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Main performance indicators of commonly used capacitors

Main performance indicators of commonly used capacitors

Nominal capacity and allowable error: the capacity of a capacitor to store electric charge. Commonly used units are F, uF, and pF. The capacitance number marked on the capacitor is the nominal capacity of the capacitor. The nominal capacity of the capacitor and its actual capacity will have an error. The allowable error levels of commonly used fixed capacitors are shown in Table 2. The nominal capacity series of commonly used fixed capacitors are shown in Table 3. Generally, the capacity is written directly on the capacitor, and numbers are also used to mark the capacity. Usually when the capacity is less than 10000pF, the unit is pF, and when it is greater than 10000pF, the unit is uF. For the sake of simplicity, capacitors larger than 100pF and smaller than 1uF are often not marked with units. If there is no decimal point, its unit is pF, and if there is a decimal point, its unit is uF. If some capacitors are marked with "332" (3300pF) with three significant digits, the first and second digits of the capacitance are given by the two digits from the left, and the third digit indicates the number with 0 after it. The unit It is pF. Rated working voltage: In the specified working temperature range, the capacitor can work reliably for a long time, and the maximum DC voltage it can withstand is the withstand voltage of the capacitor, also called the DC working voltage of the capacitor. If it is in an AC circuit, it should be noted that the maximum value of the applied AC voltage cannot exceed the DC working voltage value of the capacitor. Commonly used fixed capacitor operating voltages are 6.3V, 10V, 16V, 25V, 50V, 63V, 100V, 2500V, 400V, 500V, 630V, 1000V.
Release time : 2020-04-09
Characteristics and selection of ceramic capacitors

Characteristics and selection of ceramic capacitors

Ceramic capacitors are currently the most widely used capacitors in electronic equipment, accounting for about 50% of the total number of capacitors used. However, due to the lack of understanding of their characteristics by many people, they lack due attention in use. In order to meet the standardization and standardization requirements of use, the following is an overview of the characteristics of ceramic capacitors and the matters needing attention in use: 1. Characteristic classification of ceramic capacitors: Ceramic capacitors have the advantages of good heat resistance, excellent insulation, simple structure, and low price. However, the characteristics of different ceramic materials are very different and must be selected correctly according to the requirements of use. Ceramic capacitors are classified into high-frequency ceramic capacitors (Class 1 ceramic) and low-frequency ceramic capacitors (Class 2 ceramic) according to frequency characteristics; high-voltage ceramic capacitors (above 1KVDC) and low-voltage ceramic capacitors (below 500VDC) , Are described as follows: 1. High-frequency ceramic capacitors (also known as Class 1 ceramic capacitors) The loss of this type of ceramic capacitor varies little with frequency in a wide range, and the high-frequency loss value is very small (tanδ≤0.15%, f=1MHz), and the maximum operating frequency can reach more than 1000MHz. At the same time, this type of ceramic capacitor has excellent temperature characteristics and is suitable for high-frequency resonance, filtering and temperature compensation circuits that require high capacity and stability. The national standard models are CC1 (low pressure) and CC81 (high pressure). At present, the commonly used temperature characteristic groups of our company are CH (NP0) and SL groups. The normal capacity ranges are as follows: The temperature coefficient αC=1/C (C2-C1/t2-t1) X106 (PPM/°C) in the table refers to the relative change rate of capacitance for every 1°C temperature change within the allowable temperature range. It can be seen from the above table that the temperature coefficient of the type 1 ceramic dielectric capacitor is very small, especially the CH characteristic, so the CH capacitance of the type 1 ceramic dielectric capacitor is often called a temperature compensation capacitor. However, due to the low dielectric constant of this type of ceramic material, its capacity value is difficult to increase. Therefore, when a capacitor with a higher capacitance value is needed, it can only be found in the 2 types of ceramic dielectric capacitors described below. 2. Low-frequency ceramic capacitors (also known as Class 2 ceramic capacitors) The ceramic material of this type of ceramic dielectric capacitor has a large dielectric constant, so the capacitor is small in size and wide in capacity, but the frequency and temperature characteristics are poor, so it is only suitable for those with low requirements for capacity, loss and temperature characteristics. The low frequency circuit is used as bypass, coupling, filtering and other circuits. The national standard models are CT1 (low pressure) and CT81 (high pressure), and their common temperature characteristic groups and normal capacity ranges correspond to the following: The temperature change rate rC/C in the table refers to the capacity change rate at the upper and lower limit temperature relative to room temperature +25°C. Among them, the 2R group is a low-loss capacitor. Because of its low temperature rise and better frequency characteristics, it can be used in higher frequency applications. For low-voltage ceramic capacitors, when the capacity is greater than 47000pF, you can only choose 3 types of ceramic capacitors (also known as semiconductor ceramic capacitors), for example: 26-ABC104-ZFX, but this type of capacitor has worse temperature characteristics and lower insulation resistance Low, just because of the high dielectric material, the volume can be made small. Therefore, it is only suitable for working circuits with lower requirements. If a larger capacity capacitor is selected, and there are higher requirements for capacity and temperature characteristics, then 27 types of organic film capacitors should be selected. 3. AC ceramic capacitor According to the requirements for the safe use of AC power supply, an AC ceramic capacitor with high insulation characteristics and high dielectric strength has been specially designed and produced in Class 2 ceramic capacitors, also known as Y capacitors, which are divided into Y1, Y2, The three major series of Y3 are classified as follows: 2. The packaging and dimensions of ceramic capacitors. Although ceramic capacitors have many of the above advantages, due to the low mechanical strength and fragility of the ceramic material itself, the geometric size of the wafer is limited. This is also the main reason why different temperature characteristics have different capacity ranges. Generally
Release time : 2020-04-09
The main parameters and circuit application of varistor

