1. [Capacitance; Capacitance is a physical quantity, which indicates the capacity of a capacitor to hold charge. The following property of a non-conductor is that when two opposite surfaces of the non-conductor maintain a certain potential difference (as in a capacitor), energy is stored in the non-conductor due to charge movement.

2. [Capacitance; Capacitor]: the common name of capacitor. Capacitance (or capacitance) is a physical quantity, which indicates the capacity of a capacitor to hold charge. We will increase the potential difference between the two plates of the capacitor by 1 volt, which is called the capacitance of the capacitor. Physically speaking, a capacitor is an electrostatic charge storage medium (just like a bucket, you can charge it). In the absence of a discharge circuit, the self-discharge effect/electrolytic capacitance is obvious, and the charge may exist forever, which is its characteristic). It has a wide range of uses and is an indispensable electronic component in the fields of electronics and electric power. Mainly used in power supply filtering, signal filtering, signal coupling, resonance, DC isolation and other circuits.

The symbol of the capacitor is C.

C=εS/4πkd=Q/U

In the international system of units, the unit of capacitance is farad, and the symbol is F. Commonly used capacitance units are millifarad (mF), microfarad (μF), nanofarad (nF) and picofarad (pF). The conversion relationship is as follows:

1 farad (F)= 1000 millifarad (mF)= 1000000 microfarad (μF)

1 microfabrication (μF)= 1000 nanofarads (nF)= 1000000 picofarads (pF).

Correlation formula:

For a capacitor, if the potential difference between two stages is 1V when charging with 1V, the capacitance of this capacitor is 1F, that is, C=Q/U, but the capacitance is not determined by q or u, that is, C=εS/4πkd. Where ε is a constant, S is the relative area of the capacitor plates, D is the distance of the capacitor plates, and K is the electrostatic force constant. The capacitance of a common parallel plate capacitor is C=εS/d, (ε is the dielectric constant of the medium between plates, S is the plate area, and D is the distance between plates. )

The formula for calculating the potential energy of capacitor is e = Cu 2/2 = Qu/2.

Multi-capacitor parallel calculation formula: C=C 1+C2+C3+…+Cn.

Calculation formula of multiple capacitors in series:1/c =1/c1+1/C2+…+1/cn.

Multi-capacitor parallel addition series C = (C1* C2 * C3)/(C1+C2+C3) 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, in letters, capacitor with C.

The second part:

Materials, expressed in letters.

The third part:

Classification, generally expressed by numbers, individual by letters.

The fourth part:

Serial number, expressed in 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-. Name: Polyester (Polyester) Capacitor (CL)

Symbol:

Capacitance: 40p-4μ

Rated voltage: 63-630 volts

Main features: small volume, large capacity, resistance to damp heat and poor stability.

Purpose: Low-frequency circuit with low requirements on stability and loss.

Name: polystyrene capacitor (CB)

Symbol:

Capacitance: 10p- 1μ

Rated voltage:100V-30KV

Main features: stability, low loss and large volume.

Application: Circuits with high requirements for stability and loss.

Name: Polypropylene Capacitor (CBB)

Symbol:

Capacitance: 1000p- 10μ.

Rated voltage: 63-2000 volts

Main features: Its performance is similar to that of polystyrene, but its volume is small and its stability is slightly poor.

Purpose: to replace most polystyrene or mica capacitors for demanding circuits.

Name: Mica Capacitor (CY)

Symbol:

Capacitance: 10p-0. 1μ

Rated voltage:100V-7KV

Main features: high stability, high reliability and low temperature coefficient.

Application: High-frequency oscillation, pulse and other demanding circuits.

Name: High Frequency Ceramic Capacitor (CC)

Symbol:

Capacitance: 1-6800p

Rated voltage: 63-500 volts

Main features: low high frequency loss and good stability.

Purpose: High frequency circuit.

