Definition 1: Capacitor, as its name implies, is a' charged container' and a device for storing charge. English name: capacitor. The figure shows some commonly used capacitors. Represented by the letter C, capacitor is one of the electronic components widely used in electronic equipment, which is widely used in DC isolation, coupling, bypass, filtering, tuning circuit, energy conversion, control circuit and so on.
Definition 2: capacitor, any two insulated conductors (including wires) constitute a capacitor.
[Edit this paragraph] The role of capacitor in the circuit
In a DC circuit, a capacitor is equivalent to an open circuit.
This has to start with the structure of the capacitor. The simplest capacitor consists of plates at both ends and an insulating dielectric in the middle. After electrification, the polar plate is charged, forming a voltage (potential difference), but it is not conductive in the middle because it is an insulating substance. 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 this substance increases to a certain extent, it can conduct electricity. We call this voltage breakdown voltage. Capacitors are no exception. After the capacitor is broken down, it is not an insulator. 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.
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, the current passes between capacitors in the form of a field.
There is a saying in the middle school stage that AC is passed and DC is blocked, which is the nature of capacitance.
[Edit this paragraph] The basic function of the capacitor-charging and discharging.
Charging and discharging are the basic functions of capacitors.
expense
The process of charging a capacitor (storing charge and electric energy) is called charging. At this time, one of the two plates of the capacitor is always positively charged, and the other plate is also negatively charged. Connect one plate of the capacitor to the positive pole of a power supply (such as a battery pack) and the other plate to the negative pole of the power supply, and the two plates will be charged with the same amount of different charges respectively. After charging, there is an electric field between the two plates of the capacitor, and the electric energy obtained from the power supply during charging is stored in the capacitor.
avoid
The process of making the charged capacitor lose charge (release charge and electric energy) is called discharge. For example, if a wire is used to connect the two poles of a capacitor, and the charges on the two poles neutralize each other, the capacitor will release charge and electric energy. After discharge, the electric field between the two plates of the capacitor disappears and the electric energy is converted into other forms of energy.
In general electronic circuits, capacitors are often used to realize bypass, coupling, filtering, oscillation, phase shift and waveform transformation, which are the evolution of their charging and discharging functions.
[Edit this paragraph] Main characteristic parameters of capacitor
1, nominal capacitance and allowable deviation
The nominal capacitance is the capacitance marked on the capacitor.
The basic unit of capacitor is farad (f), but this unit is too large to be used for field marking.
Other unit relationships are as follows:
1F= 1000mF
1mF= 1000μF
1μF= 1000nF
1nF= 1000pF
The deviation between the actual capacitance and the nominal capacitance of the capacitor is called error, and within the allowable deviation range is called accuracy.
Correspondence between accuracy grade and allowable error: 00 (0 1)- 1%, 0 (02)-2%, I-5%, II- 10%, III-20%, IV-(+20%-.
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.
2. Rated voltage
The effective value of the highest DC voltage that can be continuously applied to the capacitor at the lowest ambient temperature and rated ambient temperature is generally directly marked on the capacitor shell. If the working voltage exceeds the withstand voltage of the capacitor, the capacitor will break down, causing irreparable permanent damage.
3. Insulation resistance
DC voltage is applied to the capacitor, resulting in leakage current. The ratio of the two is called insulation resistance.
When the capacitance is small, it mainly depends on the surface state of the capacitor. When the capacitance > > 0.1uf, it mainly depends on the performance of the medium. The smaller the insulation resistance, the better.
Capacitor time constant: In order to correctly evaluate the insulation of large-capacity capacitors, a time constant is introduced, which is equal to the product of insulation resistance and capacitor capacity.
Step 4 fail
Under the action of electric field, the energy consumed by heat per unit time is called loss. All kinds of capacitors have their allowable loss values in a certain frequency range. The loss of capacitor is mainly caused by dielectric loss, conductance loss and the resistance of all metal parts of capacitor.
