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What is the function of electrolytic capacitors in the PCB ?

                                                  What is the function of electrolytic capacitors in the PCB ?

First, let's take a look at the functions of electrolytic capacitors in the PCB:

1. Filtering function: the rectifier circuit turns the alternating current into a pulsating direct current in the power supply circuit, and then the rectifier circuit is connected to a larger capacity electrolytic capacitor, the charging and discharging characteristics are used to make the rectified pulsating DC voltage become relatively stable DC voltage. In practice, in order to prevent the supply voltage of each part of the PCB circuit from changing due to load changes, tens to hundreds μF of electrolytic capacitors are generally connected to the output end of the power supply and the power input end of the load. Since large-capacity electrolytic capacitors generally have a certain inductance, the high-frequency and pulse-interference signals cannot be effectively filtered out, so a capacitor with a capacity of 0.001-0.lpF is connected in parallel at both ends to filter high-frequency and pulse interference. 

2. Coupling function: in the process of transmission and amplification of low-frequency signals, in order to prevent the static working points of the front and rear two-pole circuits from interacting with each other, capacitor coupling is often used. In order to prevent excessive loss of low frequency components in the signal, electrolytic capacitors with a large capacity are often used.


Then let's take a look at the precautions for using electrolytic capacitors:

1. Electrolytic capacitors have positive and negative polarity, so they cannot be reversed when used in a circuit. In the power supply circuit, when the positive voltage is output, the positive electrode of the electrolytic capacitor is connected to the output end of the power supply, and the negative electrode is grounded. When a negative voltage is output, the negative terminal is connected to the output terminal, and the positive terminal is grounded.

When the polarity of the filter capacitor in the power supply circuit is reversed, the filtering function of the capacitor is greatly reduced. On the one hand, the output voltage of the power supply fluctuates, and on the other hand, due to the reverse energization, the calorific value of the electrolytic capacitor equivalent to one resistor. When the reverse voltage exceeds a certain value, the reverse leakage resistance of the capacitor will become small, so that the power supply will work shortly, and the capacitor will be broken due to overheating.

2. The voltage applied to the two ends of the electrolytic capacitor should not exceed the allowable working voltage. There should be a certain margin according to the specific conditions when designing the actual circuit . When designing the filter capacitor of the regulated power supply, if the AC power supply voltage is 220V, the transformer secondary rectification voltage will be reach 22V, and we should select an electrolytic capacitor with a withstand voltage of 25V for the PCB to meet the requirements. However, it is best to select an electrolytic capacitor with a withstand voltage of 30V or higher if the AC power supply voltage fluctuation is very large and may rise to more than 250V.

3. The electrolytic capacitor should not be close to the high-power heating components in the circuit to prevent the electrolyte from drying up due to heat.

4. For the filtering of signals with positive and negative polarity, two electrolytic capacitors can be used in series with the same polarity as a non-polar capacitor.

5. The capacitor case and auxiliary lead-out terminals must be completely isolated from the positive and negative terminals and the PCB.

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