[Repost] The working principle and application of humidity sensitive resistor
[Copy link]
Humidity resistors are made by using the principle that humidity-sensitive materials absorb moisture in the air and cause their own resistance value to change. The popular humidity resistors in industry are mainly lithium chloride humidity resistors and organic polymer film humidity resistors. The popular humidity resistors in industry are mainly: 1. Semiconductor ceramic humidity sensor; 2. Lithium chloride humidity resistor; 3. Organic polymer film humidity resistor. The characteristic of humidity sensitive resistor is that a film made of humidity sensitive material is covered on the substrate. When water vapor in the air is adsorbed on the humidity sensitive film, the resistivity and resistance value of the component change. This characteristic can be used to measure humidity. Humidity sensitive capacitors are generally made of polymer film capacitors. Commonly used polymer materials include polystyrene, polyimide, butyric acid acetate fiber, etc. When the ambient humidity changes, the dielectric constant of the humidity sensitive capacitor changes, causing its capacitance to change as well. The change in capacitance is proportional to the relative humidity. The working principle of humidity sensitive resistor: The humidity sensitive resistor sensor is mainly composed of a humidity sensitive layer 3, an electrode 4 and an insulating substrate 2 with a certain mechanical strength, as shown in Figure 1. After the humidity sensitive layer absorbs moisture in the environment, it causes a change in the resistance value between the two electrodes, thus converting the change in relative humidity into a change in resistance value. Schematic diagram of humidity sensitive resistor structure Humidity sensitive resistor can only use AC. DC will cause humidity sensitive failure because the DC electric field will cause the charged particles in the polymer material to deviate to the two poles. After a certain period of time, the humidity sensitive resistor will fail. Therefore, AC must be used to maintain its dynamic balance. This is why a bridge is used to measure the resistance of a humidity sensitive resistor instead of an ordinary multimeter. Water molecules are polar molecules. They will decompose into H2 and O2 in a DC electric field, which will affect the measurement. In addition, there are conductive ions in the humidity sensor. In high humidity conditions, if DC is used, it will drift, causing conductivity drift and affecting the service life of the sensor. RC charging and discharging can be used for timing. Some chips have RFC functions, which can also be used. Alternatively, IC can be used to directly generate a 1KHZ square wave, which is superimposed on the sampling resistor and the humidity sensor. Through AD, the voltage division can be measured in the positive cycle. Alternatively, an analog circuit can be used to perform R/V conversion through logarithmic operations, and then AD sampling can be performed. Application of humidity sensitive resistor: Humidity sensitive resistor is widely used in household appliances such as washing machines, air conditioners, video recorders, microwave ovens, as well as in industry and agriculture for humidity detection and humidity control. As shown in the figure, this is the schematic diagram of the humidity detector circuit. Schematic diagram of humidity detector circuit In the circuit shown in Figure 2, a multivibrator is composed of a 555 time base circuit, a humidity sensor CH, etc. A capacitor C2 is connected to the output end of the oscillator, which converts the square wave signal output by the multivibrator into a triangular wave. When the relative humidity changes, the capacitance of the humidity sensor CH will change accordingly, which will cause the frequency and amplitude of the triangular wave output by the multivibrator to change accordingly. After the output signal is rectified by VD1 and VD2 and filtered by C4, the index corresponding to the relative humidity can be directly read from the voltmeter. The Rp potentiometer is used to adjust the zero of the instrument. Lithium chloride humidity resistor The multi-chip resistor combined lithium chloride humidity sensor is a sensor that uses the electrical characteristics of the humidity-sensitive element (such as resistance value) to measure humidity. The humidity-sensitive element is generally made by impregnating a hygroscopic substance on an insulating material, or by evaporation, coating and other processes to make a layer of metal, semiconductor, polymer film and powder particles. During the moisture absorption and dehumidification process of the humidity-sensitive element, the conduction state of the ion H+ decomposed from the water molecules changes, so that the resistance value of the element changes with the humidity. Lithium chloride humidity sensor has many important advantages, such as stability, temperature resistance and long service life. Lithium chloride humidity sensor has a history of more than 50 years of production and research, with a variety of product types and manufacturing methods, all of which apply the various advantages of lithium chloride humidity sensor, especially the strongest stability. Lithium chloride humidity sensor belongs to electrolyte humidity sensing material. Among many humidity sensing materials, it was first noticed and used in the manufacture of humidity sensing devices. Lithium chloride electrolyte humidity sensor is based on the principle that the equivalent conductivity decreases with the increase of solution concentration. The electrolyte dissolves in water to reduce the water vapor pressure on the water surface to achieve humidity sensing. The structure of lithium chloride humidity sensing substrate is to select the substrate of insulating material, make a pair of metal electrodes on the top, and coat a layer of electrolyte solution humidity sensing film. Lithium chloride is a typical ionic crystal and belongs to non-affinity electrolyte. In lithium chloride solution, Li+ has a strong attraction to polar water molecules and the ion moisture content is the highest. The lithium chloride humidity sensing film is made of lithium chloride and polyvinyl alcohol. Its main characteristics are as follows: (1) It can work stably in a high temperature environment of 120 degrees, which is unmatched by other polymer capacitors and humidity sensors; 51)](2) The linear humidity measurement range of lithium chloride humidity sensor is relatively narrow, about 20%RH. Within this measurement range, its linear error is less than 2%RH. Therefore, in order to achieve high-precision humidity measurement in a full-range humidity measurement environment, the currently commonly used single-chip humidity sensor measurement method is difficult to achieve. Organic polymer film humidity resistor The characteristic of humidity resistor is that a film made of humidity-sensitive material is covered on the substrate. When water vapor in the air is adsorbed on the humidity-sensitive film, the resistivity and resistance value of the component change. This characteristic can be used to measure humidity. Humidity-sensitive capacitors are generally made of polymer film capacitors. Commonly used polymer materials include polystyrene, polyimide, butyric acid acetate fiber, etc. When the ambient humidity changes, the dielectric constant of the humidity-sensitive capacitor changes, causing its capacitance to change as well. The change in capacitance is proportional to the relative humidity. The accuracy of electronic humidity sensors can reach 2-3%RH, which is higher than the accuracy of dry-bulb and wet-bulb humidity measurement. The linearity and pollution resistance of humidity-sensitive elements are poor. When detecting ambient humidity, the humidity-sensitive elements must be exposed to the environment to be measured for a long time, and are easily contaminated, affecting their measurement accuracy and long-term stability. This is not as good as the dry-bulb and wet-bulb method.
| 
提升卡
变色卡
千斤顶
京公网安备 11010802033920号