Under the action of DC electric field, the loss of capacitor exists in the form of leakage conductance loss, which is generally small. Under the action of alternating electric field, the loss of capacitor is not only related to leakage conductance, but also to the process of periodic polarization establishment.
5. Frequency characteristics
With the increase of frequency, the capacitance of general capacitors presents a decreasing law.
6. Commonly used formulas
C=εS/4πkd in parallel plate capacitor formula
[Edit this paragraph] Model name and label of capacitor.
Naming method of 1. capacitor model
The model of domestic capacitors generally consists of four parts (not suitable for pressure sensitive, variable and vacuum capacitors). In turn, it represents name, material, classification and serial number respectively.
The first part: name, expressed in letters, capacitance is C.
Part II: Materials, expressed in letters.
The third part: classification, generally expressed by numbers, and individually expressed by letters.
The fourth part: serial number, expressed by numbers.
The materials of the products are indicated by letters: nonpolar films such as A- tantalum electrolysis, B- polystyrene, C- high frequency ceramics, D- aluminum electrolysis, E- other materials electrolysis, G- alloy electrolysis, H- composite medium, I- glass glaze, J- metallized paper, polar organic films such as L- polyester, N- niobium electrolysis, O- glass film, Q-.
2. Capacitor capacity label
1, direct standard method
Directly marked with numbers and unit symbols. For example, 1uF means 1 microfarad, and some capacitors use "r" to represent decimal points, for example, R56 means 0.56 microfarad.
2, the text symbol method
Capacity is represented by a regular combination of numbers and text symbols. For example, p 10 means 0. 1pF, 1p0 means 1pF, 6P8 means 6.8pf, and 2u2 means 2.2uF.
3. Color coding method
The main parameters of the capacitor are represented by color rings or color points. The color coding method of capacitors is the same as that of resistors.
Symbols of capacitor deviation:+100%-0-H,+100%-1r, +50%- 10%-T,+30%-1.
4. Mathematical counting method: As shown in the above figure, the nominal value of ceramic capacitor is 272, and the capacity is 27X 100pf=2700pf. If the nominal value is 473, it is 47X 1000pf=0.047uf (the following 2 and 3 both represent the power of 10). Another example is: 332=33X 100pf=3300pf.
[Edit this paragraph] Classification of capacitors
1. According to the structure, it can be divided into three categories: fixed capacitance, variable capacitance and fine-tuning capacitance.
2. According to electrolyte classification: organic dielectric capacitor, inorganic dielectric capacitor, electrolytic capacitor and air dielectric capacitor.
3. According to the use, it can be divided into high-frequency bypass, low-frequency bypass, filtering, tuning, high-frequency coupling, low-frequency coupling and small capacitance.
4. According to the different manufacturing materials, it can be divided into ceramic capacitors, polyester capacitors, electrolytic capacitors, tantalum capacitors, advanced polypropylene capacitors and so on.
5. High frequency bypass: ceramic capacitors, mica capacitors, glass film capacitors, polyester capacitors and glass glaze capacitors.
6. Low frequency bypass: paper capacitor, ceramic capacitor, aluminum electrolytic capacitor and polyester capacitor.
7. Filtering: aluminum electrolytic capacitor, paper dielectric capacitor, composite paper dielectric capacitor and liquid tantalum capacitor.
8. Tuning: ceramic capacitors, mica capacitors, glass film capacitors, polystyrene capacitors.
9. Low coupling: paper capacitors, ceramic capacitors, aluminum electrolytic capacitors, polyester capacitors and solid tantalum capacitors.
10. Small capacitors: metallized paper capacitors, ceramic capacitors, aluminum electrolytic capacitors, polystyrene capacitors, solid tantalum capacitors, glass glaze capacitors, metallized polyester capacitors, polypropylene capacitors and mica capacitors.
[Edit this paragraph] Common capacitors
aluminium electrolytic capacitor
A capacitor made of absorbent paper impregnated with paste electrolyte is sandwiched between two aluminum foils, and a thin oxide film is used as the dielectric. Because the oxide film has unilateral conductivity, the electrolytic capacitor has polarity.
