Editor's note: A voltage is applied between the metal electrodes at both ends of the photoresistor, and a current is passed therethrough. When irradiated with light of a suitable wavelength, the current becomes larger as the light intensity increases, thereby realizing photoelectric conversion. The photoresistor has no polarity and is purely a resistive device. It can be used with either a DC voltage or an AC voltage. The photoresistor is a photovoltaic element fabricated from a semiconductor material that operates using an internal photoelectric effect. Its resistance is often reduced under the action of light. This phenomenon is called light guiding effect. Therefore, the photoresistor is also called light pipe. The materials used to make the photoresistor are mainly semiconductors such as sulfides, selenides and tellurides. Generally, a thin photoresistor and a comb-shaped ohmic electrode are formed on the insulating substrate by coating, spraying, sintering, etc., and then the lead is taken out and packaged in a sealed case having a light-transmitting mirror to protect it from moisture. The principle structure of the photoresistor is shown in the figure. In the dark environment, its resistance value is very high. When the light is received, as long as the photon energy is greater than the forbidden band width of the semiconductor material, the electrons in the valence band absorb the energy of a photon and can transition to the conduction band and in the valence band. A positively charged hole is generated, and the electron-hole pair generated by the illumination increases the number of carriers in the semiconductor material, making the resistivity small, thereby causing the photoresistor resistance to decrease. The stronger the light, the lower the resistance. After the incident light disappears, the electron-hole pairs generated by the photon excitation will gradually recombine, and the resistance of the photoresistor will gradually return to the original value. A voltage is applied between the metal electrodes at both ends of the photoresistor, and a current is passed therethrough. When irradiated with light of a suitable wavelength, the current becomes larger as the light intensity increases, thereby realizing photoelectric conversion. The photoresistor has no polarity and is purely a resistive device. It can be used with either a DC voltage or an AC voltage. Basic characteristics and its main parameters 1, dark resistance, bright resistance Photosensitive resistance measured at room temperature and full dark conditions is called dark resistance, or dark resistance. The current flowing at this time is called a dark current. For example, the MG41-21 type photoresistor has a dark resistance of 0.1M or more. The value of the stabilizing resistance of the photoresistor measured at room temperature and under certain light conditions is called bright resistance or bright resistance. The current flowing at this time is called a bright current. The MG41-21 type photoresistor has a light resistance of 1k or less. The difference between bright current and dark current is called photocurrent. Obviously, the larger the dark resistance of the photoresistor, the better, and the smaller the bright resistance, the better, that is, the dark current is small, and the bright current is large, so the sensitivity of the photoresistor is high. 2. Volt-ampere characteristics Under a certain illumination, the relationship between the voltage applied across the photoresistor and the current flowing through the photoresistor is called the volt-ampere characteristic. As can be seen from Figure 2.6.2, the volt-ampere characteristics of the photoresistor are approximately straight and there is no saturation. Limited by the power dissipation, the voltage across the photoresistor should not exceed the maximum operating voltage during use. The dotted line in the figure is the allowable power consumption curve, which determines the normal operating voltage of the photoresistor. ![]()
Figure 2.6.2 Volt-ampere characteristics of the photoresistor Figure 2.6.3 Photoelectric characteristics of the photoresistor Figure 2.6.4 Spectral characteristics of the photoresistor 3. Photoelectric characteristics The relationship between the photocurrent and the illuminance of the photoresistor is called the photoelectric property. As shown in Figure 2.6.3, the photoelectric characteristics of the photoresistor are nonlinear. Therefore, it is not suitable for the detecting element, which is one of the shortcomings of the photoresistor. In the automatic control, it is commonly used as a switching photoelectric sensor. 4. Spectral characteristics The relative sensitivities of the photoresistors are different for incident light of different wavelengths. The spectral properties of the various materials are shown in Figure 2.6.4. It can be seen from the figure that the peak of cadmium sulfide is in the visible region, and the peak of lead sulfide is in the infrared region. Therefore, when the photoresistor is selected, the combination of the component and the source should be considered in order to obtain satisfactory results. 5. Frequency characteristics When the photoresistor is subjected to pulsed illumination, the photocurrent will reach a steady state value after a period of time. When the light suddenly disappears, the photocurrent is not immediately zero. This shows that the photoresistor has a time-delay characteristic. Due to the different delay characteristics of the photoresistors of different materials, their frequency characteristics are also different. Figure 2.6.5 shows the relationship between the relative sensitivity Kr and the change frequency f of the light intensity. It can be seen that the use frequency of lead sulfide is much higher than that of barium sulfide. However, most photoresistors have large delays and therefore cannot be used in applications requiring fast response, which is a defect of the photoresistor. ![]()
Figure 2.6.5 Frequency characteristics of the photoresistor Figure 2.6.6 Spectral temperature characteristics of lead sulfide 6. Temperature characteristics Photosensitive resistors, like other semiconductor devices, are greatly affected by temperature, and their dark resistance decreases as the temperature increases. The change in temperature also has a large effect on the spectral characteristics. Figure 2.6.6 shows the spectral temperature characteristics of lead sulfide photoresistors. As can be seen from the figure, its peak shifts toward a shorter wavelength as the temperature rises. Therefore, it is sometimes necessary to take measures to reduce the temperature in order to increase the sensitivity or to receive far-infrared light. A commonly used photoresistor is a cadmium sulfide photoresistor made of a semiconductor material. The resistance of the photoresistor varies with the intensity of the incident light (visible light). Under dark conditions, its resistance (dark resistance) can reach 1~10MΩ; under strong light conditions (100LX), its resistance (Light resistance) is only a few hundred to several thousand ohms. The sensitivity of the photoresistor to light (ie, spectral characteristics) is very close to the response of the human eye to visible light (0.4~0.76) μm. As long as the light that can be felt by the human eye, it will cause its resistance change. Therefore, when designing a light control circuit, an incandescent light bulb (small bead) light or natural light is used as a control light source, which greatly simplifies the design. The change of the corresponding resistance of the photoresistor with the intensity of the incident light is not linear, and it cannot be used as a linear transformation of the photoelectric. This is the place that the user should pay attention to. Beginners can purchase a photoresistor (MG45 type), point a 60~100W incandescent lamp at night, and directly measure the resistance of the photoresistor with a multimeter. When measuring, the light should be placed against the incandescent lamp, and then the distance from the lamp (from near to far) should be gradually opened. Observe the resistance change indicated by the multimeter, and the special characteristic of the photoresistor can be visually verified to deepen it. Sensual understanding. Commonly used photoresistor models are sealed MG41, MG42, MG43 and unsealed MG45 (cheap price). Their rated power is below 200mW.
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