JPH0473676A - Temperature detection circuit for fixing device - Google Patents

Abstract

PURPOSE:To detect the occurrence of the disconnection of a lead wire reaching to a thermistor and the missing of a connector, etc., by detecting a resultant resistance value of the thermistor and a resistor which is connected with the thermistor in parallel and deciding the temperature of a fixing device and the disconnection of a circuit. CONSTITUTION:The resultant resistance value of the thermistor 16 and the resistor 20 is detected so as to decide the temperature of the fixing device and the disconnection of the circuit. The resistor 20 which is connected with the thermistor 16 in parallel is capable of detecting the occurrence of the missing of the connector 1 and the disconnection of the lead wire reaching to the thermistor 16, to put it concretely, the disconnection of the lead wire between the junction A with a DC power supply and the junction B for an IC 2. That means, by applying the value applicable to the temperature and resistance value characteristic of the thermistor 16 as the resistance value of the resistor 20, the voltage at the normal time can be made largely different from the voltage at the disconnection time. Thus, the temperature of the fixing device and the disconnection of the circuit are decided while retaining the modification of the constitution of a hardware and the increase of the cost to a minimum.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to a temperature detection circuit for a fixing device provided in an image forming apparatus such as a copying machine or a laser beam printer.

2. Description of the Related Art Image forming apparatuses such as copying machines and laser beam printers are equipped with a fixing device that fixes toner images formed on recording paper. This fixing device always needs to be controlled to a certain temperature. This control is performed by a temperature control device as shown in FIG.

In the figure, ICI is a microcomputer that primarily functions as a control device for controlling image forming operations, but also has a function of controlling the temperature of the fixing device, which will be described later. Current is supplied to the heater 10 from an AC power source 12. Energization to the heater 10 is controlled via a phototriac coupler 14 and a triac 15 that operate in response to signals output from the ICI. Specifically, the ICI OUT output is at the old GH level and 0LIT.
When the output is at the LOW level, the phototriac 14 becomes conductive, and accordingly the triac 15 also becomes conductive, power from the AC power supply 12 is supplied to the heater 10, current flows through the heater 10, and the fuser is activated. heated.

Note that the heater 10 described above is for heating the fixing device, and a halogen lamp or the like is used.

IC2 is connected to this ICI via eight ports. This IC2 converts the analog signal from the thermistor 16 that detects the temperature of the fixing device into a digital signal,
It has a function as an A/D converter that outputs the converted signal to the ICI. This thermistor 16 is placed near the heater 10 to detect the temperature of the fixing device, and is connected to the IC 2 via a connector 17.

A resistor 18 is connected in series to the thermistor 16 via a connector 17. IC2 calculates the divided voltage value of resistor 18 in the series circuit of thermistor 16 and resistor 18,
Output to ICI as a digital value. A DC power supply Vcc is applied to this series circuit, and a divided voltage value is formed by this Vcc. Further, this Vcc is also used as a reference voltage of IC2.

With this configuration, the ICI controls the heater 10 on and off based on the digital value output from the IC2, that is, based on the temperature of the fixing device, and controls the temperature of the fixing device within a specified range. It looks like this.

By the way, the temperature-resistance characteristic diagram of the thermistor 16 is as follows:
It is expressed as shown in FIG. For example, the above characteristics of a thermistor used for boat fishing include approximately IMΩ at 0°C, 500Ω at 10°C, 15Ω at 100°C, and IMΩ at 200°C. 1 becomes Ω.

Since the thermistor changes its resistance value in this way as the temperature changes, the analog value input to IC2 in the temperature control device shown in FIG.

That is, the voltage value Vx changes approximately as shown in FIG. 5 as the temperature changes (the resistance value of the resistor 18 is assumed to be IKΩ).

For example, if the voltage of the DC power supply is 5V, the voltage value Vx at 0°C is 5×1Ω/(1MΩ+1Ω) = 5rnV, and 2
Similarly, the voltage value Vx at 00° C. is 5×1Ω/(IKΩ+1×Ω)=2.5V.

In this way, when the temperature changes from 0℃ to 200℃,
The voltage value Vx input to IC2 will change from 5 mV to 2.5V.

Problems to be Solved by the Invention By the way, in copiers, printers, and the like, it is desired to be able to detect failures in advance, minimize the spread of damage, and take prompt action. Moreover, the cost involved is kept to a minimum. As an example, consider a case where the lead wire leading to the thermistor 16 is broken for some reason. If such a disconnection occurs, the temperature of the fuser cannot be controlled and the heater 10 remains energized. However, even in such a case, there is a system that can detect the disconnection of the lead wire and notify the occurrence of the disconnection. It is desirable to do so.

In conventional temperature control devices, when trying to detect occurrences such as breakage of the lead wire leading to the thermistor 16 or disconnection of the connector 17, there is a problem that this detection cannot be done easily and inexpensively. In other words, if a conventional temperature control device is to perform this detection, an IC 2 of extremely high precision must be used. With conventional temperature control devices, this ranges from 0°C to 20°C as mentioned above.
The voltage will change in the range of 5mV to 2.5V until it reaches 0℃, but in the unlikely event that the lead wire is disconnected or the connector is disconnected, the value of the voltage Vx input to IC2 will change. becomes OV, IC2 is at least 5m
This is because precision is required to be able to distinguish between V and OV.

In this case, it is impossible to distinguish between them using a commonly used 8-bit A/D converter, and a minimum of 10-bit A/D converter must be used, which increases costs due to disconnection detection. Therefore, it is not practical.

