What Are the Characteristics of Light Dependent Resistance?

What Are the Characteristics of Light Dependent Resistance?

Light dependent resistance is a kind of resistor made by the photoconductive effect of semiconductor, whose resistance value changes with the intensity of incident light. It is also called a photoconductive detector. When the incident light is strong, the resistance decreases. When the incident light is weak, the resistance increases. There is another kind of weak incident light, the resistance is reduced, and the incident light is strong, the resistance increases.


Ⅰ. Dark resistance, light resistance and photocurrent of light dependent resistance


The resistance of the light dependent resistance when it is not exposed to light is called the dark resistance (MΩ level), and the current flowing at this time is called the dark current. The resistance when irradiated with light is called bright resistance (a few KΩ or less), and the current at this time is called bright current. The difference between the bright current and the dark current is called the photocurrent.


Generally, the larger the dark resistance, the better, the smaller the bright resistance, the higher the sensitivity of the light dependent resistance. The resistance value of the dark resistance of the light dependent resistance is generally in the order of megohms, and the light resistance is below several thousand ohms. The ratio of dark resistance to light resistance is generally 100 to 1000000.


Ⅱ. The volt-ampere characteristics of the light dependent resistance


The relationship curve between the voltage applied at both ends of the light dependent resistance and the current passing through it is called the volt-ampere characteristic of the light dependent resistance.


When the light is constant, the resistance of the light dependent resistance has nothing to do with the applied voltage. The higher the applied voltage, the greater the photocurrent (linear), and there is no saturation. Under a given voltage, the value of the photocurrent will increase as the illumination increases.


Ⅲ. The light characteristics of the light dependent resistance


The illumination characteristic of the light dependent resistance is used to describe the relationship between the photocurrent I and the light intensity, and most of the light dependent resistance's illumination characteristic curve is non-linear. The illumination characteristics of different photoresistors are different. Generally, the stronger the illumination, the greater the photocurrent. Due to the non-linearity of the illumination characteristics of the light-dependent resistor, it is not suitable for linear measurement elements, and is generally used as a switch-type photoelectric converter.


Ⅳ. The spectral characteristics of the light dependent resistance


For incident light of different wavelengths, the sensitivity of the light-dependent resistor is different. The peak of cadmium sulfide is in the visible region, while the peak of lead sulfide is in the infrared region. Therefore, when selecting photoresistors, the types of components and light sources should be considered in order to obtain satisfactory results.


Ⅴ. The response time and frequency characteristics of the light-dependent resistor


The photocurrent of the light-dependent resistor cannot be changed immediately with the change of the amount of light, that is, the photocurrent generated by the light-dependent resistor has a certain inertia, which is usually described by the time constant t. The so-called time constant is the time required for the light-dependent resistor to decrease to 63% of the current since the light is stopped. Therefore, the smaller the time constant, the faster the response. But the time constant of most photoresistors is relatively large, which is one of its shortcomings. 


The use frequency range of lead sulfide is the largest, and the others are inferior. Currently, process improvements are being used to improve the frequency characteristics of photoresistors of various materials.


Ⅵ. The temperature characteristics of the light dependent resistance


As the temperature continues to rise, the dark resistance and sensitivity of the light-dependent resistor will decrease, and temperature changes also affect its spectral characteristic curve. The peak temperature of the spectrum of lead sulfide moves to a shorter wavelength as the temperature rises. Therefore, some cooling measures are sometimes taken to improve the sensitivity of the component or to be able to accept longer wavelength infrared radiation.

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