Function of capacitor: mainly used in AC circuit and pulse circuit. In DC circuits, capacitors generally play a role in blocking DC.

2. The capacitor neither produces energy nor consumes energy, and it is a kind of energy storage element.

3. Capacitor is an important equipment to improve power factor in power system; In electronic circuits, it is the main element to obtain oscillation, filtering, phase shift, bypass and coupling.

4. Because the industrial load is mainly motor inductive load, it is necessary to combine capacitive load to balance the power grid.

5. On the grounding wire, why do some of them have to pass through the capacitor to be grounded?

Answer: In the DC circuit, it is anti-interference, and the interference pulse is grounded through the capacitor (in this case, it is important to isolate the DC-potential relationship in the circuit); Ac circuits are also grounded by capacitors, which are generally small in capacity and have anti-interference and potential isolation functions.

6. What is the power factor of capacitance compensation?

Answer: Because the voltage on the capacitor needs a charging process first, with the charging process, the voltage on the capacitor will gradually increase, so that there will be current first and then the voltage will be established. Usually, we call the current 90 degrees ahead of the voltage (when there is no resistance and inductance in the capacitive current loop, it is called a pure capacitive circuit). Inductive circuits with coils, such as motors and transformers, are just the opposite of capacitors, because the current through inductors does not change suddenly. There is no current until the voltage is established at both ends of the coil (when there is no resistance and capacitance in the inductive current loop, it is called a pure inductive circuit), and the current of the pure inductive circuit lags behind the voltage by 90 degrees. Because power is voltage multiplied by current, when voltage and current are not generated at the same time (for example, when the voltage on the capacitor is the largest, the electricity is fully charged and the current is 0; When there is a voltage on the inductor first, the inductor current is also 0), so the product (power) obtained is also 0! This is reactive power. Then the relationship between the voltage and current of the capacitor is just the opposite to that of the inductor, so the reactive power generated by the inductor is compensated by the capacitor, which is the principle of reactive power compensation.

Lecture 1: characteristics of capacitors (cross-DC AC)

A capacitor is a container that can store charge. It consists of two pieces of metal that are close together, separated by an insulating substance. According to different insulating materials, it can be made into various capacitors, such as mica, porcelain, paper, electrolytic capacitors and so on. Structurally, it can be divided into fixed capacitance and variable capacitance. The capacitor has infinite resistance to DC, that is, the capacitor has the function of isolating DC. The resistance of capacitor to alternating current is affected by alternating current frequency. That is, capacitors with the same capacity present different capacitive reactance to alternating current with different frequencies. Why do these phenomena occur in opportunities?' This is because the capacitor works by its charging and discharging function, as shown in figure 1. When the power switch S is not closed, the two metal plates of the capacitor and other common metal plates are not charged. When the switch s is closed, as shown in fig. 2, the free electrons on the positive plate of the capacitor are attracted by the power supply and pushed to the negative plate. Because there is an insulating material between the two plates of the capacitor, free electrons running out of the positive plate will accumulate on the negative plate. The positive plate will be positively charged due to the decrease of electrons, and the negative plate will be negatively charged due to the gradual increase of electrons. There is a potential difference between the two plates of the capacitor. When the potential difference is equal to the power supply voltage, the charging of the capacitor stops. At this time, if the power supply is cut off, the capacitor can still maintain the charging voltage. For a charged capacitor, if we connect two plates with wires, because there is a potential difference between the two plates, electrons will go back to the positive plate through the wires until the potential difference between the two plates is zero. The capacitor will return to the neutral state without charge, and there will be no current flowing in the wire. The discharge process of the capacitor is shown in Figure 3. The frequency of alternating current applied to the two plates of the capacitor is high, and the charging and discharging times of the capacitor will increase. The charging and discharging current is also enhanced; That is to say, the blocking effect of the capacitor on high-frequency alternating current is reduced, that is, the capacitive reactance is small, while the capacitive reactance generated by the capacitor on low-frequency alternating current is large. For alternating current with the same frequency, the larger the capacitor capacity, the smaller the capacitive reactance and the larger the capacitive reactance.

Lecture 2: Parameters and Classification of Capacitors

In electronic products, capacitance is an essential electronic device, which plays the role of smoothing filter of rectifier, decoupling of power supply, bypass of AC signal and AC coupling of AC /DC circuit. Because there are many types and structures of capacitors, we should not only know the performance indexes and general characteristics of various capacitors, but also know the advantages and disadvantages of various components in a given use, as well as the mechanical or environmental constraints. Here, the main parameters of the capacitor and its application will be briefly explained.

