments, point P is coupled to ground through a resistor R 3  in the consumable-component sensing section  100 , and another resistor R 4  is inserted in series between point P and transistor TR 1 . The ID unit  24  has an internal fuse F that is coupled between point P and ground when the ID unit is installed, but does not have a resistor inserted in series between the fuse and point P. 
     In the drawing, fuse F is shown as grounded within the ID unit  24 , but fuse F may be connected to ground in the consumable-component sensing section  100  as in the preceding embodiments. 
     Next, the operation of the fifth embodiment will be described. For simplicity, the resistance value of resistor R 4  will be ten ohms (10 Ω), the resistance values of resistors R 1  and R 3  will both be twenty kilohms (20 Ω), and the power-supply voltage (Vcc) will be 5 V. The fuse F has a room-temperature resistance of 2 L and a current rating of one hundred twenty-five milliamperes (125 mA), and is specified to blow within five seconds at 200% of the rated current. The signal input at the analog input port of the CPU  101  will be denoted HFU, and the signal output from the output port will be denoted IDFU. 
     Referring to FIG. 8, when the printer&#39;s power is switched on or its cover (not shown) is opened and then closed (step S 100 ), the printer begins an initial sequence of operations in preparation for printing. As part of the initial sequence, the analog input signal HFU is sampled and compared with a predetermined value of, for example, 1.5 V (step S 102 ). Output signal IDFU is held at the low output level at this time. If the voltage of HFU is equal to or greater than the predetermined value (1.5 V), indicating that the fuse F is already blown and thus that the ID unit  24  is not a new unit, step S 117  (described below) is carried out. Since resistors R 1  and R 3  have the same resistance value, if the fuse F is blown, the voltage at point P is approximately 2.5 V; a predetermined value of 1.5 V allows a margin for resistor tolerances. 
     If the HFU voltage is lower than the predetermined value, indicating that the fuse F is not blown, the CPU  101  switches output signal IDFU from the low to the high logic level, turning on transistor TR 3 , and at the same time starts a timer (step S 103 ). The timer may be internal to the CPU  101 , or an external timer may be used. Transistor TR 3  conducts current from the base of transistor TR 1 , which therefore turns on, sending current from the 5-V power supply Vcc to the fuse F through resistor R 4 . 
     Since resistor R 4  has much less resistance than resistor R 1 , the current value is determined substantially by the resistance value of resistor R 4  (for simplicity, the V CE  effect of transistor TR 1  is ignored). Since the resistance value of resistor R 4  is 10 Ω and the resistance value of fuse F is 2 Ω at room temperature, more than 400 mA flows through fuse F, exceeding 200% of its current rating. If fuse F is normal, it will blow within five seconds. To decide whether fuse F is normal or not, the timer outputs a trigger signal when one hundred milliseconds (100 ms) has elapsed (step S 104 ). 
     Referring to FIG. 9, on receiving the trigger signal, the CPU  101  reads the HFU voltage value again and compares it with another predetermined value, such as 0.5 V (step S 105 ). 
     When current flows through a fuse, its temperature rises due to resistive heating. The increased temperature increases the resistance of the fuse, generating still more heat and raising the temperature still further, until finally the fuse blows. At room temperature the 2-Ω resistance of the fuse F should yield an HFU voltage value of approximately 0.8 V, so after 100 ms has elapsed, the voltage should exceed 0.8 V. 
     If the HFU voltage value after 100 ms has elapsed is found to be lower than 0.5 V in step S 105 , the fuse F is assumed to have too little resistance to blow, so the CPU  101  returns output signal IDFU to the low logic level (step S 106 ), displays a fuse-error alarm warning (step S 107 ), and terminates the initial sequence. 
     If the HFU voltage after 100 ms is found to be equal to or greater than 0.5 V in step S 105 , the fuse F is assumed to be normal, that is, to be capable of blowing. While the fuse F is blowing, the HFU voltage should rise together with the resistance of the fuse F, becoming approximately 5 V after the fuse F has blown. In step S 108 , the HFU voltage is monitored and compared with another predetermined value; a value of 3.5 V is used here. If the HFU voltage exceeds 3.5 V, indicating that the fuse F has blown or substantially blown, the CPU  101  returns the output signal IDFU to the low logic level (step S 109 ), clears a fuse defect bit in the EEPROM  102  (step S 110 ), resets the counter that measures the service life of the ID unit  24  (step S 111 ), and proceeds with other parts of the initial sequence (not shown). 
