01 Electrolytic Capacitors

Electrolytic capacitors are capacitors that use an oxide layer formed on the electrode via an electrolyte as the insulating layer, typically offering large capacitance. The electrolyte is a liquid or gel-like substance rich in ions. Most electrolytic capacitors are polarized, meaning during operation, the voltage at the positive terminal must always be higher than that at the negative terminal.

The high capacitance of electrolytic capacitors is achieved at the expense of other characteristics, such as larger leakage current, higher equivalent series inductance and resistance, significant capacitance tolerance, and shorter lifespan.

Besides polarized electrolytic capacitors, non-polarized electrolytic capacitors also exist. For example, two 1000μF, 16V electrolytic capacitors are shown—the larger one is non-polarized, while the smaller one is polarized.

The internal structure of electrolytic capacitors may use liquid electrolytes or solid polymers, with electrode materials commonly being aluminum or tantalum. The structure of a typical polarized aluminum electrolytic capacitor consists of two electrode layers separated by fiber paper soaked in electrolyte, along with an insulating layer, all wound into a cylinder and sealed in an aluminum casing.

When dissected, the basic structure becomes clear. To prevent evaporation and leakage of the electrolyte, the leads are secured with sealing rubber. The image also shows the difference in internal volume between polarized and non-polarized electrolytic capacitors: under the same capacitance and voltage rating, the non-polarized capacitor is about twice the size of the polarized one.

This size difference primarily arises from the significant disparity in the electrode area inside the two capacitor types. The electrode of the non-polarized capacitor is wider, while the polarized capacitor’s electrode is thinner.

02 Capacitor Explosion

When the voltage applied to a capacitor exceeds its rated voltage, or when the polarity is reversed for a polarized electrolytic capacitor, the leakage current increases sharply. This causes a rise in internal temperature, leading the electrolyte to generate a large amount of gas.

To prevent explosions, the capacitor casing is typically stamped with three grooves at the top. These grooves allow the top to rupture first under high pressure, releasing internal pressure.

However, if these grooves are improperly manufactured, the internal pressure may force out the sealing rubber at the bottom. The sudden release of pressure then results in an explosion.

1. Explosion of a Non-Polarized Electrolytic Capacitor

   A non-polarized electrolytic capacitor (1000μF, 16V) was tested. When the applied voltage exceeded 18V, the leakage current surged, causing a rapid increase in internal temperature and pressure. Eventually, the rubber sealing ring at the bottom exploded, and the internal electrodes loosened like popcorn.

   By attaching a thermocouple, the temperature change during voltage increase was measured. The graph shows that once the voltage exceeded the rated value, the temperature continued to rise.

   The current change during this process was also recorded. The current increase was the main cause of the temperature rise. As the current spiked, the voltage dropped due to the power supply’s internal resistance. When the current exceeded 6A, the capacitor exploded with a loud noise.

   Due to the larger internal volume and more electrolyte in non-polarized capacitors, the pressure generated was immense. This caused the bottom seal to blow open instead of the top grooves rupturing.

2. Explosion of a Polarized Electrolytic Capacitor

   For a polarized electrolytic capacitor (1000μF, 16V), when the voltage exceeded its rating, the leakage current also increased sharply, leading to overheating. In this case, the pressure was released through the top grooves, avoiding an explosion.

   The temperature and leakage current changes were similarly recorded. The leakage current began to rise sharply once the voltage exceeded 15V.

   These experiments highlight the importance of leaving sufficient voltage margin when using electrolytic capacitors to avoid breakdown due to voltage fluctuations.

03 Series Connection of Electrolytic Capacitors

In certain situations, parallel or series connections of capacitors are used to achieve larger capacitance or higher voltage tolerance, respectively.

In applications where the voltage across the capacitor is alternating current (e.g., speaker coupling capacitors, AC phase compensation, motor phase-shifting capacitors), non-polarized electrolytic capacitors are required. Some manufacturers suggest using traditional polarized capacitors in a back-to-back series connection (i.e., connecting two capacitors in series with opposite polarities) to achieve a non-polarized effect.

The leakage current under three conditions was compared:

1. Forward Voltage and Leakage Current: For a polarized electrolytic capacitor (1000μF, 16V) within its rated voltage range, the leakage current remained below 0.5mA.

2. Reverse Voltage and Leakage Current: When the reverse voltage exceeded 4V, the leakage current increased rapidly. The reversed capacitor behaved like a 1Ω resistor.

3. Back-to-Back Series Connection: Two identical capacitors (1000μF, 16V) were connected in series with opposite polarities. When the voltage exceeded 4V, the leakage current increased but remained below 1.5mA. Surprisingly, the leakage current of the series combination was higher than that of a single capacitor under forward voltage. This anomaly might be due to prior damage to one of the capacitors during reverse voltage testing.