What is the capacitance
As the name implies, is the ability to hold power.
Capacitance characterization of conductor peculiar properties of conductors, on value is equal to the potential of the conductor as a unit when the conductor of electricity.
In the international system of units, the unit of capacitance for ferrari.If a conductor of electricity for 1 c, the potential for IV, the conductor capacitance is the 1F.
Different capacity capacitor unit conversion relation, and the representation method for the capacity of the deviation as below
1F=106 uF=109 nF=1012 pF
1uF=103 nF=106 pF
What is a capacitor
As the name suggests, is to hold charge device.When the conductor in the presence of other objects around the conductor capacitance will be affected.Therefore, it is necessary to design a kind of conductor, its capacitance value is big, and geometry size is not big, and, of course, not affected by other objects.This combination of conductor is capacitors.
The capacitor dielectric
Relative permittivity of the medium.
Can be found in the physics experiment, open to the outside world and charger to charge the capacitor of electric
field force between the plates After inserted into the medium between the two plates, the electric field force between the two plate smaller.This phenomenon shows that human medium electric potential between the two plates of capacitor is reduced, under the condition of capacitive charge unchanged, the capacitance between the plate voltage decrease of capacitance of the result is increased.Therefore, the capacitance between the plate and plunges dielectric can increase the capacitance.To increase the capacitance, usually in the capacitance between the plate to join the relative dielectric coefficient of large medium.
Capacitor dielectric loss
Under the applied voltage and dielectric in the part of the phenomenon of electric energy is converted to heat energy, become a dielectric loss.Generally to say, the dielectric has a weak electrical conductivity, the energy loss is caused by leakage current and smaller.Main interface mass loss at high frequency alternating voltage, high frequency electric field makes the dielectric polarization occurred in the process of repeatedly.The higher the frequency rate, fever, the more significant.If severe fever, will make the dielectric loss of insulation and cause damage.
In strong electric field, the dielectric polarization feature will be lost as conductor, finally cause the damage of the dielectric (such as lattice cracks, oxidation, melting, etc.) phenomenon, this phenomenon is called the dielectric breakdown phenomenon.Dielectric breakdown has three forms, namely thermal breakdown, chemical
breakdown and electrical breakdown.
Thermal breakdown of dielectric loss, when more heat than the dielectric loss what had happened to pass around the amount of heat, the temperature of the dielectric rapidly rising, the conductivity increase, and even lead to the dielectric thermal damage.Therefore, thermal breakdown is always in the most bad parts of the capacitors.
Chemical breakdown is dielectric following a long-term work under high pressure.Strong electric field in the dielectric surface or internal holes near cause local air collision ionization, causing the dielectric corona, ozone generation carbon and oxygen.That gas is harmful to the organic insulation materials, can make the material of insulation performance, and damage the electric medium.
Electric breakdown is dielectric under strong electric field, are free electrons.At this point, the dielectric out now in the electronic current increases sharply with the increase of the electric field, which destroys the dielectric insulation performance.
Does not exist if the capacitor dielectric absorption, is connected to a dc power supply when the initial charging or polarization current will be
Formula ι as the polarization current of capacitor;V as the applied voltage;R is the resistance of the capacitor in series;E to the bottom of the natural logarithm of (2.718);C for capacitor;T is time.
When time t tends to infinity, polarization current tends to zero.If the R value is small, the process will be finished in a short time, and the capacitor is charged.But in fact, for solid dielectric capacitor, the capacitor will be full of electricity instantaneous discharge after open circuit, over a period of time will be absorbed by the medium, because some of the original charge "on the capacitor plate" will accumulate new charges, of course, this leads to become a dielectric absorption effect, the corresponding capacitor voltage is called the residual voltage.For liquid electrode or the dielectric, the generation of residual voltage is caused by resistance, the resistance is not only the ESR, but rough electrode in the depth of the parasitic resistance, the resistance is usually outside.
The relationship between capacitor parameters
Current and voltage in the capacitor in the time domain shown by the following relationship,
Under the voltage sine wave voltage of capacitor current said as follows
The capacitor connection
Parallel capacitor capacitor:C=C1+C2+......
Like the resistance in series features, the capacitive reactance of shunt capacitor
With the same resistor in parallel features, the capacitive reactance of series capacitor is
_Xc = XC1+XC2+XC3
The main purpose of the capacitor
the main function of capacitor energy storage, filter, coupling (blocking), the bypass (decoupling, decades ago called cross-linking), resonance, phase shift and keep (for sampling keeping circuit and integral circuit).
The main parameters of capacitors
Main parameters of the capacitor is the basic foundation of choose to use a capacitor, the capacitor of the main parameters as follows.
