JPS61261921A - Light emitting element driving circuit - Google Patents

Abstract

PURPOSE:To make a light emitting output of a light emitting element constant by impressing a voltage to a base of an output transistor through between a base and an emitter of a transistor having a temperature characteristic being reverse to a characteristic of the light emitting output of the light emitting element and an ambient temperature. CONSTITUTION:Transistors Q1, Q2 are connected to a transistor Q. These transistors Q1, Q2 have a characteristic being reverse to a characteristic of a light emitting output of a light emitting element D. A voltage is impressed to a base of the transistor through between the base and the emitter of these transistors Q1, Q2. In this way, the temperature characteristic is offset against a temperature variation consequently, and the light emitting output of the light emitting element becomes constant.

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field] The present invention relates to a light emitting element drive circuit used, for example, in a photoelectric switch.

[Background Art 1] FIG. 2 shows a conventional light emitting element drive circuit, which is composed of a series circuit of a light emitting element made of, for example, a light emitting diode, an output transistor Q, and a resistor R. Here, the voltage applied to the base of transistor Q is Vi,) the base-emitter dark voltage of Rannosph Q is VBI! %) Rannosph Q emitter potential V
If it is an LED, the driving current of the light emitting element is I LFiD, and the current amplification factor of the transistor Q is hfe, then VLpo=Vi VB□ I tEn=V+-Eo/[Ri(1/hfe)+11
]. Here, if hfe>>1, I LED=
It can be approximated as V LED/R ==(V i VBE)/R.

Here, the relative light emission output PO of the light emitting element and the ambient temperature Ta
The relationship between (forward current I p =const) is shown in FIG. 3, where the horizontal axis is linear zero and the vertical axis is logarithmic zero. As is clear from FIG. 2, it can be seen that the relative light emission output PO changes greatly due to temperature changes. For example, in a certain light emitting diode, when the ambient temperature Ta is 25°C, P
If o=100%, when Ta=25℃, Po=
160%, and when Ta=60°C, Po=71%. The temperature characteristics when this light emitting diode is used will be described below as an example.

Also, the base-emitter voltage VBH of transistor Q
Assuming the temperature coefficient T, C (Ib V / "C), ■ When Ta = 25℃ (standard) Itpo = (Vi VBE) / R ■ When Ta = -25℃ ILEo = (Vi (VBE T , CX50) l/
R■ When Ta=60℃ ILro=iVi (VBp+T, CX35))/R
The general formula when Ta=T is IcEn=[Vi IVsE T, CX(25T)l
/R.

Here, Vi=3V VB! =0.65V R=10Ω T , C= 2 Ill V / '(:: ■ When Ta = 25℃ ■ L2D = 235 mA ■ When Ta = -25℃ r LPD = 225 mA ■ Ta = 60 ℃, ILED=242A.The temperature drift F of the resistor R is so small that it can be ignored compared to the temperature drift of the resistor R.

Now, let us assume that the relative IP at Ta"25 is 100%.

Furthermore, since the relationship between I and Po can be regarded as linear between the above-mentioned ambient temperature Ta and relative light emission, the relationship between Ta and Po considering the temperature characteristics of I is as shown in the table below.

Considering the case of a photoelectric switch, the relationship between the relative light emission output Po and the detection output α is p6ocQ'', so as a result,
This means that the distance set when Ta = 25°C increases by 25% when Ta = -25°C, and the string separation becomes 15% shorter when Ta = 60°C. If the detection distance changes significantly with respect to temperature as described above, it may cause malfunction and become unusable as a photoelectric switch.

[Object of the Invention] The present invention has been provided in view of the above-mentioned point, and is capable of compensating for changes in the light emitting output of a light emitting element due to temperature and supplying constant light emitting output and power in response to temperature changes. What do you want to do? ! 11-? It provides a -0 to half V life route.

[Disclosure of invention] Embodiments of the present invention will be described below with reference to the drawings.

Figure 1 shows a specific circuit diagram, and shows the transistor Q.
A voltage Vi is applied to the bases of transistors Q, , Q2 between their bases and emitters. Although three transistors are used in the embodiment, any number of transistors may be used. The number may be determined depending on the driving current, the characteristics of the light emitting element, which is a light emitting diode, and the like. Note that a transistor is used that has characteristics opposite to those of the light emitting element in terms of light emission output and ambient temperature. In FIG. 1, consideration similar to the conventional example is attempted.

Vshito=Vi-Vo-VBE2 I Lpn=V LED/R=(Vi-VayuIVBE□)/R Now, the temperature coefficient of the base-emitter voltage VBEllVBE2 of transistors Q, Q, respectively T, C1
, T, and C2, the general formula of I LED when Ta=T can be expressed as follows.

rcEo”4Vi (Vst+T, CIX(25
-T)1-(VaE□-T, C2X(25-T)+1
/RHere, Vi=3V ■B=0.65V VBE□=1.5V T, C1=-2mV/'C T, C2=-4mV/'C R=3,6Ω Then, the dog's It becomes like this.

■ When Ta=25°C I LED = 236 mA ■ When Ta=25℃ r　LE111=　153　mA ■ When Ta=60℃ I Lpo=29411+A Here, considering the relationship between Ta and Po, it becomes.

Considering the case of a photoelectric switch, the table below shows a comparison with the conventional example, which shows a significant improvement in the change in detection distance with respect to temperature changes. In other words, even if the temperature changes, the detection distance changes. In the above table, the middle row shows the conventional example, and the lower row shows the example.

In addition, in the past, large increases and decreases in the light emitting power (output) due to temperature changes caused saturation of the light receiving circuit in the photoelectric switch, or malfunctions due to insufficient amount of light received. The increase and decrease of , becomes low, and saturation of the light receiving circuit in the photoelectric switch and malfunction due to insufficient amount of light received can be prevented. Therefore,
This reduces changes in detection distance due to temperature changes and enables stable operation.

[Effect of the Invention 1] As described above, the present invention provides a light emitting device in which a light emitting element such as a light emitting diode and an output transistor are connected in series, and a voltage is applied to the base of the output transistor to cause the light emitting element to emit light. In the element drive circuit, at least one transistor has temperature characteristics opposite to the characteristics of the light emission output and ambient temperature of the light emitting element, and applies a voltage to the base of the output transistor via the base input and emitter. As described above, by applying a voltage to the base of the output transistor through the base-emitter of the transistor, which has temperature characteristics opposite to the characteristics of the light emitting output of the light emitting element and the ambient temperature, temperature changes can be suppressed. On the other hand, since the temperature characteristics are in phase J5-, it becomes possible to supply a constant light emitting output from the light emitting element. Therefore, for example, if the light emitting element of the present invention is used in a photoelectric switch, the light emitting element will change due to temperature changes. By reducing the change in the detection distance due to the reduction in the change in the light emission output of the sensor, a stable detection distance can be set against temperature changes, and malfunctions can be prevented.

[Brief explanation of drawings]

FIG. 1 is a specific circuit diagram of an embodiment of the present invention, FIG. 2 is a specific circuit diagram of a conventional example, and FIG. 3 is a characteristic diagram showing the relationship between ambient temperature and relative light output. D represents a light emitting element, Q represents an output transistor, and QIIQ2 represents a transistor.

Claims (1)

[Claims] (1) In a light emitting element drive circuit in which a light emitting element such as a light emitting diode and an output transistor are connected in series and a voltage is applied to the base of the output transistor to cause the light emitting element to emit light, A light emitting element drive circuit comprising at least one transistor having a temperature characteristic opposite to that of an output and an ambient temperature, and applying a voltage to the base of an output transistor via its base and emitter.