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Dec 24 2019

Why Can't Aluminum Electrolytic Capacitor Withstand Reverse Voltage?

Ⅰ. Introduction

As we all know, 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 integration, the "channel" in oscillating circuits, bypass and power filter, etc. Aluminum electrolytic capacitor is made of anodized aluminum foil, corroded cathode aluminum foil and electrolytic paper in the middle, then impregnated with working electrolyte and sealed in aluminum shell.

. Common problems of electrolytic capacitor

1. Why can't aluminum electrolyte capacitor withstand reverse voltage?

Due to the polarity of electrolytic capacitors, it is necessary to pay attention to the correct connection of positive and negative electrodes in use, otherwise not only the capacitors can not play a role, but also the leakage current is very large. In a short time, the inside of the capacitors will heat up, damage the oxide film, and then damage. As shown in the figure, the basic structure of the aluminum electrolytic capacitor is composed of anode, aluminum layer attached to the insulating medium, cathode aluminum layer of the receiving electrode and the real cathode composed of electrolyte. The electrolyte is soaked in the paper between the two aluminium layers. Aluminum oxide layer is plated on the aluminum layer, which is very thin compared with the voltage applied on it, and it is easy to be broken down, leading to capacitor failure.

The alumina layer can withstand the forward DC voltage. If it bears the reverse DC voltage, it is easy to fail in 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 oxide layer, the non-polar capacitor will be formed.

Aluminum Electrolytic Capacitor Structure

Many articles report the mechanism of the threshold phenomenon of the reverse voltage of aluminum electrolytic capacitor, which is called the hydrogen ion theory. When the electrolytic capacitor bears the reverse DC voltage, that is, the cathode of the electrolyte bears the positive voltage while the oxide bears the negative voltage, The hydrogen ions gathered in the oxide layer will pass through the medium and reach the boundary between the medium and the metal layer, and then they will be converted into hydrogen. And the expansion force of the gas causes the oxide layer to fall off.

Therefore, the current flows directly through the capacitor after breaking through the electrolyte, and the capacitor fails. This DC voltage is very small. Under the 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 positive voltage is applied to the electrolytic capacitor, the negative ions are concentrated between the oxide layers. Because the diameter of the negative ions is very large, it can not break through the oxide layer, so it can withstand higher voltage.

2. What are the similarities and differences between nonpolar capacitance and polar capacitance?

Are nonpolar capacitors the same as nonpolar electrolytic capacitors? Most kinds of capacitors are nonpolar, only the electrolytic capacitors have polarity. Among them, there are very special nonpolar electrolytic capacitors. Compared with ordinary capacitors, electrolytic capacitors have larger capacity, lower price and smaller volume than other capacitors, but electrolytic capacitors generally have polarity, and their working reliability, withstand voltage, temperature resistance, dielectric loss and other indicators are not as good as other capacitors.

Non Polar Radial Electrolytic Capacitors

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

3. What will happen when a polar capacitor is reversed?

If the capacitance capacity is very small, the withstand voltage is very high, and the working voltage is low, there will be nothing wrong with the reverse connection. If the capacity is slightly large (above 100uF) and the withstand voltage is close to the working voltage, the capacitance can just work for nearly 10 minutes, then it will bulge and burst.

4. The reverse connection of polar capacitor will explode. Does it mean that it can't be directly connected to the AC power supply?

It can't be connected to the AC power supply, because the polar capacitor is designed to be used in the DC power supply for filtering, and there is special material inside the polar capacitor, which can not bear the back pressure. If it is connected to the AC power supply, it will breakdown reversely or explode.

Capacitor with fixed capacitance

5. If the polarity capacitor is reversed, why is it short circuited?

The internal structure of polar capacitor is divided into positive electrode, dielectric layer and negative electrode. The dielectric layer has the property of unidirectional conduction. Of course, the dielectric layer of the product will not play the role of insulation after being connected reversely, and the capacitor will be short circuited.

6. Why does the resistivity of electrolytic capacitor become smaller when the positive and negative poles are reversed?

It involves the principle of electrolytic capacitor. When the positive electrode of the capacitor is connected positively, a very thin oxide film (alumina) will be formed as the dielectric. When the negative electrode of the capacitor is connected reversely, H2 will be produced without forming the oxide film, and the other electrode will not form the oxide film which can be used as the dielectric due to different materials.

7. Why can we only use nonpolar capacitor in pure AC circuit?

In the circuit of DC voltage superposing AC signal, if we can ensure that the lowest voltage after superposing will not become negative, we can use a capacitor with polarity. In the case of the same capacity, the volume and cost of the polar capacitor are far less than that of the nonpolar capacitor, so when we need a larger capacity, the volume of the capacitor is a big contradiction. We usually replace non-polar capacitors with polar ones, which not only solves the volume problem, but also reduces the cost. Large capacitance can filter the AC signal with lower frequency and above, while small capacitance can only filter the signal with higher frequency and above.

8. What is electrolytic capacitance?

Electrolytic capacitor is a kind of capacitor. Its medium is coated with electrolyte. It can be divided into positive and negative electrodes and cannot be connected wrongly. The capacitance is composed of two metal poles and the insulating material (medium) sandwiched between them.

