capacitance

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One of electrical physical quantities

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synonym Capacitance (Capacitance) generally refers to capacitance (one of electrical physical quantities)

This entry is made by Compilation and application of scientific encyclopedia entries of "Science Popularization China" to examine.

Capacitance, also known as "capacitance", refers to the storage capacity of free charge under a given potential difference. It is recorded as C, and the SI unit is Farad (F). In general, charge It will move under force in the electric field. When there is a medium between conductors, it will prevent the movement of charges and make the charges accumulate on the conductors, resulting in the accumulated storage of charges. The amount of stored charges is called capacitance.

Capacitance refers to the ability to hold electric charges. whatever electrostatic field It is composed of many capacitors. If there is electrostatic field, there will be capacitance. Capacitance is described by electrostatic field. It is generally believed that isolated conductors form capacitance with infinity, and conductor grounding is equivalent to being connected to infinity and connected to the earth as a whole.

Capacitance (or electric capacity) is a physical quantity that represents the ability of a capacitor to hold electric charges. From the physical point of view, capacitance is a static charge storage medium, which may permanently exist. This is its feature. It has a wide range of applications. It is an indispensable electronic component in the field of electronics and power. It is mainly used in power filter, signal filter, signal coupling, resonance, filter, compensation, charge discharge, energy storage, DC isolation and other circuits.

Chinese name: capacitance
Foreign name: Capacitance
Alias: Capacitance
International unit: Farad

Unit symbol: F
application area: Electronics, power
Role: Filtering, coupling, resonance, filtering, compensation, charging and discharging, energy storage, DC isolation and other circuits
Physical quantity symbol: C

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definition

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AC circuit with pure capacitance^[2]

The ratio of the charged quantity Q of the capacitor to the voltage U between the two poles of the capacitor is called the capacitance of the capacitor. In circuit science, given the potential difference, the capacity of a capacitor to store charge is called capacitance, which is marked as C. The international system of units is adopted, and the unit of capacitance is farad, marked with F.

The symbol of capacitance is C.

^{[3 ]}

Units and conversion

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stay International System of Units The unit of capacitance is Farad , short for method, the symbol is F. Because the unit of farad is too large, the commonly used capacitance units are millifa (mF) Microfluid (μ F), nano method (nF) and pico method (pF). The conversion relationship is:

1 Farad (F)=10 ^ 3 milli method (mF)=10 ^ 6 micro method (μ F)=10 ^ 9 nano method (nF)=10 ^ 12 pico method (pF)=10 ^ 15fF

Relationship between capacitance and battery capacity:

1 VA h=1 Wh=3600 joule

W=0.5CUU

Calculation formula

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A capacitor, if the power with 1 bank is between two stages Potential difference It is 1 volt, and the capacitance of this capacitor is 1 farad, that is, C=Q/U. However, the capacitance is not determined by Q (charged quantity) or U (voltage), that is, the determinant of capacitance is: C=ε rS/4 π kd. Where, ε r is the relative dielectric constant, S is the positive area of the capacitor plate, d is the distance of the capacitor plate, and k is electrostatic force constant 。 common Parallel plate capacitor The capacitance is C=ε S/d (ε is the dielectric constant of the dielectric between the plates, ε=ε r ε 0, ε 0=1/4 π k, S is the area of the plates, and d is the distance between the plates).

Definition formula:

Capacitor Electric potential energy Calculation formula: E=C * (U ^ 2)/2=QU/2=(Q ^ 2)/2C

Multi capacitor parallel connection Calculation formula: C=C1+C2+C3+...+Cn

Multi capacitor series connection Calculation formula: 1/C=1/C1+1/C2+...+1/Cn

Three capacitors in series: C=(C1 * C2 * C3)/(C1 * C2+C2 * C3+C1 * C3)^[1]

Function of capacitance

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1） bypass

The bypass capacitor provides energy for local devices Energy storage Device, which can make the output of voltage regulator uniform and reduce load Demand. Like a small rechargeable battery, the bypass capacitor can be charged and discharged to the device. To minimize the impedance, the bypass capacitor should be as close as possible to the power supply pin and ground pin of the load device. This can well prevent ground potential rise and noise caused by excessive input value. The ground potential is the voltage drop when the ground connection passes through a large current burr.

2） decoupling

Decoupling is also called decoupling. In terms of the circuit, it can always be distinguished into the source of the drive and the load being driven. If the load capacitance is relatively large, the drive circuit needs to charge and discharge the capacitance to complete the signal jump. When the rising edge is relatively steep, the current is relatively large, so the drive current will absorb a large power supply current. Because of the inductance in the circuit, resistance (Especially the inductance on the chip pin) will rebound. This current is actually a kind of noise relative to the normal situation, which will affect the normal operation of the front stage. This is called "coupling".

Decoupling capacitor It acts as a "battery" to meet the current changes of the drive circuit, avoid mutual coupling interference, and further reduce the high-frequency interference impedance between the power supply and the reference ground in the circuit.