1. Nominal capacitance (C R). Capacitance value marked by capacitor products. The capacitance of mica and ceramic dielectric capacitors is low (about below 5000pF); The capacitors in the form of paper, plastic and some ceramic media are in the middle (about 0.005 UF ~1.0 UF); Generally, the capacity of electrolytic capacitors is relatively large. This is a rough classification.

2. Category temperature range. Ambient temperature range of continuous operation determined by capacitor design. This range depends on the temperature limit of its corresponding category, such as the upper category temperature, the lower category temperature, the rated temperature (the highest ambient temperature where the rated voltage can be continuously applied) and so on.

3. Rated voltage. At any temperature between the lower limit temperature and the rated temperature, the effective value of the maximum DC voltage or the maximum AC voltage or the peak value of the pulse voltage can be continuously applied to the capacitor. When capacitors are used to sum high voltage electric fields, we must pay attention to the influence of corona. Corona is caused by the gap between dielectric layer and electrode layer, which will not only produce parasitic signals that damage equipment, but also lead to dielectric breakdown of capacitors. Corona is particularly prone to occur under AC or pulsating conditions. For all capacitors, the sum of DC voltage and AC peak voltage should not exceed the rated voltage of the capacitors used.

4. Tangent of loss angle (tg δ). Under the sine voltage of specified frequency, the loss power of capacitor divided by the reactive power of capacitor is the loss tangent. In practical application, the capacitor is not a pure capacitor, and there is an equivalent resistance inside it. Its simplified equivalent circuit is shown in the attached figure. For electronic equipment, the smaller the R S, the better, that is to say, the smaller the loss power, the smaller the angle with the capacitor power.

5. Temperature characteristics of capacitors. It is usually expressed as the percentage of the capacitance at the reference temperature of 20℃ to the capacitance at the relevant temperature.

6. service life. The service life of capacitors decreases with the increase of temperature. The main reason is that the temperature accelerates the chemical reaction and degrades the medium with time.

7. Insulation resistance. Due to the increase of electron activity caused by temperature rise, the insulation resistance will decrease with the increase of temperature.

Capacitors include fixed capacitors and variable capacitors. Fixed capacitors can be divided into mica capacitors, ceramic capacitors, paper/plastic film capacitors,

Lecture 3: Types and symbols of capacitors

There are many kinds of capacitors. In order to distinguish them, several Latin letters are often used to indicate the types of capacitors, as shown in figure 1. The first letter c stands for capacitance, the second letter stands for dielectric material, and the third letter stands for shape, structure, etc. The picture above shows a small paper capacitor, and the picture below shows a vertical rectangular sealed paper capacitor. Table 1 lists the types and symbols of capacitors. Table 2 shows several characteristics of common capacitors.

Lecture 4: Discrimination of polarity of electrolytic capacitor

Without knowing the polarity of electrolytic capacitor, its polarity can be measured by the resistance of multimeter.

As we know, the leakage current of electrolytic capacitor is very small (the leakage resistance is very large) only when the positive terminal of electrolytic capacitor is connected with a positive power supply (black stylus in case of power plugging) and the negative terminal is connected with a negative power supply (red stylus in case of power plugging). On the contrary, the leakage current of the electrolytic capacitor increases (the leakage resistance decreases).

When measuring, one pole is assumed to be "+"pole, which is connected with the black probe of multimeter, and the other pole is connected with the red probe of multimeter. Write down the stop scale of the stylus (the resistance of the stylus is large to the left), then discharge the capacitor (that is, the two leads touch each other), switch the two stylus, and measure again. In the two measurements, the last time the pointer stays on the left side (with large resistance), and the black stylus is connected to the positive electrode of the electrolytic capacitor.

It is best to choose R* 100 or R* 1K for measurement. Judge the quality of capacitor with multimeter

Lecture 5: Judge the quality of capacitor with multimeter.

According to the different capacities of electrolytic capacitors, R× 10, R× 100 and R× 1K of multimeter are usually selected for testing and judgment. The red and black probes are respectively connected to the negative electrode of the capacitor (the capacitor needs to be discharged before each test), and the quality of the capacitor is judged by the deflection of the probe. Generally speaking, if you swing your hands to the right quickly and then slowly return to the original position to the left, the capacitor is good. If the pointer does not turn after swinging, the capacitor has been damaged. If the pointer swings back to a certain position and stops, it means that the capacitor is leaking. If you can't swing your hands, it means that the capacitor electrolyte has dried up and lost its capacity.