     If the HFU voltage value is less than 3.5 V in step S 108 , the elapsed time is compared with five seconds (step S 112 ). If the elapsed time is less than five seconds, step S 108  is repeated. The CPU  101  loops between steps S 108  and S 112 , continuously monitoring the HFU voltage (the voltage at point P) until it reaches or exceeds 3.5 V, or until five seconds have elapsed. 
     If the HFU voltage value has not reached 3.5 V by the time five seconds have elapsed, the fuse F is assumed to have failed to blow, and the CPU  101  returns output signal IDFU to the low logic level (step S 113 ). Next, the CPU  101  checks the fuse defect bit in the EEPROM  102  (step S 114 ). If the fuse defect bit is in the cleared state, it can be inferred that the ID unit  24  is a newly installed unit. The CPU  101  now sets the fuse defect bit (step S 115 ), resets the counter (step S 111 ), and terminates the processing. If the fuse defect bit is found to be already set in step S 114 , indicating that the fuse F also failed to blow the last time this process was performed, the CPU  101  does not reset the counter, displays a fuse error alarm (step S 116 ), and terminates the initial sequence. 
     If the ID unit  24  has a blown fuse, as determined in step S 102  in FIG. 8, indicating that the ID unit  24  is not new, the CPU  101  clears the fuse defect bit in the EEPROM  102  (step S 117 ) and proceeds with other parts of the initial sequence (not shown) without resetting the counter. 
     In the fifth embodiment, when a new ID unit  24  with a non-blown fuse F is installed, if the fuse F does not have an abnormally low resistance, the counter that keeps track of the service life of the ID unit  24  is reset automatically, and an attempt is made to blow the fuse F. If the attempt fails, this is recorded by setting the fuse defect bit in the EEPROM  102 , and a second attempt is made the next time the printer&#39;s power is switched on or its cover is opened and closed. If the second attempt to blow the fuse succeeds, the fuse defect bit is cleared and normal use of the ID unit  24  continues. If the second attempt also fails, the fuse F is considered defective and a fuse error alarm is indicated. 
     Various actions can be taken in response to the fuse error alarm. For example, the user may replace the ID unit  24 , or continue to use the ID unit  24  but be alert for possible printing quality problems later, since the counter may not indicate the service life of the ID unit  24  correctly. In any case, the fifth embodiment enables an ID unit with a defective fuse to be used at least once before being discarded. 
     Next, a sixth embodiment of the invention will be described. FIG. 10 is a block diagram showing the structure of the consumable-component sensing section  120  of a printer according to the sixth embodiment. The sixth embodiment adds an A/D converter  103  and a voltage-dividing circuit  104  to the structure of the fifth embodiment. The voltage-dividing circuit  104  divides a transfer voltage output by a high-voltage power source  105  to a transfer roller  106  that faces the photosensitive drum  107  in the ID unit  24 . The divided transfer voltage is converted to digital form by the A/D converter  103  and supplied to the CPU  101 . Alternatively, the divided transfer voltage may be supplied directly to an analog input port of the CPU  101 . 
     The operation of the sixth embodiment will be described with reference to the flowchart in FIGS. 11 and 12, assuming the same resistance values and fuse specifications as in the fifth embodiment. This flowchart differs from the flowchart in the fifth embodiment in that step S 101  is inserted between steps S 100  and S 102 . 
     When the printer&#39;s power is switched on or its cover is opened and closed (step S 100 ), as part of the initial sequence, the CPU  101  activates the motor (not shown) that rotates the photosensitive drum  107  in the ID unit  24 , and controls the high-voltage power source  105  so as to charge the photosensitive drum  107  to a fixed potential. During these operations, the high-voltage power source  105  operates as a constant-current source, and the CPU  101  monitors the transfer voltage to determine whether the transfer roller  106  and photosensitive drum  107  are in contact and rotating or not. The reason why this can be determined is as follows. 