Name: low frequency ceramic capacitor (CT)

Symbol:

Capacitance: 10p-4.7μ

Rated voltage: 50V 50V-100V

Main features: small size, low price, large loss and poor stability.

Purpose: Low frequency circuit with low requirements.

Name: Glass Glaze Capacitor (CI)

Symbol:

Capacitance: 10p-0. 1μ

Rated voltage: 63-400 volts

Main features: good stability, low loss and high temperature resistance (200 degrees).

Application: Pulse, coupling, bypass and other circuits.

Name: aluminum electrolytic capacitor (CD)

Symbol:

Capacitance: 0.47- 10000μ

Rated voltage: 6.3-450 volts

Main features: small volume, large capacity, large loss and large leakage.

Applications: power supply filtering, low frequency coupling, decoupling, bypass, etc.

Name: Tantalum electrolytic capacitor (CA) Niobium electrolytic capacitor (CN).

Symbol:

Capacitance: 0. 1- 1000μ

Rated voltage: 6.3-125 v.

Main features: loss and leakage are less than aluminum electrolytic capacitance.

Purpose: to replace aluminum electrolytic capacitors in demanding circuits.

Name: air dielectric variable capacitor

Symbol:

Variable capacitance: 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 needs.

Applications: electronic instruments, radio and television equipment, etc.

Name: thin film dielectric variable capacitor

Symbol:

Variable capacitance: 15-550p

Main features: small size and light weight; The loss is greater than that of air medium.

Application: communication, broadcast receiver, etc.

Name: Thin Film Dielectric Trimming Capacitor

Symbol:

Variable capacitance: 1-29P

Main features: large loss and small volume.

Application: Radio recorders, electronic instruments and other circuits are used for circuit compensation.

Name: ceramic dielectric trimming capacitor

Symbol:

Variable capacitance: 0.3-22p

Main features: small loss and small volume.

Purpose: Fine tuning high frequency oscillation circuit.

Name: Monolithic Capacitor

Capacity range: 0.5 pf- 1 μ f

Withstand voltage: twice the rated voltage.

Usage: widely used in electronic precision instruments. Resonance, coupling, filtering and bypass of various small electronic devices.

Monolithic capacitors are characterized by large capacitance, small volume, high reliability, stable capacitance, good high temperature and humidity resistance, etc.

The biggest disadvantage is that the temperature coefficient is very high, so it is unbearable to make the oscillator drift steadily. The capacitor of a 555 oscillator we made is next to 7805. After starting, the frequency of oscilloscope changes slowly, so it is much better to switch to polyester capacitor later.

As far as temperature drift is concerned, the whole ceramic has a positive temperature of about+130, while CBB has a negative temperature coefficient of -230. When used in parallel in an appropriate proportion, the temperature drift can be reduced to a very small level.

In terms of price, Tantalum-niobium capacitors are the most expensive, monolith and CBB are cheaper, and ceramics are the lowest, but some high-frequency zero-temperature drift black-spot ceramics are slightly more expensive, and mica capacitors have higher Q value and are slightly more expensive.

It is said that monolithic capacitors are also called multilayer ceramic capacitors, which are divided into two types. 1 model has good performance, but its capacity is small, generally less than 0. 2U, and the other is called Type II, which has large capacity but average performance. In many electronic products, capacitor is an essential electronic component, which plays the role of smoothing filter of rectifier, power supply and decoupling, bypass of AC signal, AC coupling of AC /DC circuit and so on. Because there are many types and structures of capacitors, users need to know not only the performance indexes and general characteristics of various capacitors, but also the advantages and disadvantages, mechanical or environmental constraints of various components in a given use. The main parameters and uses of capacitors are introduced below for readers to use when choosing capacitor types.

1. Nominal capacitance (CR): the capacitance value marked on the capacitor product.

The capacitance of mica and ceramic dielectric capacitors is low (about below 5000pF); The capacitance of paper, plastic and some ceramic media is in the middle (about 0.005 μ f10 μ f); Generally, the capacity of electrolytic capacitors is relatively large. This is a rough classification.