The combination of bypass capacitance and decoupling capacitance will be easier to understand. The bypass capacitor is actually decoupled, but generally refers to high frequency Bypass, that is, to provide a low impedance discharge path for high-frequency switching noise. The high-frequency bypass capacitance is generally small, and it is generally 0.1 μ F, 0.01 μ F, etc. according to the resonant frequency; The capacity of decoupling capacitor is generally large, which may be 10 μ F or more, and it is determined according to the distribution parameters in the circuit and the change of driving current. Bypass refers to the interference in the input signal as the filtering object, while decoupling refers to the interference in the output signal as the filtering object to prevent the interference signal from returning to the power supply. This is their essential difference.

3） wave filtering

Theoretically (assuming that the capacitance is pure), the larger the capacitance, the smaller the impedance, and the higher the passing frequency. However, in fact, most of the capacitors above 1 μ F are electrolytic capacitors with large inductance, so the impedance will increase when the frequency is high. Sometimes it can be seen that an electrolytic capacitor with a large capacitance is connected in parallel with a small capacitor. At this time, the large capacitor filters the low frequency and the small capacitor filters the high frequency. The function of capacitor is to connect AC and isolate DC, connect high-frequency resistance and low-frequency resistance. The larger the capacitance, the easier the high frequency pass. Specifically used in filtering, large capacitance (1000 μ F) filters low frequency and small capacitance (20pF) filters high frequency. Some netizens once vividly compared the filter capacitor to a "pond". Since the voltage at both ends of the capacitor will not change suddenly, it can be seen that the higher the signal frequency is, the greater the attenuation will be. It can be said that the capacitor is like a pond and will not change the water volume due to the addition or evaporation of a few drops of water. It converts the change of voltage into the change of current. The higher the frequency, the greater the peak current, thus buffering the voltage. Filtering is the process of charging and discharging.

4） Energy storage

The energy storage capacitor collects the charge through the rectifier, and transmits the stored energy to the output terminal of the power supply through the converter lead. The rated voltage is 40~450VDC, and the capacitance is 220~150 000 μ F Aluminum electrolytic capacitor It is commonly used. According to different power supply requirements, components may sometimes be in series, parallel or combination. For power supply with power level more than 10KW, tank shaped spiral terminal capacitor with large volume is usually used.

Multimeter detection capacitance

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use Digital multimeter The following methods can be used to detect capacitors.

1、 Direct detection with capacitive gear

Some digital multimeters have the function of measuring capacitance, and their measuring ranges are divided into 2000p, 20n, 200n, 2 μ and 20 μ. During measurement, the two pins of the discharged capacitor can be directly inserted into the Cx jack on the meter board, and the display data can be read after the appropriate range is selected.

2000p, it is suitable to measure capacitance less than 2000pF; 20n, suitable for measuring capacitance between 2000pF and 20nF; 200n, suitable for measuring capacitance between 20nF and 200nF; 2 μ, suitable for measuring capacitance between 200nF and 2 μ F; 20 μ, suitable for measuring capacitance between 2 μ F and 20 μ F.

Experience has proved that some models of digital multimeter (such as DT890B+) have large errors when measuring small capacity capacitors below 50pF, and there is almost no reference value when measuring capacitors below 20pF. At this time, the parallel connection method can be used to measure the small capacitance. The method is: first find a capacitance of about 220pF, use a digital multimeter to measure its actual capacity C1, then connect the small capacitance to be measured with it and measure its total capacity C2, then the difference between the two (C1-C2) is the capacity of the small capacitance to be measured. This method is very accurate for measuring small capacitance of 1~20pF.

2、 Test with resistance gear

Practice has proved that the charging process of capacitor can also be observed by using digital multimeter, which actually reflects the change of charging voltage with discrete digital quantity. If the measurement rate of the digital multimeter is n times/second, then during the observation of the charging process of the capacitor, n independent and increasing readings can be seen every second. According to this display feature of the digital multimeter, the quality of the capacitor can be detected and the capacitance can be estimated. The following describes the method of using the resistance range of the digital multimeter to detect the capacitor, which is very practical for instruments without a capacitance range. This method is suitable for measuring large capacity capacitors of 0.1 μ F~several thousand micro method.

3、 Detect with voltage range

Using the DC voltage range of the digital multimeter to detect the capacitor is actually a Indirect measurement This method can measure 220pF~1 μ F small capacity capacitors and accurately measure the leakage current of capacitors.

Type of capacitance

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The types of capacitance can be divided into: non-polar variable capacitance, non-polar fixed capacitance, polar capacitance, etc. in principle, and can be divided into: CBB capacitor (Polyethylene), Polyester capacitor 、 Porcelain chip capacitor 、 Mica capacitor 、 Monolithic capacitor 、 Electrolytic capacitor 、 Tantalum capacitor Etc.