     The surface of the photosensitive drum  107  is coated with a photosensitive substance, forming a photosensitive layer, the resistance value of which decreases under optical illumination. While being charged in the initial sequence, the photosensitive drum is not illuminated, so it acts substantially as a capacitor, storing charge on the surface of the photosensitive layer. The charge is supplied as current from the high-voltage power source  105  through the resistance of the transfer roller  106 , provided the photosensitive drum  107  and transfer roller  106  are in contact. If the photosensitive drum  107  is rotating, the current keeps flowing at a substantially constant rate, as new areas of the surface of the photosensitive drum  107  are continuously brought into contact with the transfer roller  106 , without requiring any change in the transfer voltage output by the high-voltage power source  105 . 
     The value of the transfer voltage during this initial operation depends on the control value of the current, the rotational speed of the photosensitive drum, and the resistance value of the transfer roller. A maximum transfer voltage of approximately 4000 V has been experimentally confirmed in a printer according to the present embodiment. 
     If the printer begins the initial sequence in the state in which the ID unit  24  is not installed, since there is no photosensitive drum  107 , no current can flow from the high-voltage power source  105 . Since the high-voltage power source  105  is being controlled for constant-current output, however, it attempts to generate current by increasing the transfer voltage to the maximum possible value, which in the present embodiment is approximately 8000 V. 
     If the photosensitive drum is installed but is not rotating, then as the area of the photosensitive drum  107  in contact with the transfer roller  106  becomes increasingly charged, it becomes increasingly difficult for more current to flow. To maintain a constant current flow, the high-voltage power source  105  must generate an increasingly high transfer voltage. After a certain time, the transfer voltage again reaches the maximum value of approximately 8000 V. 
     Thus by monitoring the transfer voltage during the initial operation of the printer, the CPU  101  can determine whether the ID unit  24  is properly installed, so that the transfer roller  106  and photosensitive drum  107  make contact, and whether the photosensitive drum  107  is rotating or not. In step S 101  in the flowchart in FIG. 11, the CPU  101  compares the value received from the A/D converter  103  with a predetermined value representing a transfer voltage of 5000 V (prior to voltage division by the voltage-dividing circuit  104 ). If the transfer voltage is lower than 5000 V, indicating that the ID unit  24  is properly installed and its photosensitive drum  107  is rotating, the process proceeds to step S 102  and continues through FIGS. 11 and 12 as in the fifth embodiment. 
     If a transfer voltage equal to or greater than 5000 V is detected in step S 101 , however, the ID unit  24  is determined not to be installed, or to have a non-rotating photosensitive drum  107 , and the CPU  101  terminates the initial sequence without resetting the counter. This prevents the fuse defect bit from being mistakenly cleared in step S 117 . It also prevents mistaken resetting of the counter in step Sill and mistaken clearing of a service-life alarm, which might otherwise occur through an incorrect or illegal operation. 
     The fifth and sixth embodiments can be modified in various ways. For example, the fuse defect bit can be checked before being cleared in step S 110 . If the fuse defect bit is set at this point, then after it is cleared in step S 110 , the resetting of the counter in step S 111  can be skipped. 
     Next, a seventh embodiment of the invention will be described. FIG. 13 is a block diagram showing the structure of a consumable component  30  and the consumable-component sensing section  130  that manages it in the seventh embodiment. 
     The consumable-component sensing section  130  includes a CPU  139  with an analog input port (A/D) having an analog-to-digital conversion function and an output port (OUT). The output signal from the output port controls a transistor TR 1  that is coupled in parallel with a resistor R 1  between a power supply (Vcc) and a point P, an additional resistor R 4  being inserted in series between transistor TR 1  and point P. A further pair of resistors R 5  and R 6  are coupled in series between point P and ground. The analog input port of the CPU  139  is connected to a point PS between resistors R 5  and R 6 . 
     The consumable component  30  makes electrical contact with the consumable-component sensing section  130  at two points  31 ,  32 , one coupled to point P, the other coupled to ground. In the consumable component  30 , a fuse F 1  and a resistor R 7  are connected in parallel between the two electrical contact points  31 ,  32 . 
