Fixing unit with cooling section and image forming apparatus with cooling section

1. Field of the Invention

The present invention relates to a fixing unit with a cooling section for an eleotrophotographic apparatus, and an image forming apparatus with a cooling section.

2. Description of the Related Art

An image forming apparatus, such as an electrophotographic apparatus, includes a fixing unit for fixing the toner image onto a print medium. The fixing roller of the fixing unit is heated up to a high temperature and the surface temperature is controlled to stay within a predetermined range.

FIG. 8 shows a temperature control circuit of a conventional fixing unit. The conventional fixing unit includes a heating section 1 for heating a fixing roller 10 , a cooling section 2 for blowing air into the interior of the unit to cool the fixing roller 10 , a temperature detection section 3 for detecting the surface temperature of the fixing roller 10 , and a control section 4 for sending a control signal to the heating section 1 to heat the fixing roller if the detected temperature of the fixing roller 10 is below a predetermined temperature or to stop heating the fixing roller 10 if the detected temperature is above the predetermined temperature.

The heating section 1 includes a series circuit of a heater 1 - 1 for heating the fixing roller 10 , a thermostat 1 - 2 for preventing overheat of the fixing roller 10 due to a malfunction, an a-c current source 1 - 3 for supplying the heater 1 - 1 with heating current, and a switching circuit 1 - 4 for turning on/off the a-c current source 1 - 3 based on control of the control section 4 . An amplifier consisting of a transistor Tr 1 , a resistor R 3 , and a resistor R 8 to receive and amplify a control signal from the control section 4 is connected to a control terminal of the switching circuit 1 - 4 .

The cooling section 2 includes a fan 2 - 1 for cooling the unit interior, an amplifier consisting of a transistor Tr 4 , a resistor R 9 , and a resistor R 10 for amplifying a control signal from the control section 4 , and a driving amplifier consisting of a transistor Tr 5 and a resistor 11 for driving the fan 2 - 1 . The temperature detection section 3 includes a thermistor 3 - 1 for detecting the temperature of a fixing roller 10 and a voltage dividing resistor R 1 . The voltage at a connection point between the thermistor 3 - 1 and the voltage dividing resistor R 1 is sent to a port P 1 of the control section 4 as the detected temperature.

The control section 4 is made of a central processing unit (CPU) and connected to a read only memory (ROM) and a random access memory (RAM). According to a readable program stored in the ROM or RAM, it receives the detected temperature from the temperature detection section 3 and executes a predetermined process, sending a control signal to the heating section 1 .

In the above conventional fixing unit, if the control section 4 becomes out of control or the switching circuit 1 - 4 is down, an unexpected heating current can be continuously supplied to the heater 1 - 1 to raise the temperature of the fixing roller above the necessary level. If the fan 2 - 1 keeps rotating under this condition, the raised temperature spreads over the entire equipment, preventing normal printing. There is a technique that the sensitivity of the thermostat 1 - 2 is enhanced to prevent the extreme increase in the temperature of a fixing roller.

In the conventional fixing unit, however, it is impossible to prevent the raised temperature from spreading over the entire unit, causing a malfunction. In addition, there is a limit to the improvement in the thermostat and it is difficult to completely prevent the continuous supply of unexpected current to the heater 1 - 1 .

Accordingly, it is an object of the invention to provide a fixing unit capable of preventing the raised temperature from spreading over the entire unit when the control section becomes out of control or the switching circuit is down.

According to the invention, current conduction to the heater and/or the fan is cutoff regardless of operation of the thermostat and control of the heater and the fan by the control section if the temperature of the fixing roller detected by the thermistor exceeds a predetermined protective temperature.

When the detected temperature exceeds the predetermined temperature, current conduction to the heater and the fan is cutoff regardless of operation of the thermostat and control of the heater and fan by the control section to prevent heating current to the heater and the raised temperature from spreading over the entire unit.

FIG. 1 is a circuit diagram of a fixing unit according to the first embodiment of the invention;

FIG. 2 is a temperature control time chart by the fixing unit;

FIG. 3 is a circuit diagram of a fixing unit according to the second embodiment of the invention;

FIGS. 4( a ) and 4 ( b ) are schematic diagrams of an air path shutting section according to the second embodiment;

FIG. 5 is a temperature control time chart by the fixing unit of the second embodiment;

FIG. 6 is a circuit diagram of a fixing unit according to the third embodiment of the invention;

FIG. 7 is a temperature control time chart by the fixing unit of the third embodiment;

FIG. 8 is a circuit diagram of a conventional fixing unit;

FIG. 9 is a perspective view of a fixing unit; and

FIG. 10 is a side view of an image forming apparatus.

