Introduction
A non-polarized capacitor is one of many capacitors. According to the polarity of the capacitor, the capacitor can be divided into non-polarized capacitor and polarized capacitor. And this article will elaborate: what is a non-polarized capacitor? What is it used for? How to choose non-polarized capacitors? What is the difference between electrolytic capacitors and non-polarized capacitors?
Polarized Capacitor Vs Non Polarized Capacitor
How to test a Non Polarized Capacitor?
Catalog
The Difference Between Non-polarized Capacitors and Polarized Capacitors |
Conception
Non-polarized capacitors are capacitors without positive or negative polarity. The two electrodes of non-polarized capacitors can be randomly inserted into the circuit, and will not leak, mainly used in circuits of coupling, decoupling, feedback, compensation, and oscillation. The figure below shows the reference diagram of the non-polarized capacitor.
Figure1. Non-polarized Capacitor
The ideal capacitor has no polarity. However, in practice, in order to obtain large capacity, some special materials and structures are used, which leads to the fact that the actual capacitors are somewhat polarized. Common polarized capacitors include aluminum electrolytic capacitors and tantalum electrolytic capacitors. Electrolytic capacitors are generally relatively large in capacity. It is not so easy to make a large-capacity non-polarized capacitor, because the volume will become very large. This is why there are so many polarized capacitors in the actual circuit. Because its size is small, and the voltage in this circuit has only one direction, polarized capacitors can come in handy.
We use polarized capacitors to avoid its shortcomings and take advantage of its advantages. We can understand it this way: A polarized capacitor is actually a capacitor that can only be used in one voltage direction. For non-polarized capacitors, both voltage directions can be used. Therefore, from the point of voltage direction alone, non-polarized capacitors are better than polarized capacitors. It is entirely possible to replace polarized capacitors with non-polarized capacitors, as long as the capacity, working voltage, volume, etc. can meet the requirements.
Function
The non-polarized capacitors applied in pure AC circuits, and because of its small capacitance, it can also be applied to high-frequency filtering. Here is an example to illustrate the application of the capacitor:
In this case, the RC spark suppression circuit is mainly introduced. When the radio and television program is received by the antenna, if the fluorescent lamp is turned on and the fluorescent lamp flashes, the irregular sound of the radio or the speaker of the television will be heard. Many strong bright lines and bright spots on the TV screen are high-frequency clutter interference caused by electric sparks.
When circuits with inductance are disconnected, spark will be generated between the contacts. As shown in the circuit at the left in figure2, the switch S is suddenly turned off, and the current quickly disappear, that is, the change of the current is large, so a large self-inductance is generated at both ends of the coil. This electromotive force may hinder current to change and its direction is consistent with that of the applied voltage. When the two are superimposed, the voltage U1 across the switch will be very high, and when the voltage is higher than a certain value, this “sharp” voltage will break down the air and form an electric spark.
The spark may causes the contacts to be ablated and oxidized to cause malfunction. Therefore, it is important to eliminate the spark between the contacts. When cutting off the circuit, as long as the current of the control coil does not plummeted, the voltage at the two ends of the coil will not be too large, so there will be no spark, as shown in cicuit on the right below, the RC spark suppression circuit is connected at both ends of the inductor. When the switch is suddenly turned off, i1 will charge the capacitor. A portion of the magnetic field energy in the inductor is dissipated on R and r, and a portion is converted into electric field energy in capacitor C, which causes capacitor C to re-discharge, thereby eliminating the spark.
Figure2. Circuit with Inductance and Spark Absorption Circuit
How to Select Non-polarized Capacitors?
Non-polarized capacitors are very convenient to be selected and used. You can directly select the capacitors with same model and the same specifications. If none of the above conditions are met, you can refer to the following methods:
1. Choose a reasonable capacitor accuracy. In most cases, the capacity requirements are not very high, and it is acceptable to have a capacity roughly the same with reference capacity. In the oscillation circuits , filtering circuits, delay circuits and tone circuits, the absolute value of the error needs to be within 0.3%-0.5%.