The present invention has been made in view of these conventional problems, and has a simple and inexpensive configuration that detects occurrences such as breakage of the lead wire leading to the thermistor and unplugging of the connector as described above. An object of the present invention is to provide a temperature detection circuit for a fixing device that can detect temperature of a fixing device.

Means for Solving the Problems To achieve the above objects, the present invention includes a thermistor that is arranged near a heat source provided in a fixing device and detects the temperature of the fixing device, and a thermistor that is connected in parallel with the thermistor. The apparatus is characterized in that it includes a resistor and a determining means that detects a combined resistance value of the thermistor and the resistor and determines the temperature of the fixing device and a break in the circuit.

Effect With this configuration, by adopting a resistance value of the resistor that matches the temperature-resistance characteristics of the thermistor, it is possible to largely control the voltage change between normal conditions and disconnection. It becomes possible to determine the temperature of the fixing device and the disconnection of the circuit while minimizing changes in the node configuration and increase in cost.

Embodiments Hereinafter, embodiments of the present invention will be described in detail based on the drawings.

FIG. 1 is a temperature detection circuit diagram of a fixing device according to the present invention. This temperature detection circuit differs from the circuit shown in FIG. 3 only in that a resistor 20 is connected in parallel with the thermistor 16. Therefore, since the operations of the other constituent elements are as described above, a description of their individual operations will be omitted.

A resistor 20 connected in parallel to the thermistor 16 prevents disconnection of the connector 17 and disconnection of the lead wire leading to the thermistor 16, specifically, the connection point A of the DC power supply shown in FIG.
This is to enable detection of disconnection of the lead wire between the connection point 8 and the IC 2. Therefore, it is desirable that this resistor 20 be connected as close to the thermistor 16 as possible. This is desirable because it would be very difficult to detect a break in the lead wire between the thermistor 16 and the resistor 20. This resistor 20
Since it is not necessary to have high precision, a material with low precision such as a carbon film resistor is sufficient. Further, the resistance value of the resistor 20 is determined by taking into consideration the temperature-resistance value characteristics of the thermistor 16 and the voltage range to be used when controlling the heater 10 on and off in the ICI.

In this embodiment, this resistance value is set to 9Ω.

The ICI, which also controls the function of detecting the temperature of the fixing device, is programmed to detect a disconnection or disconnection of a connector after the power is turned on and before the heater 10 starts heating. Also, even if normal temperature control is being performed,
The program is designed to immediately determine a disconnection when the voltage changes. Note that the ICI functions as a determining means.

When the resistor 18 is connected in parallel with the thermistor 16 as shown in FIG. 1, the change in the voltage vy input to the IC 2 is 0.
It changes as shown in Figure 2 from ℃ to 200℃.

For example, approximately 1MΩ at 0℃, 50Ω at 10℃
A commonly used thermistor 16 with characteristics such as 0 Ω, 15 Ω at 100°C, and 1 Ω at 200°C is used, and a resistor is connected in parallel with this thermistor 16. Assume that a resistor 9 is used with a resistance value of Ω, and a resistor 18 with a resistance value of IKQ is used. Furthermore, if the voltage of the DC power supply is 5V, the voltage value vy input to IC2 when the thermistor 16 detects 0°C is Vy=IKQ x 5/f1MQx9 Ω/(
IMQ +9 to Ω) +1 to Ω1-11.5V and 200
Voltage value vy input to IC2 when detecting °C
is, Vy= 5/1lKQx9Ω/(IKQ+9Ω)+
1, Ω+ = 2.63 V.

In this way, when the temperature changes from 0°C to 200°C, the voltage value Vy input to IC2 changes from 0.5V to 2.6V.
It will change within a range of 3V.

Therefore, in order to detect disconnection of the lead wire leading to the thermistor 16 or disconnection of the connector 17, the minimum
It is sufficient to be able to distinguish between V and 0, so an 8-bit A/D converter is sufficient (if the DC power supply voltage is 5 V)
The resolution of an 8-bit A/D converter is approximately 20
(about mV). Then, before issuing a heating command to the heater]0, the ICI determines whether the lead wire leading to the thermistor 16 is disconnected or the connector 17 is disconnected, based on the digital value output from the IC2. Based on the determination, either normal heating control is started or a disconnection process is performed in which the heater 10 is not energized.

Effects of the Invention As is clear from the above explanation, according to the present invention, the disconnection of the lead wire leading to the thermistor can be prevented with a simple and inexpensive configuration of simply connecting a resistor in parallel to the thermistor.
This makes it possible to detect occurrences such as disconnection of connectors.

[Brief explanation of the drawing]

FIG. 1 is a temperature detection circuit diagram of a fuser according to the present invention, FIG. 2 is a graph showing the temperature-voltage characteristics input to IC2 in the circuit of FIG. 1, and FIG. 3 is a diagram of a conventional fuser. The temperature detection circuit diagram, Figure 4, is
- A graph showing the temperature-resistance characteristics of a thermistor used for boat fishing. FIG. 5 is a graph showing the temperature-voltage characteristics input to IC2 in the circuit of FIG. C1... Microcomputer C2... A/D converter, 0... Heater (heat source), 6... Thermistor, 0... Resistor. (Determination means)

Claims (1)

[Claims] a thermistor disposed near a heat source provided in the fixing device to detect the temperature of the fixing device; a resistor connected in parallel with the thermistor; detecting a combined resistance value of the thermistor and the resistor; A temperature detection circuit for a fixing device, comprising determining means for determining the temperature of the fixing device and a break in the circuit.