2. Category temperature range: The ambient temperature range that can work continuously as determined by the capacitor design depends on the temperature limit of its corresponding category, such as the upper category temperature, the lower category temperature and the rated temperature (the highest ambient temperature that can continuously apply rated voltage).

3. Rated voltage (UR): the effective value of the maximum DC voltage or the maximum AC voltage or the peak value of the pulse voltage that can be continuously applied to the capacitor at any temperature between the lower limit temperature and the rated temperature.

When capacitors are used in high voltage situations, 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 DC voltage rating in use.

4. Tangent of loss angle (tanδ): Under the sinusoidal voltage with specified frequency, the loss power of capacitor is divided by the reactive power of capacitor.

What needs to be explained here is that 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 following figure. In the figure, C is the actual capacitance of the capacitor, Rs is the series equivalent resistance of the capacitor, Rp is the insulation resistance of the medium, and Ro is the absorption equivalent resistance of the medium. For electronic equipment, the smaller the Rs, the better, that is to say, the smaller the power loss and the smaller the angle δ with the capacitor power.

This relationship is expressed by the following formula: tanδ=Rs/Xc=2πf×c×Rs. Therefore, this parameter should be carefully selected in application to avoid excessive self-heating and reduce equipment failure.

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

Supplement:

1. Capacitance is generally represented by "c" plus a number in the circuit (for example, C 13 is represented by the number 13). Capacitor is an element with two metal films attached to each other and separated by insulating material. The characteristics of capacitors are mainly blocking DC and circulating AC.

The size of the capacitor means the amount of electric energy that can be stored. The blocking effect of capacitance on AC signal is called capacitive reactance, which is related to the frequency and capacitance of AC signal.

Capacitance XC= 1/2πf c (f stands for the frequency of AC signal, and c stands for capacitance) The commonly used capacitor types in telephones are electrolytic capacitor, ceramic capacitor, patch capacitor, monolithic capacitor, tantalum capacitor and polyester capacitor.

2. Identification method: The identification method of capacitance is basically the same as that of resistance, which is divided into three methods: direct standard method, color standard method and number standard method. The basic unit of capacitance is farad (f), and other units are millifarad (mF), microfarad (μF)/mju:/, nanofarad (nF) and picofarad (pF). Where: 1 Farah = 1000 millifarads (mF), 1 millifarads = 1000 microfarads (μF), 1 microfarads = 1000 nanofarads (nF).

The capacitance value of a large capacitor is directly indicated on the capacitor, such as 10 μF/ 16V.

The capacitance value of a capacitor with a small capacity is represented by letters or numbers on the capacitor.

Letter symbol:1m =1000μ f1p 2 =1.2pf1n =1000pf.

Digital representation: the representation of three digits is also called the digital representation of capacitance. The first two digits of the three digits are the significant digits of the nominal capacity, and the third digit represents the number of zeros after the significant digits, all in pF.

For example, 102 means that the nominal capacity is 1000pF.

22 1 means that the nominal capacity is 220pF.

224 denotes a nominal capacity of 22x 10(4)pF.

There is a special case of this representation, that is, when the third digit is represented by 9, the effective number is multiplied by 10- 1 to represent the capacity.

For example, 229 means that the nominal capacity is 22x (10-1) pf = 2.2 pf. ..

Allowable error1%2% 5%10%15% 20%

For example, the capacitance of a ceramic chip is 104J, which means the capacitance is 0. 1 μF, and the error is 5%.

6 service life: the service life of capacitor 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: As the electronic activity increases with the increase of temperature, the insulation resistance will decrease with the increase of temperature.

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. 1, nominal capacitance and allowable deviation

2. Rated voltage

3. Insulation resistance

Step 4 fail

5. Frequency characteristics

Fault characteristics analysis of electronic components

Although there are a large number of electronic components in electrical equipment, their failures are regular.

