Deep analysis of aluminum electrolytic capacitors that cannot withstand reverse voltage

The figure below shows the basic structure of an aluminum electrolytic capacitor consisting of an anode (anode), an aluminum layer of alumina attached to an insulating medium, a cathode aluminum layer of the receiving pole, and a true cathode composed of an electrolyte. The electrolyte is saturated on the paper between the two aluminum layers.

The aluminum oxide layer is plated on the aluminum layer and is very thin relative to the voltage applied thereto, which is easily broken down, resulting in capacitor failure.

The aluminum oxide layer can withstand a positive DC voltage, and if it is subjected to a reverse DC voltage, it can easily fail within a few seconds. This phenomenon is called 'Valve Effect', which is why the aluminum electrolytic capacitor has polarity. If both electrodes of the electrolytic capacitor have an oxide layer, a non-polar capacitor is formed.

Many articles have reported the mechanism of the threshold phenomenon of the reverse voltage of aluminum electrolytic capacitors, called Hydrogen ion theory. When the electrolytic capacitor is subjected to reverse DC voltage, the cathode of the electrolyte is subjected to the forward voltage and the oxide layer is subjected to Negative voltage, the hydrogen ions collected in the oxide layer will pass through the medium to the boundary between the medium and the metal layer, and converted into hydrogen. The expansion force of the hydrogen causes the oxide layer to fall off. Therefore, the current directly flows through the capacitor after breaking through the electrolyte, and the capacitor fails. This DC voltage is very small. Under the action of the reverse DC voltage of 1~2V, the aluminum electrolytic capacitor will immediately fail due to the hydrogen ion effect in a few seconds. Conversely, when the electrolytic capacitor is subjected to a forward voltage, negative ions are concentrated between the oxide layers. Because the negative ions are very large in diameter, they do not penetrate the oxide layer and can withstand higher voltages.

Glossary:

1. Anode (anode): The anode aluminum layer, which is the positive electrode of the electrolytic capacitor. 2. Cathode: The electrolyte layer. 3. Dielectric di: A layer of aluminum oxide attached to the surface of the aluminum layer. 4. Cathode Foil: Connects the electrolyte to the outer layer. This layer does not require oxidation during fabrication, but in practice it is naturally oxidized due to the easy oxidation of aluminum during the etching process. Oxide layer, this oxide layer can withstand a voltage of 1~2v.

5. Spacer paper: Isolation of the cathode and anode so that they are not directly shorted and adsorb a certain amount of electrolyte.

What happens when there is a polarity capacitor reversed?

If the capacity of the capacitor is small, the withstand voltage is very high, and the working voltage is low, the reverse connection cannot be seen. If the capacity is slightly larger (above 100UF), the withstand voltage is close to the working voltage, and the capacitor will not be more than 10 minutes. The form of expression is: first drum, then blow, then burst.

If there is a polarity capacitor, the reverse connection will explode. Does it mean that it cannot be directly connected to the AC power supply?

Can not be connected to the AC power supply, because this polar capacitor design is used on the DC power supply for filtering. I have also asked this question for a long time. I have been asking for a long time, "Is the capacitor not blocked?" How can a polar capacitor be used on an AC power supply?" Because this polar capacitor has a special substance inside, this material cannot withstand back pressure. If it is connected to AC, it will reverse breakdown or explosion.

If there is a polar capacitor that cannot be reversed, why is it allowed to pass the negative half cycle?

The AC signal can be used as a short circuit under certain conditions. How to solve the negative half cycle of the AC signal? Is it going to be pulled into DC?

The AC signal must be carried on the DC current, which is to be pulled up into DC!

When a polar capacitor is working, the positive pole potential must be higher than the negative pole. Otherwise, the capacitor leaks----the light circuit will not work, and the capacitor will explode.

Why is the polarity capacitor reversed?

The internal structure of the polar capacitor is divided into a positive electrode, a dielectric layer and a negative electrode. The dielectric layer has the property of unidirectional conduction. Of course, after the reverse connection, the dielectric layer of the product does not function as an insulator, and the capacitor is naturally short-circuited.

