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Lecture on Capacitor knowledge(II)

Views: 0     Author: Site Editor     Publish Time: 2018-04-09      Origin: Site

Parameters and classification of capacitors. 
Capacitor is an indispensable electronic device in electronic products. It acts as smooth filter of rectifier, decoupling of power supply, bypass of AC signal, AC / DC circuit coupling and so on. Since there are many types and structures of capacitors, we need to understand not only the performance and general characteristics of all types of capacitors, but also the advantages and disadvantages of various components for a given purpose. And mechanical or environmental constraints, etc.The main parameters of the capacitor and its application will be briefly described here. 

1.Nominal capacitance (C R). Capacitor product marked capacitance value. The capacitance of mica and ceramic dielectric capacitors is lower (about below 5000pF; paper, plastic, and some ceramic dielectric forms) are intermediate (about in 0.005uF~1.0uF; usually electrolytic capacitors have a larger capacity). This is a rough classification. 

2.Category temperature range. The range of ambient temperatures that the capacitor design can determine to work continuously. The range depends on the temperature limit of its corresponding category, such as the upper temperature category, the lower limit temperature, the rated temperature (the maximum ambient temperature at which the rated voltage can be continuously applied), and so on. 

3.Rated voltage (U R). At any temperature between the lower class temperature and the rated temperature, the maximum DC voltage or the maximum AC voltage RMS or the peak pulse voltage may be continuously applied to the capacitor. When capacitors are used in high voltage fields, the effects of corona must be observed. Corona is created due to the presence of voids between the dielectric/electrode layers. It can, in addition to generating spurious signals that can damage the device, also cause breakdown of the capacitor dielectric.Under alternating or pulsating conditions, corona is particularly easy to occur. For all capacitors, the sum of the DC voltage and AC peak voltage should not exceed the rated voltage of the capacitor. 

4.Loss tangent (tg δ ). At a sinusoidal voltage of the specified frequency, the loss power of the capacitor divided by the reactive power of the capacitor is the loss tangent. In the practical application, the capacitor is not a pure capacitor, its internal equivalent resistance, its simplified equivalent circuit as shown in the drawings. For electronic devices, it is required that the smaller R S is, the better, that is, the required power loss is small, and the angle between the capacitor and the power of the capacitor is small. 

5.The temperature characteristics of the capacitor. It is usually expressed as a percentage of the capacity of the reference temperature at 20 °C and the capacity of the relevant temperature. 

6.Service life. The service life of the capacitor decreases as the temperature increases. The main reason is that the temperature accelerates the chemical reaction and degrades the medium over time. 

7.Insulation resistance. Because the temperature rise causes the electronic activity to increase, therefore the temperature rise will cause the insulation resistance to reduce. 

Capacitors include fixed capacitors and variable capacitors. The fixed capacitors can be further divided into mica capacitors, ceramic capacitors, paper/plastic film capacitors according to their dielectric materials


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