JPH01116811A - Overheat preventing device for heater - Google Patents

Abstract

PURPOSE:To prevent thermal runaway of a heater by constituting a device so that power is not supplied to the heater independently of the output of a first comparator when disconnection of a connector is detected by a second comparator. CONSTITUTION:When a connector 4 is disconnected, the apparent temperature of a heat roll 10 extremely falls and a first comparator 5 outputs '1', but a second comparator 6 outputs '0' because an input voltage V1 of the second comparator 6 is V0 and is higher than the voltage generated by voltage division of a second resistor 3 and a first resistor 2. Then, the output of an AND circuit 7 goes to '0', and power is not supplied to a heater 9 from a heater driver 8 to prevent thermal runaway of the heater 9 due to disconnection of the connector 4.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a thermal fixing device for fixing toner onto paper such as an electronic printer device or a copying device. The present invention relates to a heater overheating prevention device that detects disconnection of a connector of a samsister and prevents thermal runaway of the heater.

[Prior Art] In conventional electronic printers and copying machines, for example, a latent image is created on a photosensitive drum, toner is attached to this latent image, and this toner is transferred onto paper using a heating roller or the like. By heating and fixing this toner on the paper,
Printing and copying are performed.

In addition, in order to maintain good fixing performance, the temperature of the heating roller is controlled to a constant value. A thermistor is usually used in which the resistance has negative temperature characteristics such that the resistance decreases as the resistance increases.

For this reason, if the apparent resistance of the thermistor is large due to a disconnection of the thermistor, etc., it is determined that the temperature of the heating roller is low, and power to the heater is continuously supplied, resulting in thermal runaway of the heater.

By the way, considering the structure of the heat fixing device, the temperature detection section consisting of a samister is connected to the main body by a connector.
If the connector is disconnected, thermal runaway of the heater occurs as described in 5 above, so it is desirable to detect disconnection of the connector and block power supply to the heater.

Therefore, in the past, indirect detection methods were used to detect disconnection of the thermistor or disconnection of the connector, such as checking whether there was a change in the resistance of the thermistor within a certain period of time after power was supplied to the heater. It is being

[Problems to be solved by the invention] As mentioned above, the temperature detection part of the heating roller is connected to the main body by a connector, so it is often forgotten to attach this connector when assembling the device or during maintenance and inspection. The conventional indirect method of checking the resistance change of the thermistor requires a certain amount of time for detection, which leads to the problem of not being able to sufficiently prevent thermal runaway of the heater due to disconnected connectors. .

In other words, the heater continues to heat up by repeating the process of turning off the power switch to reset the detection circuit before detecting that the connector is disconnected, and then turning on the power switch again to start energizing the heater.

The present invention aims to solve the above problems. That is, an object of the present invention is to provide a heater overheat prevention device that can relatively easily and directly detect disconnection of the samister connector and prevent power supply to the heater. It is something.

[Means for solving the problem] Thermal fixing that detects changes in the resistance value of the thermistor, supplies power to the heater of the heating roller, maintains the heating roller at a predetermined temperature, and fixes the toner on the paper. In the apparatus, a resistor inserted between the thermistor and the connector in parallel with the thermistor, a first comparator that detects a change in resistance value of the thermistor near a predetermined temperature and outputs a binary value, and the connector a second comparator for detecting that the apparent resistance value of the thermistor and the resistor becomes infinite; a logic circuit that determines whether or not to supply power to the heater, and when the second comparator detects a disconnection of the connector, the power is supplied to the heater regardless of the output of the first comparator. It was configured so that no power was supplied.

[Function] According to the present invention, if the connector of the samister is disconnected, power is not supplied to the heater, so thermal runaway of the heater is prevented, and it is possible to avoid forgetting to attach the connector during maintenance and inspection of the heat fixing device. It is possible to prevent failures in the heat fixing device due to human errors.

[Embodiment] FIG. 1 is a block diagram of a circuit showing a first embodiment of the present invention. .

In FIG. 1, 1 is the thermistor, 2 is the first resistor, 3
is the second resistor, 4 is the connector, 5 is the first comparator-6
7 is a second comparator; 7 is an AND circuit that performs the logical product of the first comparator 5 and the second comparator 6; 8 is a heater driver that drives the heater; 9 is a heater; This is the heating roller inside. The samister 1 and heating roller 10 are in thermal contact with each other.

Further, the first comparator 5 is configured such that the samister 1 is, for example, at a temperature of 18
When the temperature is below 0°C, that is, when the resistance of the thermistor 1 is large, the output is set to be 1"°, and the second comparator 6 When the value is less than the resistance value of the second resistor 3, the output is “°1”°
Therefore, when the connector 4 is disconnected and the apparent resistance value of the thermistor 1 and the second resistor 3 becomes infinite, the output becomes °
It is set to be O゛°.

In a heater overheat prevention device having a circuit configured as shown in FIG. 1, the resistance of the samister 1 decreases as the temperature rises, and the resistance is obtained by dividing the power supply voltage v0 with the first resistor 2. vl, ie the input voltage of the first comparator 5, decreases. For example, assuming that the temperature of the heating roller 10 is 180°C, if this temperature is exceeded,
The first comparator 5 outputs "0", and outputs "1" when the temperature is below this temperature.

On the other hand, since the second resistor 3 is inserted in parallel with the thermistor 1, the heating roller 10 is at any temperature, that is,
No matter what resistance value Samista 1 takes, Samista 1
The parallel resistance value of the resistor 3 and the second resistor 3 is less than the resistance value of the second resistor 3, and the second comparator 6 always outputs ゛°1p. Therefore, normally the temperature of the heating roller 10 is 18
When the temperature drops below 0° C., the output of the first comparator 5 becomes “1”, the output of the AND circuit 7 also becomes “1”, and the heater driver 8 energizes the heater 9.

