First, the meaning of the capacitor
Capacitor (Capacitance) also referred to as "capacitor amount" means reserves the charge at a given potential difference, denoted by C, International farads (F). Generally, the electric charge moves in the electric field. When there is a medium between the conductors, the electric charge is hindered and the electric charge accumulates on the conductor, causing the accumulated storage of the electric charge. The stored electric charge amount is called a capacitance.
The formula for the capacitor is: C = εS / 4πkd. Where ε is a constant, S is the facing 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 plate area, and d is the distance between the plates).
When the reference direction of the voltage u across the capacitive element is given, if the amount of charge on the reference positive potential plate is represented by q, q=Cu is satisfied between the amount of charge of the capacitive element and the voltage. The current is equal to the amount of charge passing through a certain cross section per unit time, so I = dq / dt is obtained, so the relationship between current and capacitance is I = dq / dt = C (du / dt). This formula shows that the magnitude and direction of the current depend on the rate of change of voltage versus time. When the voltage increases, du/dt “0, then dq/dt “0, iâ€0, the charge on the plate increases, the capacitor is charged; the voltage is lowered. When du/dt "0, then dq/dt" 0, i "0, the charge on the plate is reduced, and the capacitor is reversely discharged. When the voltage does not change with time, du/dt=0, then the current I=0, at which time the current of the capacitive element is equal to zero, which is equivalent to an open circuit. Therefore, the capacitor element has the function of blocking DC.
Second, the capacitance of the capacitor
The symbol of the capacitor is C. In the International System of Units, the unit of capacitance is Farah, abbreviated as French, and the symbol is F. Since the unit of Farah is too large, the commonly used capacitance units are millifarad (mF) and microfarad (μF). Nafa (nF) and picofarad (pF), etc., the conversion relationship is as follows
1 Farad (F) = 1000 millifarads (mF) = 1000000 microfarads (μF);
1 microfarad (μF) = 1000 nanofarads (nF) = 1,000,000 picofarads (pF).
Third, the parameters of the capacitor
1. Nominal capacitance and error
Capacitance is the ability of a capacitor to store a charge after it is charged. The capacitance error refers to the deviation between the actual capacity and the nominal capacity, usually ±10%, ±20%, and the capacitance used in the PI matching in the RF circuit is ±0.5%, ±0.75% of the small error capacitance.
2. Rated voltage
The rated operating voltage is the maximum DC voltage (also known as withstand voltage) that the capacitor can withstand long-term reliable operation in the circuit without being broken down. It is related to the structure of the capacitor, the dielectric material and the thickness of the medium. Generally speaking, for capacitors of the same structure and medium, the capacitors with the same capacity have higher withstand voltage and larger volume.
When a voltage is applied between the two plates of the capacitor, the electrolyte between the plates is in an electric field, which is originally a neutral dielectric. Due to the external electric field force, the positive and negative charges in the dielectric molecules will slightly deviate in the spatial position. Shift (such as a negative charge moving in the direction of the electric field) forms a so-called electric dipole, that is, an electric field appears inside the medium, destroying the original electrical neutral state. This phenomenon is called polarization of the electrolyte. It can be seen that the medium in the polarized state is negatively charged, but these charges are still bound by the medium itself and cannot move freely. The dielectric properties of the medium have not been destroyed, and only a small amount of charge is decoupled to form a small leakage current. If the applied voltage is continuously strengthened, the polarized charge will be largely decoupled from the binding, and the leakage current will be greatly increased. Therefore, the dielectric properties of the dielectric are destroyed, and the two plates are short-circuited, completely losing the capacitance. This phenomenon is called dielectric breakdown. After the dielectric breakdown, the capacitor is destroyed. Therefore, the operating voltage of the capacitor must be limited and cannot be increased arbitrarily.
3. Temperature coefficient
The capacitance capacitance changes with temperature by the temperature coefficient (in a certain temperature range, the temperature changes by 1 ° C, the relative change in capacitance), this is the same as the resistance.
4. Insulation resistance
The size of the capacitor leakage is measured by the insulation resistance. The smaller the capacitor leakage, the better, that is, the larger the insulation resistance, the better. In general, small capacitors have large insulation resistances of up to several hundred megaohms or several gigaohms. The insulation resistance of electrolytic capacitors is generally small.
5. Loss
Under the action of an electric field, the energy consumed by the capacitor during heating per unit time is called the loss of the capacitor. An ideal capacitor should not consume energy in the circuit, but in practice, the capacitor consumes more or less energy. Its energy consumption is mainly composed of dielectric loss and loss of metal parts, and is usually expressed by the loss tangent.
6. Frequency characteristics
The frequency characteristic of a capacitor generally refers to the nature of the electrical parameters of the capacitor (eg, capacitance, loss tangent, etc.) as a function of the frequency of the electric field. Capacitors operating at high frequencies, since the dielectric constant is lower at high frequencies than at low frequencies, the capacitance will be correspondingly reduced. At the same time, its loss will increase with increasing frequency. In addition, when operating at high frequencies, the distribution parameters of the capacitor, such as the resistance of the pole piece, the contact resistance of the lead and the pole piece, the inductance of the pole piece, the inductance of the lead, etc., will affect the performance of the capacitor. Due to the influence of these factors, the capacitor is The frequency of use is limited.
7. Media
The parameters describe the dielectric material type, temperature characteristics, and error parameters used in the capacitor. Different values ​​also correspond to a certain range of capacitance capacity. For example, X7R is often used for capacitors with a capacity of 3300pF~0.33uF. Such capacitors are suitable for filtering, coupling, etc., and the dielectric constant is relatively large. When the temperature changes from 0°C to 70°C, the capacitance change is ±15%. ;
Y5P and Y5V are commonly used for capacitors with a capacity of 150pF~2nF. The temperature range is wide. With temperature changes, the capacitance varies from ±10% or +22%/-82%.
For other coding and temperature characteristics, you can refer to Table 4-1. For example, X5R means that the normal operating temperature of the capacitor is -55 ° C ~ +85 ° C, the corresponding capacitance capacity changes ± 15%
8. Package size
Mainly for chip capacitors, the package and resistor package size are the same.
Fourth, the classification of capacitors
There are many different types of capacitors according to different ways. The following summarizes several common types.
1. According to the material, there are different types of mica capacitors, electrolytic capacitors, ceramic capacitors, tantalum capacitors, etc.
2. According to the purpose, there are filter capacitors, bypass capacitors, coupling capacitors, load capacitors, etc.;
3. According to the polarity, there are no polar capacitors and polar capacitors.
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