First, the role of electrolytic capacitor in the circuit
1, filtering function, in the power supply circuit, the rectifier circuit changes alternating current into pulsating DC, and the rectifier circuit is connected with a large-capacity electrolytic capacitor, and the rectified pulsating DC voltage is changed into a relatively stable DC voltage by using its charging and discharging characteristics. In practice, in order to prevent the power supply voltage of each part of the circuit from changing due to the load change, electrolytic capacitors of tens to hundreds of microfarads are generally connected between the output end of the power supply and the input end of the load. Because electrolytic capacitors with large capacity generally have a certain inductance and cannot effectively filter out high-frequency and pulse interference signals, a capacitor with a capacity of 0.00 1-0. The two ends of LPF are connected in parallel 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 working points of the front and rear circuits from influencing each other, capacitive coupling is often used. In order to prevent excessive loss of low-frequency components in the signal, electrolytic capacitors with large capacity are generally used.
Second, the judgment method of electrolytic capacitance
The common faults of electrolytic capacitors include capacity reduction, capacity disappearance, breakdown short circuit and leakage, in which the capacity change is caused by the gradual drying of electrolyte in electrolytic capacitors during use or placement, while breakdown and leakage are generally caused by excessive applied voltage or poor quality. To judge the quality of power supply capacitance, the resistance of multimeter is generally used to measure it. The specific method is: short-circuit discharge the two pins of the capacitor, and connect the black probe of the multimeter to the positive electrode of the electrolytic capacitor. The red stylus is connected to the negative electrode (for pointer multimeter, the intermodulation of the stylus is measured with a digital multimeter) and the normal timepiece.
The pointer should swing in the direction of low resistance, and then gradually return to infinity. The larger the swing amplitude of the hand or the slower the return speed, the larger the capacitance and the smaller the capacitance. If the pointer does not change somewhere in the middle, it means that the capacitor is leaking. If the indicated resistance value is small or zero, it means that the capacitor has broken down and shorted. Because the battery voltage used by multimeter is generally low, it is more accurate to measure the capacitance with low withstand voltage. When the capacitor withstand voltage is high, the measurement is normal, but when the voltage is increased.
Three, the use of electrolytic capacitor matters needing attention
1, electrolytic capacitor has positive and negative polarity, so it can't be reversed when used in the circuit. In the power supply circuit, when a positive voltage is output, the anode of the electrolytic capacitor is connected to the output end of the power supply, and when a negative voltage is output, the cathode is connected to the output end and the anode is grounded. When the polarity of the filtering capacitor in the power supply circuit is reversed, the filtering effect of the capacitor is greatly reduced, which on the one hand causes the output voltage of the power supply to fluctuate, and on the other hand causes the electrolytic capacitor equivalent to a resistor to heat up. When the reverse voltage exceeds a certain value, the reverse leakage resistance of the capacitor will become very small, making the power supply work quickly.
2. The voltage applied to the electrolytic capacitor should not 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 regulated power supply, if the voltage of AC power supply is 220~, the secondary rectification voltage of transformer can reach 22V, and the electrolytic capacitor with voltage resistance of 25V can generally meet the requirements. However, if the voltage of AC power supply fluctuates greatly, it may rise to more than 250 volts, and it is best to choose electrolytic capacitors with withstand voltage greater than 30 volts. ..
3. The electrolytic capacitor should not be close to the high-power heating element in the circuit to prevent the electrolyte from drying due to heating.
4. In order to filter positive and negative polarity signals, two electrolytic capacitors can be connected in series, with the same polarity as the nonpolar capacitor.