It is not easy to accurately judge the quality of some capacitors with leakage by the above methods. When the withstand voltage value of the capacitor in the multimeter is greater than the battery voltage value, according to the characteristics that the leakage current of the electrolytic capacitor is small when charging in the forward direction and large when charging in the reverse direction, R× 10K can be used to charge the capacitor in the reverse direction, and observe whether the stop of the hands is stable (that is, whether the reverse leakage current is constant), so as to judge the quality of the capacitor with high accuracy. The black stylus is connected to the negative electrode of the capacitor, and the red stylus is connected to the positive electrode of the capacitor. When your hands swing up quickly and then gradually retreat to a certain place, it means that the capacitance is good. Any capacitor whose pointer stays in a certain position and is unstable or gradually moves to the right after staying has leaked electricity and can't be used any more. The pointer generally stays and stabilizes in the scale range of 50-200 K.

Lecture 6: Talking about Electrolytic Capacitors

First, the role of electrolytic capacitor in the circuit

1, filtering function, in the power supply circuit, the rectifier circuit changes alternating current into pulsating DC, and the rectifier circuit is connected with a large-capacity electrolytic capacitor, and the rectified pulsating DC voltage is changed into a relatively stable DC voltage by using its charging and discharging characteristics. In practice, in order to prevent the power supply voltage of each part of the circuit from changing due to the load change, electrolytic capacitors of tens to hundreds of microfarads are generally connected between the output end of the power supply and the input end of the load. Because electrolytic capacitors with large capacity generally have a certain inductance and cannot effectively filter out high-frequency and pulse interference signals, a capacitor with a capacity of 0.00 1-0. The two ends of LPF are connected in parallel to filter out high frequency and pulse interference.

2. Coupling effect: In the process of low-frequency signal transmission and amplification, in order to prevent the static working points of the front and rear circuits from influencing each other, capacitive coupling is often used. In order to prevent excessive loss of low-frequency components in the signal, electrolytic capacitors with large capacity are generally used.

Second, the judgment method of electrolytic capacitance

The common faults of electrolytic capacitors include capacity reduction, capacity disappearance, breakdown short circuit and leakage, in which the capacity change is caused by the gradual drying of electrolyte in electrolytic capacitors during use or placement, while breakdown and leakage are generally caused by excessive applied voltage or poor quality. To judge the quality of power supply capacitance, the resistance of multimeter is generally used to measure it. The specific method is: short-circuit discharge the two pins of the capacitor, and connect the black probe of the multimeter to the positive electrode of the electrolytic capacitor. The red contact pin is connected to the negative electrode (for pointer multimeter, the contact pin is intermodulation when measured with a digital multimeter). Under normal circumstances, the stylus should swing in the direction of low resistance and then gradually return to infinity. The larger the swing amplitude of the hand or the slower the return speed, the larger the capacitance and the smaller the capacitance. If the pointer does not change somewhere in the middle, it means that the capacitor is leaking. If the indicated resistance value is small or zero, it means that the capacitor has broken down and shorted. Because the battery voltage used by multimeter is generally low, it is more accurate to measure the capacitance with low withstand voltage. When the capacitor withstand voltage is high, the measurement is normal, but when the voltage is increased.

Three, the use of electrolytic capacitor matters needing attention

1, electrolytic capacitor has positive and negative polarity, so it can't be reversed when used in the circuit. In the power supply circuit, when a positive voltage is output, the anode of the electrolytic capacitor is connected to the output end of the power supply, and when a negative voltage is output, the cathode is connected to the output end and the anode is grounded. When the polarity of the filtering capacitor in the power supply circuit is reversed, the filtering effect of the capacitor is greatly reduced, which on the one hand causes the output voltage of the power supply to fluctuate, and on the other hand causes the electrolytic capacitor equivalent to a resistor to heat up. When the reverse voltage exceeds a certain value, the reverse leakage resistance of the capacitor will become very small, making the power supply work quickly.

2. The voltage applied to the electrolytic capacitor should not exceed its allowable working voltage. When designing the actual circuit, a certain margin should be left according to the specific situation. When designing the filter capacitor of regulated power supply, if the voltage of AC power supply is 220~, the secondary rectification voltage of transformer can reach 22V, and the electrolytic capacitor with voltage resistance of 25V can generally meet the requirements. However, if the voltage of AC power supply fluctuates greatly, it may rise to more than 250 volts, and it is best to choose electrolytic capacitors with withstand voltage greater than 30 volts. ..

