Basic Concepts of Capacior
Capacitor is the charge storage in a given potential difference, recorded as C, the international unit is farad (F). Generally speaking, the charge in the electric field will be forced to move, when the conductor between the medium, the charge is hindered to move and make the charge accumulated in the conductor, resulting in the accumulation of charge storage, the amount of stored charge is called capacitor.
The formula for capacitance is: C=εS/4πkd . Where ε is a constant, S is the square area of the capacitor plate, d is the distance of the capacitor plate, and k is the electrostatic force constant. A common parallel plate capacitor has a capacitance of C = εS/d (ε is the dielectric constant of the dielectric between the plates, S is the area of the plates, and d is the distance between the plates).
Capacitive elements have the effect of isolating DC.
Capacitance
The symbol of capacitance is C. In the International System of Units, the unit of capacitance is farad, referred to as law, the symbol is F. Since the unit of farad is too large, the commonly used units of capacitance are millifarad (mF), microfarad (μF), nano-farad (nF) and pi-farad (pF), etc. The conversion relationship is as follows:
1 farad (F) = 1000 millifarads (mF) = 1000000 microfarads (μF)
1 microfarads (μF) = 1000 nanofarads (nF) = 1000000 picofarads (pF)
Parameters of capacitor
Nominal capacitance and error
Capacitance that is, the capacitor plus the charge after the ability to store the size of the charge. Capacitance error refers to the deviation between its actual capacity and the nominal capacity, usually ± 10%, ± 20%, used in RF circuits in PI matching capacitance ± 0.5%, ± 0.75% of the small error capacitance.
Rated voltage
Rated voltage is the capacitor in the circuit can work reliably for a long time without being breakdown can withstand the maximum DC voltage (also known as withstand voltage). It is related to the structure of the capacitor, the dielectric material and the thickness of the dielectric, in general, for the structure, the same medium, equal capacity capacitor, the higher the withstand voltage value, the larger the volume.
When the capacitor is applied between the two plates after the voltage, the electrolyte between the plates will be in the electric field, originally neutral dielectric, due to the role of the external electric field force, the positive and negative charges within the dielectric molecules will be in the spatial position of a small offset (such as negative charges against the direction of the electric field to move), the formation of the so-called electric dipole, that is, within the medium appeared in the electric field, destroying the original electro-neutral state. This phenomenon is called electrolyte polarization. It can be seen, the polarized state of the medium is negatively charged, but these charges are still bound by the medium itself and can not move freely, the insulating properties of the medium has not yet been destroyed, only a few charges out of the bondage and the formation of a very small leakage current. If the applied voltage is constantly strengthened, and finally will make the polarized charge from the bound, causing a significant increase in leakage current, so the insulation of the medium has been destroyed, so that the two plates short, the complete loss of the role of capacitance. This phenomenon is called dielectric breakdown. After dielectric breakdown, the capacitor is destroyed. Therefore, the working voltage of the capacitor should have a certain limit, can not be increased arbitrarily.
Temperature coefficient
Capacitor capacitance with the size of the temperature change with the temperature coefficient (in a certain temperature range, the temperature changes every 1 ℃, the relative change in capacitance) to indicate that this is the same as the same resistance.
Insulation resistance
The size of the capacitor leakage is measured by insulation resistance. Capacitor leakage the smaller the better, that is, the greater the insulation resistance the better. General small capacitor insulation resistance is very large, up to several hundred megohms or several gigohms. Electrolytic capacitors generally smaller insulation resistance.
Loss
Under the action of the electric field, the capacitor unit time heating and consumption of energy called capacitor loss. Ideal capacitor in the circuit should not consume energy, but in practice, capacitors are more or less to consume energy, its energy consumption is mainly composed of dielectric loss and loss of metal parts, usually with the loss of angle tangent value to indicate.
Frequency characteristics
Capacitor frequency characteristics usually refers to the capacitor's electrical parameters (such as capacitance, loss angle tangent, etc.) with the frequency of the electric field and the nature of the change. Capacitors working at high frequencies, due to the dielectric constant at high frequencies than at low frequencies, so the capacitance will be reduced accordingly. At the same time, its loss will increase as the frequency rises. In addition, in high-frequency operation, the distribution parameters of the capacitor, such as chip resistance, lead and chip contact resistance, chip inductance, lead inductance, etc., will affect the performance of the capacitor, due to the impact of these factors, so that the frequency of use of capacitors is limited.
Dielectric
This parameter describes the type of dielectric material used for the capacitor, the temperature characteristics, and the error, etc. The different values correspond to a certain range of capacitance. For example, X7R is commonly used in the capacity of 3300pF ~ 0.33uF capacitors, this type of capacitor is suitable for filtering, coupling and other occasions, the dielectric constant is relatively large, when the temperature changes from 0 ° C to 70 ° C, the capacitance capacity of the change of ±15%;.
Y5P and Y5V are commonly used in capacitors with a capacity of 150pF~2nF, the temperature range is relatively wide, and the capacitance capacity change range is ±10% or +22%/-82% as the temperature changes.
For the relationship between other codes and temperature characteristics, you can refer to Table 1. For example, X5R means that the normal operating temperature of the capacitor is -55°C~+85°C, corresponding to the capacitance capacity change of ±15%.
Table 1
Package Size
Mainly for chip capacitors, package and resistor package size is the same.
The classification of capacitors
According to different ways of classification of capacitors there are many kinds, the following summarizes a few common types :
According to the material, there are different types of mica capacitors, electrolytic capacitors, ceramic capacitors, tantalum capacitors, etc.
According to the purpose, there are filter capacitors, bypass capacitors, coupling capacitors, load capacitors, etc.
According to the polarity, there are non-polarized capacitors, polarized capacitors.
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