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Capacitors are passive components frequently used in circuits. Common types include aluminum electrolytic capacitors, filter capacitors, tantalum capacitors, and surface-mount ceramic capacitors. Because the characteristics of each type of capacitor determine its corresponding application, this article first introduces the basics of capacitors, then compares the differences and characteristics of several types, and summarizes techniques for selecting capacitors in practical circuits.
Basic knowledge of capacitors
1. Classification and Functions of Capacitors
A capacitor consists of two metal electrodes sandwiched between an insulating material (dielectric). Different types of capacitors are made from different insulating materials.
(1) According to the structure, it can be divided into: fixed capacitor, variable capacitor, and fine-tuning capacitor.
(2) According to the dielectric material, capacitors can be divided into: gas dielectric capacitors, liquid dielectric capacitors, inorganic solid dielectric capacitors, organic solid dielectric capacitors, and electrolytic capacitors.
(3) According to polarity, capacitors are divided into polarized capacitors and non-polarized capacitors. The most common type we see is the electrolytic capacitor.
(4) Capacitors in circuits have the function of blocking direct current and passing alternating current.
2. The symbol for a capacitor
Capacitor symbols are also divided into domestic and international electronic symbol representations. However, the domestic and international representations of capacitor symbols are similar. The only difference is that for polarized capacitors, the domestic symbol is an empty frame with a horizontal line below, while the international symbol is a regular capacitor with a "+" symbol to represent the positive terminal.
In circuit diagrams, capacitors are generally identified by the symbol C.
3. Unit of capacitance
The basic unit of resistance is the farad (F). Other units include microfarads (μF), nanofarads (nF), and picofarads (pF). Because capacitance (F) is very large, we usually see μF, nF, and pF, not F. The specific conversions between them are as follows:
1F = 1,000,000 μF
1μF = 1000nF = 1000000pF
4. The unit of voltage rating for capacitors: V (volt).
Every capacitor has its voltage rating, which is one of the important parameters of a capacitor. The nominal voltage ratings of common non-polarized capacitors include: 63V, 100V, 160V, 250V, 400V, 600V, 1000V, etc. Polarized capacitors generally have lower voltage ratings than non-polarized capacitors, with common nominal voltage ratings including: 4V, 6.3V, 10V, 16V, 25V, 35V, 50V, 63V, 80V, 100V, 220V, 400V, etc.
5. Types of capacitors
There are many types of capacitors. They can be classified by principle as: non-polarized variable capacitors, non-polarized fixed capacitors, and polarized capacitors. They can also be classified by material as: CBB capacitors (polyethylene), polyester capacitors, ceramic capacitors, mica capacitors, monolithic capacitors, electrolytic capacitors, and tantalum capacitors.
Tips for selecting capacitors in circuits
With the foundation of basic capacitor knowledge laid above, let's now compare the differences and characteristics of several types of capacitors and summarize some techniques for selecting capacitors in practical circuits.
(1) Aluminum electrolytic capacitors. Their main components are aluminum foil and electrolyte. A simple understanding of the manufacturing process of aluminum electrolytic capacitors is that aluminum foil is rolled into a cylindrical shape, liquid electrolyte is injected, positive and negative terminals are led out, and then the capacitor core is sealed in a metal casing. A certain proportion of water is present in the liquid electrolyte. When leakage current flows through the capacitor, the water can be decomposed into hydrogen and oxygen. The oxygen can form a new oxide film with the anode through an oxidation reaction, while the hydrogen is discharged through the capacitor's rubber stopper. This prevents damage to the capacitor. Simple manufacturing process and low cost are characteristics of aluminum electrolytic capacitors. In addition, aluminum electrolytic capacitors also have the following characteristics:
a. Because the sealing shell is not completely sealed, the electrolyte can easily dry out, thus limiting the lifespan of aluminum electrolytic capacitors;
b. The presence of water in the electrolyte affects the performance of aluminum electrolytic capacitors in high and low temperature environments;
c. Due to the characteristics of the manufacturing process, it is difficult to reduce the ESR and ESL of aluminum electrolytic capacitors. Therefore, the self-resonant frequency of aluminum electrolytic capacitors is usually relatively low, roughly in the range of tens of kHz to several MHz.
d. The capacitance of aluminum electrolytic capacitors is positively correlated with the size of the aluminum foil. The capacitance can be made very large, and the larger the capacitance, the larger the capacitor size.
Based on these characteristics, aluminum electrolytic capacitors are widely used in low-frequency filtering applications, especially in environments ranging from tens of kHz to several MHz, such as power supply output filtering. When using aluminum electrolytic capacitors, it's crucial that the capacitor's voltage rating meets the circuit requirements. Additionally, when other requirements are less stringent, a larger capacitance can be chosen; a larger capacitance results in a lower ESR, making it easier to meet the circuit's target impedance requirements. In high-temperature environments, avoid selecting small-sized, low-capacity aluminum electrolytic capacitors, as excessive heat can cause the electrolyte to evaporate and dry out, leading to capacitor failure and affecting the entire circuit's operation.