     The resistance value of resistor R 7  varies depending on the type and specifications of the consumable component  30 , but is high enough to enable the fuse F 1  to be blown. Resistors R 1 , R 5 , and R 6  also have comparatively high resistance values, while resistor R 4  has a comparatively low resistance value. 
     If the consumable component  30  is not installed, the A/D input value corresponds to the power-supply voltage Vcc divided at point PS by the resistances of resistors R 1 , R 5 , and R 6 . If the consumable component  30  is installed and fuse F 1  is blown, the A/D input will have a lower value, since the resistance between point P and ground is reduced by the parallel path through resistor R 7 . This lower value will vary depending on the resistance of resistor R 7 , thus on the type and specifications of the consumable component  30 . If the consumable component  30  is installed and its fuse F 1  is not blown, point P is pulled down substantially to ground level through the fuse F 1 , so the A/D input value is substantially zero. 
     The operation of the seventh embodiment will be described with reference to the flowchart in FIG.  14 . 
     When the printer&#39;s power is switched on or a cover (not shown) is opened and then closed, the CPU  139  reads the A/D-converted input value at the analog input port, representing the voltage at point PS (step S 121 ), and compares it with a first predetermined value, such as hexadecimal ‘10’, representing a voltage close to ground level (step S 122 ). If the A/D input value is less than this first predetermined value, indicating that the consumable component  30  is installed and its fuse has not yet been blown, the CPU  139  resets the counter that manages the service life of the consumable component (step S 123 ), then sends a ‘0’ pulse out from the output port OUT (step S 124 ), switching on transistor TR 1  for a certain interval to blow the fuse F 1 . Next, the CPU  139  reads the A/D input again (step S 125 ), compares it with the first predetermined value (step S 126 ), and returns to step S 124  if the input value is still less than the first predetermined value. Steps S 124  to S 126  are repeated until the A/D input value becomes equal to or greater than the first predetermined value, indicating that fuse F 1  has blown, or until a limit number of repetitions is reached. If fuse F 1  does not blow within the limit number of repetitions, the CPU  139  generates a fuse error alarm, although this is not indicated in the drawing. 
     When the A/D input becomes equal to or greater than the first predetermined value (e.g., ‘10’) in step S 126 , the CPU  139  compares the A/D input value with a prescribed value that should be obtained if the correct type of consumable component  30  is installed and resistor R 7  has the prescribed resistance value (step S 127 ). If the A/D input reveals that resistor R 7  does not have the prescribed resistance value, an out-of-specification alarm is generated (step S 128 ). If resistor R 7  has the prescribed resistance value, the procedure ends. 
     If the A/D input value is equal to or greater than the first predetermined value in step S 122 , it is compared with a second predetermined value such as hexadecimal ‘80’ (step S 129 ). The second predetermined value is greater than any A/D input value that should be obtained if the consumable component  30  is installed, but less than the A/D input value obtained when the consumable component  30  is not installed. If the A/D input value is less than this second predetermined value, then step S 127  is carried out to decide whether resistor R 7  has the prescribed resistance value. If the A/D input value is equal to or greater than the second predetermined value, the user is informed by a control-panel display, an audible alarm, or the like that the consumable component  30  is not installed (step S 130 ). 
     By reading the A/D input value, the CPU  139  indirectly measures the resistance between the electrical contact points  31 ,  32 . From this resistance measurement, the CPU  139  can determine whether the consumable component  30  is installed or not; if installed, whether its fuse F 1  is blown or not; and if the fuse is blown, and whether the consumable component  30  is of the correct type or not. 
     The procedure shown in FIG. 14 can be modified in various ways. For example, the A/D input value can be compared with the second predetermined value before being compared with the first predetermined value. 
     Next, an eighth embodiment of the invention will be described. FIG. 15 is a block diagram showing the structure of a consumable component  40  and the consumable-component sensing section  130  that manages it in the eighth embodiment. 
     The consumable-component sensing section  130  in the eighth embodiment is substantially identical to the consumable-component sensing section in the seventh embodiment. The consumable component  40  has a positive-temperature-coefficient (PTC) thermistor T 1  coupled in parallel with the internal fuse F 1 . The PTC thermistor T 1  is a type of resistor having a resistance that increases rapidly as its temperature rises. 
