Exposure Safeguarding Device For Photocopying Machine

Abstract

This invention provides a safety device for preventing under exposure in a photocopying device. The device includes a sensitive element for detecting the temperature around the exposure light source, and delay means for varying the delay time according to the temperature difference detected by the sensitive element.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to a safety device for an electrophotographic copying machine and more particularly an exposure safeguarding device for preventing under-exposure which would result in a poor quality copy.

2. Description of the Prior Art

In the conventional photocopying machines fluorescent lamps have been generally used as a light source for exposure, but the rise time of brightness is varied depending upon the environmental temperature as shown in FIG. 1 in which the maximum brightness of a fluorescent lamp is plotted as 100% at 25.degree. C. In general the conventional photocopying machines are designed so that a copy may be produced if the brightness of a light source is in excess of 60% (indicated by the two-dot line in FIG. 1). However, depending upon the environmental temperature, the rise time required for a light source to reach the 60% threshold level is varied. For example, it takes 1 minute when the environmental temperature is 25.degree. C, about 2 minutes at 15.degree. C and about 3 minutes at 5.degree. C. Therefore, in some cases the photocopying operation is started even before the brightness of a light source reaches the 60% threshold level so that an underexposure occurs. To overcome this problem there has been proposed a method for maintaining the temperature around a light source or fluorescent lamps and at a constant level as some experiments have been conducted in a thermostatic oven in a laboratory at the same temperature. A method based on this principle is disclosed for example in U.S. Pat. No. 3,330,180, filed Oct. 14, 1964 by Laurence A. Ferguson et al. and patented July 11, 1967. According to this method both heating and cooling means are provided so as to maintain the temperature around a light source at a constant level.

The photocopying machines of the type generally used in offices and the like are required to be compact in size, easy and simple in maintenance and free from noise and heat so that the environmental conditions may not be adversely affected. Therefore, the above method which requires heating and cooling means has a defect that the photocopying machine becomes inevitably large in size and complex in construction. The above method may be used in a large-sized photocopying machine, but when it is applied to a small-sized photocopying machine for office use the cost and the dimensions are increased, the maintenance becomes complex, and the office environment is adversely affected because heat is generated. Thus it is seen that there has not been proposed an effective method for compact photocopying machines for office use for overcoming the problem caused by the variation in rising time of a light source reaching a predetermined threshold level.

SUMMARY OF THE INVENTION

One of the objects of the present invention is therefore to provide an exposure safeguarding device for an electrophotographic copying machine of the type using fluorescent lamps as a light source for exposure, which is simple in construction and may prevent under-exposure which would result in poor quality copies.

Another object of the present invention is to provide an exposure safeguarding device for an electrophotographic copying mahcine which permits the photocopying operation to start at the optimum time depending upon the environmental temperature.

Another object of the present invention is to provide an exposure safeguarding device best adapted to be incorporated in a compact photocopying machine.

Briefly stated the present invention provides an exposure safeguarding device for a photocopying machine of the type using an exposure light source such as fluorescent lamps in which the time delay obtained by a time-delay circuit for delaying the start of the driving system of the photocopying machine is varied in response to the detected environmental temperature so that when the brightness of a light source reaches the optimum level the photocopying operation may be started.

The above and other objects, features and advantages of the present invention will become more apparent from the following description of some preferred embodiments thereof taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a graph used for the explanation of variations in rise time of the brightness of a light source of a photocopying machine depending upon the environmental temperature;

FIG. 2 is a sectional view of a small-sized electrophotocopying machine to which is applied an exposure safeguarding device in accordance with the present invention;

FIGS. 3 and 4 are diagrams of an electric circuit of the electrophotographic copying machine shown in FIG. 1 including an electric circuit of the exposure safeguarding device; and

FIG. 5 is an electric circuit diagram of another embodiment of the present invention adapted for continuously controlling the delay time.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 2 illustrating a compact-sized electrophotographic copying machine, an original which is transported over a glass plate 2 in an illumination stage 1 is illuminated by lamps 3 so that the image of the original may be focused upon a sensitive drum 7 through an exposure stage 6 consisting of a reflecting mirror 4 and a mirror lens 5. The photosensitive member upon the surface of the drum 7 comprises a photosensitive layer coated with a transparent insulating layer. The sensitive drum 7 which is rotated in the direction indicated by the arrow is charged positively by a first charger 9 electrically connected to a high-voltage source 8. As the drum continues to rotate the image of the original is focused upon the charged surface and, simultaneously an AC discharging operation is performed by means of a discharger 10 to which is applied high AC voltage from the power source 8. Then, the drum is subjected to whole-surface-exposure by means of a lamp 11, whereby an electrostatic latent image may be formed upon the surface of the sensitive drum 7. An electrostatic latent image may be also formed by the first DC charging, the simultaneous projection of the original image with a second charging of opposite polarity, and the whole-surface-exposure or application of the second charging is applied just after projection of the original image.

The latent image is developed by a developing agent 13 in a developer 12, and then applied with corona discharge by the high negative voltage applied to a post charger 14 so that the excess developing agent may be removed without the developed image being adversely affected. A copy sheet P which is fed from a copy sheet stand or the like by a feed roller 15 is brought into contact with the sensitive drum 7 so that the developed image thereupon may be transferred to the copy sheet P when positive corona discharge is applied by a charger 16. Thereafter the copy sheet P is separated from the sensitive drum 7 by a separating belt 17, and is transported into a fixing stage 19 by means of an endless belt conveyer 18.

