PCB technology Electrolytic capacitors and capacitors
one. Fixed capacitor: refers to a capacitor whose capacitance cannot be changed once it is made.
Capacitors can generally be divided into non-polar ordinary capacitors and polar electrolytic capacitors. Common capacitors are divided into fixed capacitors, semi-adjustable capacitors (trimming capacitors), and variable capacitors.
1. Classification of capacitors: Capacitors are generally classified by dielectric.
1) Paper-membrane capacitors: The general capacity is from a few tens of picofarads (pF) to a few microfarads (uF), and the withstand voltage is 250V, 400V, 630V, etc. The capacity error is generally: ±5%, ±10%, ±20 %. There is also a metallized paper capacitor, the biggest feature is the limited self-healing ability. Generally can not be used in high-frequency circuits, the operating frequency is only tens of KHZ.
2) Polyester capacitors:
3) Polystyrene capacitors:
4) Polypropylene capacitors:
5) PTFE capacitor:
6) Polyimide film capacitors:
7) Polycarbonate film capacitors:
8) Composite film capacitors:
9) Film capacitor:
10) Laminated metallized polycarbonate capacitors:
11) Mica capacitors:
12) Porcelain capacitors: inexpensive and widely used. Divided into two kinds of low voltage low power and high voltage and high power.
Low-voltage low-power ceramic capacitors can be classified into types I and II according to the properties and characteristics of the materials used. Type I is characterized by low dielectric loss, capacitance for temperature, frequency, voltage, and time stability are better, often used in high-frequency circuits. Type II is characterized by small size, but poor stability, large dielectric loss, commonly used in low-frequency circuits. UHF ceramic capacitors can be used in high frequency circuits with frequencies up to 500MHz.
High-voltage, high-power ceramic capacitors are generally only suitable for use in low-loss, low-power circuits.
13) Glass Glaze Capacitors:
2. The model name of the capacitor:
1) The naming of capacitors in various countries is not uniform, and the naming of domestic capacitors is composed of four parts:
The first part: Names are represented by letters and capacitors are C.
Part II: Use letters to represent materials.
The third part: use digital representation classification.
Part IV: Numbers are numbered.
2) Capacitance marking method:
(1) Straight mark method:
Mark the model and specification directly on the shell with letters and numbers.
(2) Text symbol method:
The capacity is represented by a regular combination of numbers and characters. The text symbol indicates the unit of its capacitance: P, N, u, m, F, and so on. The same way as resistors. The nominal allowable deviation is also the same as the representation of the resistance. For capacitors smaller than 10pF, the allowable deviations are replaced by letters: B ± 0.1pF, C ± 0.2pF, D ± 0.5pF, F ± 1pF.
(3) Color labeling method:
The same as the resistor, the unit is generally pF. The withstand voltage of a small electrolytic capacitor is also color-coded and located close to the base of the positive lead. The meanings indicated here are as follows:
Color Black Brown Red Orange Yellow Green Blue Purple Purple
Withstand Voltage 4V 6.3V 10V 16V 25V 32V 40V 50V 63V
(4) Marking method for imported capacitors:
Imported capacitors generally have 6 components.
The first item:
Use letters to indicate categories:
Second item:
Use two digits to indicate its shape, structure, package method, lead start and relationship with the axis.
Item 3: The temperature characteristics of the temperature compensation capacitor are useful for letters and colors. The meanings are as follows:
No. Letter Color Temperature Coefficient Allowable Deviation Letter Color Temperature Coefficient Allowable Deviation
1 A gold +100 R yellow -220
2 B gray +30 S green -330
3 C Black 0 T Blue -470
4 G ±30 U Violet -750
5 H Brown -30 ±60 V -1000
6 J ±120 W -1500
7 K ±250 X -2200
8 L Red -80 ±500 Y -3300
9 M ±1000 Z -4700
10 N ±2500 SL +350~-1000
11 P Orange -150 YN -800~-5800
Remark: The unit of temperature coefficient is 10e -6/°C; the allowable deviation is %.
Item 4: Withstand the pressure with numbers and letters, the letter represents a valid number, and the number represents a power of 10 of the multiplicand.
Item 5: Nominal capacity, represented by three digits, the first two being valid values and the third being a power of 10. When there are decimals, they are represented by R or P. The unit of ordinary capacitor is pF, and the unit of electrolytic capacitor is uF.
Item 6: Permissible deviations. With a single letter, the meaning is the same as that of domestic capacitors. It is also useful for color labeling, meaning the same as the logo method for domestic capacitors.
3. The main characteristic parameters of the capacitor:
(1) Capacity and error: The maximum allowable deviation range of actual capacity and nominal capacity. Generally divided into 3 levels: I level ± 5%, II level ± 10%, III ± 20%. In some cases, there are 0 levels with an error of ±20%. The allowable error of the precision capacitor is small, and the error of the electrolytic capacitor is large, and they use different error levels.