     At room temperature, the resistance of the PTC thermistor T 1  is less than the resistance of resistor R 7  in the seventh embodiment. Consequently, there is a greater difference between the potential at point PS when the consumable component  40  is installed and the potential at point PS when the consumable component  40  is not installed than in the seventh embodiment, making the installed state easier to distinguish from the not-installed state. 
     When transistor TR 1  is turned on to blow the fuse F 1 , initially, less current flows through fuse F 1  than in the seventh embodiment, because more current is shunted through the PTC thermistor T 1 , but resistive heating quickly causes the resistance of the PTC thermistor T 1  to rise to a value higher than the resistance of resistor R 7  in the seventh embodiment. More current then flows through fuse F 1  than in the seventh embodiment, so fuse F 1  is blown more effectively than in the seventh embodiment. 
     After the fuse F 1  has been blown, the temperature dependence of the resistance of the PTC thermistor T 1  can be used to monitor the temperature in the consumable component  40 . 
     The operation of the eighth embodiment will be described below with reference to the flowchart in FIG.  16 . Steps S 121  to S 126 , S 129 , and S 130  are identical to the corresponding steps in the seventh embodiment (FIG.  14 ), so descriptions of these steps will be omitted. 
     If the A/D input value is greater than the first predetermined value (‘10’) in step S 126  or less than the second predetermined value (‘80’) in step S 129 , indicating in either case that the consumable component  40  is installed and the fuse F 1  is blown, the CPU  139  leaves transistor TR 1  switched off and begins monitoring the printer&#39;s temperature by reading the A/D input value (step S 131 ) and comparing it with a third predetermined value (step S 132 ). 
     Since transistor TR 1  is turned off, the current flowing through the PTC thermistor T 1  is limited by the comparatively large resistance of resistor R 1 . Resistive heating is therefore slight, the temperature and resistance of the PTC thermistor T 1  are comparatively low, and the A/D input value is correspondingly low. The third predetermined value is selected so that if the temperature inside the printer is normal, the A/D input will be below the third predetermined value, and if the temperature rises to an unsafe level, the resulting increase in the resistance of the PTC thermistor T 1  will raise the A/D input above the third predetermined value. In the drawing, the third predetermined value is hexadecimal ‘50’, although of course this value is only shown as an example. 
     If the A/D input value is less than the third predetermined value in step S 132 , the CPU  139  takes no particular action, but repeats steps S 131  and S 132  at suitable intervals thereafter to continue monitoring the printer&#39;s temperature. If the A/D input value is equal to or greater than the third predetermined value in step S 132 , the CPU  139  issues a thermal alarm (step S 133 ) and disables further use of the printer until the A/D input value is reduced below the third predetermined value. 
     By connecting a PTC thermistor instead of a resistor in parallel with the fuse F 1 , the eighth embodiment both facilitates the blowing of the fuse and provides a convenient way to monitor the printer&#39;s temperature, thereby improving the safety of the printer. 
     Although various types of PTC thermistors may be used in the eighth embodiment, a polymer PTC thermistor is preferable, because this type of thermistor has a large positive temperature coefficient and responds quickly to temperature changes. Use of a polymer PTC thermistor thus enables the fuse F 1  to be blown rapidly and reliably, and also enables temperature changes in the printer to be detected quickly and sensitively. 
     In the preceding embodiments, the analog voltage at point P (or PS) was converted to, for example, an eight-bit digital value, but it is also possible to employ comparators that compare the analog voltage with various preset threshold voltages or slice levels, and output one-bit signals indicating whether the analog voltage is above or below the corresponding slice levels. These one-bit signals can be received at digital input ports of the CPU. 
     In any of the preceding embodiments, when a service-life alarm is displayed to indicate the need for replacement of the consumable component, the service-life alarm may be cleared at the point at which the counter is reset. 
     Although the present invention has been described in relation to a tandem color electrophotographic printer, it can also be practiced in a monochrome electrophotographic printer, in electrophotographic printers used as components in other image-forming devices such as photocopiers and facsimile machines, and more generally in any type of device having a consumable component. 
     A few variation of the above embodiments have been mentioned, but those skilled in the art will recognize that further variations and modifications are possible within the scope of the appended claims.