(2) Filtering capacitors. Since the capacitance of the capacitors used for filtering after rectification is relatively large, electrolytic capacitors must be used. When used in a power amplifier, the value of the filtering capacitor should be above 10000μF; when used in a preamplifier, a capacitance of around 1000μF is sufficient. When the power supply filtering circuit directly supplies power to the amplifier, the larger the capacitance, the better the sound quality. However, large-capacity capacitors will cause the impedance to rise from around 10kHz. In this case, several smaller capacitors should be connected in parallel to form a large capacitor, and several film capacitors should also be connected in parallel next to the large capacitor to suppress the rise in high-frequency impedance. The characteristics of filtering capacitors include the following aspects:
a. The harmonic filter circuit consists of a capacitor and a reactor in series. It forms the lowest impedance at a certain harmonic order to absorb a large amount of harmonic current. The quality of the capacitor will affect the stable absorption effect of the harmonic filter. The service life of the capacitor is closely related to the temperature. The higher the temperature, the shorter the service life. The filter full-film capacitor has the characteristics of low temperature rise, which can ensure its service life.
b. Low loss, dielectric loss tangent (tgδ): ≤0.0003;
c. Complies with GB and IEC standards, and each internal individual capacitor is equipped with a protection device;
d. Small in size and light in weight, making it extremely convenient to transport and install.
Based on the characteristics of filter capacitors, we know that they are energy storage devices connected in parallel at the output of rectifier power supply circuits to reduce AC ripple coefficients and smooth DC output. In electronic circuits that convert AC to DC power, filter capacitors not only stabilize the DC output of the power supply and reduce the impact of alternating ripple on the electronic circuits, but also absorb current fluctuations generated during the operation of the electronic circuits and interference introduced via AC power, making the operating performance of the electronic circuits more stable.
(3) Tantalum capacitors. Tantalum capacitors are another widely used type of capacitor. Like aluminum electrolytic capacitors, tantalum capacitors are also a type of electrolytic capacitor. The main process involves pressing tantalum powder into a porous solid block, sintering it, anodizing it to form an oxide film, coating it with a solid electrolyte, then coating it with a layer of graphite and lead-tin, and finally encapsulating it with resin to form a solid tantalum capacitor. Tantalum capacitors have the following characteristics:
a. Unlike aluminum electrolytic capacitors, tantalum capacitors use a solid electrolyte, so there is no problem of the electrolyte drying out, which improves their lifespan.
b. Because of the solid electrolyte used, its capacity-temperature characteristics are relatively stable, so temperature has little effect on the capacitance, and its high and low temperature performance is better than that of aluminum electrolytic capacitors;
c. Tantalum electrolytic capacitors can achieve large capacitance in small packages, so ESR and ESL can be controlled to be relatively small, and their self-resonant frequency is higher than that of aluminum electrolytic capacitors.
d. The manufacturing process is more complex and the cost is higher than that of aluminum electrolytic capacitors.
By comparison, we can see that tantalum capacitors have many advantages that aluminum electrolytic capacitors lack, and in certain filtering applications, tantalum capacitors can effectively replace aluminum electrolytic capacitors. However, there are a few points to note: Due to the structure of tantalum capacitors, their voltage rating is generally not high. In actual circuits, it is important to pay attention to the voltage rating requirements of tantalum capacitors and leave a certain margin. Tantalum capacitors are not as good as aluminum electrolytic capacitors in handling large current surges and large voltage transients during power-on. Temperature has a very small effect on tantalum capacitors; in practical applications, we can ignore the influence of temperature on tantalum capacitors.
(4) Ceramic capacitors. Ceramic capacitors are the most commonly used capacitors in practical circuits. Their structure is relatively simple, consisting of alternating layers of ceramic sheets sintered together. They are small in size, low in cost, and widely used. Their main characteristics include:
a. Small size. Ceramic capacitors have a simple structure and can be made very small. 0402 or even 0201 packaged ceramic capacitors are widely used in applications with strict size requirements, such as mobile phones;
b. Stable electrical performance with minimal temperature influence;
c. The ESR and ESL values are very low, and the self-resonant frequency is relatively high, which can be applied to the filtering requirements of several MHz to 1 GHz on PCB.
d. Due to the multi-layered structure and packaging, the PCB has poor resistance to bending, and bending deformation may cause capacitors to crack and fail.
Based on the above characteristics, ceramic capacitors, in addition to their application in filtering scenarios, are also widely used in various applications such as DC blocking, coupling, and bypassing. Their operating frequency is significantly higher than that of electrolytic capacitors, meeting the requirements of applications ranging from a few MHz to 1 GHz.