The rated voltage and dielectric strength
On both ends of the capacitor voltage can be sustained, general capacitor for the dc voltage, dedicated to alternating current (ac) for ac RMS voltage.Rated voltage of capacitor (rated voltage) is lower than the medium inside the capacitor dielectric strength (dielectric strength) (breakdown voltage), the craft that make different, the breakdown voltage and the rated voltage of the differential capacitor is endless also and same.Such as electrolytic capacitor anodic can control the very accurate, so the general breakdown electric pressure for the rated voltage of 1. 1 ~ 1. 3 times;Other media are usually is 1. 75 ~ 2 times;Power supply of electromagnetic interference suppression with capacitor need higher ratio, in order to ensure electrical safety.
Electric capacity of capacitor (capacitance) is decided by measure the impedance of the exchange capacity presented.Usually communicate with the method of measuring frequency, voltage and capacitance change and change, it's just different specifications of the capacitor is not the same as the degree of change, unless required precise capacitance and temperature characteristics of special stability, general electric capacity of capacitor of capacitance change with the rate of frequency is lower than the accuracy tolerance.
And frequency, the temperature at which the measurement has a certain influence on the capacity of the capacitor.As the measured temperature drop, electric capacity changes.
The capacity of the error
Capacitor in the process of manufacture can't ensure that each electric capacity of capacitor and the design value (or nominal value)
All agree, there is always a certain deviation, the capacitor has a capacity of error (how).Capacitor more expressed as a percentage of the capacity of the error, said method is as follows:
J=+/-5% ，K=+/-10% ，M=+/-20% ，S=+50-20% ，Z=+80-20% ，B=+/-0.1pF
C=+/-0 .25 pF ，D=+/-0. 5 pF，F=+/-1%
The loss factor of capacitors
Due to the leakage current, dielectric absorption, the equivalent series resistance loss and the working frequency of the cause.For dielectric absorption, usually as long as the loss is low, the change of the dielectric coefficient can be ignored.Media under electric field polarization process make the collision between the child and consumption of energy, resulting in a loss, and thus caused the fall of dielectric coefficient.In electrical solutions for capacitor equivalent series resistance is the main cause of the loss, and leakage current, caused by dielectric absorption loss is negligible.Thus, the loss factor of electrolytic capacitor (dissipation factor) with equivalent series resistance ESR with capacitive reactance 1 / wC, sometimes called loss tangent
Tan delta said.Therefore, electrolytic capacitor loss tangent tan delta is decreasing with frequency.Loss factor marks the size of the loss in the capacitor itself on the job itself, the size of the loss _ can pronounce righteous: when the capacitor is applied alternating current (ac), each cycle of capacitor loss and the ratio of the power of each cycle capacitor stored.
The equivalent series resistance
Capacitor electrodes to lead the resistance, general foil capacitor equivalent series resistance (ESR) is smaller than metallization electricity container of ESR, double metal and increase the ESR metallization is smaller than general metallization of ESR, many lead the ESR is smaller than the ESR of the single lead, the ESR than rough plates of the flat plate electrode of the ESR is small, and so on.
The temperature coefficient of capacitors
Temperature coefficient (temperature characteristics) is the extent of the capacitance change with temperature, the dielectric coefficient of some media along with the rise of temperature change is big, fall into this category, most of the media is often the change range is less than the tolerance range;And some media decreases with temperature rise, such as polypropylene;Some media may be in different temperature range And have different change, like a class II ceramic capacitor.
Working temperature range of capacitors
Any medium are working temperature range (operation temperature), high temperature can make a change in the physical properties of medium such as melting, dielectric strength and chemical changes (such as coking) and no longer meet the requirements of capacitor dielectric quality performance;Electrolytic capacitor is to avoid the evaporation of electrolyte capacitor caused permanent damage.
Leakage current of capacitors
Capacitors leakage (leakage current) insulation resistance is not infinite of mainly medium and medium deposit in defect (impurities), as a result of the different medium, the leakage current is different, such as aluminium electrolytic capacitor leakage current is mainly due to the presence of chlorine, iron, copper ion and the damage to the alumina medium and mini caused by galvanic effect.
Life of capacitors
Most of the capacitor in theory there is no life (life test), only the liquid medium (electrode) or medium dielectric coefficient decline after the applied voltage and life problems.The most obvious is that aluminum electrolytic capacitor, due to the negative electrode of electrolytic capacitors is electrolyte, when electrolyte dried up after aluminum electrolytic capacitor of the negative electrode area is greatly small, make the electric capacity drops greatly, when the electric capacity down to the end of life value, aluminum electrolytic capacitor is throughout the life.Usually, aluminum electrolytic capacitors are marked the highest working temperature and the service life of the under this temperature, such as: 105 t / 2000 ho
1.6 parameter representation
The classification of the capacitor
According to the classification of medium, the capacitor can be divided into: air dielectric capacitors, mica capacitor, paper condenser, organic membrane electric container [organic membrane mainly include: polyester film (previously known as the polyester), polypropylene film, polycarbonate membrane, ptfe membrane, etc.], ceramic capacitor, electrolytic capacitors, electrolytic capacitor, tantalum capacitor, ferroelectric capacitors and electric double layer capacitors (supercapacitor), etc.
The representation of electric capacity
1F=106 uF=109 nF=1012 pF
1uF=103 nF=106 pF