Why can’t aluminum electrolytic capacitors withstand reverse voltage?

[Guide]We all know that capacitors have always played a very important role in Electronic circuits. They are responsible for the coupling of signals in electronic circuits, the differentiation of volt-ampere characteristics in RC circuits, such as integrals, “tanks” in oscillating circuits, bypasses, and power supplies. Filtering, etc.

We all know that capacitors have always played a very important role in electronic circuits. They are responsible for the coupling of signals in electronic circuits, the differentiation of volt-ampere characteristics in RC circuits, such as integrals, “tanks” in oscillating circuits, bypasses, and power supplies. Filtering, etc.

The aluminum electrolytic capacitor is made of an anode aluminum foil that has been corroded and formed with an oxide film, a corroded cathode aluminum foil, wound with electrolytic paper in the middle, and then immersed in working electrolyte, and then sealed in an aluminum shell.

1. Why can’t aluminum electrolyte capacitors withstand reverse voltage?

Due to the polarity of electrolytic capacitors, you must 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. In a short time, the inside of the capacitor will heat up, destroy the oxide film, and then be damaged.

The figure below shows the basic structure of an aluminum electrolytic capacitor. It consists of an anode, an aluminum layer made of aluminum oxide attached to an insulating medium, a cathode aluminum layer of the receiver, and a real cathode made of electrolyte. The electrolyte soaks the paper between the two aluminum layers. The aluminum oxide layer is electroplated on the aluminum layer, which is very thin relative to the voltage applied to it, and can easily be broken down, causing the capacitor to fail.

Why can’t aluminum electrolytic capacitors withstand reverse voltage?

The aluminum oxide layer can withstand a positive DC voltage. If it withstands a reverse DC voltage, it will easily fail within a few seconds. This phenomenon is called the Valve Effect. This is why aluminum electrolytic capacitors have polarity. If both electrodes of the electrolytic capacitor have oxide layers, a non-polar capacitor is formed.

Many articles have reported the mechanism of the reverse voltage threshold phenomenon of aluminum electrolytic capacitors, called hydrogen ion theory. When the electrolytic capacitor is subjected to a reverse DC voltage, that is, the cathode of the electrolyte bears the forward voltage and the oxide layer bears With a 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 be converted into hydrogen gas. The expansion force of the hydrogen gas makes the oxide layer fall off.

Therefore, the current flows through the capacitor directly after the electrolyte breaks down, and the capacitor fails. This DC voltage is very small. Under the action of a reverse DC voltage of 1~2V, the aluminum electrolytic capacitor will immediately fail due to the hydrogen ion effect in a few seconds. . On the contrary, when the electrolytic capacitor is subjected to a positive voltage, the negative ions gather between the oxide layers. Because the diameter of the negative ions is very large, they cannot break down the oxide layer, so they can withstand higher voltages.

2. What are the terms related to electrolytic capacitors?

●Anode: The anode aluminum layer, that is, the positive electrode of the electrolytic capacitor.

●Cathode: Electrolyte layer.

●Dielectric di: The aluminum oxide layer attached to the surface of the aluminum layer.

●Cathode Foil: The layer that connects the electrolyte and the outside. This layer does not need to be oxidized during production, but in practice, because aluminum is easily oxidized during the etching process, it forms a naturally oxidized layer. The oxide layer, this oxide layer can withstand a voltage of 1~2v.

●Insulation paper (spacer paper): Isolate the cathode and anode so that they are not directly short-circuited, and absorb a certain amount of electrolyte.

Why can’t aluminum electrolytic capacitors withstand reverse voltage?

3. What is the difference between a non-polarized capacitor and a polarized capacitor?

Are non-polar capacitors the same as non-polar electrolytic capacitors? Most types of capacitors are non-polar. Only electrolytic capacitors have polarity. 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 that other capacitors cannot match. However, electrolytic capacitors generally have polarity, and their working reliability, voltage resistance, temperature resistance, dielectric loss and other indicators are not as good as those of other capacitors. Other capacitors.

The so-called non-polar electrolytic capacitors are actually encapsulating two identical electrolytic capacitors back to back. This kind of capacitance has large loss, low reliability, and low withstand voltage, and can only be used in a few occasions with low requirements.

Why can’t aluminum electrolytic capacitors withstand reverse voltage?

4. What happens when a polarized capacitor is reversely connected?

If the capacity of the capacitor is small, the withstand voltage is high, and the working voltage is low, nothing can be seen from reverse connection; if the capacity is slightly larger (above 100UF), the withstand voltage is close to the working voltage, and the capacitor will not be broken for more than 10 minutes. The manifestation is: first bulge, then blow, and then burst.

