Capacitance plays an important role in circuits such as tuning, bypass, coupling and filtering. It is used in the tuning circuit of transistor radio, and also in the coupling circuit of color TV and fiber channel.
With the rapid development of electronic information technology, the upgrading speed of digital electronic products is getting faster and faster. The production and sales of consumer electronic products, mainly flat-panel TVs (LCD and PDP), notebook computers, digital cameras and other products, continued to grow, which led to the growth of the capacitor industry.
Introduction editing
Capacitors are elements that store electricity and electric energy (electric potential energy). One conductor is surrounded by another conductor, or all the electric field lines emitted by one conductor terminate in the conductive system of another conductor. This conductor is called a capacitor.
Capacitance formula of parallel plate capacitor:
Where UA-UB is the potential difference between two parallel plates, εr is the relative dielectric constant, K is the electrostatic force constant, S is the area facing the two plates, and D is the distance between the two plates. Note: The electric field in parallel plate capacitor is uniform. [3]
Capacitance is different from capacitor. Capacitance is a basic physical quantity, with symbol c and unit f (farad). [3]
General formula C=Q/U, special formula for parallel plate capacitor: electric field intensity between plates e = U/d[3]
Editor of main parameters of capacitor
(1) The nominal capacitance is the capacitance marked on the capacitor. But the actual capacitance of the capacitor is the same as
Capacitor charging process
Capacitor charging process
There is a deviation in the nominal capacitance, and the accuracy level has a corresponding relationship with the allowable error. Generally, I, II and III grades are commonly used for capacitors, and IV, V and VI grades are used for electrolytic capacitors, which are selected according to the purpose. The capacitance of electrolytic capacitor depends on the impedance when it works under AC voltage, and it will change with the change of working frequency, temperature, voltage and measurement method. The unit of capacitance is f (method). [4]
Because the capacitor is a kind of "container" for storing charge, there is a problem of "capacity". In order to measure the storage capacity of the capacitor, the physical quantity of the capacitor is determined. Capacitors can only store charge under the action of applied voltage. Under the action of voltage, different capacitors can store different charges. It is internationally stipulated that when a DC voltage of 1 volt is applied to a capacitor, the amount of charge it can store is the capacitance of the capacitor (that is, the amount of electricity under unit voltage), which is represented by the letter C, and the basic unit of the capacitance is Farah (F). Under the DC voltage of 1V, if the charge stored in the capacitor is 1 coulomb, the capacitance will be defined as 1 farad, which is represented by the symbol f,1f =1q/v. In practical applications, the capacitance of capacitors is often much smaller than 1 farad, and smaller units are often used, such as millifarad (mF), microfarad (μF) and nanofarad (nF). Their relationship is: 1 microfarad equals one millionth of farad; 1 picofarad is equal to one millionth microfarad, that is, [5]
1 farad (F)= 1000 millifarad (MF); 1 millifarad (mF)= 1000 microfarad (μ f); 1 microfabrication (μF)= 1000 nanofabrication (nf); 1 nf =1000 pf; Namely:1f =1000000μ f; 1μF= 1000000pF .[5]
(2) The rated voltage is the highest DC voltage that can be continuously applied to the capacitor at the lowest ambient temperature and rated ambient temperature. If the working voltage exceeds the withstand voltage of the capacitor, the capacitor will be damaged by breakdown. In practice, with the increase of temperature, the pressure resistance value will become lower. [4]
(3) Insulation resistance. The DC voltage is applied to the capacitor, resulting in leakage current, and the ratio of the two is called insulation resistance. When the capacitance is small, its value mainly depends on the surface state of the capacitance; When the capacity is greater than 0. 1μF, its value mainly depends on the medium. Generally, the greater the insulation resistance, the better. [4]
(4) losses. Under the action of electric field, the energy consumed by heat per unit time is called loss. Loss is related to frequency range, medium, conductance, resistance of metal part of capacitor, etc. [4]
(5) Frequency characteristics. With the increase of frequency, the capacitance of general capacitors presents a decreasing law. When the capacitor works below the resonant frequency, it is capacitive; When it exceeds its resonant frequency, it is inductive, and at this time it is not capacitance but inductance. Therefore, it is necessary to avoid the capacitor working above the resonant frequency. [4]
Action editing
capacitor
capacitor
In a DC circuit, a capacitor is equivalent to an open circuit. Capacitor is a kind of component that can store charge, and it is also one of the most commonly used electronic components. [6]
