Why can't aluminum electrolytic capacitors withstand reverse voltage?
Due to the polarity of electrolytic capacitors, you must pay attention to the correct connection of the positive and negative poles when using them. Otherwise, not only will the capacitor not function, but the leakage current will be very large, and the inside of the capacitor will heat up in a short period of time, destroying the oxide film and then being damaged.
The figure shows the basic structure of an aluminum electrolytic capacitor, which consists of an anode, an aluminum layer composed of aluminum oxide attached to an insulating medium, a cathode aluminum layer at the receiving pole, and a cathode composed of an electrolyte. The electrolyte is soaked in the paper between the two aluminum layers. The aluminum oxide layer is electroplated on the aluminum layer, and is very thin relative to the voltage applied to it, and is easily broken down, causing the capacitor to fail.
The aluminum oxide layer can withstand a positive DC voltage. If it is subjected to a reverse DC voltage, it will easily fail within seconds. This phenomenon is called the 'Valve Effect', which is why aluminum electrolytic capacitors have polarity. If both electrodes of the electrolytic capacitor have an oxide layer, a non-polarized capacitor is formed.
Many articles have reported the mechanism of the threshold phenomenon of reverse voltage in aluminum electrolytic capacitors, which is called the hydrogen ion theory. When the electrolytic capacitor is subjected to a reverse DC voltage, that is, the cathode of the electrolyte is subjected to a positive voltage and the oxide layer is subjected to a negative voltage, the hydrogen ions gathered in the oxide layer will pass through the medium to reach the boundary between the medium and the metal layer and be converted into hydrogen. The expansion force of the hydrogen 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 fail immediately in a few seconds due to the hydrogen ion effect. On the contrary, when the electrolytic capacitor is subjected to a forward voltage, the negative ions gather between the oxide layers. Because the diameter of the negative ions is very large, they cannot break through the oxide layer, so they can withstand higher voltages.
What are the common terms related to electrolytic capacitors?
1. Anode: The anode aluminum layer, which is the positive electrode of the electrolytic capacitor.
2. Cathode: electrolyte layer.
3. Dielectric: Aluminum oxide layer attached to the surface of aluminum layer.
4. Cathode Foil: The layer connecting the electrolyte and the outside. This layer does not need to be oxidized during production, but in practice, since aluminum is easily oxidized during the etching process, it forms a naturally oxidized oxide layer that can withstand a voltage of 1~2V.
5. Spacer paper: Isolates the cathode and anode to prevent them from short-circuiting directly, and absorbs a certain amount of electrolyte.
What are the similarities and differences between non-polarized capacitors and polarized capacitors?
Are non-polar capacitors the same as non-polar electrolytic capacitors?
Most types of capacitors are non-polar, only electrolytic capacitors have polarity. Among 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, but electrolytic capacitors generally have polarity, and their working reliability, voltage resistance, temperature resistance, dielectric loss and other indicators are not as good as other capacitors.
The so-called non-polar electrolytic capacitor is actually two identical electrolytic capacitors packaged together back to back. This type of capacitor has large loss, low reliability, and low withstand voltage, and can only be used in a few occasions with low requirements.
What will happen if a polarized capacitor is reversely connected?
If the capacitor capacity is very small, the withstand voltage is very high, and the working voltage is low, the reverse connection will not cause any damage; if the capacity is slightly larger (above 100UF), the withstand voltage is close to the working voltage, and the capacitor will fail in less than 10 minutes. The manifestation of failure is: first bulging, then blowing, and then bursting.
If a polarized capacitor is connected in reverse, it will explode. Does that mean it cannot be connected directly to an AC power source?
It cannot be connected to an AC power source because this polarized capacitor is designed to be used on a DC power source for filtering. Because this polarized capacitor has a special material inside that cannot withstand reverse pressure, it will reversely break down or explode if it is connected to AC power.
Why does a reverse polarity capacitor short-circuit?
The internal structure of a polarity capacitor is divided into a positive pole, a dielectric layer, and a negative pole. The dielectric layer has the property of unidirectional conductivity. Of course, after the reverse connection, the dielectric layer of the product will not play an insulating role, and the capacitor will naturally short-circuit.
Why does the resistivity decrease when the positive and negative poles of an electrolytic capacitor are connected in reverse?