In FIG. 1 , a fixing unit includes a heating section 1 , a cooling section 2 , a temperature detection section 3 , a control section 4 , and a conduction cut-off section 5 .

The heating section 1 heats a heat member (hereinafter “fixing roller”) and includes a heater 1 - 1 for heating the fixing roller, a thermostat 1 - 2 for preventing overheat of the fixing roller 10 upon malfunction, an a-c current source 1 - 3 , a switching circuit 1 - 4 for turning on/off the a-c current source 1 - 3 based on control of the control section 4 , and an amplifier consisting of a transistor Tr 1 and resistors R 6 and R 8 for receiving a control signal from the control section 4 .

One end of the a-c current source 1 - 3 is connected to an end of the heater 1 - 1 via the thermostat 1 - 2 . The other end of the heater 1 - 1 is connected to the switching circuit 1 - 4 to which the other end of the a-c current source 1 - 3 and the collector of the transistor Tr 1 are connected. The emitter and base of the transistor Tr 1 are connected to the ground and the port P 2 of the control section 4 , respectively, and the collector is connected to 3.3 Vd-c via the resistor R 6 . The resistor R 8 is connected between the emitter and the base.

The cooling section 2 includes a fan 2 - 1 for cooling the unit interior, an amplifier consisting of a transistor Tr 4 and resistors R 9 and R 10 for amplifying a control signal from the control section 4 , and a driving amplifier consisting of a transistor Tr 5 and a resistor R 11 for driving the fan 2 - 1 to blow air into the unit interior to cool the fixing roller 10 . The base of the transistor Tr 4 is connected to a port P 3 of the control section 4 via the resistor R 10 . The emitter of the transistor Tr 4 is connected to the ground and the collector is connected to the base of the transistor Tr 5 via the resistor R 11 . The emitter and collector of the transistor Tr 5 are connected to 24 Vd-c and an end of the fan 2 - 1 , respectively. The other end of the fan 2 - 1 is grounded. The resistor R 10 is connected to the conduction cut-off section 5 .

The temperature detection section 3 includes a thermistor 3 - 1 for detecting the temperature of the fixing roller 10 and the voltage dividing resistor R 1 to detect the surface temperature of the fixing roller 10 . The voltage at the connection point between the thermistor 3 - 1 and the dividing resistor R 1 is sent to the port P 1 of the control section 4 as the detected temperature by the temperature detecting section 3 . An end of the thermistor 3 - 1 is connected to 5 Vd-c and the other end is grounded via the voltage dividing resistor R 1 . The connection point between the thermistor 3 - 1 and the voltage dividing resistor R 1 is connected to not only the port P 1 but also the negative terminal of a comparator 5 - 1 .

The control section 4 is made of a CPU connected to a ROM and a RAM. According to a readable program stored in the ROM or RAM, it receives a detected temperature from the temperature detecting section 3 and executes a predetermined process. If the fixing roller 10 is below a predetermined temperature, it sends a control signal to the heating section 1 for heating the fixing roller 10 and, if the fixing roller 10 is above a predetermined temperature, it sends a control signal to stop heating the fixing roller 10 . The ports P 1 , P 2 , and P 3 are connected to the connection point between the thermistor 3 - 1 and the voltage dividing resistor R 1 , the base of the transistor Tr 1 , and the base of the transistor Tr 4 via the resistor R 10 , respectively.

The conduction cut-off section 5 cuts off current conduction to both the heating section 1 and the cooling section 2 if the temperature detected by the temperature detecting section 3 exceeds a predetermined temperature (protecting temperature). It includes a pulse forming circuit consisting of voltage dividing resistors R 2 and R 3 , the comparator 5 - 1 , transistors Tr 2 and Tr 3 , and resistors R 4 , R 5 , and R 7 .

As shown in the figure, the protecting voltage is inputted to the positive terminal of the comparator 5 - 1 to prevent an abnormal rise of the temperature of the fixing roller 10 . The protecting voltage is set by the voltage dividing ratio of the voltage dividing resistors R 2 and R 3 based on the protective temperature. The output terminal of the comparator 5 - 1 is connected to both the base of the transistor Tr 2 and 3.3 Vd-c via the resistor R 4 . The emitter and collector of the transistor Tr 2 are connected to the ground and the base of the transistor Tr 3 via the transistor R 5 , respectively. The emitter and collector of the transistor Tr 3 are connected to 3.3 Vd-c and the base of the transistor Tr 1 via the resistor R 7 , respectively. The connector of the transistor Tr 3 is connected to both the port P 3 of the control section 4 via the resistor R 9 and the resistor R 10 .

