Main parameters and basic characteristics of the photoresistor

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(1) Dark resistance, bright resistance, photocurrent, dark current of the photoresistor: The resistance value of the photoresistor measured at room temperature under full darkness (no light irradiation) after a certain period of time is called dark resistance. The current flowing at a given voltage at this time.
Bright current: The resistance of a photoresistor under a certain illumination is called the bright resistance under this illumination. The current flowing at this time.
Photocurrent: The difference between bright current and dark current.
The dark resistance of the photoresistor is larger, and the smaller the bright resistance is, the better the performance is. That is to say, the smaller the dark current, the larger the photocurrent, and the higher the sensitivity of such a photoresistor.
The practical photoresistor's dark resistance often exceeds 1MΩ, even up to 100MΩ, while the bright resistance is below a few kΩ. The ratio of dark resistance to bright resistance is between 102 and 106. The sensitivity of visible light sensitive resistor is very high.
(2) Illumination characteristics of the photoresistor The graph below shows the illumination characteristics of the CdS photoresistor. The relationship between the photocurrent of the photoresistor and the luminous flux at a given applied voltage. Different types of photoresistors have different illumination characteristics, but the illumination characteristics are nonlinear. Therefore, it is not suitable for quantitative detection components, which is a deficiency of the photoresistor. It is generally used as a photoelectric switch in an automatic control system.
(3) Spectral characteristics of the photoresistor The spectral characteristics are related to the material of the photoresistor. As can be seen from the figure, the lead sulfide photoresistor has high sensitivity in a wide spectral range, and the peak value is in the infrared region; the peak of cadmium sulfide and cadmium selenide is in the visible light region. Therefore, when a photoresistor is selected, the material of the photoresistor and the type of the light source should be considered together to obtain a satisfactory effect.
(4) Volt-ampere characteristics of the photoresistor Under certain illumination, the relationship between the voltage applied to the photoresistor and the current is called the volt-ampere characteristic. Curves 1 and 2 in the figure indicate volt-ampere characteristics when the illuminance is zero and the illuminance is a certain value. It can be seen from the curve that under a given bias voltage, the illuminance is large and the photocurrent is also larger. Under a certain illuminance, the larger the applied voltage, the larger the photocurrent and the absence of saturation. However, the voltage cannot be increased indefinitely because any photoresistor is limited by the rated power, the maximum operating voltage and the rated current. Exceeding the maximum operating voltage and the maximum rated current may cause permanent damage to the photoresistor.

(5) Frequency characteristics of the photoresistor When the photoresistor is irradiated by pulsed light, the photocurrent will reach a stable value after a period of time, and after stopping the illumination, the photocurrent is not immediately zero, which is the time delay characteristic of the photoresistor. . Due to the light sensitivity of different materials,
The resistance delay characteristics are different, so their frequency characteristics are also different, as shown in the figure. Lead sulfide is used at a much higher frequency than cadmium sulfide, but most photoresistors have a relatively large time delay, so it cannot be used where fast response is required.

(6) Stability of Photosensitive Resistor Curves 1 and 2 show the stability of two types of CdS photoresistors, respectively. The initially fabricated photoresistor is not stable enough due to the unstable operation of the internal mechanism and the balance of the resistor and its medium. However, under artificial heating, light and load, the performance can be stabilized after one to two weeks of aging. During the aging process of the photoresistor, some samples have increased resistance, and some samples have decreased resistance, but they will not change after reaching a stable value. This is the main advantage of photoresistors.
The life of the photoresistor is almost infinitely long when it is sealed and used properly.

(7) Temperature characteristics of the photoresistor The performance (sensitivity, dark resistance) is greatly affected by the temperature. As the temperature increases, the dark resistance and sensitivity decrease, and the peak of the spectral characteristic curve shifts in the direction of short wavelength. The relationship between photocurrent I and temperature T of cadmium sulfide is shown in the figure. Sometimes the components are cooled down in order to increase sensitivity or to be able to receive longer wavelengths of radiation. For example, a refrigerator can be utilized to lower the temperature of the photoresistor.