Why is the resistivity small when the positive and negative electrodes of the electrolytic capacitor are connected?

The principle of electrolytic capacitor is related: when the positive electrode is connected, the positive electrode of the capacitor will form a very thin oxide film (alumina) as a dielectric; when the metal foil (positive capacitor) is connected to the negative pole of the power supply, the H2 will be electrolyzed without An oxide film is formed, and the other electrode does not form an oxide film which can serve as a dielectric depending on the material.

The aluminum electrolytic capacitor is an anode aluminum foil which has been etched and forms an oxide film, a corroded cathode aluminum foil,

After the electrolytic paper is wound, the working electrolyte is immersed, and then sealed in an aluminum casing. Since the electrolytic capacitor has polarity, it is necessary to pay attention to the correct connection of the positive and negative electrodes during use. Otherwise, not only the capacitor will not function, but also the leakage current is large, and the inside of the capacitor will heat up in a short time, destroying the oxide film, and then being damaged.

The electrolytic capacitor is a kind of capacitor. The medium has an electrolyte coating, which has polarity, and can be positively and negatively connected. Electric capacity consists of two metal poles with an insulating material (medium) interposed therebetween. Characteristics of electrolytic capacitors: The capacitance per unit volume is very large, tens to hundreds of times larger than other types of capacitors. Electrolytic capacitors feature two: the rated capacity can be very large, can easily achieve tens of thousands of μf or even a few f (but not with the double layer capacitance). Electrolytic capacitors have three characteristics: the price is overwhelmingly superior to other types, because the constituent materials of electrolytic capacitors are common industrial materials, such as aluminum. The equipment for manufacturing electrolytic capacitors is also an ordinary industrial equipment, which can be mass-produced and has relatively low cost. Electrolytic capacitors are usually made of metal foil (aluminum/bismuth) as a positive electrode, an insulating oxide layer of aluminum foil (aluminum oxide/yttrium oxide) as a dielectric, and electrolytic capacitors are divided into aluminum electrolytic capacitors and tantalum electrolysis by their positive electrodes. Capacitor. The negative electrode of the aluminum electrolytic capacitor is composed of a thin paper/film or an electrolyte polymer immersed in an electrolyte liquid (liquid electrolyte); the negative electrode of the tantalum electrolytic capacitor is usually manganese dioxide. Since the electrolyte is used as the negative electrode (note that it is distinguished from the dielectric), the electrolytic capacitor is named after it. Polar electrolytic capacitors usually function as power supply filtering, decoupling, signal coupling, time constant setting, and DC blocking in power supply circuits or intermediate frequency and low frequency circuits. Generally, it can not be used in the AC power supply circuit. When used as a filter capacitor in the DC power supply circuit, the anode (positive electrode) should be connected to the positive terminal of the power supply voltage, and the cathode (negative electrode) should be connected to the negative terminal of the power supply voltage. Otherwise it will damage the capacitor.

Non-polar electrolytic capacitors are commonly used in speaker divider circuits, television S correction circuits, and start-up circuits for single-phase motors. Electrolytic capacitors are widely used in household appliances and various electronic products. Their capacities range from 1 to 1000 μF and rated operating voltages range from 6.3 to 450V. The disadvantage is that the dielectric loss and the capacity error are large (the maximum allowable deviation is +100%, -20%), the high temperature resistance is poor, and the storage time is long and easy to be invalid.

There are differences in performance and principle structure between polar and non-polar capacitors.

A polar capacitor refers to a capacitor such as an electrolytic capacitor, which is formed by an aluminum foil of an anode and an electrolyte of a cathode, respectively, and an aluminum oxide film produced on the anode aluminum foil is used as a dielectric capacitor. The structure has a polarity. When the capacitor is positively connected, the aluminum oxide film will remain stable due to the electrochemical reaction. When the connection is reversed, the aluminum oxide layer will become thinner, so that the capacitor is easily broken down by breakdown. Therefore, the electrolytic capacitor is The polarity must be paid attention to in the circuit. The ordinary capacitor is non-polar, and the two electrolytic capacitor anodes or cathodes can be connected in series to form a non-polar electrolytic capacitor.

