Some analog sensors constructed with infrared light sources require high stability of the light source. The common arsenic-increasing infrared light-emitting diodes vary in luminous intensity with changes in ambient temperature, working time, etc., and it is difficult to obtain a light source with stable light intensity. Therefore, a constant light source circuit must be used.
First, the basic principle of constant light source circuit
Figure 1 shows the light source circuit powered by the precision regulated power supply TL431. The stable voltage at the z-point is:
Where Vref is the reference voltage of TL431, which is approximately 2.5V. The operating current I of the infrared light emitting diode V2 is
I=(V2-VD)/R3(2)
Where VD is the forward voltage drop of the infrared light-emitting diode, which is a parameter that varies with temperature. It can be seen from the formula (2) that the change in temperature inevitably causes a change in I to cause a change in the luminous intensity of the light-emitting diode. According to formulas (1) and (2), it can be seen that if the resistance of R1 can be controlled by the intensity of light Iv of V2, and R1 and I2 are approximately inversely proportional, when the light intensity Iv is weakened for some reason, The following changes process:
As a result, the light intensity is stabilized at the original value. vice versa.
Accordingly, a block diagram of the constant light source circuit shown in FIG. 2 is assumed. In the figure, E/O is the light-emitting diode V1 in FIG. The photoelectric conversion circuit O/E intercepts a part of the light of the light source and converts it into a voltage signal. After voltage amplification, voltage and current conversion circuit VI/outputs a current to control the resistance of the variable resistor R1, the R1 is proportional to the light source intensity. Thereby, a closed loop system of optical feedback is formed, which outputs infrared light whose intensity is almost constant.
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