In FIG. 2 , (a) is the detected temperature of the thermistor 3 - 1 ( FIG. 1 ) or the input to the port P 1 (FIG. 1 ), (b) is the output at the port P 2 ( FIG. 1 ), (c) is the output of the comparator 5 - 1 ( FIG. 1 ), (d) is the on/off of the transistor Tr 2 , (e) is the on/off of the transistor Tr 3 ( FIG. 1 ), (f) is the output at the port P 3 ( FIG. 1 ), (g) is the on/off of the transistor Tr 4 ( FIG. 1 ), (h) is the on/off of the transistor Tr 5 ( FIG. 1 ), (i) is the rotation/stop of the fan 2 - 1 ( FIG. 1 ), (j) is the on/off of the transistor Tr 1 ( FIG. 1 ), and (k) is the time common to all the items.

At a time t 1 , printing is started and the temperature control of the fixing unit is started. It is assumed that the initial value T 0 of the detected temperature for the thermistor 3 - 1 is lower than the optimum fixing temperature T 1 . Consequently, the voltage (corresponding to the initial value T 0 ) at the contact point between the thermistor 3 - 1 and the voltage dividing resistor R 1 is lower than the voltage (corresponding to the protective temperature T 2 ) at the contact point between the voltage dividing resistors R 2 and R 3 so that the output of the comparator 5 - 1 is on as shown in (c), turning on the transistors Tr 2 and Tr 3 as shown in (d, e). Consequently, the transistors Tr 1 and Tr 4 are normally biased by the resistors R 7 and R 9 for operation.

The control section 4 , which has determined that the initial value TO at the port P 1 is lower than the optimum fixing temperature T 1 , turns on the port P 2 (high level) to raise the temperature of the fixing roller 10 (b). This action corresponds to sending a control signal to the switching circuit 1 - 4 . Consequently, the transistor Tr 1 is turned on (j), conducting heating current in a loop consisting of the switching circuit 1 - 4 , the heater 1 - 1 , the thermostat 1 - 2 and the a-c current source 1 - 3 so that the surface temperature of the fixing roller starts to rise (a). At the same time, the control section 4 turns on the port P 3 to rotate the fan 2 - 1 (f).

At a time t 2 , the surface temperature of the fixing roller 10 exceeds the optimum fixing temperature T 1 . The control section 4 , which has determines that the detected temperature T from the port P 1 is higher than the optimum fixing temperature T 1 , turns off the port P 2 to lower the temperature of the fixing roller 10 (b). This action corresponds to sending a control signal to the switching circuit 1 - 4 . Consequently, the transistor Tr 1 is turned off (j), cutting off the loop consisting of the switching circuit 1 - 4 , the heater 1 - 1 , the thermostat 1 - 2 , and the a-c current source 1 - 3 to bring the heating current to 0 so that the surface temperature of the fixing roller 10 starts to fall (a). At this point, however, the detected temperature T does not exceed the protective temperature T 2 so that the items other than the port P 2 (b) and the transistor Tr 1 (j) keep the conditions at t 1 .

At a time t 3 , the surface temperature of the fixing roller 10 falls below the optimum fixing temperature T 1 . The control section 4 , which has determines that the detected temperature T from the port P 1 is lower than the optimum fixing temperature T 1 , turns on the port P 2 to raise the temperature of the fixing roller 10 (b). This action corresponds to sending a control signal to the switching circuit 1 - 4 . Consequently, the transistor Tr 1 is turned on (j), conducting heating current in the loop consisting of the switching circuit 1 - 4 , the heater 1 - 1 , the thermostat 1 - 2 , and the a-c source 1 - 3 so that the surface temperature of the fixing roller starts to rise (a).

At a time t 8 , the surface temperature of the fixing roller 10 exceeds the optimum fixing temperature T 1 . Consequently, the control section 4 , which has determined that the detected temperature T from the port P 1 is higher than the optimum fixing temperature T 1 , is supposed to turn off the port P 2 to lower the temperature of the fixing roller 10 . However, it is assumed that a failure of the control section 4 or the switching circuit 1 - 4 keeps the on conditions (b). Consequently, the heating current continues to run, raising the surface temperature of the fixing roller 10 above the protective temperature T 2 at a time t 9 .