This is related to the principle of electrolytic capacitors: when connected in the positive direction, the positive pole of the capacitor will form an extremely thin oxide film (aluminum oxide) as a dielectric; when connected in the reverse direction, the metal aluminum sheet (the positive pole of the capacitor) is connected to the negative pole of the power supply, and H2 will be electrolyzed without forming an oxide film. The other electrode will not form an oxide film that can serve as a dielectric due to different materials.
Why can only non-polar capacitors be used in pure AC circuits?
In a circuit where DC voltage is superimposed on AC signals, and it can be guaranteed that the lowest voltage after superposition will not become a negative value, polar capacitors can be used. Under the condition of the same capacity, the volume and cost of polar capacitors are much smaller than non-polar capacitors, so when a larger capacitance is required, the volume of the capacitor is a big contradiction. In situations where non-polar capacitors can be used, polar capacitors will naturally be used instead, which not only solves the volume problem, but also reduces the cost a lot. How unhappy. Large capacitors can filter out AC signals above lower frequencies, while small capacitors can only filter out signals above higher frequencies.
What is an electrolytic capacitor?
An electrolytic capacitor is a type of capacitor. The dielectric has an electrolyte coating and has polarity. It can be connected in positive and negative directions. Capacitors (electric capacity) are composed of two metal poles with an insulating material (dielectric) in between.
Feature 1 of electrolytic capacitors: The capacitance per unit volume is very large, dozens to hundreds of times larger than other types of capacitors.
The second characteristic of electrolytic capacitors: the rated capacity can be very large, easily reaching tens of thousands of μF or even several F (but cannot be compared with double-layer capacitors).
Feature 3 of electrolytic capacitors: The price has an overwhelming advantage over other types because the components of electrolytic capacitors are common industrial materials, such as aluminum and so on.
The equipment used to manufacture electrolytic capacitors is also common industrial equipment that can be mass-produced at a relatively low cost. Electrolytic capacitors usually use metal foil (aluminum/tantalum) as the positive electrode and the insulating oxide layer (aluminum oxide/tantalum pentoxide) of the metal foil as the dielectric. Electrolytic capacitors are divided into aluminum electrolytic capacitors and tantalum electrolytic capacitors according to their positive electrodes. The negative electrode of an aluminum electrolytic capacitor is made of thin paper/film or electrolyte polymer soaked in electrolyte (liquid electrolyte); the negative electrode of a tantalum electrolytic capacitor is usually manganese dioxide. Since both use electrolytes as negative electrodes (note the difference from dielectrics), electrolytic capacitors get their name.
Polarized electrolytic capacitors usually play the role of power supply filtering, decoupling, signal coupling, time constant setting, DC isolation, etc. in power supply circuits or medium frequency and low frequency circuits. Generally, they cannot be used in AC power supply circuits. When used as filter capacitors in DC power supply circuits, their 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. They cannot be connected in reverse, otherwise the capacitor will be damaged.
What are the important differences and similarities between polar capacitors and non-polar capacitors in terms of performance, principle and structure?
Polar capacitors refer to electrolytic capacitors, which are capacitors with two electrodes formed by the anode aluminum foil and the cathode electrolyte, and a layer of aluminum oxide film produced on the anode aluminum foil as the dielectric.
Due to this structure, it has polarity. When the capacitor is connected in the forward direction, the aluminum oxide film will remain stable due to the electrochemical reaction. When it is connected in the reverse direction, the aluminum oxide layer will become thinner, making the capacitor easily broken down and damaged. Therefore, the polarity of electrolytic capacitors must be paid attention to in the circuit. Ordinary capacitors are non-polar, and two electrolytic capacitors can also be connected in series with their anodes or cathodes facing each other to form a non-polar electrolytic capacitor.
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Same principle
Both store and release charges; the voltage on the plates (here the electromotive force of accumulated charges is called voltage) cannot change suddenly.
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Different media
What is a dielectric? To put it bluntly, it is the material between the two plates of a capacitor. Most polar capacitors use electrolytes as dielectric materials, and polar capacitors usually have larger capacitances of the same volume. In addition, polar capacitors made of different electrolyte materials and processes will have different capacities of the same volume. In addition, the withstand voltage is closely related to the dielectric material used. There are also many dielectric materials for non-polar capacitors, most of which use metal oxide films, polyester, etc. The reversible or irreversible properties of the dielectric determine the use environment of polar and non-polar capacitors.