Large capacity, able to withstand large pulsating current.
Large capacity error and large leakage current; Ordinary ones are not suitable for high-frequency and low-temperature applications and should not be used at frequencies above 25kHz.
Low frequency bypass, signal coupling, power supply filtering.
Tantalum electrolytic capacitor
Sintered tantalum blocks are used as positive electrodes, and solid manganese dioxide is used as electrolyte.
The temperature characteristics, frequency characteristics and reliability are better than those of ordinary electrolytic capacitors, especially the leakage current is very small, the storage is good, the capacity error is small, the volume is small, and the maximum capacitance-voltage product can be obtained per unit volume.
The tolerance to pulsating current is poor, and it is easy to short circuit if it is damaged.
Ultra-small and highly reliable devices.
thin film capacitor
The structure is similar to that of paper capacitor, but low-loss plastic materials such as polyester and polystyrene are used as dielectrics.
Good frequency characteristics and low dielectric loss.
Can not be made into large capacity, poor heat resistance.
Filter, integral, oscillation, timing circuit.
ceramic capacitor
One electrode of a ceramic capacitor having a through hole or columnar structure is a mounting screw. The lead inductance is very small,
Good frequency characteristics, low dielectric loss and temperature compensation.
Can not be made into a large capacity, vibration will cause capacity changes.
Especially suitable for high frequency bypass.
Monolithic capacitor (multilayer ceramic capacitor)
Several ceramic membrane blanks are covered with electrode paddle material, which are wound into an inseparable whole after overlapping, and then encapsulated with resin.
New capacitors with small volume, large capacity, high reliability and high temperature resistance and low-frequency monolithic capacitors with high dielectric constant also have the characteristics of stable performance, small volume and high Q value.
The capacity error is very large
Noise bypass, filtering, integration and oscillation circuit
Paper dielectric
Generally, two pieces of aluminum foil are used as electrodes, and capacitor paper with a thickness of 0.008 ~ 0.0 12 mm is wound separately.
The manufacturing process is simple, the price is cheap, and larger capacitance can be obtained.
Generally, in low-frequency circuits, it cannot be used at frequencies higher than 3 ~ 4 MHz. Oil-immersed capacitors have higher withstand voltage and better stability than ordinary paper capacitors, and are suitable for high-voltage circuits.
Trimming capacitor (semi-variable capacitor)
The capacitance can be adjusted in a small range and can be fixed at a certain capacitance value after adjustment.
Ceramic dielectric trimming capacitors have high Q value and small volume, and are usually divided into two types: circular tube type and circular plate type.
Mica and polystyrene media usually adopt spring structure, which is simple in structure but poor in stability.
Wire-wound ceramic trimming capacitor changes the capacitance by removing the copper wire (external electrode), so it can only reduce the capacitance and is not suitable for repeated debugging.
ceramic capacitor
Barium titanate-titanium oxide, a capacitor ceramic with high dielectric constant, was extruded into a round tube, a disk or a wafer as a medium, and silver was plated on the ceramic as an electrode through sintering infiltration. Divided into high-frequency porcelain and low-frequency porcelain.
Capacitors with small positive capacitance temperature coefficient are used as loop capacitors and pad capacitors in high stability oscillation circuits.
Low-frequency ceramic capacitors are limited to bypass or DC isolation in circuits with low working frequency or circuits that do not require stability and loss (including high frequency). Such capacitors are not suitable for pulse circuits because they are easily broken down by pulse voltage.
High-frequency ceramic capacitors are suitable for high-frequency circuits.
mica capacitor
In terms of structure, it can be divided into foil type and silver type. Silver-plated electrode is formed by direct silver plating on mica by vacuum evaporation or combustion infiltration. Because the air gap is eliminated, the temperature coefficient is greatly reduced and the capacitance stability is higher than that of foil.
Good frequency characteristics, high Q value and low temperature coefficient.