When the surface temperature of the fixing roller 10 exceeds the protective temperature T 2 , the voltage at the contact point of the thermistor 3 - 1 and the voltage dividing resistor R 1 is higher than the protective voltage (corresponding to the protective temperature T 2 ) at the contact point between the voltage dividing resistors R 2 and R 3 , turning off the output of the comparator 5 - 1 (b), which in turn turns off the transistors Tr 2 and Tr 3 (d, e). Consequently, the transistors Tr 1 and Tr 4 are turned off by the resistors R 7 and R 9 . Consequently, no matter what the outputs of the ports P 2 and P 3 are, the heating current becomes 0 and the fan 2 - 1 is stopped.

As has been described above, when the fixing roller 10 is heated up above the protective temperature T 2 , supply of the heating current to the heater 1 - 1 is stopped regardless of control by the control section 4 . Thus, even if the control section 4 becomes out of control or the switching circuit is broken down, there is no danger that unexpected heating current is continuously supplied to the heater to raise the temperature of the fixing roller above the necessary temperature or the raised temperature spreads over the unit, causing a malfunction.

Second Embodiment

In FIG. 3 , the temperature control circuit for the fixing unit which includes the heating section 1 , the cooling section 2 , the temperature detection section 3 , the conduction cutoff section 5 , a control section 21 , and a shutter driving section 22 . Only those different from the first embodiment will be described and the components identical with those of the first embodiment are given the same reference numbers as those of the first embodiment.

The control section 21 is made of a central processing unit (CPU) and connected to a read only memory (ROM) and a random access memory (RAM). According to the program read from the ROM or RAM, it receives a detected temperature from the temperature detection section 3 , performs a predetermined process, and sends a control signal to the heating section 1 to heat the fixing roller 10 if the fixing roller 10 is below a predetermined temperature and to stop heating of the fixing roller 10 if the fixing roller 10 is above the predetermined temperature. Simultaneously, it sends a control signal to the shutter driving section 22 to keep a hereinafter described air path shutter 23 closed.

The port P 1 is connected to the connection point between the thermistor 3 - 1 and the voltage dividing resistor R 1 . The port P 2 is connected to the base of the transistor Tr 1 . The port P 3 is connected to the base of the transistor Tr 4 via the resistor R 10 . The port P 4 is connected to the base of a transistor Tr 6 via a resistor R 13 .

The shutter driving section 22 closes the unit if the temperature inside the unit rises extremely high to prevent damage to the unit. It includes an air path shutter 23 , an amplifier consisting of the transistor Tr 6 , a resistor R 12 , and a resistor R 13 to amplify the control signal from the control section 21 , and a driver amplifier consisting of a transistor Tr 7 and a resistor R 14 to drive the air path shutter 23 .

The port P 4 of the control section 21 is connected to the base of the transistor Tr 6 via the resistor R 13 . The emitter of the transistor Tr 6 is grounded and the collector is connected to the base of the transistor Tr 7 via a resistor R 14 . The emitter of the transistor Tr 7 is connected to 24 Vd-c and the collector is connected to an end of the air path shutter 23 . The other end of the air path shutter 23 is grounded. An end of the resistor R 12 is connected to the conduction cutoff section 5 . An example of-the air path shutter 23 will be described below.

FIGS. 4( a ) and 4 ( b ) show the air path shutter 23 for opening and closing the air path between the fixing roller 10 and the fan 2 - 1 , respectively. The air path shutter 23 includes a shutter member 23 - 1 , a spring 23 - 2 , and a solenoid 23 - 3 .

When the fixing roller 10 operates at a normal temperature, the shutter member 23 - 1 is attracted by the solenoid 23 - 3 , keeping the spring 23 - 3 compressed. Under this condition, the air path from the fan 2 - 1 to the fixing roller 10 is opened. If the fixing roller 10 reaches an abnormally high temperature, the shutter member 23 - 1 is pushed by the spring 23 - 2 to close the air path from the fan 2 - 1 to the fixing roller 10 .

FIG. 5 is the temperature control time chart of the fixing unit according to the second embodiment, wherein (a) is the temperature detected by the thermistor 3 - 1 ( FIG. 3 ), (b) the output of the port P 2 ( FIG. 3 ), (c) the output of the comparator 5 - 1 ( FIG. 3 ), (d) the on/off of the transistor Tr 2 ( FIG. 3 ), (e) the on/off of the transistor Tr 3 ( FIG. 3 ), (f) the output of the port P 3 ( FIG. 3 ), (g) the on/off of the transistor Tr 4 ( FIG. 3 ), (h) the on/off of the transistor Tr 5 ( FIG. 3 ), (i) the rotation/stop of the fan 2 - 1 ( FIG. 3 ), (j) the on/off of the transistor Tr 1 , (k) the output of the port P 4 , ( 1 ) the on/off of the transistor Tr 6 ( FIG. 3 ), (m) the on/off of the transistor Tr 7 , (n) the open/close of the air path shutter 23 ( FIG. 3 or 4 ), and (o) the time common to all the items.