3. The electrolytic capacitor should not be close to the high-power heating element in the circuit to prevent the electrolyte from drying due to heating.

4. In order to filter positive and negative polarity signals, two electrolytic capacitors with the same polarity can be connected in series to form a nonpolar capacitor.

Overview of the role of capacitance:

Various capacitors are needed in electronic manufacturing, which play different roles in the circuit. Similar to a resistor, it is usually referred to as a capacitor for short, and is represented by the letter C. As the name implies, a capacitor is a "container for storing charge". Although there are many kinds of capacitors, their basic structures and principles are the same. A capacitor consists of two closely spaced metal sheets separated by a substance (solid, gas or liquid). Two pieces of metal are called boards, and the substance in the middle is called medium. Capacitors are also divided into fixed capacity and variable capacity. But the common ones are constant capacity capacitors, and the most common ones are electrolytic capacitors and ceramic capacitors.

Different capacitors have different abilities to store charge. It is stipulated that the amount of charge stored when a DC voltage of 1 volt is applied to the capacitor is called the capacitance of the capacitor. The basic unit of capacitance is farad (f). But in fact, farad is a very unusual unit, because the capacity of capacitor is often much smaller than 1 farad. Commonly used methods are micron method (μF), nano method (nF) and micro method (micro method is also called micro method), and their relationship is: 1 farad (f) = 100000.

In electronic circuits, capacitors are used to block direct current through alternating current, and also serve as filters to store and release charges, so as to smoothly output pulsating signals. Capacitors with small capacity are usually used in high-frequency circuits, such as radios, transmitters and oscillators. Large-capacity capacitors are usually used to filter and store charges. Moreover, there is another feature. Generally, capacitors above 1μF are electrolytic capacitors, and capacitors below 1μF are mostly ceramic capacitors. Of course, there are others, such as monolithic capacitors, polyester capacitors and small-capacity mica capacitors. The electrolytic capacitor has an aluminum shell, which is filled with electrolyte, and two electrodes are led out as a positive electrode (+) and a negative electrode (-). Unlike other capacitors, their polarity in the circuit cannot be wrong, while other capacitors have no polarity.

Connect the two electrodes of the capacitor to the positive electrode and the negative electrode of the power supply respectively. After a period of time, even if the power supply is turned off, there will still be residual voltage between the two pins (you can observe it with a multimeter after learning the tutorial). We say that a capacitor stores charge. When a voltage is generated between the plates of a capacitor, electric energy is accumulated. This process is called capacitor charging. There is a certain voltage across the charged capacitor. The process that the charge stored in the capacitor is released into the circuit is called the discharge of the capacitor.

For example, in real life, we can see that after unplugging the plug, the LED on the commercial rectifier power supply will continue to light for a while, and then gradually go out, because the capacitor inside stores the electric energy in advance and then releases it. Of course, this capacitor was originally used for filtering. As for capacitive filtering, I don't know if you have heard of the Walkman with rectifier power supply. Generally, low-quality power supply buzzes in headphones because manufacturers use small-capacity filter capacitors in order to save costs. At this time, a large-capacity electrolytic capacitor (1000μF, note that the positive electrode is connected to the positive electrode) can be connected in parallel at both ends of the power supply, which can generally improve the effect. Enthusiasts should at least use 1000 microfabrication to filter when making HiFi audio. The larger the filter capacitor is, the closer the output voltage waveform is to DC. The energy storage function of the large capacitor makes the circuit have enough energy to convert into powerful audio output when the sudden big signal comes. At this time, the function of large capacitor is a bit like a reservoir, which makes the original turbulent water flow output smoothly, and can also ensure the supply when a large amount of water is used downstream.

In an electronic circuit, current can only flow when the capacitor is charged. After the charging process, the capacitor can't pass DC, which plays a role of "blocking DC" in the circuit. In circuits, capacitors are usually used for coupling, bypassing, filtering, etc. All of them use its characteristics of "communicating and isolating DC". So why can alternating current pass through a capacitor? Let's first look at the characteristics of alternating current. Alternating current not only changes alternately in direction, but also changes regularly in size. The capacitor is connected to the AC power supply, and the capacitor is constantly charged and discharged, so that the charging current and discharging current which conform to the AC variation law flow in the circuit.

The choice of capacitor involves many problems. The first is the problem of compression resistance. When the voltage across the capacitor exceeds its rated voltage, the capacitor will be broken down and damaged. Generally, the breakdown voltages of electrolytic capacitors are 6.3V, 10V, 16V, 25V, 50V, etc.