5. Reverse connection of a polarized capacitor will explode, so it cannot be directly connected to the AC power supply?

It cannot be connected to an AC power supply, because this polarized capacitor is designed to be used on a DC power supply for filtering. Because there is a special substance inside this polarized capacitor, this substance cannot withstand back pressure, and if it is connected to alternating current, it will breakdown or explode in the reverse direction.

As for why the reverse connection of the polar capacitor will cause a short circuit, the reason is that 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. , The capacitor will naturally short-circuit.

6. Why does the resistivity decrease when the positive and negative poles of the electrolytic capacitor are reversely connected?

Involving the principle of electrolytic capacitors: when the positive connection of the capacitor is positive, a very thin oxide film (alumina) will be formed as the dielectric; when reverse connection, the metal aluminum sheet (the positive electrode of the capacitor) is connected to the negative electrode of the power supply, which will electrolyze H2. No oxide film is formed, and the other electrode does not form an oxide film that can be used as a dielectric due to different materials.

7. Why can only non-polarized capacitors be used in pure AC circuits?

In the circuit where the DC voltage is superimposed on the AC signal, and it can be ensured that the minimum voltage after superposition will not become a negative value, a polarized capacitor can be used.

In the case of the same capacity, the volume and cost of a polarized capacitor are much smaller than that of a non-polarized capacitor. Therefore, when a larger capacitance is required, the volume of the capacitor is a big contradiction. A non-polarized capacitor can be used. In such occasions, they will naturally be replaced by polarized capacitors, which not only solves the volume problem, but also costs much lower, which is unpleasant.

Large capacitors can filter out AC signals above lower frequencies; small capacitors can only filter out signals above higher frequencies.

Why can’t aluminum electrolytic capacitors withstand reverse voltage?

8. What is an electrolytic capacitor?

Electrolytic capacitor is a kind of capacitor. The medium has electrolyte coating, has polarity, and cannot be connected wrongly. The electric capacity (Electric capacity) consists of two metal poles with an insulating material (medium) sandwiched in between.

Features of electrolytic capacitors:

●The capacitance per unit volume is very large, tens to hundreds of times larger than other types of capacitors;

●The rated capacity can be very large, it can easily achieve tens of thousands of μf or even a few f (but not comparable to the electric double layer capacitance);

●The price has an overwhelming advantage over other types, because the constituent materials of electrolytic capacitors are common industrial materials, such as aluminum.

The equipment for manufacturing electrolytic capacitors is also common industrial equipment, which can be produced on a large scale with relatively low cost. Electrolytic capacitors are usually made of metal foil (aluminum/tantalum) as the positive electrode, and the insulating oxide layer of the metal foil (aluminum oxide/tantalum pentoxide) as the dielectric. Electrolytic capacitors are divided into aluminum electrolytic capacitors and tantalum based on the difference in their positive electrodes. Electrolytic capacitors.

The negative electrode of aluminum electrolytic capacitors is composed of thin paper/film or electrolytic polymer impregnated with electrolyte (liquid electrolyte); the negative electrode of tantalum electrolytic capacitors usually uses manganese dioxide. Since the electrolyte is used as the negative electrode (note that it is distinguished from the dielectric), the electrolytic capacitor gets its name.

Polarized electrolytic capacitors usually play a role in power supply circuits or intermediate frequency and low frequency circuits for power supply filtering, decoupling, signal coupling, time constant setting, and DC blocking. Generally cannot be used in AC power circuits. When used as filter capacitors in DC power circuits, the 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. , Otherwise it will damage the capacitor.

Why can’t aluminum electrolytic capacitors withstand reverse voltage?

9. What are the similarities and differences between polarized capacitors and non-polarized capacitors in terms of performance, principle and structure?

A polarized capacitor refers to a type of electrolytic capacitor. It is a capacitor in which the aluminum foil of the anode and the electrolyte of the cathode respectively form two electrodes, and a layer of aluminum oxide film produced on the anode aluminum foil is used as the dielectric.

Due to this structure, it has polarity. When the capacitor is connected directly, the aluminum oxide film will remain stable due to the electrochemical reaction; when the capacitor is connected reversely, the aluminum oxide layer will become thin, making the capacitor easy to be damaged by breakdown. Therefore, the polarity of electrolytic capacitors must be paid attention to in the circuit.

Ordinary capacitors are non-polar, and two electrolytic capacitor anodes or cathodes can also be connected in series to form non-polar electrolytic capacitors.

●The same principle

Both are storing charge and releasing charge; the voltage on the plate (here the electromotive force accumulated by the charge is called voltage) cannot be changed suddenly.

