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Detailed analysis of common sense knowledge of electrolytic capacitors

elecfans | 21/04/2021

With the rapid development of society, our electrolytic capacitors are also developing rapidly. Electrolytic capacitors are capacitors in which an oxide layer formed on an electrode by an electrolyte acts as an insulating layer, and usually has a larger capacity. Electrolyte is a liquid, jelly-like substance rich in ions. Most electrolytic capacitors are polarized, that is, when working, the voltage of the positive electrode of the capacitor must always be higher than the voltage of the negative electrode.

The high capacity of electrolytic capacitors is also sacrificed because of many other characteristics, such as large leakage current, large equivalent series inductance and resistance, large capacitance error and short life. In addition to polar electrolytic capacitors, there are also non-polar electrolytic capacitors. There are two types of 1000uF, 16V electrolytic capacitors, the larger one is non-polar, and the smaller one is polar. The inside of an electrolytic capacitor can be a liquid electrolyte or a solid polymer, and the electrode material is usually aluminum (aluminum) or tantalum (tantalum). There is a layer of fiber paper impregnated with electrolyte between the two layers of electrodes, and a layer of insulating paper is added to make it cylindrical and sealed in an aluminum shell.

Electrolytic capacitors are widely used in household appliances and various electronic products, with a large capacity range, generally 1~33000μF, and a rated working voltage range of 6.3~700V. Its disadvantages are dielectric loss, large capacity error (the maximum allowable deviation is +100%, -20%), poor high temperature resistance, and prone to failure due to long-term storage. The polarity of the electrolytic capacitor, pay attention to the side of the electrolytic capacitor with "-" indicating negative pole, and "+" indicating positive pole. If the positive and negative poles are not marked on the electrolytic capacitor, it can also be judged based on the length of its pins. It is the positive pole, and the short leg is the negative pole.

If you want to know the cause of the explosion of an electrolytic capacitor, you must first know what an electrolytic capacitor is. Electrolytic capacitor is a type of capacitor. The metal foil is the positive electrode (aluminum or tantalum), and the positive electrode is closely attached to the metal oxide film (aluminum oxide or tantalum pentoxide). It is a dielectric. The cathode is composed of conductive materials, electrolyte (the electrolyte can be liquid or solid) and other materials. Since the electrolyte is the main part of the cathode, it is named after the electrolytic capacitor. At the same time, the positive and negative electrolytic capacitors cannot be connected by mistake. Aluminum electrolytic capacitors can be divided into four categories: lead-type aluminum electrolytic capacitors. Amplifier-type aluminum electrolytic capacitors; bolt-type aluminum electrolytic capacitors; solid aluminum electrolytic capacitors.

When the voltage applied to the capacitor exceeds its withstand voltage, or when the voltage polarity of a polarized electrolytic capacitor is reversed, the leakage current of the capacitor will rise sharply, resulting in an increase in the internal heat of the capacitor, and the electrolyte will generate a large amount of gas. In order to prevent the capacitor from exploding, three grooves are pressed on the top of the capacitor case so that the top of the capacitor can rupture first under high pressure and release the internal pressure.

The electrolyte will expand when heated, and when it expands to a certain extent, it will open the capacitor case and leak out. This is usually called a leak. When the temperature rises slowly, the electrolyte in the electrolytic capacitor will slowly leak; but when the temperature rises very fast, the inside of the electrolytic capacitor will expand rapidly, which will immediately stretch the shell of the electrolytic capacitor, or even destroy the entire shell, which is an explosion .

Polarized electrolytic capacitors usually play a role in power circuits or intermediate and low frequency circuits for power supply filtering, decoupling, signal coupling, time constant setting and DC blocking. Usually cannot be used in AC power circuits. When used as a filter capacitor in a DC power supply circuit, its anode (positive) should be connected to the positive terminal of the power supply voltage, and the cathode (negative) should be connected to the negative terminal of the power supply voltage, not reversed. , Otherwise it will damage the capacitor.

The reason for the increase in the temperature of the electrolytic capacitor is that its own power consumption increases abnormally, and exceeds the power consumption that it can withstand. The reasons for the increase in power consumption may be: electrolytic capacitor overvoltage; large ripple current; reverse polarity.

However, in the manufacturing process of some capacitors, the top groove is unqualified, and the pressure inside the capacitor will cause the sealing rubber at the bottom of the capacitor to pop out. At this time, the pressure inside the capacitor is suddenly released and an explosion will occur. Once an electrolytic capacitor with a larger capacity explodes, its power supply may endanger personal safety. Therefore, not only should a large-capacity electrolytic capacitor be designed from the perspective of performance, but also a large-capacity electrolytic capacitor should be designed from the perspective of safety. Many electrolytic capacitors are designed with explosion-proof tanks on the top of the capacitors, and some electrolytic tanks have explosion-proof holes beside the pins. When it increases, the explosion-proof valve will open to release the pressure to avoid an explosion.

The above is a detailed analysis of the relevant knowledge of electrolytic capacitors. It is necessary for everyone to continue to accumulate experience in practice in order to design better products.
 
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