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Different performance
Performance is the requirement for use, and maximizing demand is the requirement for use. If metal oxide film capacitors are used for filtering in the power supply part of a TV, and the capacitor capacity and withstand voltage required for filtering must be met, then the case can only contain a power supply. Therefore, only polarized capacitors can be used for filtering, and polarized 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, and are used for coupling, decoupling, power supply filtering, etc. Non-polar capacitors are mostly below 1 microfarad, and are involved in resonance, coupling, frequency selection, current limiting, etc. Of course, there are also large-capacity and high-voltage ones, which are mostly used in power reactive compensation, motor phase shifting, variable-frequency power supply phase shifting, etc.
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Different capacity
We have already mentioned that capacitors of the same volume have different capacities due to different dielectrics, so I will not go into details here.
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Different structures
In principle, if tip discharge is not considered, any shape of capacitor can be used in the environment. The commonly used electrolytic capacitors (polarized capacitors) are round, and square ones are rarely used. Non-polarized capacitors come in a variety of shapes. They can be tube-shaped, deformed rectangular, sheet-shaped, square-shaped, round-shaped, combined square-shaped and round-shaped, etc., depending on where they are used. Of course, there are also invisible ones, and the invisible here refers to distributed capacitors.
Distributed capacitance should never be ignored in high-frequency and medium-frequency devices. The functions are the same. The main difference is in capacity. Affected by the material structure, the capacity of non-polarized capacitors is generally small, usually below 10uF, while the capacity of polarized capacitors is generally large. For example, when filtering the power supply, you have to use large-capacity polarized capacitors.
A basic principle of circuit design is that designers should fully understand and master the components in reality. The components used should be standard and universal as much as possible, preferably the most common models on the market (the more universal the components are, the easier it is to purchase, the larger the supplier's output is, the lower the purchase cost is). For the components used in the drawings, if the materials can only be obtained by custom-made, the cost must be high. If they cannot be obtained even by custom-made, then this design drawing is equivalent to waste paper.
In addition, large capacitors are suitable for filtering low-frequency signals, while small capacitors are suitable for filtering high-frequency signals (see the circuit basics, the relationship between capacitive reactance and frequency for the principle). However, decoupling is only one function of capacitors. Capacitors have other functions. Different types of capacitors have very different characteristics and uses. The capacitor on the schematic diagram is just a symbol. There are many skills behind it. This aspect is closely related to experience. It is impossible to learn it quickly, and it can only be accumulated slowly through practice.
How to classify capacitors?
According to the dielectric inside the capacitor, it is divided into:
Air capacitor: A capacitor that uses air as the dielectric, such as
the variable capacitor used for "tuning" in a radio.
Paper capacitor: A capacitor that uses a special capacitor paper as the dielectric.
Electrolytic capacitor: A capacitor that uses an electrolyte as the dielectric.
Mica capacitor: A capacitor using natural mica as the dielectric.
Ceramic chip capacitor: A capacitor that uses a single layer of ceramic material as the dielectric.
Monolithic capacitor: It is also a capacitor using ceramic material as dielectric. In order to solve the shortcoming of small capacity of single-layer ceramic capacitor, it is actually a capacitor with multiple ceramic capacitors connected in series.
Polyester power capacitor: a capacitor using nylon material as the dielectric.
Niobium capacitor: It uses metal niobium as the positive electrode, dilute sulfuric acid and other liquids as the negative electrode, and the oxide film generated on the surface of niobium as the dielectric.
Tantalum capacitor: a capacitor made of metal tantalum (Ta) as the anode material.
Wire-wound capacitor: A capacitor that uses metal wire wound around a dielectric as an electrode. The electrode area and thus the capacity can be adjusted by changing the number of turns of the metal wire.
Oil-immersed paper capacitor: A capacitor that uses a neutral petroleum oil as a dielectric, mostly used in power systems......
According to the adjustability of capacitance, it can be divided into:
Fixed capacitance: A capacitor with a constant capacitance value.
Variable capacitor: A variable capacitor is a capacitor that can be freely adjusted within a certain capacity range. For example, a variable capacitor is used to manually tune and select channels in a radio.
Adjustable capacitor: Adjustable capacitor (also called semi-variable capacitor) is a capacitor that can be adjusted within a certain range, such as ceramic micro-carved capacitor and wirewound capacitor.
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