Simple analysis of the selection of capacitors in practical and reliable circuits

Capacitors are passive devices that we often use in circuits. Several types of aluminum electrolytic capacitors, filter capacitors, tantalum capacitors, and ceramic capacitors are frequently seen. Since the characteristics of each type of capacitor determine the corresponding use of different occasions. Therefore, this article first introduces the basic knowledge of the capacitor, and then compares the differences and characteristics of several types of capacitors, and summarizes the techniques for selecting capacitors in an actual circuit.

The symbol of the capacitor is also divided into the national standard representation and the international electronic symbol representation. However, the capacitance symbol is similar in both domestic and international representations. The only difference is that in the case of a polar capacitor, the domestic one is an empty basket below a horizontal line. , And the international is the ordinary capacitor plus a "+" symbol for positive.

There are many types of capacitors, which can be divided into two types: non-polar variable capacitors, non-polar fixed capacitors, and polar capacitors. The materials can be divided into: CBB capacitors (polyethylene), polyester capacitors, ceramic capacitors , mica capacitors, monolithic capacitors, electrolytic capacitors, tantalum capacitors.

With the basic knowledge of the above capacitors as a basis, let's compare the differences and characteristics of several types of capacitors and summarize some of the techniques for selecting capacitors in an actual circuit.

Aluminum electrolytic capacitors. Its main component is aluminum foil and electrolyte. A simple understanding of the production process of aluminum electrolytic capacitors, that is rolled into a column of aluminum foil, into the liquid electrolyte, and then lead out the positive and negative terminals, and then seal the capacitor core material in the metal housing. A certain proportion of water exists in the liquid electrolyte. When a 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 the oxidation reaction, and the hydrogen can pass through the rubber plug of the capacitor. discharge. This will not damage the capacitor. The simple production process and low cost are a feature of aluminum electrolytic capacitors. In addition, aluminum electrolytic capacitors have the following features:

According to the above characteristics, aluminum electrolytic capacitors are widely used in low-frequency filtering applications, especially in the environment of tens of KHz to several MHz, such as the output filtering of power supplies. We often see aluminum electrolytic capacitors. When using aluminum electrolytic capacitors, it should be noted that the capacitor withstand voltage meets the circuit requirements. In addition, in the case of other requirements are not strict, the capacity can be chosen as large as possible, the larger the capacity is, the smaller the ESR is, and it is easier to meet the requirements of the circuit target impedance. In certain high-temperature environments, try to avoid the use of small-capacity aluminum electrolytic capacitors to avoid the effect of capacitance failure due to the excessively high temperature of the electrolyte causing the entire circuit to work.

 (Filtering capacitor. Since the capacity of the capacitor used for filtering is large after rectification, an electrolytic capacitor must be used. When the filter capacitor is used in a power amplifier, its value should be 10000 μF or more, and for a preamplifier, the capacity can be about 1000 μF. When the power filter circuit supplies the amplifier directly, the larger the capacity is, the better the sound quality is, but the large-capacity capacitor will cause the impedance to rise from around 10KHz, and then several small capacitors should be used in parallel to form a large capacitor and should also be connected in parallel. A thin film capacitor, next to the bulk capacitor, suppresses the rise of high-frequency impedance.The characteristics of the filter capacitor include the following: According to the characteristics of the filter capacitor, we can understand that the filter capacitor is connected in parallel at the output of the rectified power circuit to reduce the AC A kind of energy storage device with fluctuating ripple factor and smooth DC output, in the use of electronic circuits that convert AC to DC power supply, the filter capacitor not only makes the DC output of the power supply stable, but also reduces the influence of the alternating pulsating ripple on the electronic circuit. Can absorb the current fluctuations generated during the operation of the electronic circuit and via the AC power supply The interference in series makes the working performance of the electronic circuit more stable.

By comparison, we can find that tantalum capacitors have many advantages that aluminum electrolytic capacitors do not have. In some filtering applications, tantalum capacitors can be a good substitute for aluminum electrolytic capacitors. However, there are several points to pay attention to: Since the structure of the tantalum capacitor is determined, the tantalum capacitor's withstand voltage is generally not high. In the actual circuit, it is important to pay attention to the tantalum capacitor's withstand voltage requirement and leave a certain margin. The tantalum capacitors do not have excellent aluminum electrolytic capacitors in dealing with large current surges and large voltage transients during power-on. Temperature has little effect on the tantalum capacitor. In practice, we can ignore the effect of temperature on the tantalum capacitor.

Based on the above device characteristics, in addition to the filtering scene, the ceramic capacitor is also widely used in applications such as blocking, coupling, and bypassing. Its operating frequency is greatly improved compared to electrolytic capacitors and can meet the application environment of a few MHz to 1 GHz.