Main Parameters and Basic Characteristics of Photoresistors
Main Parameters and Basic Characteristics of Photoresistors
(1) Dark Resistance, Bright Resistance, and Photocurrent of Photoresistors Dark Current: The resistance value of a photoresistor measured after a certain period of time in complete darkness (no light irradiation) at room temperature is called dark resistance. The current flowing through it under a given voltage at this time is called the light resistance. Bright Current: The resistance value of a photoresistor under a certain light irradiation is called bright resistance under that light irradiation. The current flowing through it at this time is called the light resistance. Photocurrent: The difference between bright current and dark current. The larger the dark resistance and the smaller the bright resistance of a photoresistor, the better its performance. That is, the smaller the dark current and the larger the photocurrent, the higher the sensitivity of such a photoresistor. The dark resistance of practical photoresistors often exceeds 1MΩ, even reaching 100MΩ, while the bright resistance is below a few kΩ. The ratio of dark resistance to bright resistance is between 10² and 10⁶, indicating that the photoresistor has very high sensitivity.
(2) Illumination Characteristics of Photoresistors The following figure shows the illumination characteristics of a CdS photoresistor. The relationship between photocurrent and luminous flux of a photoresistor under a certain applied voltage. Different types of photoresistors have different illumination characteristics, but the illumination characteristic curves are all nonlinear. Therefore, it is not suitable as a quantitative detection element, which is a drawback of photoresistors. They are generally used as photoelectric switches in automatic control systems.
(3) Spectral characteristics of photoresistors Spectral characteristics are related to the material of the photoresistor. As shown in the figure, lead sulfide photoresistors have high sensitivity over a wide spectral range, with peak values in the infrared region; cadmium sulfide and cadmium selenide have peak values in the visible light region. Therefore, when selecting a photoresistor, the material of the photoresistor and the type of light source should be considered together to obtain satisfactory results.
(4) I-V characteristics of photoresistors The relationship between the voltage and current applied across a photoresistor under a certain illuminance is called the volt-ampere characteristic. Curves 1 and 2 in the figure represent the volt-ampere characteristics when the illuminance is zero and when the illuminance is a certain value, respectively. As shown by the curves, under a given bias voltage, the larger the illuminance, the larger the photocurrent. Under a certain illuminance, the higher the applied voltage, the greater the photocurrent, and there is no saturation phenomenon. However, the voltage cannot be increased indefinitely because any photoresistor is limited by its rated power, maximum operating voltage, and rated current. Exceeding the maximum operating voltage and maximum rated current may cause permanent damage to the photoresistor.
(5) Frequency Characteristics of Photoresistors When a photoresistor is irradiated by pulsed light, the photocurrent takes a period of time to reach a stable value, and after the light stops, the photocurrent does not immediately become zero. This is the time delay characteristic of the photoresistor. Due to the different time delay characteristics of different photoresistors, their frequency characteristics are also different, as shown in the figure. Lead sulfide is used much more frequently than cadmium sulfide, but most photoresistors have a relatively large time delay, so it cannot be used in applications requiring fast response.
(6) Stability of Photoresistors Curves 1 and 2 in the figure represent the stability of two types of CdS photoresistors, respectively. Newly manufactured photoresistors are not stable in performance because the internal mechanism is unstable and the interaction between the resistive element and the dielectric has not yet reached equilibrium. However, under artificial heating, light exposure, and load conditions, the performance can stabilize after one to two weeks of aging. During the initial aging process, the resistance of some photoresistors increases while that of others decreases, but eventually reaches a stable value and remains unchanged. This is the main advantage of photoresistors. The lifespan of a photoresistor is almost infinite under proper sealing and use.
(7) Temperature Characteristics of Photoresistors Their performance (sensitivity, dark resistance) is greatly affected by temperature. As the temperature increases, their dark resistance and sensitivity decrease, and the peak of the spectral characteristic curve shifts towards shorter wavelengths. The relationship between the photocurrent I and temperature T of cadmium sulfide is shown in the figure. Sometimes, to improve sensitivity or to receive radiation in longer wavelength bands, the element is cooled. For example, a cooler can be used to lower the temperature of the photoresistor.