Non-polar variable capacitor	Manufacturing process	The rotatable moving piece is a ceramic surface coated with a metal film, and the fixed piece is a ceramic bottom coated with a metal film; The moving sheet is coaxial metal sheet, and the fixed sheet is organic film sheet as medium
	advantage	Easy to produce, low technical content.
	shortcoming	Large volume and small capacity
	purpose	Change the oscillation and resonant frequency circuit. FM, AM, transmit/receive circuit
Non polar and non inductive CBB capacitor	Manufacturing process	Two layers of polypropylene plastic and two layers of metal foil are alternately mixed and then bundled.
	advantage	Non inductive, good high-frequency characteristics, small size
	shortcoming	Not suitable for large capacity, high price, poor heat resistance.
	purpose	Coupling/oscillation, audio, analog/digital circuit, high-frequency power filtering/decoupling
Non polar CBB capacitor	Manufacturing process	2nd floor Polyethylene plastic It is mixed with two layers of metal foil alternately and then bundled.
	advantage	Inductive, good high-frequency characteristics, small size
	shortcoming	Not suitable for large capacity, high price, poor heat resistance.
	purpose	Coupling/oscillation, analog/digital circuit, power filter/decoupling
Non polar ceramic chip capacitor	Manufacturing process	The thin porcelain piece is made by crossing the metal film silver on both sides.
	advantage	Small size, high withstand voltage, low price and high frequency (one is high-frequency capacitor)
	shortcoming	Fragile, low capacity
	purpose	High frequency oscillation, resonance, decoupling, audio
Nonpolar mica capacitor	Manufacturing process	Two layers of metal film on mica
	advantage	Easy to produce, low technical content.
	shortcoming	Large volume, small capacity,
	purpose	Oscillation, resonance, decoupling and circuit without high requirements The volume of non-polar monolithic capacitor is smaller than that of CBB. Others are the same as CBB. Inductive analog/digital circuit signal bypass/filtering, audio
Polar electrolytic capacitor	Manufacturing process	Two pieces of aluminum strips and two layers of insulating film are stacked together, and then they are immersed in electrolyte after being transferred.
	advantage	Large capacity.
	shortcoming	Poor high-frequency characteristics.
	purpose	Low frequency interstage coupling, bypass, decoupling, power filter, audio
Tantalum capacitor	Manufacturing process	Tantalum is used as the positive electrode, and metal is sprayed on the electrolyte as the negative electrode.
	advantage	Good stability, large capacity, good high-frequency characteristics.
	shortcoming	High cost.
	purpose	High precision power filter, signal level coupling, high-frequency circuit, audio circuit
Polyester (polyester) capacitor	Symbol	CL
	Capacitance	40p--4u
	Rated voltage	63--630V
	main features	Small size, large capacity, heat and moisture resistance, poor stability
	application	Low frequency circuit with low requirements for stability and loss
polystyrene capacitor	Symbol	CB
	Capacitance	10p--1u
	Rated voltage	100V--30KV
	main features	Stable, low loss, large volume
	application	Circuits requiring high stability and loss
Polypropylene capacitor	Symbol	CBB
	Capacitance	1000p--10u
	Rated voltage	63--2000V
	main features	Similar to polystyrene in performance, but small in size and slightly poor in stability
	application	Replace most polyphenyl or mica capacitors for circuits with high requirements
Mica capacitor	Symbol	CY
	Capacitance	10p--0.1u
	Rated voltage	100V--7kV
	main features	High stability, high reliability, small temperature coefficient
	application	High frequency oscillation, pulse and other circuits with high requirements
High frequency porcelain dielectric capacitor	Symbol	CC
	Capacitance	1--6800p
	Rated voltage	63--500V
	main features	Low high-frequency loss and good stability
	application	High frequency circuit
Low frequency porcelain dielectric capacitor	Symbol	CT
	Capacitance	10p--4.7u
	Rated voltage	50V--100V
	main features	Small size, low price, high loss, poor stability
	application	Low frequency circuit with low requirements
Glass glaze capacitance	Symbol	CI
	Capacitance	10p--0.1u
	Rated voltage	63--400V
	main features	Good stability, low loss, high temperature resistance (200 ℃)
	application	Pulse, coupling, bypass and other circuits
Aluminum electrolytic capacitor	Symbol	CD
	Capacitance	0.47--10000u
	Rated voltage	6.3--450V
	main features	Small size, large capacity, large loss, large leakage
	application	Power filtering, low-frequency coupling, decoupling, bypass, etc
tantalum electrolytic capacitor (CA), Niobium electrolytic capacitor (CN)	Capacitance	0.1--1000u
	Rated voltage	6.3--125V
	main features	Loss and leakage are less than aluminum electrolytic capacitor
	application	Replacing aluminum electrolytic capacitors in demanding circuits
Air dielectric variable capacitor	Variable electrical capacity	100--1500p
	main features	Low loss and high efficiency; It can be made into linear type, linear wavelength type, linear frequency type and logarithmic type according to requirements
	application	Electronic instruments, radio and television equipment, etc
Film dielectric variable capacitor	Variable electrical capacity	15--550p
	main features	Small size and light weight; The loss is greater than that of air medium
	application	Communication, broadcast receiver, etc
Thin film dielectric trimming capacitor	Variable electrical capacity	1--29p
	main features	Large loss and small size
	application	Circuit compensation for radio recorder, electronic instrument and other circuits
Ceramic dielectric trimming capacitor	Variable electrical capacity	0.3--22p
	main features	Small loss and volume
	application	Precisely tuned high-frequency oscillation circuit

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