As one of the passive components, the capacitor has the following functions:

1, which is applied to power supply circuit to realize bypass, decoupling, filtering and energy storage functions, is specifically divided into the following categories:

1) bypass

Bypass capacitor is an energy storage device that provides energy for local equipment. It can make the output of voltage regulator uniform and reduce the load demand. Just like a small rechargeable battery, the bypass capacitor can charge and discharge the device. In order to minimize the impedance, the bypass capacitor should be as close as possible to the power supply pin and the ground pin of the load device. This can well prevent the noise caused by the rise of ground potential and excessive input value. Grounding elasticity refers to the voltage drop when the grounding connection passes through the high current burr.

2) Take out the lotus root

Going to lotus root is also called going to lotus root. As far as the circuit is concerned, it can always be divided into driven source and driven load. If the load capacitance is relatively large, the driving circuit needs to charge and discharge the capacitance to complete the signal jump. When the rising edge is steep, the current is relatively large, so the driving current will absorb a large power supply current. Because the inductance and resistance in the circuit (especially the inductance on the chip pin will rebound), this current is actually a kind of noise compared with the normal situation, which will affect the normal work of the previous stage. This is coupling.

Decoupling capacitor acts as a battery to meet the change of driving circuit current and avoid mutual coupling interference.

It will be better understood by combining bypass capacitor and decoupling capacitor. Bypass capacitor is actually decoupled, but generally refers to high-frequency bypass, which is a low-impedance leakage prevention method to improve high-frequency switching noise. The high-frequency bypass capacitance is generally very small, which is generally 0. 1u, 0.0 1u and so on according to the resonance frequency. However, the decoupling capacitance is generally larger, above 10uF, which is determined according to the distribution parameters in the circuit and the change of driving current.

Bypass takes the interference in the input signal as the filtering object, and decoupling takes the interference in the output signal as the filtering object to prevent the interference signal from returning to the power supply. This should be their essential difference.

3) filtering

Theoretically (that is, assuming that the capacitance is pure capacitance), the larger the capacitance, the smaller the impedance and the higher the pass frequency. But in fact, most of the capacitors exceeding 1uF are electrolytic capacitors, and the inductance component is large, so the impedance will increase after the frequency is high. Sometimes we can see that the electrolytic capacitor of a large capacitor is connected in parallel with a small capacitor, and then the large capacitor is connected to the low frequency and the small capacitor is connected to the high frequency. The function of capacitor is to pass high resistance and low resistance and pass high frequency and low frequency. The larger the capacitance, the easier it is for low frequency to pass, and the larger the capacitance, the easier it is for high frequency to pass. Specifically used for filtering, large capacitor (1000uF) filters out low frequency, and small capacitor (20pF) filters out high frequency.

Some netizens compared the filter capacitor to a "pond". Because the voltage across the capacitor does not suddenly change, it can be seen that the higher the signal frequency, the greater the attenuation. It can be said vividly that the capacitor is like a pond, and the water quantity will not change because of adding or evaporating a few drops of water. It changes the voltage into the 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 end of the power supply through the converter lead. Commonly used aluminum electrolytic capacitors (such as B43504 or B43505 of EPCOS) have a rated voltage of 40 ~ 450VDC and a capacitance value of 220 ~ 15000UF. According to different power requirements, devices are sometimes connected in series, parallel or in combination. For the power supply with power level exceeding 10KW, a large can spiral terminal capacitor is usually used.

2. Used in signal circuit, which mainly completes the functions of coupling, oscillation/synchronization and time constant:

1) coupling

For example, the emitter of a transistor amplifier has its own bias resistor, and at the same time, it makes the signal generate a voltage drop and feed it back to the input end, forming the coupling of input and output signals. This resistor is a coupling element. If a capacitor is connected in parallel at both ends of this resistor, the coupling effect caused by the resistor will be reduced because the impedance of the capacitor with appropriate capacitance to AC signal is small, so it is called decoupling capacitor.

2) oscillation/synchronization

Load capacitance including RC, LC oscillator and crystal all belong to this category.

3) Time constant

This is an ordinary integrating circuit composed of R and C in series. When the input signal voltage is applied to the input terminal, the voltage on the capacitor (C) gradually rises. However, the charging current decreases with the increase of voltage. The current characteristic through the resistor (r) and the capacitor (c) is described by the following formula:

i = (V/R)e-(t/CR)