2. Select the capacitor according to the circuit requirements. The paper capacitor is generally used for the low frequency AC bypass circuit. The mica capacitor or the ceramic capacitor are generally used in circuits of high frequency or high voltage.
3. Capacitors can be selected with rated voltage greater or equal than the actual needs.
4. High frequency capacitors cannot be replaced by low frequency capacitors.
5. Consider the operating temperature, working range, temperature coefficient of the capacitor according to the application occasion.
6. The series or parallel method can be used when the nominal capacity cannot be met, but the voltage added to the capacitor should be less than the withstand voltage of the capacitor.
The Difference Between Non-polarized Capacitors and Polarized Capacitors
Both polarity and non-polarized capacitors have same principles, that is, storing and releasing charges; the voltage on the plate (here the electromotive force of charge accumulation is called voltage) cannot change suddenly
The different media, different performance, different capacity and different structure results in different using environment and usage. Conversely, more excellent and diversified capacitors will emerge with the development of science and technology and the discovery of new materials.
Figure3. Different Capacitor Types
1. Different dielectric
What is the dielectric? In other words, it is the substance between two capacitor plates. Most of the polarity capacitors use electrolytes as dielectric, which makes polarity capacitor has larger capacitance compared to other capacitors that has same volume. In addition, polarity capacitors produced by different electrolyte materials and processes will have different capacitance. Meanwhile, voltage withstand is related mostly to the dielectric material. And there are also many non-polarized materials, including the most used metal oxide film and polyester, the use of polarity and non-polarized capacitors is determined by whether the nature of the dielectric is reversible.
Figure4. Non-polarized Capacitor and Polarized Capacitor
2. Different performance
Performance and the demand maximization is the requirement of use. If the power supply of the TV use metal oxide film capacitor as filter, and if the capacitance and voltage withstand are required to meet the filter, I'm afraid only a power supply can be installed inside the shell. Therefore, filter can only use polarity capacitor, and polarity capacitance is irreversible. Generally, the electrolytic capacitor is above 1 MF, which participates in coupling, decoupling, power supply filtering and so on. The non-polar capacitor is mostly below 1 MF, which is involved in resonance, coupling, frequency selection, current limiting and so on. Of course, there are also non-polar capacitors with large capacity and high voltage, mostly used in reactive power compensation, motor phase shifting, frequency conversion power phase shifting and other purposes. There are many kinds of non-polarized capacitors.
Figure5. Capacitors
3. Different capacity
As mentioned before, capacitors of the same volume have different capacitance under different dielectric .
4. Different structure
In principle, it is possible to use a capacitor of any shape in the environment without considering the point discharge. The most using electrolytic capacitors are circular, and the square type is rare. The shape of capacitors are varied, such as tubular, deformed rectangular, sheet, square, circular, combined square or circular and so on, depending on where they are used. Of course, there is also invisible ones called distributed capacitor, which must not be ignored in high frequency and intermediate frequency devices.
5. Using environment and usage
Because of the internal material and structure, polarity capacitor (such as aluminum electrolysis) capacity can be very large, but its high-frequency characteristics are not good, so it is suitable for power filter and other occasions. There are also polarity capacitors of good high-frequency characteristics - tantalum electrolysis, whose price is relatively high;
Including ceramic capacitors, monolithic capacitors, polyethylene (CBB) capacitors and so on, these non polarized capacitors are small in size, low in price and good in high-frequency characteristics, but they are not suitable for the large capacity. Ceramic capacitors are generally used in high-frequency filtering, oscillation circuit.
Figure6. Different Capacitors
The magnetic dielectric capacitors use ceramic material as meson and use silver layer on the surface as electrode. Having stable performance and small leakage, the magnetic dielectric capacitors are suitable to apply in high frequency and high voltage circuits.
Generally speaking, according to the insulating material between the two poles of capacitor. Material with large dielectric permittivity (such as ferroelectric ceramics, electrolyte) is suitable for capacitors of large capacity and small volume, whose loss is also large. Material with small dielectric permittivity (such as ceramics) has a low loss and is suitable for high frequency applications.
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Re:Capacitance and leakage current are not related, so the amount of leakage current has no effect on capacitance.