1. Characteristics of resistance damage

Resistance is the most numerous component in electrical equipment, but it is not the component with the highest damage rate. Resistance damage is the most common in open circuit, but the resistance increases very little and the resistance decreases very little. Common are carbon film resistance, metal film resistance, wire wound resistance and safety resistance. The first two types of resistors are the most widely used, and their damage characteristics are as follows: First, the damage rate of low resistance (below100Ω) and high resistance (above100Ω) is high, and the medium resistance (such as hundreds of ohms to tens of thousands of ohms) is rarely damaged; Second, when the low resistance resistor is damaged, it is often blackened and easy to find, while when the high resistance resistor is damaged, there are few traces. Wire wound resistance is generally used for large current limit, and its resistance is not large. After the cylindrical wire wound resistor is burnt out, some will be black or the surface will burst, and some will have no trace. Cement resistance is a kind of wire-wound resistance, which may break when burned out, otherwise no trace can be seen. The safety resistor is burnt out, and some surfaces will blow off a piece of skin, while others will have no trace, but they will never burn black. According to the above characteristics, we can focus on checking the resistance and quickly find out the damaged resistance.

2. The characteristics of electrolytic capacitor damage

Electrolytic capacitors are widely used in electrical equipment with high failure rate. The damage of electrolytic capacitor has the following manifestations: first, the capacity is completely lost or reduced; Second, slight or serious leakage; Third, the capacity is lost or reduced, and there is leakage. The method to find the damaged electrolytic capacitor is as follows:

(1) Look: some capacitors will leak electricity when damaged, and there will be a layer of oil stains on the surface of the circuit board or even the surface of the capacitor below the capacitor. This capacitor can never be used again; Some capacitors will swell up after being damaged, so this capacitor can no longer be used.

(2) Touch: Some electrolytic capacitors with serious leakage will generate heat after starting, and even touch with fingers will generate heat. The capacitor must be replaced;

(3) There is electrolyte in the electrolytic capacitor, and baking for a long time will dry the electrolyte and reduce the capacitance. Therefore, it is very important to check the capacitance near the heat sink and high-power components. The closer the distance, the greater the possibility of damage.

3. Second, the characteristics of semiconductor devices such as triodes are damaged.

Second, the damage of triode is generally PN junction breakdown or open circuit, of which breakdown short circuit is the majority. In addition, there are two kinds of damage performance: first, the thermal stability becomes worse, it is normal at start-up, and soft breakdown appears after working for a period of time; The other is the deterioration of PN junction characteristics. When measured by multimeter R× 1k, all PN junctions are normal, but they can't work normally after being put on the computer. If we use a low-range instrument, R× 10 or R× 1, we will find that the forward resistance of PN junction is larger than normal. Measure two. The triode can be measured by pointer multimeter on the road. The more accurate method is: set the multimeter at R× 10 or R× 1 (generally use R× 10, and then use R× 1 when it is not obvious) to measure the forward and backward resistance of the PN junction of II and triode on the road. If the forward resistance is not too large, it is because the peripheral resistance of diodes and triodes in general circuits is mostly above hundreds or thousands of ohms. When measuring with a multimeter with low resistance, the influence of peripheral resistance on the resistance of PN junction can be basically ignored.

4. The characteristics of integrated circuit damage

The integrated circuit has complex internal structure and many functions, and any part of it can't work normally if it is damaged. There are also two kinds of damage to integrated circuits: complete damage and poor thermal stability. When it is completely damaged, it can be removed. Compared with normal integrated circuits of the same model, it is always found that the positive and negative resistance of each pin to ground is abnormal. For those with poor thermal stability, the suspected integrated circuit can be cooled with anhydrous alcohol when the equipment is working. If the fault occurs later or no longer, it can be judged. Usually it can only be eliminated by replacing a new integrated circuit.