This has to start with the structure of the capacitor. The simplest capacitor consists of plates at both ends and an insulating medium (including air) in the middle. After electrifying, the polar plate is charged, forming a voltage (potential difference), but because there is an insulating substance in the middle, the whole capacitor is not conductive. However, such a situation is under the premise of not exceeding the critical voltage (breakdown voltage) of the capacitor. We know that any substance is relatively insulated. When the voltage across the substance increases to a certain extent, the substance can conduct electricity. We call this voltage breakdown voltage. Capacitance is no exception. They are not insulators when they are decomposed. But in the middle school stage, such a voltage can't be seen in the circuit, so working below the breakdown voltage can be regarded as an insulator. [6]
However, in AC circuit, the direction of current changes with time in a certain functional relationship. The process of capacitor charging and discharging has time. At this time, a changing electric field is formed between the plates, and this electric field is also a function of time. In fact, current flows between capacitors in the form of electric field. [6]
The role of the capacitor:
● Coupling: Capacitors used in coupling circuits are called coupling capacitors, which are widely used in capacitive coupling circuits such as resistance-capacitance coupling amplifiers to block DC- AC current. [6]
● Filtering: The capacitor used in the filtering circuit is called the filtering capacitor. This capacitor circuit is used in power supply filtering and various filtering circuits, and the filtering capacitor removes the signal of a certain frequency band from the total signal. [6]
● Decoupling: The capacitor used in decoupling circuit is called decoupling capacitor, which is used in DC power supply circuit of multistage amplifier to eliminate harmful low-frequency cross-connection between amplifiers at all levels. [6]
● High-frequency vibration elimination: Capacitors used in high-frequency vibration elimination circuits are called high-frequency vibration elimination capacitors. In the audio negative feedback amplifier, in order to eliminate the possible high-frequency self-excitation, this capacitor circuit is adopted to eliminate the possible high-frequency howling of the amplifier. [6]
● Resonance: Capacitors used in LC resonant circuits are called resonant capacitors, which are needed in both LC parallel and series resonant circuits. [6]
● Bypass: Capacitors used in Fibre Channel are called bypass capacitors. If the signal of a certain frequency band needs to be removed from the signal, a bypass capacitor circuit can be used. According to the frequency of the removed signal, there are full frequency domain (full AC signal) bypass capacitor circuit and high frequency bypass capacitor circuit. [6]
● Neutralization: The capacitor used in the neutralization circuit is called neutralization capacitor. This neutralizing capacitor circuit is used in radio high-frequency and intermediate-frequency amplifiers and TV high-frequency amplifiers to eliminate self-excitation. [6]
● Timing: The capacitor used in the timing circuit is called timing capacitor. Timing capacitor circuit is used in the circuit that needs time control through capacitor charging and discharging, and the capacitor plays the role of controlling the size of time constant. [6]
● Integration: The capacitor used in the integration circuit is called the integration capacitor. In the synchronous separation circuit of field scanning, the field synchronization signal can be extracted from the field composite synchronization signal by using this integrated capacitor circuit. [6]
● Differential: Capacitors used in difference channel are called differential capacitors. In order to get the spike trigger signal in the trigger circuit, this differential capacitor circuit is used to get the spike trigger signal from various (mainly rectangular pulse) signals. [6]
● Compensation: The capacitor used in the compensation circuit is called compensation capacitor. In the bass compensation circuit of the card holder, this low-frequency compensation capacitor circuit is used to improve the low-frequency signal in the playback signal. In addition, there is a high-frequency compensation capacitor circuit. [6]
● Bootstrap: The capacitor used in the bootstrap circuit is called bootstrap capacitor, which is often used in the output stage circuit of OTL power amplifier to slightly improve the positive half-cycle amplitude of the signal through positive feedback. [6]
● Frequency division: The capacitance in the frequency division circuit is called frequency division capacitance. In the speaker frequency division circuit of the sound box, the high frequency speaker works in the high frequency band, the intermediate frequency speaker works in the middle frequency band and the low frequency speaker works in the low frequency band by using the frequency division capacitor circuit. [6]
● Load capacitance: refers to the effective external capacitance that determines the load resonance frequency together with the quartz crystal resonator. Common standard values of load capacitance are 16pF, 20pF, 30pF, 50pF, 100pF. The load capacitance can be properly adjusted according to the specific situation, and the working frequency of the resonator can generally be adjusted to the nominal value by adjustment.