At a time t 1 , printing is started and the temperature control of the fixing unit is started. It is assumed that the initial value T 0 of temperature detected by the thermistor 3 - 1 is lower than the optimum fixing temperature T 1 . Consequently, the voltage (corresponding to the initial value T 0 ) at the connection point between the thermistor 3 - 1 and the voltage dividing resistor R 1 is lower than the protective voltage (corresponding to the protective temperature) at the connection point between the voltage dividing resistors R 2 and R 3 so that the output of the comparator 5 - 1 becomes on (b), turning on the transistors Tr 2 and Tr 3 (d) and (e). Consequently, the resistors R 7 , R 9 , and R 12 normally bias the transistors Tr 1 , Tr 4 , and Tr 6 for operation.

The control section 21 , which determined that the initial value T 0 is lower than the optimum fixing temperature T 1 , turns on the port P 2 to raise the temperature of the fixing roller 10 (b). This corresponds to sending out a control signal to the switching circuit 1 - 4 . Consequently, the transistor Tr 1 is turned on (j), conducting heating current in the loop consisting of the switching circuit 1 - 4 , the heater 1 - 1 , the thermostat 1 - 2 , and the a-c source 1 - 3 so that the surface temperature of the fixing roller 10 starts to rise (a). Simultaneously, the control section 21 turns on not only the port P 3 to rotate the fan 2 - 1 (f) but also the port P 4 to open the air path shutter 23 (k).

At a time t 2 , the surface temperature of the fixing roller 10 exceeds the optimum fixing temperature T 1 . At this point, the control section 21 , which has determined that the detected temperature T from the port P 1 is higher than the optimum fixing temperature T 1 , turns off the port P 2 to lower the temperature of the fixing roller 10 (b). This corresponds to the issue of a control signal to the switching circuit 104 . Consequently, the transistor Tr 1 is turned off (j), cutting off the loop consisting of the switching circuit 1 - 4 , the heater 1 - 1 , the thermostat 1 - 2 , and the a-c source 1 - 3 , bringing the heating current to zero (0) so that the surface temperature of the fixing roller 10 starts falling (a). At this point, the detected temperature T does not exceed the protective temperature T 2 so that the items other than the port P 2 (b) and the transistor Tr 1 (j) keep the same conditions at the time t 1 .

At a time t 3 , the surface temperature of the fixing roller 10 falls below the optimum fixing temperature T 1 . The control section 21 , which has determined that the detected temperature T from the port P 1 is lower than the optimum fixing temperature T 1 , turns on the port P 2 to raise the temperature of the fixing roller 10 (b). This corresponds to the issue of a control signal to the switching circuit 1 - 4 . Consequently, the transistor Tr 1 is turned on (j), conducting heating current in the loop consisting of the switching circuit 1 - 4 , the heater 1 - 1 , the thermostat 1 - 2 , and the a-c source 1 - 3 , so that the surface temperature of the fixing roller starts to rise (a).

In subsequent period between t 4 and t 7 , it continues the similar operation to control the temperature of the fixing roller 10 .

At a time t 8 , the surface temperature of the fixing roller 10 exceeds the optimum fixing temperature T 1 . The control section 21 , which has determined that the detected temperature T from the port P 1 is higher than the optimum fixing temperature T 1 , is supposed to turn off the port P 2 to lower the temperature of the fixing roller 10 , but it is assumed that the “on” state continues for a failure of the control section 21 or the switching circuit 1 - 4 (b). Consequently, the heating current continues to flow so that the surface temperature of the fixing roller 10 continues to rise and exceeds the protective temperature T 2 at a time t 9 .

Consequently, the voltage at the connection point between the thermistor 3 - 1 and the voltage dividing resistor R 1 becomes higher than the protective voltage at the connection point between the voltage dividing resistors R 2 and R 3 so that the comparator 5 - 1 is turned off (b), turning off the transistors Tr 2 and Tr 3 (d) and (e). Consequently, the resistors R 7 and R 9 shit the bias point to such an extent that the transistors Tr 4 and Tr 6 become inoperable. As a result, not only the heating current becomes zero (0) regardless of the outputs of the port P 2 (b), P 3 (f), and P 4 (k) but also the fan 2 - 1 is stopped (i) and the air path shutter 23 is closed.