Can't be made into large capacity
Widely used in high-frequency electrical appliances, and can be used as standard capacitors.
Glass glaze capacitor
It is made by spraying a special mixture with a proper concentration into a thin film, and the dielectric and the silver electrode are sintered to form a monolithic structure.
Its performance is equivalent to that of mica capacitor, and it can withstand various climatic environments. Generally, it can work above 200℃, the rated working voltage can reach 500V, and the loss TG δ is 0.0005 ~ 0.008.
Capacitor: Electronic components in electronic equipment, such as smoothing filter of rectifier, power supply and decoupling, bypass of AC signal and AC coupling of AC and DC circuits, are called capacitors. Capacitors include fixed capacitors and variable capacitors, in which fixed capacitors can be divided into mica capacitors, ceramic capacitors, paper/plastic film capacitors, electrolytic capacitors and glass glaze capacitors according to the dielectric materials used. Variable capacitors can also be glass, air or ceramic dielectric structures.
The loss of capacitance has a great relationship with leakage and the temperature of the environment! ! !
Fixed capacitor
Detection method of fixed capacitor
A. Detection of small capacitance below 10pF Because the fixed capacitance below 10pF is too small, we can only qualitatively check whether there is leakage, internal short circuit or breakdown. When measuring, you can choose to use a multimeter R× 10k, and use two probes to connect the two pins of the capacitor at will, and the resistance value should be infinite. If the measured resistance (the pointer swings to the right) is zero, it indicates that the capacitor is damaged due to leakage or internal breakdown.
B. Check whether the fixed capacitor of 10pf ~ 00 1μ f is charged, and then judge whether it is good or bad. Select R× 1k range for multimeter. The β values of both transistors are above 100, and the penetration current is very small. Silicon triodes of 3DG6 and other models can be selected to form composite pipes. The red and black probes of the multimeter are respectively connected with the emitter E and the collector C of the composite pipe. Due to the amplification of the compound triode, the charging and discharging process of the measured capacitor is amplified, which makes the pointer swing of the multimeter larger and easier to observe.
It should be noted that in the test operation, especially when measuring a small capacity capacitor, it is necessary to exchange the A and B contacts of the capacitor pin to be measured repeatedly, so as to obviously see the swing of the multimeter pointer. C For a fixed capacitor above 00 1μF, the R× 10k of the multimeter can be used to directly test whether the capacitor has a charging process, whether there is an internal short circuit or leakage, and the capacity of the capacitor can be estimated according to the amplitude of the pointer swinging to the right.
[Edit this paragraph] What safety should be paid attention to when dealing with faulty capacitors?
Because the two poles of the capacitor have the characteristics of residual charge, we must first try to discharge the charge, otherwise it is easy to get an electric shock accident. When dealing with the faulty capacitor, the circuit breaker of the capacitor bank and its upper and lower isolating switches should be opened first. If fuse protection is used, the fuse tube should be removed first. At this time, although the capacitor bank has been discharged by the overdischarge resistor, there will still be some residual charges, which must be discharged manually. When discharging, first fix the grounding end of the grounding wire with the grounding grid, then discharge the capacitor several times with the grounding rod until there is no spark and discharge sound, and finally fix the grounding wire. At the same time, it should be noted that there may be residual charge between the two poles of the capacitor if there is internal disconnection, fuse blowing or poor lead contact, and these residual charges will not be discharged during automatic discharge or manual discharge. Therefore, before touching the faulty capacitor, operators or maintenance personnel should also wear insulating gloves and short-circuit the two poles of the faulty capacitor to discharge it. In addition, the capacitors in series should be discharged separately.
[Edit this paragraph] What abnormal conditions often occur during capacitor operation?
When the compensation capacitor is in operation, it is often prone to shell bulging and oil leakage in the shell or oil tank. The main reason is that the capacitor temperature is too high. Excessive temperature rise is caused by the following factors. 1, the ambient temperature is too high and the ventilation is poor.
2. The power supply voltage exceeds the rated value, causing overload and fever.