The main parameters and circuit application of varistor

Introduction: Varistors are generally used in parallel in the circuit. When the voltage across the resistor changes sharply, the resistor short-circuits the current fuse to fuse it, which plays a protective role. Varistors are often used in power supply overvoltage protection and voltage stabilization in circuits. Parameters that need to be understood in circuit design: 1. Varistor voltage UN (U1mA): usually the voltage when a 1mA direct current is passed on the varistor to indicate whether it is conductive, this voltage is called the varistor voltage UN. Varistor voltage is also commonly represented by the symbol U1mA. The error range of the varistor voltage is generally ±10%. In the test and actual use, a 10% drop in the varistor voltage from the normal value is usually used as the criterion for the failure of the varistor.   2. Maximum continuous working voltage UC: refers to the maximum AC voltage (effective value) Uac or the maximum DC voltage Udc that the varistor can withstand for a long time. Generally Uac≈0.64U1mA, Udc≈0.83U1mA   3. Maximum clamping voltage (limiting voltage) VC: The maximum clamping voltage value refers to the voltage appearing on the varistor when the specified 8/20μs wave impulse current IX (A) is applied to the varistor.   4. Leakage current Il: The current that flows when the maximum DC voltage Udc is applied to the varistor. When measuring the leakage current, usually add Udc=0.83U1mA voltage to the varistor (sometimes 0.75U1mA is also used). Generally, the static leakage current Il ≤ 20μA (there are also requirements ≤ 10μA). In actual use, what is more concerned about is not the size of the static leakage current itself, but its stability, that is, the rate of change after the impact test or under high temperature conditions. After the impact test or under high temperature conditions, the rate of change does not exceed one time, which is considered stable   Methods and steps   1. Calculation of varistor voltage:    can generally be calculated by the following formula:   U1mA=KUac Where: K is a coefficient related to power quality, generally K=(2~3), cities with better power quality may be smaller, and rural areas with poor power quality (especially mountainous areas) may be larger. Uac is the effective value of AC power supply voltage. For 220V~240V AC power surge arresters, a varistor with a varistor voltage of 470V~620V should be used. Choosing a varistor with a higher varistor voltage can reduce the failure rate and prolong the service life, but the residual voltage will increase slightly.   2. Calculation of the nominal discharge current:    The nominal discharge current of the varistor should be greater than the required surge current or the maximum surge current that may occur every year. The nominal discharge current should be calculated by pressing the value of more than 10 impacts in the surge life rating curve of the varistor, which is about 30% of the maximum impact flow rate (ie 0.3 IP)    3. Parallel connection of varistors:    When a varistor cannot meet the requirements of the nominal discharge current, multiple varistors should be used in parallel. Sometimes in order to reduce the limit voltage, even if the nominal discharge current meets the requirements, multiple varistors are used in parallel. Special attention should be paid to the fact that when varistors are used in parallel, they must be strictly selected with the same parameters (for example: ΔU1mA≤3V, Δα≤3) for pairing to ensure uniform current distribution. Precautions The temperature fuse should have a good thermal coupling with the varistor. When the varistor fails (high impedance short circuit), the heat generated by it will fuse the temperature fuse to separate the failed varistor from the circuit to ensure the equipment Safety. When the high power frequency temporary overvoltage acts on the varistor, it may cause the varistor to instantaneously break down and short-circuit (low-impedance short-circuit), and the temperature fuse is too late to blow, and it may catch fire. In order to avoid this phenomenon, an impact-resistant power frequency fuse can be connected in series with each varistor (single-use power frequency fuse may not blow when aging failure)
Release time : 2020-04-09
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