As described above, if the fixing roller 10 is heated up above the protective temperature T 2 , the supply of heating current to the heater 1 - 1 is stopped, the rotation of the fan 2 - 1 is stopped, and the air path shutter 23 is closed regardless of the control by the control section 21 . Since not only the fan 2 - 1 is stopped but also the air path shutter is closed, the raised temperature is prevented to spread over the entire unit, causing a malfunction. In addition, even if the fan 2 - 1 is not stopped due to an abnormality of the driving circuit of the cooling section, the raised temperature is prevented to spread over the entire unit.

Alternatively, the solenoid 23 - 3 and the shutter member 23 - 1 may be replaced by any mechanism capable of opening/closing the air path between the fixing roller 10 and the fan 2 - 1 . If the air path has a rectangular section, the air path is closed by a rectangular shutter member that is rotatable 90 degrees to open the air path.

Third Embodiment

FIG. 6 shows the temperature control circuit for the fixing unit which includes the heating section 1 , the cooling section 2 , the temperature detection section 3 , the control section 4 , a heating current cutoff section 31 , and a cooling current reducing section 32 . Only those different from the first embodiment will be described and the components identical with those of the first embodiment are given the same reference numbers as those of the first embodiment.

The heating current cutoff section 31 cuts off current conduction to the heating section 1 regardless of control of the control section 4 if the temperature detected by the temperature detection section 3 exceeds a predetermined temperature (protective temperature). It includes the voltage dividing resistors R 2 and R 3 , and a pulse forming circuit consisting of the comparator 5 - 1 , the transistors Tr 2 and Tr 3 , and the resistors R 4 , R 5 , and R 7 .

The positive terminal of the comparator 5 - 1 is provided with the protective voltage to avoid an abnormal temperature increase in the fixing roller 10 . The protective voltage is defined by the ratio of the voltage dividing resistors R 2 and R 3 . The output terminal of the comparator 5 - 1 is connected to not only the base of the transistor Tr 2 but also the 3.3 Vd-c voltage source via the resistor R 4 . The emitter of the transistor Tr 2 is grounded and the collector is connected to the base of the transistor Tr 3 via the resistor R 5 . The emitter and collector of the transistor Tr 3 are connected to the 3.3 Vd-c source and the base of the transistor Tr 1 via the resistor R 7 , respectively. The collector of the transistor Tr 3 is connected to the base of the transistor Tr 6 for the cooling current reducing section 32 via the resistor R 12 .

The cooling current reducing section 32 reduces the cooling current flowing through the fan 2 - 1 of the cooling section 2 if the temperature detected by the temperature detection section 3 exceeds the protective temperature T 2 , making the heating current cutoff section 31 cut off the current conduction to the heating section 1 . It includes the transistor Tr 6 , the resistor R 12 connecting the base of the transistor Tr 6 and the collector of the transistor Tr 3 , and the resistor R 9 connecting the collector and emitter of the transistor Tr 6 . The emitter and collector of the transistor Tr 6 are connected to the 24 Vd-c source and the emitter of the transistor Tr 5 for supplying cooling current to the fan 2 - 1 , respectively.

Until the temperature detected by the temperature detection section 3 exceeds the protective temperature, the transistor Tr 3 is on so that the transistor Tr 6 is on. At this point, the cooling current to the fan 2 - 1 is supplied from the 24 Vd-c source to the transistor Tr 5 via the transistor Tr 6 . When the temperature detected by the temperature detection section 3 exceeds the protective temperature, the transistor Tr 3 is turned off, which in turn turns off the transistor Tr 6 . Consequently, the cooling current for the fan 2 - 1 is supplied from the 24 Vd-c source to the transistor Tr 5 via the resistor R 9 . That is, the cooling current is reduced to decrease the number of rotations of the fan 2 - 1 , resulting in the reduced amount of air.

FIG. 7 shows the temperature control by the fixing unit according to the third embodiment, wherein (a) is the temperature detected by the thermistor 3 - 1 ( FIG. 6 ), (b) the output of the port P 2 ( FIG. 6 ), (c) the output of the comparator 5 - 1 ( FIG. 6 ), (d) the on/off of the transistor Tr 2 ( FIG. 6 ), (e) the on/off of the transistor Tr 3 ( FIG. 6 ), (f) the output of the port P 3 ( FIG. 6 ), (g) the on/off of the transistor Tr 4 ( FIG. 6 ), (h) the on/off of the transistor Tr 5 ( FIG. 6 ), (i) the on/off of the transistor Tr 1 ( FIG. 6 ), (j) the on/off of the transistor Tr 6 ( FIG. 6 ), (k) the number of rotations of the fan 2 - 1 , and (l) the time common to all the items.

