The Role of Electrolytic Capacitors in Circuit Design

When talking about the electrolytic capacitors, we have to learn more about its function.

1. Filtering Effect:
In the power supply circuit, the rectifier circuit converts the AC into a pulsating DC, and a large-capacity electrolytic capacitor is connected after the rectifier circuit, and its charge and discharge characteristics (energy storage effect) are used to make the rectified capacitor. The pulsating DC voltage becomes a relatively stable DC voltage. In practice, in order to prevent the power supply voltage of each part of the PCB manufacturing 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 supply input end of the load. Since large-capacity electrolytic capacitors generally have a certain inductance and cannot effectively filter out high-frequency and pulse interference signals, a capacitor with a capacity of 0.001–0.lpF is connected in parallel at both ends to filter out high-frequency signals, and pulse interference.

2. Coupling Effect:
In the process of transmission and amplification of low-frequency signals, capacitive coupling is often used to prevent the static operating points of the front and rear circuits from affecting each other. In order to prevent excessive loss of low-frequency components in the signal, electrolytic capacitors with larger capacity are generally used.

Next, we need to understand the judgment method of electrolytic capacitors.

Common faults of electrolytic capacitors include capacity reduction, capacity disappearance, breakdown short-circuit and leakage. The change in capacity is caused by the gradual drying of the electrolyte inside the electrolytic capacitor during use or placement, while breakdown and leakage are generally added because the voltage is too high or the quality itself is not good. Judging the quality of the PCB proofing power supply capacitor is generally measured by the resistance gear of the multimeter.
The specific method is: short-circuit the two pins of the capacitor to discharge, and use the black test lead of the multimeter to connect the positive electrode of the electrolytic capacitor. The red test lead is connected to the negative pole (for an analog multimeter, the test lead is intermodulated when measuring with a digital multimeter). Normally, the test needle should swing in the direction of small resistance, and then gradually return to infinity. The larger the swing of the needle or the slower the return speed, the larger the capacity of the capacitor, and the smaller the capacity of the capacitor. If the pointer does not change somewhere in the middle, it means that the capacitor is leaking. If the resistance indication value is small or zero, it means that the capacitor has been broken down and short-circuited. Because the voltage of the battery used by the multimeter is generally very low, it is more accurate to measure the capacitor with low withstand voltage. When the withstand voltage of the capacitor is high, although the measurement is normal, there may be leakage or shock when high voltage is added. wear phenomenon.

More need to know the precautions for the use of electrolytic capacitors.

1. Since electrolytic capacitors have positive and negative polarities, they cannot be connected upside down when used in circuits. In the power supply circuit, when the positive voltage is output, the positive pole of the electrolytic capacitor is connected to the output terminal of the power supply, and the negative pole is grounded. When outputting a negative voltage, the negative pole is connected to the output terminal, and the positive pole is grounded.

When the polarity of the filter capacitor in the power circuit is reversed, the filtering effect of the capacitor is greatly reduced. On the one hand, it causes the output voltage of the power supply to fluctuate. When the reverse voltage exceeds a certain value, the reverse leakage resistance of the capacitor will become very small, so that the capacitor can be burst and damaged due to overheating for a short time 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 regulated power supply, if the AC power supply voltage is 220~, the rectified voltage of the transformer secondary can reach 22V, and the electrolytic capacitor with a withstand voltage of 25V can generally meet the requirements for PCB proofing. However, if the AC power supply voltage fluctuates greatly and may rise to more than 250V, it is best to choose an electrolytic capacitor with a withstand voltage of more than 30V.

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

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

5. The capacitor shell, auxiliary lead-out terminals, positive and negative electrodes and circuit boards must be completely isolated.