Capacitor Terminal's Identification

9. The characteristics of electrolytic capacitors are as follows:

①The capacitance per unit volume is dozens to hundreds of times larger than other kinds of capacitance.

Rated capacity can easily reach tens of thousands of μ f or even several F, but it is not as good as double electric layer capacitance.

③The price is much lower than other kinds, because the components of electrolytic capacitors are ordinary industrial materials, such as aluminum.

The equipment for manufacturing electrolytic capacitors are also common industrial equipment, which can be mass produced with relatively low cost. Electrolytic capacitors are usually made up of metal foil (aluminum / tantalum) as the positive electrode, and the insulating oxide layer (alumina / tantalum pentaoxide) of metal foil as the dielectric. The negative electrode of aluminum electrolytic capacitor is composed of thin paper / film or electrolyte polymer soaked in electrolyte, the negative electrode of tantalum electrolytic capacitor is usually manganese dioxide. As both of them use electrolyte as negative electrode, electrolytic capacitor gets its name.

The polar electrolytic capacitor usually plays the role of power filter, decoupling, signal coupling, time constant setting, DC isolation and so on in the power circuit or IF and LF circuits. It can't be used in AC power circuit. When it is used as filter capacitor in DC power circuit, its anode (positive) should be connected with the positive end of the power voltage, and the cathode (negative) should be connected with the negative end of the power voltage. It can't be reversed, or it will be damaged.

10. What are the similarities and differences that cannot be ignored between polar and nonpolar capacitors in performance, principle and structure?

Electrolytic Capacitor

Polar capacitance is a kind of electrolytic capacitance. It consists of two electrodes formed by the anode aluminum foil and the cathode electrolyte . A layer of aluminum oxide film produced on the anode aluminum foil is used as the dielectric of capacitance. As a result of this structure, it has polarity. When the capacitance is directly connected, the aluminum oxide film will remain stable due to the electrochemical reaction. When the reverse connection is made, the aluminum oxide layer will become thinner,which makes the capacitor easy to be broken down and damaged. Therefore, we must pay attention to the polarity of electrolytic capacitor in the circuit. Ordinary capacitors are nonpolar. We can also connect two anode or cathode of electrolytic capacitors in series to form nonpolar electrolytic capacitors.


①The same principle

They both store and release charges.The voltage on the plate shall not change suddenly. (voltage here refers to the electromotive force of charge accumulation)

②Different media

What's the medium? It's the material between the two plates of the capacitor. Most of the polar capacitors use electrolyte as dielectric material, and the capacity of the polar capacitor is larger than that of the same volume. In addition, the capacity of the same volume of polar capacitance produced by different electrolyte materials and processes will be different. The withstand voltage of the capacitor is closely related to the dielectric materials used. There are also many dielectric materials for non-polar capacitance, most of which are metal oxide film, polyester and so on. The reversibility or irreversibility of dielectric determines the use environment of polar and nonpolar capacitors.

③Different performance.

Performance and maximization of requirements are the requirements for use. If a metal oxide film capacitor is used for filtering in the power supply part of the TV set, and the capacity and withstand voltage of the capacitor should meet the requirements of filtering, a power supply must be installed in the shell. For a filter, only the polar capacitance can be used, which is irreversible.

The positive terminal must be connected to the high potential terminal and the negative terminal to the low potential terminal. Generally, when the electrolytic capacitance is more than 1 microfarad, it is used for coupling, decoupling, power filtering, etc. Most of the nonpolar capacitors are below 1 microfarad, which participate in resonance, coupling, frequency selection, current limiting, etc. Large capacity and high withstand voltage capacitors are usually used for reactive power compensation, motor phase-shifting and frequency conversion power supply phase-shifting. There are many kinds of nonpolar capacitors.

④Different capacity

For capacitors of the same volume, when the medium is different, the capacity is also different.

Different structure

We can use capacitors of any shape without considering the tip discharge. The polar capacitance is usually round, and there are few polar capacitances of square type. There are many shapes of nonpolar capacitors, including tube type, deformed rectangle, sheet type, square type, circular type, combined square type and circular type, and of course there are intangible ones. Here intangible refers to distributed capacitance.

Aluminum Electrolytic Capacitor

The distributed capacitance in HF and IF devices should not be ignored. The function is the same. The main difference is their capacity. Due to the influence of material structure, the capacity of non-polar capacitance is relatively small, generally below 10uF, while the capacity of polar capacitance is usually large. When filtering the power supply, you have to use a polar capacitor of large capacity.

Ⅲ. Summary

One of the basic principles of circuit design is to require the designer to fully understand and master the real components. The components used should be standard parts, general parts, and the most common models on the market (the better the versatility of components, the easier the procurement, the larger the supplier's output, and the lower the procurement cost). For the components used in the drawings, if the materials can only be obtained by customization, the cost is certainly not low. If you can't get the customized material, this design is wastepaper.

In addition, large capacitance is suitable for filtering low frequency signals and small capacitance for filtering high frequency signals. However, decoupling is only one function of capacitance. Different kinds of capacitance have different characteristics and usages. This aspect has a lot to do with experience. It is impossible to achieve it quickly. It can only be accumulated through practice.

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