Commonly used capacitors have the same accuracy level as resistors. With letters: D – 005 level – ± 0.5%; F – 01 level – ± 1%; G – 02 level – ± 2%; J – I level – ± 5%; – Class II – ± 10%; M – Class III – ± 20%.
(2) Rated operating voltage: Capacitors in the circuit can be long-term stable, reliable work, the maximum DC voltage withstand, also known as pressure. For devices with the same structure, medium, and capacity, the higher the withstand voltage, the larger the volume.
(3) Temperature coefficient: The relative change in capacitance for each temperature change of 1°C within a certain temperature range. The smaller the temperature coefficient, the better.
(4) Insulation resistance: used to indicate the size of leakage. Generally small-capacity capacitors have large insulation resistances of several hundred megaohms or several gigaohms. The electrolytic capacitor’s insulation resistance is generally small. Relatively speaking, the larger the insulation resistance, the better, and the leakage current is also small.
(5) Loss: The energy consumed by a capacitor to heat up in a unit of time under the influence of an electric field. These losses mainly come from dielectric loss and metal loss. It is usually expressed as a loss tangent.
(6) Frequency characteristics: The properties of the capacitor’s electrical parameters vary with the frequency of the electric field. Capacitors operating at high frequencies have a corresponding decrease in capacitance as the dielectric constant is lower at high frequencies than at low frequencies. Loss also increases with frequency. In addition, in the high-frequency operation, the distribution parameters of the capacitor, such as the resistance of the pole piece, the resistance between the lead and the pole piece, the self inductance of the pole piece, the lead inductance, etc., will affect the performance of the capacitor. All of this makes the use of capacitors limited.
Different types of capacitors have different maximum frequencies of use. Small mica capacitors are within 250MHZ; wafer-type ceramic capacitors are 300MHZ; round-tube ceramic capacitors are 200MHZ; disc-type porcelain is up to 3000MHZ; small paper capacitors are 80MHZ; and medium paper capacitors are only 8MHZ.
4. Circuit symbol:
5. Use of capacitors:
1) Select the appropriate model:
2) Reasonably determine the accuracy of the capacitor:
3) Determine the rated working voltage of the capacitor: For the general circuit, the working voltage of the circuit shall be 10%~20% of the rated voltage of the capacitor; when there is a pulsating voltage, the working voltage shall be the highest voltage of the pulsation. When applied to AC, the rated voltage increases with frequency. When the temperature environment is relatively high, the rated voltage must be chosen larger.
4) Try to choose a capacitor with a large insulation resistance:
5) Consider the temperature coefficient and frequency characteristics:
6) Note the use of the environment:
two. Electrolytic capacitor:
Electrolytic capacitors generally have positive and negative polarity, ie they have polarity. Therefore, positive and negative electrodes cannot be connected incorrectly when used in a circuit. It is now possible to manufacture non-polarized or electrolytic capacitors for AC circuits, called bipolar electrolytic capacitors or nonpolar electrolytic capacitors. In the role of the applied voltage, due to a certain cause of local damage to the device, with its own repair role, this phenomenon is called electrolytic capacitor self healing.
Electrolytic capacitors can only be used within the range of -20°C to +70°C.
Characteristics are greatly affected by temperature and frequency. Aluminum electrolytic capacitors are commonly used. Aluminum electrolytic capacitor type is generally CDXX, capacity, pressure, positive and negative are marked on the shell. Sometimes the length of the lead is used to indicate that the long line is positive and the short line is negative.
Aluminum electrolytic capacitors have large losses, poor temperature and frequency characteristics, and limit their application in AC circuits.
three. Tantalum electrolytic capacitors:
The tantalum electrolytic capacitor is a capacitor using a metal tantalum as an electrode and tantalum oxide as a dielectric. Features are: high chemical stability, high rated pressure, high temperature performance, high mechanical strength, small size. Commonly used CA logo, its capacity from 0.47uF to 1000uF, rated voltage is mainly 6.3V, 10V, 16V, 63V several. The performance is much better than aluminum electrolytic capacitors, but the price is more expensive.
Other electrolytic capacitors include tantalum electrolytic capacitors, which have better performance and smaller size. Titanium electrolytic capacitors, tantalum-niobium electrolytic capacitors.
four. Miniature capacitors:
1. Chip ceramic capacitors: surface mount capacitors. The capacity range is 1pF to 47800pF and the withstand voltage is 25V and 50V. Package mounting dimensions are the same as surface mount resistors.
2. Chip tantalum capacitors: small size, large capacity. The welding temperature is limited to 280°C and the time is less than 10S. Its capacity range is 0.1uF ~ 100uF, DC operating voltage is 4 ~ 50V, commonly used voltage is 16V, 35V and so on.
3. Semiconductor capacitors:
4. Double layer electrolytic capacitor: Between the battery and the capacitor, it has a large capacity and is called a super capacitor. It can only be used under DC or low frequency conditions and is often used as a storage element.