Then, when the temperature of the heating roller 10 exceeds 180°C,
The output of the first comparator 5 becomes 0'°, the output of the AND circuit 7 also becomes "0", and the energization from the heater driver 8 to the heater 9 is stopped. In reality, the heating roller 1 is heated with a certain temperature ripple due to a delay in heat conduction, etc.
At 0, the temperature is maintained around 180°C.

Now, if we consider the case where the connector 4 is disconnected, the apparent temperature of the heating roller 10 has dropped extremely, and the first comparator 5 outputs "°1". but,
The input terminal Vl of the second comparator 6 is V. Since the voltage is higher than the voltage generated by the voltage division between the second resistor 3 and the first resistor 2, the second comparator 6 outputs zero.

As a result, the output of the AND circuit 7 becomes "0" and the heater 9 is not energized by the heater driver 8. In this way, thermal runaway of the heater 9 due to disconnection of the connector 4 is prevented.

It is preferable to set the resistance value of the second resistor 3 to be sufficiently larger than the resistance value of the thermistor 1 near the specified temperature of the heating roller 10, since this will not impair the sensitivity of the thermistor 1 to temperature changes. 4 is missing and the second
When input terminal 1 of comparator 6 becomes vo, and
The power supply voltage ■ is set by the resistor 2 and the second resistor 3. Since the second comparator 6 must be able to sufficiently distinguish between the case where the voltage generated by dividing the voltage is applied to the input terminal of the second comparator 6, it is unnecessary to It is not preferable to increase the resistance value of the second resistor 3.

In this embodiment, the resistance value of the first resistor 2 is R1,
The resistance value of the second resistor 3 is R2, and the resistance value of the thermistor 1 at 180°C is RTH1aO5, and the power supply voltage is ■. Then, the threshold level V□ of the first comparator 5 and that Vs2 of the second comparator 6 are as follows. However, 〆 indicates the parallel resistance value.

The energization conditions for the heater 9 are as follows: V□≦V, ≦Vs2 It is.

FIG. 2 is a block diagram of a circuit showing a second embodiment of the present invention.

In FIG. 2, circuits having the same functions as those shown in FIG. 1 are given the same reference numerals.

However, the circuit shown in FIG. 2 differs from the circuit shown in FIG. , although the logic is reversed from that in FIG. 1, it produces exactly the same effect.

FIG. 3 is a block diagram of a circuit showing a third embodiment of the present invention, and FIG. 4 is a flowchart thereof.

That is, FIG. 3 shows an example in which the functions of the first comparator 5, the second comparator 6, and the AND circuit 7 described above are realized by a microcomputer 11 that substantially incorporates them.

In recent years, with the spread of microcomputers, the temperature control of the heating roller 10 is mostly carried out using microcomputers and heaters, but in the case of Fig. 3, only a small resistance of 1 is used.
The connector 4 can be disconnected by adding a book or a small program.

FIG. 5 is a block diagram of a circuit showing a fourth embodiment of the present invention.

In FIG. 5, the connector 4 is
When a omission is detected, the display driver 12 and display 13 are used to display the omission. By doing this, the indicator 13 indicates that the connector 4 is disconnected.
You can notify immediately by

[Effects of the Invention] As explained above, according to the present invention, a resistor inserted in parallel with the thermistor between the thermistor and the connector, and a change in the resistance value of the thermistor near a predetermined temperature are detected and a binary value is generated. a first comparator that outputs , a second comparator that detects that the apparent resistance value of the thermistor and the resistor becomes infinite due to the disconnection of the connector; a logic circuit that determines whether or not to supply power to the heater based on the output of the second comparator, and when the second comparator detects that the connector is disconnected, the first Since the configuration is such that power is not supplied to the heater regardless of the output of the comparator, if the connector connecting the thermistor is disconnected, the power supply to the heater cannot be stopped. Therefore, thermal runaway of the heater can be prevented.

[Brief explanation of the drawing]

Fig. 1 is an explanatory diagram showing the first embodiment of the present invention, Fig. 2 is an explanatory diagram also showing the second embodiment, and Fig. 3 is an explanatory diagram showing the third embodiment.
FIG. 4 is an explanatory diagram showing the flowchart of FIG. 3, and FIG. 5 is an explanatory diagram showing the fourth embodiment of the present invention. 1... Samista 2... First resistor 3...
- Second resistor 4... Connector 5... First comparator 6... Second comparator 7... AND circuit
8... Heater driver 9... Heater
10...Heating roller 11...Microcomputer 12...Display driver 13...Display device Fig. 3

Claims (1)

[Claims] In a heat fixing device that detects a change in the resistance value of a samister, supplies power to a heater of a heating roller, and maintains the heating roller at a predetermined temperature to fix the toner on the paper, the shimsistor is connected to the connector. a resistor inserted in parallel with the thermistor; a first comparator that detects a change in the resistance value of the thermistor near a predetermined temperature and outputs a binary value; a second comparator that detects that the resistance value of the resistor has become infinite;
and a logic circuit that determines whether or not to supply power to the heater based on the outputs of the first comparator and the second comparator, and the second comparator prevents disconnection of the connector. Overheating of a heater, comprising a heater driver that prevents power supply to the heater by a signal from the logic circuit regardless of the output of the first comparator when overheating of the heater is detected. Prevention device.