1. The principle is the same. (1) Both store the charge and release the charge; (2) The voltage on the plate (where the electric potential of the charge is called the voltage) cannot be abruptly changed. (3) The difference is that the medium is different, the performance is different, the capacity is different, and the structure is different. Conversely, according to the needs of production practice, people have experimentally manufactured capacitors with various functions to meet the normal operation of various electrical appliances and the operation of new equipment. With the development of science and technology and the discovery of new materials, higher quality and diversified capacitors will continue to emerge.

2. The media is different. What is the medium? To put it bluntly is the substance between the two plates of the capacitor. Most of the polar capacitors use electrolyte as the dielectric material, and usually the same volume of capacitor has a large capacitance capacity. In addition, different electrolyte materials and processes produce polar capacitors with the same volume capacity. There is also a close relationship between pressure resistance and the use of dielectric materials. There are also many non-polar capacitor dielectric materials, mostly using metal oxide film, polyester and so on. Due to the reversible or irreversible performance of the medium, the environment with extreme and non-polar capacitance is determined.

3. Performance is different. Performance is the requirement for use, and maximizing demand is the requirement for use. If the power supply part of the TV is filtered with a metal oxide film capacitor, and the capacitor capacity and withstand voltage required for filtering are to be achieved. I am afraid that I can only install a power supply inside the case. Therefore, as a filter, only polar capacitors can be used, and polar capacitors are irreversible. That is to say, the positive pole must be connected to the high potential end, and the negative pole must be connected to the low potential end. Generally, the electrolytic capacitor is above 1 microfarad, and it is used for coupling, decoupling, and power supply filtering. Non-polar capacitors are mostly below 1 microfarad, participating in resonance, coupling, frequency selection, current limiting, and so on. Of course, there are also large capacity and high withstand voltage, which are used in the reactive power compensation of electric power, the phase shifting of the motor, and the variable frequency power supply. There are many types of non-polar capacitors, not to mention them one by one.

4, the capacity is different. As mentioned above, capacitors of the same volume have different capacities and are not described one by one. 5. The structure is different. In principle, it is possible to use a capacitor of any shape in the environment in which the tip discharge is not considered. Commonly used electrolytic capacitors (having a polar capacitor) are circular, and the square shape is rarely used. The shape of the non-polar capacitor is very varied. Like tube type, deformed rectangle, sheet type, square type, round type, combined square type and round type, etc., it is used where it is used. Of course, there is invisible, here the invisible refers to the distributed capacitance. For distributed capacitors, high frequency and intermediate frequency devices must not be ignored.

The function is the same. The main difference is that in terms of capacity, due to the influence of material structure, the capacity of general non-polar capacitors is relatively small, generally below 10uF, and the capacity of polar capacitors is generally large. For example, when power filtering, you have to use a large capacity of polar capacitors.

A basic principle of circuit design is to require designers to fully understand and master the real-world components. The components used are standard parts and common parts. It is best to be the most common model on the market (the versatility of components is better. The easier it is to purchase, the higher the supplier's output and the lower the procurement cost. For the components used in the drawings, if the materials are only available for customization, the cost is certainly not low. If it is not available, then this design is equivalent to waste paper.

PS: You are only talking about the power supply decoupling capacitor. The large capacitor is suitable for filtering low frequency signals, and the small capacitor is for filtering high frequency signals (see the circuit foundation, the relationship between capacitive reactance and frequency).

However, decoupling is only a function of the capacitor. Capacitors have other functions. Different types of capacitor characteristics have different usages. The capacitor on the schematic is just a symbol. There are many techniques behind it. This aspect has a lot to do with experience. It can't be quick, and it can only be accumulated slowly through practice.

In pure AC circuits, only non-polar capacitors can be used.

In a circuit in which a DC voltage is superimposed on an AC signal, and it is ensured that the lowest voltage after superposition does not become a negative value, a polar capacitor can be used.