At a time t 1 , printing is started and the temperature control of the fixing unit is started. It is assumed that the initial value T 0 of temperature detected by the thermistor 3 - 1 is lower than the optimum fixing temperature T 1 . Consequently, the voltage (corresponding to the initial value T 0 ) at the connection point between the thermistor 3 - 1 and the voltage dividing resistor R 1 is lower than the protective voltage (corresponding to the protective temperature T 2 ) at the connection point between the voltage dividing resistors R 2 and R 3 so that the output of the comparator 5 - 1 becomes on (b), turning on the transistors Tr 2 and Tr 3 (d) and (e). Consequently, the resistors R 7 and R 9 make the transistors Tr 1 and Tr 6 normally biased for operation.

The control section 4 ( FIG. 6 ), which has determined that the initial value T 0 is lower than the optimum fixing temperature T 1 , turns on the port P 2 to raise the temperature of the fixing roller 10 ( FIG. 6 ) (b). This corresponds to the issue a control signal to the switching circuit 1 - 4 . Consequently, the transistor Tr 1 is turned on (i), conducting heating current in a loop consisting of the switching circuit 1 - 4 , the heater 1 - 1 , the thermostat 1 - 2 , and the a-c source 1 - 3 so that the surface temperature of the fixing roller 10 starts rising (a). Simultaneously, the control section 4 turns on the port P 3 to rotate the fan 2 - 1 (f).

At a time t 2 , the surface temperature of the fixing roller 10 exceeds the optimum fixing temperature T 1 . At this point, the control section 4 , which has determined that the detected temperature T from the port P 1 is higher than the optimum fixing temperature T 1 , turns off the port P 2 to lower the temperature of the fixing roller 10 (b). This corresponds to the issue of a control signal to the switching circuit 1 - 4 . Consequently, the transistor Tr 1 is turned off (i), cutting off the loop consisting of the switching circuit 1 - 4 , the heater 1 - 1 , the thermostat 1 - 2 , and the a-c current source 1 - 3 , bringing the heating current to zero (0) so that the surface temperature of the fixing roller 10 starts to fall (a). At this point, the detected temperature T does not exceed the protective temperature T 2 so that the items other than the port P 2 (b) and the transistor Tr 1 (i) keep the same conditions at the time t 1 .

At a time t 3 , the surface temperature of the fixing roller 10 falls below the optimum fixing temperature T 1 . The control section 4 , which has determined that the detected temperature T from the port P 1 is lower than the optimum fixing temperature T 1 , turns on the port P 2 to raise the temperature of the fixing roller 10 (b). This corresponds to the issue of a control signal to the switching circuit 1 - 4 . Consequently, the transistor Tr 1 is turned on (i), conducting heating current in the loop consisting of the switching circuit 1 - 4 , the heater 1 - 1 , the thermostat 1 - 2 , and the a-c current source 1 - 3 , so that the surface temperature of the fixing roller starts rising (a).

In subsequent period between t 4 to t 7 , it continues the similar operation to control the temperature of the fixing roller 10 .

At a time t 8 , the surface temperature of the fixing roller 10 exceeds the optimum fixing temperature T 1 . The control section 4 , which has determined that the detected temperature T from the port P 1 is higher than the optimum fixing temperature T 1 , is supposed to turn off the port P 2 to lower the temperature of the fixing roller 10 , but it is assumed that the on state continues for a failure of the control section 4 or the switching circuit 1 - 4 (b). Consequently, the heating current continues to flow so that the surface temperature of the fixing roller 10 continues to rise and exceeds the protective temperature T 2 at a time t 9 .

Consequently, the voltage at the connection point between the thermistor 3 - 1 and the voltage dividing resistor R 1 becomes higher than the protective voltage T 2 at the connection point between the voltage dividing resistors R 2 and R 3 so that the comparator 5 - 1 is turned off (b), turning off the transistors Tr 2 and Tr 3 (d) and (e). Consequently, the resistors R 7 and R 12 shit the bias point to such an extent that the transistors Tr 1 and Tr 6 become inoperable. As a result, the heating current becomes zero (0) regardless of the output of the port P 2 (b). The cooling current for the fan 2 - 1 is supplied to the transistor Tr 5 from the 24 Vd-c source to the transistor Tr 5 via the resistor R 9 regardless of the output of Port P 3 (f). That is, the cooling current is reduced to decrease the number of rotations of the fan 2 - 1 , resulting in the reduced amount of air blown.

As described above, if the fixing roller 10 is heated up above the protective temperature T 2 , not only the supply of heating current to the heater 1 - 1 is stopped but also the number of rotations of the fan 2 - 1 is reduced regardless of the control by the control section 4 . For a certain type of the fixing unit, it is better to reduced the number of rotations of the fan 2 - 1 to an appropriate number than to stop the rotation of the fan 2 - 1 for minimizing the occurrence of a malfunction.

