1. **PCB aluminum electrolytic capacitors** use an aluminum cylindrical shell as the negative electrode, which is filled with a liquid electrolyte. The positive electrode is connected via an aluminum strip and electrode. After applying DC voltage, an oxide film dielectric forms on the anode aluminum foil. PCB aluminum electrolytic capacitors can have large capacitance values and are relatively inexpensive, making them suitable for low-frequency filtering applications.
2. The capacitance values of aluminum electrolytic capacitors range from a fraction of a microfarad to several tens of thousands of microfarads, with voltage ratings typically spanning from 5V to 630V. The tolerance for capacitance in electrolytic capacitors used on PCB circuit boards is generally ±20%.
3. Due to the specific manufacturing process of aluminum electrolytic capacitors, PCB manufacturers cannot treat them as ideal capacitors during production. This means that, beyond just capacitance, factors such as ESR (Equivalent Series Resistance), ESL (Equivalent Series Inductance), and leakage current must also be considered.
1. The term **ESR (Equivalent Series Resistance)** refers to the fact that a real capacitor is effectively modeled as the series combination of an ideal capacitor and a resistor. When a large AC current flows through the capacitor, this current also passes through the series-equivalent resistance, thereby reducing the capacitor’s ability to filter power supply ripple effectively. This impact is further compounded by heat generated from the ESR, which can also shorten the capacitor’s lifespan.
2. In addition to ESR, actual capacitors also exhibit inductive characteristics that can impede AC voltage and current. The higher the frequency, the more pronounced this effect becomes, leading to reduced filtering effectiveness for high-frequency noise. Furthermore, aluminum electrolytic capacitors have some inherent leakage current, which becomes more significant as voltage and temperature increase.
3. To mitigate the negative impact of ESR, circuit designers often parallel multiple aluminum electrolytic capacitors. They also add small capacitors such as ceramic or monolithic types in parallel to filter out high-frequency noise, complementing the behavior of the aluminum electrolytic capacitors.
4. The volatilization of the electrolyte in aluminum electrolytic capacitors is an inevitable process, meaning these capacitors will eventually fail, with the only variable being when it occurs.
5. Under typical operating conditions, temperature is the primary factor that affects the lifespan of aluminum electrolytic capacitors. For every 10°C increase in temperature, the capacitor’s lifespan is typically halved. Based on the author’s maintenance records, high-quality capacitors from brands like NICHICON or RUBYCON usually last over 10 years, while lower-quality capacitors tend to fail within 3 to 5 years.
6. When replacing aluminum electrolytic capacitors, it’s crucial to account for the voltage derating. It’s recommended to leave a voltage margin of at least 15%. For instance, a 24V power supply using a 25V-rated capacitor may function fine initially, but over time, the capacitor’s lifespan will be significantly reduced.
7. Aluminum electrolytic capacitors are polarized, so it’s important to install them with the correct polarity. Reversing the polarity can lead to serious hazards, especially with high-voltage capacitors, as reversed polarity can result in dangerous explosions. The negative terminal of the capacitor is typically marked on the case, and this must be aligned with the correct polarity on the circuit board during replacement. In some industrial PCBs, polarity markings may not be present, so it’s important to mark the positions before disassembly to ensure proper orientation during replacement.
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