In the case of the same capacity, the volume and cost of a capacitor with a polarity are much smaller than those of a non-polar capacitor. Therefore, when a large capacitance is required, the volume of the capacitor is a large contradiction, and a non-polar capacitor can be used. In the occasion, it will naturally be replaced by a polar capacitor, which not only solves the volume problem, but also has a much lower cost.

Large capacitors can filter out AC signals above the lower frequency, while small capacitors can only filter out signals above the higher frequencies. How large a capacitor is needed depends on the frequency of the signal to be filtered and the decibel to be filtered out.

In general, as long as there is an electric field between the two conductors, a capacitance is generated between the two conductors, and the capacity of this capacitor is large, and the electric field strength, distance, dielectric and power frequency between the two conductors related. In an electronic circuit, if the voltage and frequency, the capacity of the capacitor, the "quality factor" of the capacitor, and the mounting conditions have been set, the capacitor of which material is selected becomes the decisive factor.

Capacitors are mainly used in electronic circuits; coupling of signals, differentiation of volt-ampere characteristics in RC circuits such as integration, "slots" in oscillating circuits, bypassing, and power supply filtering.

The type of capacitor is divided according to the dielectric inside the capacitor, there are;

1. an air capacitor; a capacitor that uses air as a dielectric, such as a variable capacitor for "tuning" in a radio

2. Paper capacitors; capacitors that use a special capacitor paper as a dielectric.

3. Electrolytic capacitors; capacitors using electrolytes as dielectrics.

4. Mica capacitor; a capacitor made of natural mica as a dielectric.

5. Ceramic chip capacitors; capacitors using a single layer of ceramic material as a dielectric.

6. Monolithic capacitors; capacitors that use ceramic materials as dielectrics. In order to solve the shortcomings of small-capacity ceramic capacitors, it is actually a capacitor that is connected in series with a plurality of ceramic capacitors;

7. Polyester work capacitors; capacitors made of nylon material as dielectric.

8. Tantalum capacitor; a capacitor made of metal niobium [ní] as a positive electrode, dilute sulfuric acid or the like as a negative electrode, and an oxide film formed on the surface of the crucible as a medium.

9. Tantalum capacitor; is a capacitor made of metal tantalum (Ta) as an anode material.

10. Winding type capacitor; it is a kind of capacitor which is wound with a wire on a dielectric as an electrode. The size of the electrode can be adjusted by changing the number of turns of the wire to adjust the capacity.

11. Oil-impregnated paper capacitors; capacitors that use a neutral mineral oil as a dielectric are used in power systems. ......

Capacitors are divided into three types: fixed capacitors, variable capacitors, and adjustable capacitors.

Most of them are made to have a constant capacity.

Variable capacitance; a capacitor that can be freely adjusted within a certain capacity range, such as a variable capacitor that can be manually tuned for selection in the radio.

Adjustable capacitors (also known as semi-variable capacitors); capacitors that can be adjusted within a certain range, such as ceramic micro-lithography capacitors and wire-wound capacitors.

It can't be said that "a capacitor with a large capacity has a polarity". This is wrong. For example, it is used in power systems for phase angle adjustment and for capacitors used in arcing in the startup network. The capacity is sometimes made large. But not polar.

Is the non-polar capacitor the same as the non-polar electrolytic capacitor? Not the same thing.

Most types of capacitors are non-polar, only the electrolytic capacitors have polarity, and among the electrolytic capacitors, there are very special non-polar electrolytic capacitors. Compared with ordinary capacitors, electrolytic capacitors have large capacity, low price, and small size, which are unmatched by other capacitors. However, electrolytic capacitors generally have polarities, and their operational reliability, withstand voltage, temperature resistance, and dielectric loss are not as good. Other capacitors. The so-called non-polar electrolytic capacitors actually pack two identical electrolytic capacitors back to back together. This type of capacitor has large loss, low reliability, and low withstand voltage, and can only be used in a few applications where the requirements are not high.