JPH05131681A - Led array device - Google Patents

Abstract

PURPOSE:To make emitted light intensity uniform in accordance with an electric current to each LED by providing an LED array obtained by series connecting of LEDs, switching elements respectively connected in parallel to the LEDs and a constant current device supplying an electric power to the LED array. CONSTITUTION:Phototransistors T1, T2,...Tn are turned ON if they make a pair with light emitting diodes E1, E2, ... En connected to output terminals of an LED driver 1 turned to L level. On the contrary, phototransistors T1, T2, ... Tn remain in OFF states if they make a pair with light emitting diodes E1, E2, ... En connected to H-level output terminals of the driver 1. In the phototransistor turned ON, an area between an emitter and a collector is clamped by a saturation voltage. However, because a forward voltage of the LED connected to the ON phototransistor in a parallel system is higher than the saturated voltage, all the currect flows to the phototransistor and does not flow to the LED. On the other hand, the current flows to the LED connected to the OFF phototransistor in a parallel system. An electric current passing through LEDs L1, L2,...Ln or the transistors T1, T2,...Tn in this manner is a constant current (is) supplied from a constant current device 2.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an LED array device used for a page printer or the like.

[0002]

2. Description of the Related Art A page printer using an LED array is disclosed in, for example, Japanese Patent Laid-Open No. 2-24164. The page printer faces the photosensitive drum with an LED array whose array direction is the drum axis length direction, forms an electrostatic latent image on the photosensitive drum by photosensitive control of each LED, develops this with toner, and transfers it to recording paper. It is a fixed thing.

FIG. 1 is a circuit diagram of a conventional LED array device. The n parallel LEDs L ₁ , L ₂ ... L _{n respectively} have current limiting resistances R ₁ , R ₂ ... R _n and a switching transistor Q.
₁ , Q ₂ ... Q _n are connected in series, and these series circuits are connected in parallel between the lines supplying the constant voltage V _CC . ON / OFF control of the transistors Q ₁ , Q ₂ ... Q _n is an LED operated by data supplied from a data source (not shown).
It is performed by being applied to the base of each transistor by the driver 1.

[0004]

[Problems to be Solved by the Invention] The current flowing through each LED,
Therefore, the emission intensity is determined by the forward conduction resistance value, the resistance values of the current limiting resistances R ₁ , R ₂ ... R _n , and the transistors Q ₁ , Q _2.
... is dominated by the conduction resistance value of Q _n . Multiple LEDs,
It is extremely difficult to make these resistance values of the resistance and the transistor the same. Also, these elements have temperature drift and change over time. For this reason, unevenness in printing occurs due to variations in light emission intensity, and printing quality is impaired.

The present invention has been made in order to solve such a problem. By connecting LEDs in series and feeding them from a constant current device, the current of each LED, that is, the emission intensity can be made uniform. It is intended to provide an LED array device.

[0006]

An LED array device according to the present invention is an LED array in which a large number of LEDs are arranged in parallel and each LE.
In an LED array device equipped with a circuit for individually controlling the conduction of D, an LED array in which LEDs are connected in series and
It comprises a switching element connected in parallel with the LED and a constant current device for supplying power to the LED array.

[0007]

[Operation] No current flows through the LED in parallel with the switching element that is turned on, but a current flows through this switching element. A current flows through the LED in parallel with the switching element that is off. The LEDs and switching elements through which these currents should flow are connected in series, and since a constant current is supplied to them from the constant current device, the current values of the LEDs through which the currents flow are all the same and do not change over time.

[0008]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below in detail with reference to the drawings showing the embodiments. The n LEDs L ₁ , L ₂ ... L _n arranged in parallel are connected in series, and each LED L ₁ , L ₂ ... L _n has a phototransistor T _{1 of a} photocoupler P ₁ , P ₂ ... P _n. ，
T ₂ ... T _n are connected in parallel. And LED L ₁ , L
Series circuit of ₂ ... L _n, that is the photo transistor T _1, T
_A direct current constant current device 2 is connected in the forward direction to the series circuit of ₂ ... T _n .

On the other hand, the anode side of the light emitting diodes E ₁ , E ₂ ... E _n of the photocouplers P ₁ , P ₂ ... P _n is connected to a constant voltage source of V _CC via resistors r ₁ , r ₂ ... R _n. Was
The cathode side is separately connected to n output terminals of the LED driver 1. Data to be printed is supplied from the CPU 3 to the LED driver 1.

In the device of the present invention as described above, the LED driver 1
Emitting diodes E ₁ output terminal becomes "L" level, E ₂ ... photo transistors T ₁ forming the E _n pair,
T ₂ ... T _n is turned on, the output terminal of the LED driver 1 in the opposite
"H" level of the light emitting diodes E _1, E ₂ ... phototransistor T _1, T ₂ ... T _n forming the E _n pair is up off. The phototransistor that is turned on is clamped to the saturation voltage between the emitter and collector, but since the forward voltage of the LED connected in parallel is higher than that, all the current flows to the phototransistor and not to the LED.

On the other hand, a current flows through the LED connected in parallel to the phototransistor which is off. LED L like this
₁ , L ₂ ... L _n or phototransistors T ₁ , T ₂ ... T
_The current flowing through _n is the constant current i _s supplied from the constant current device 2. Therefore, all the currents flowing through the emitting LEDs are driven by i _s . Since the constant current device 2 supplies the current i _s regardless of the temperature drift of the load impedance, each LED is always driven by i _s .

Although the phototransistor of the photocoupler is used as the switching element connected in parallel with the LED in the above-described embodiment, the present invention is not limited to this, and other appropriate switching elements may be used.

[0013]

As described above, in the case of the present invention, each LED is
Since they are all driven with the same current, and there is no variation in emission intensity or change with time, the print quality is improved when the device of the present invention is applied to a page printer.

[Brief description of drawings]

FIG. 1 is a circuit diagram of a conventional LED array device.

FIG. 2 is a circuit diagram of an LED array device of the present invention.

[Explanation of Codes] 1 LED driver 2 Constant current device 3 CPU L ₁ , L ₂ ... L _n LED T ₁ , T ₂ ... T _n phototransistor

Front Page Continuation (72) Inventor Koji Riko, 3-13 Azuchi-cho, Chuo-ku, Osaka-shi, Osaka Osaka International Building Minolta Camera Co., Ltd.

Claims (1)

[Claims] 1. In an LED array device comprising an LED array having a large number of LEDs arranged in parallel and a circuit for individually controlling the conduction of each LED, an LED array in which each LED is connected in series and each LED are connected in parallel. An LED array device comprising: a connected switching element; and a constant current device for supplying power to the LED array.