1. The role of electrolytic capacitors in the circuit
1. Filtering function. In the power circuit, the rectifier circuit turns AC into pulsating DC, and a larger capacity electrolytic capacitor is connected after the rectifier circuit. Using its charge and discharge characteristics, the rectified pulsating DC voltage becomes Relatively stable DC voltage. In practice, in order to prevent the supply voltage of each part of the circuit from changing due to load changes, electrolytic capacitors of tens to hundreds of microfarads are generally connected to the output end of the power supply and the power input end of the load. Because large-capacity electrolytic capacitors generally have a certain inductance, they cannot effectively filter out high-frequency and pulse interference signals, so a capacitor with a capacity of 0.001--0.lpF is connected in parallel at both ends to filter out high-frequency And pulse interference.
2. Coupling effect: In the process of low-frequency signal transmission and amplification, in order to prevent the static operating points of the front and rear two-stage circuits from interacting with each other, capacitor coupling is often used. In order to prevent excessive loss of low-frequency components in the signal, the total capacity is generally larger. Large electrolytic capacitor.
2. Judgment method of electrolytic capacitor
Common faults of electrolytic capacitors include capacity reduction, capacity disappearance, short-circuit breakdown, and current leakage. The capacity change is caused by the gradual drying of the electrolyte inside the electrolytic capacitor during use or placement, and breakdown and leakage are generally added. Caused by high voltage or poor quality. To judge the quality of the power supply capacitor, the resistance profile of a multimeter is generally used for measurement. The specific method is: short-circuit the two pins of the capacitor to discharge, and connect the black test pen of the multimeter to the positive electrode of the electrolytic capacitor. Connect the red test lead to the negative pole (for an analog multimeter, the test leads are inter-adjusted when measuring with a digital multimeter). Normally, the test needle should first swing in the direction of low resistance, and then gradually return to infinity. The greater the swing of the hands or the slower the return speed, the greater the capacitance of the capacitor, otherwise, the smaller the capacitance of the capacitor. If the pointer does not change somewhere in the middle, it indicates that the capacitor is leaking. If it is small or zero, it indicates that the capacitor has broken down and shorted. Because the voltage of the battery used by the multimeter is generally very low, it is more accurate when measuring low withstand voltage capacitors. When the withstand voltage of the capacitor is high, although the measurement is normal, when the high voltage is added, leakage or shock may occur. Wear phenomenon.
Three, the use of electrolytic capacitors
1. Since electrolytic capacitors have positive and negative polarity, they cannot be connected upside down when used in a circuit. In the power supply circuit, when outputting a positive voltage, the positive electrode of the electrolytic capacitor is connected to the output terminal of the power supply, and the negative electrode is grounded. When a negative voltage is output, the negative electrode is connected to the output terminal and the positive electrode is grounded. When the filter capacitor in the power circuit is connected in reverse polarity, the The filtering effect is greatly reduced. On the one hand, it causes the output voltage of the power supply to fluctuate. On the other hand, the electrolytic capacitor, which is equivalent to a resistor, heats up due to the reverse energization. When the reverse voltage exceeds a certain value, the reverse leakage resistance of the capacitor will change. It must be very small, so that the capacitor will burst and be damaged due to overheating shortly after power on.
2. The voltage applied to both ends of the electrolytic capacitor cannot exceed its allowable working voltage. When designing the actual circuit, a certain margin should be left according to the specific situation. When designing the filter capacitor of the stabilized power supply, if the AC power supply voltage is 220~h The rectified voltage of the transformer secondary can reach 22V. At this time, choosing an electrolytic capacitor with a withstand voltage of 25V can generally meet the requirements. However, if the AC power supply voltage fluctuates greatly and may rise above 250V, it is best to choose a withstand voltage of 3V or more Of electrolytic capacitors.
3. The electrolytic capacitor should not be close to the high-power heating element 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 in series with the same polarity can be used as a non-polar capacitor.
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