The copy sheet P in the fixing stage 19 is heated by a heater 20 and by the heated air discharged from a duct 21 through a discharge port 21.sub.2 around the heater 20 so that the copy sheet P is fixed and dried. Thereafter the charge remaining on the copy sheet P is removed by a discharger 22, and the copy sheet is deposited upon a tray 26 through a discharge opening 24 and a shoot 25 by a discharge roller 23.

FIGS. 3 and 4 are the diagrams of an electric circuit of the photocopying machine shown in FIG. 2 including a safeguarding device in accordance with the present invention.

Door switches 27 and 28 connected to AC lines are closed when a door of the housing of the photocopying machine is closed in a normal manner, and when a main switch SW is closed the power is supplied to a DC circuit (See Fig. 4) and the heaters H1 and H2 in the fixing stage are energized. The heater circuit includes a thermoswitch SM1 for controlling the temperature and a fuse 29 for preventing an excessive temperature rise. A motor M2 for the developer, and fan motors FM1 and FM2 are also energized.

When the DC power source is energized, the charging of a capacitor C1 in a time-delay circuit for safeguarding the normal exposure is started. Assuming for example that the thermoswitch SM2 is set to 15.degree. C. Then when the environmental temperature is lower than 15.degree. C, the thermoswitch SM2 is opened, and the setting time T1 is dependent upon the values of and the capacitor and resistances VR1 + VR2 + R connected in series therewith. However, when the environmental temperature rises above 15.degree. C, the thermoswitch SM2 is closed so that the resistor VR1 is short-circuited. Therefore, the setting time T2 is dependent upon the resistance VR2 + R1 and the capacitance C1.

During the setting time T1 or T2, an N-gate thyristor Q1 is cut off and a diode D1 is sufficiently biased so that as soon as the switch SW is closed the capacitor C3 is immediately charged through a resistor R7, the diode D1 and a resistor R9. Since the time constant (R7 + R9) .sup.. C3 is sufficiently smaller than that of the delay circuit, transistors Q3 and Q4 connected through a resistor R11 to the capacitor C3 are made to conduct so that a relay K1 is energized and the illumination or exposure lamps 3 which are operatively coupled to the relay K1 are turned on. Furthermore, when the relay K1 is energized a transistor Q5 is cut off so that the base of a transistor Q6 is forward biased through the device RL1 and resistors R15, R16, and R17. As a result the transistor Q6 may conduct, but its conduction is also controlled by the thyristor Q2 so that only when the latter conducts after the time delay, the transistor Q6 conducts and a relay K2 is energized. The main motor M1, the high-voltage sources 8(AC) and 8(DC) and the lamp 11 for the whole-surface illumination are then energized so that the photocopying operation can commence.

FIG. 5 is a circuit diagram of another embodiment of a temperature detector in which is used a temperature-sensitive element such as thermistor Th whose resistance R(t) is substantially linearly varied in response to temperature variations so that the time required for charging a capacitor C2 in a time-delay circuit may be varied. The time delay To(T) is dependent upon the resistance R(T) of the thermistor, and the resistances of a variable resistor VR3 and a fixed resistor R1, and the capacitance C1 as in the case of the first embodiment described above.

From the foregoing description it is seen that according to the present invention a time delay to be attained by a time-delay circuit is controlled in response to detection of the environmental temperature so that the photocopying operation may be started only after the brightness of the fluorescent lamps has risen above a predetermined level. Therefore, an image of desired quality may be obtained from a first copy by utilizing and the exposure safeguarding device in accordance with the present invention, which device is simple in construction.

Claims

We claim:

1. In an electrophotographic copying machine of the type comprising an original holder, an exposure light source, a sensitive member upon which is reproduced the image of an original placed upon said original holder, an optical system for focusing said original upon said sensitive member, processing means disposed around said sensitive means for producing a copy of said original, means for driving said machine to effect copy processing, and means for controlling energization of said driving means, an improvement comprising an exposure safeguarding device having a temperature sensing element for sensing the temperature around said exposure light source, time-delay means coupled to said temperature sensing element and having an output for producing a signal after a delay time which commences with actuation of said exposure light source, and which changes in response to temperature variations sensed by said temperature sensing element, and means coupled between said time-delay means output and said driving means for controlling the energization of said driving means.

2. An exposure safeguarding device as defined in claim 1 wherein said temperature sensing element comprises switch means adapted to turn on or off at a predetermined threshold temperature.

3. An exposure safeguarding device as defined in claim 2 wherein said time-delay means comprises a time constant circuit consisting of resistor means and capacitor means and said temperature sensing element is connected in parallel with one of said resistor means.

4. An exposure safeguarding device as defined in claim 2 wherein said temperature sensing element is a thermostat.

5. An exposure safeguarding device as defined in claim 1 wherein said temperature sensing element comprises an element having a resistance which varies in response to temperature variations.

6. An exposure safeguarding device as defined in claim 5 wherein said temperature sensing element is a thermistor.

7. An exposure safeguarding device as defined in claim 5 wherein said time-delay means comprises a time constant circuit consisting of a series circuit including resistor means, capacitor means and said temperature sensing element.

8. An exposure safeguarding device as defined in claim 1 wherein said temperature sensing element comprises switch means adapted to turn on or off at a predetermined threshold temperature, said time-delay means comprises a time constant circuit including capacitor means and a plurality of resistor means and has said temperature sensing element connected in parallel with one of said resistor means, and wherein said energization control means includes an N-gate thyristor connected to said time-delay means output for actuation to control said driving means, and said thyristor having principal conducting electrodes coupled to said capacitor means to provide a discharge path therefor.