●Different media

What is the medium? To put it bluntly, it is the substance between the two plates of the capacitor. Most of the polar capacitors use electrolyte as the dielectric material, usually the same volume of the capacitor has a large capacity. In addition, polarized capacitors manufactured by different electrolyte materials and processes have different capacities with the same volume. Furthermore, the pressure resistance is also closely related to the use of dielectric materials. There are also many non-polar capacitor dielectric materials, most of which use metal oxide film, polyester and so on. Due to the reversible or irreversible properties of the medium, the use environment of polar and non-polar capacitors is determined.

●Different performance

Performance is the requirement of use, and maximum demand is the requirement of use. If a metal oxide film capacitor is used for filtering in the power supply part of the TV, and the capacitor capacity and withstand voltage required for filtering must be achieved. I am afraid that only one power supply can be installed in the case. Therefore, only polar capacitors can be used as filters, 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, electrolytic capacitors are above 1 microfarad for coupling, decoupling, power filtering, etc. Most of the non-polar capacitors are below 1 microfarad and participate in resonance, coupling, frequency selection, current limiting, and so on. Of course, there are also large-capacity and high withstand voltage, which are mostly used for reactive power compensation of electric power, phase shifting of motors, and variable frequency power shifting. There are many types of non-polar capacitors, so I won’t repeat them one by one.

●Different capacities

I have already mentioned that capacitors of the same volume have different capacities, so I won’t repeat them one by one.

●Different structure

In principle, without considering the tip discharge, any shape of capacitor can be used in the environment. The commonly used electrolytic capacitors (with polarized capacitors) are round, and square ones are rarely used. The shapes of non-polar capacitors vary. Like tube type, deformed rectangle, sheet type, square type, round type, combined square type and round type, depending on where it is used. Of course, there are also intangibles. The intangibles here refer to distributed capacitance.

Distributed capacitance must not be ignored in high-frequency and intermediate-frequency devices. The function is the same. The main difference is in the capacity, affected by the material structure, generally the capacity of non-polar capacitors is relatively small, generally below 10uF, while the capacity of polar capacitors is generally larger. For example, when filtering the power supply, you have to use large-capacity polar capacitors.

A basic principle of circuit design is to require designers to fully understand and master the components in reality. The components used should be standard parts and general parts as much as possible, preferably the most common type on the market (the more common the components are, the better, The easier the purchase, the greater the supplier’s output, and the lower the purchase cost). For the components used in the drawings, if the materials can only be obtained by order, the cost is certainly not low. If it is custom-made and cannot be obtained, then this design drawing is equivalent to waste paper.

In addition, large capacitors are suitable for filtering low-frequency signals, and small capacitors are suitable for filtering high-frequency signals. However, decoupling is only one function of capacitors. Capacitors have other functions. The characteristics of different types of capacitors are very different in usage. The capacitor on the schematic diagram is just a symbol, and there are many tricks behind it. This aspect has a lot to do with experience, and it is impossible to make it fast, only to accumulate slowly through practice.

Why can’t aluminum electrolytic capacitors withstand reverse voltage?

10. How to classify capacitors?

Air capacitors are classified according to the dielectric in the capacitor: capacitors that use air as the dielectric, such as variable capacitors used in radios for “tuning”.

Paper capacitor: A capacitor that uses a special capacitor paper as a dielectric.

●Electrolytic capacitor: a capacitor that uses an electrolyte as a dielectric.

●Mica Capacitor: A capacitor that uses natural mica as a dielectric.

●Porcelain chip capacitor: a capacitor that uses a single-layer ceramic material as a dielectric.

●Monolithic capacitor: It is also a capacitor that uses ceramic materials as the dielectric. In order to solve the shortcomings of the small capacity of single-layer ceramic capacitors, it is actually a capacitor that uses multiple ceramic capacitors in series.

●Polyester power capacitor: a capacitor that uses nylon material as a dielectric.

●Niobium capacitor: it uses metal niobium[ní]It is a kind of capacitor made of positive electrode, dilute sulfuric acid and other solution as negative electrode, and the oxide film formed on the surface of niobium as the dielectric.

●Tantalum capacitor: It is a kind of capacitor made with metal tantalum (Ta) as the anode material.

●Wire-wound capacitor: It is a kind of capacitor that uses a metal wire to wind on the dielectric as an electrode. The area of ​​the electrode can be adjusted by changing the number of turns of the metal wire to adjust the capacity.

●Oil-impregnated paper capacitors: capacitors that use a neutral oil as the dielectric are mostly used in power systems

According to the adjustability of the capacitance, it is divided into: fixed capacitance: a capacitor with a constant capacitance value.

●Variable capacitance: a capacitor whose variable capacitance can be adjusted freely within a certain capacity range. For example, in the radio, the variable capacitor can be used for manual tuning and channel selection.

●Adjustable capacitors: adjustable capacitors (also called semi-variable capacitors) can be adjusted within a certain range, such as: ceramic micro-carved capacitors and wire-wound capacitors.

(Source: 21ic Electronic Network)


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