In FIG. 9 , the fixing unit includes the fixing roller 10 rotatably supported by a frame (not shown) and a pressure roller 21 which is rotated by a driving device (not shown) in contact with the fixing roller 10 under pressure. The fixing roller 10 has a three-layer structure made by coating a tubular core of aluminum or the like with a heat resistant, elastic layer of silicone rubber or the like to a thickness of 1–2 mm, then a release layer of Teflon (trade name) or the like to a thickness of 30 μm, and finally an elastic layer of silicone rubber or the like to a thickness of 70 μm. The heater 1 - 1 made of a halogen lamp or the like is provided within the tubular core to heat the fixing roller 10 by heat conduction.

The pressure roller 21 is made by coating a tubular core of aluminum or stainless steel with a release layer of Teflon, such as PFT or ETFE, to a thickness of 3–100 μm. The pressure roller 21 has no elastic layer and is made very hard and the fixing roller 10 is provided with a relatively thick elastic layer so as to form a press contact section (nip section) between the pressure and fixing rollers 21 and 10 .

The thermistor 3 - 1 is provided in contact with the fixing roller 10 to detect the surface temperature of the fixing roller 10 . Based on the temperature detected by the thermistor 3 - 1 , the control section 4 controls current conduction to the heater 1 - 1 to keep the fixing roller 10 at a predetermined surface temperature. A thermostat 22 is provided at a predetermined distance from the fixing roller 10 to cut off current conduction to the heater 1 - 1 if the fixing roller 10 is heated up abnormally.

In FIG. 10 , an image forming apparatus 31 includes an electrostatic latent image carrier or photosensitive drum 32 , a charging unit 33 , an exposure unit 34 , a developing unit 35 , and a transfer unit 36 . The fixing unit is provided on the downstream of a medium transporting path 37 which runs through an abutment section between the photosensitive drum 32 and the transfer unit 36 .

The charging unit 33 applies a high negative voltage to the charging roller to provide the surface of the photosensitive drum 32 with a uniform negative charge. The exposure unit 34 turns on a light emitting element according to the printing data to bring the charged surface to approximately zero (0) V to form an electrostatic latent image.

The developing unit 35 includes a developing roller 38 , a developing blade 39 , a toner transport roller 40 , and a developer or toner 41 and applies a high negative voltage lower than the voltage on the surface of the photosensitive drum 32 to the developing roller 38 and the developing blade 39 to provide the toner 41 with a negative charge and the developing roller with a thin layer, respectively.

Where the developing roller 38 is of the contact type, the roller surface is made of a smooth rubber roller whereas for the non-contact type or the magnetic developing system, for example, the roller surface is not necessarily a rubber layer. The toner transport roller 40 is made of a sponge roller.

The transfer unit 36 applies a high positive voltage to a transfer roller to provide a positive charge for the printing medium transported along the medium transport path 37 . The fixing unit fuses the toner onto the printing medium 42 when the printing medium with the transferred toner passes through the nip section between the rollers 10 and 21 .

The photosensitive drum 32 , the charging roller 33 , the developing roller 38 , the toner transport roller 40 , the fixing roller 10 , and the pressure roller 21 are rotated in the directions of arrows by motors (not shown) which are controlled by an image forming and control unit (not shown).

In operation, when the image forming and control unit receives a print command and print data from an upper device (not shown), it rotates the photosensitive drum 32 , the charging roller 33 , the developing roller 38 , the toner transport roller 40 , the fixing roller 10 , and the pressure roller 21 in the respective directions.

Then, the image forming and control unit applies a high negative voltage to the charging roller 33 via the charging unit to provide the surface of the photosensitive drum 32 with a uniform negative charge, turns on the light emitting element via the exposure unit, forming an electrostatic latent image on the photosensitive drum 32 . Then, it applies a high negative voltage to the developing roller 38 and the toner transport roller 40 via the developing unit to provide the toner 41 with a negative charge so that the toner 41 adheres to the electrostatic latent image by electrostatic effects for making development.

Then, it supplies the transfer roller 36 with a high positive voltage via the transfer unit to provide the print medium 42 on the medium transporting path 37 with a positive charge. The developed toner image is then transferred to the print medium 42 from the photosensitive drum 32 . The toner image transferred to the print medium 42 is passed through the nip section between the pressure roller 21 and the fixing roller 10 that is heated by the control section 4 for fusion. The print medium 42 , on which the toner image has been fixed, is discharged into an outside stacker (not shown).