Electrolytic Capacitor: Aluminum Type and Polarity Explained

An electrolytic capacitor is a type of capacitor that uses an extremely thin layer of aluminum oxide as its dielectric. These aluminum electrolytic capacitors are known for being inexpensive and offering very large capacitance values, up to 150,000 µF. They’re commonly found in power supplies.

Polarity Matters!

Electrolytic capacitors are polarized components. This means they have a designated positive (+) terminal, clearly marked, and the capacitor’s case itself often serves as the negative terminal.

The DC leakage current within the capacitor plays a vital role in maintaining the oxide layer. If the capacitor is reverse biased, the oxide layer will deteriorate.

In low-voltage electronic circuits requiring capacitance values less than 100 µF, tantalum electrolytic capacitors are often preferred over aluminum ones.

The Importance of Forward Bias

Due to their construction, electrolytic capacitors must be forward biased. In other words, the voltage at the positive terminal must be higher than the voltage at the negative terminal.

It’s crucial to pay attention to electrolytic capacitor polarity when connecting it in a circuit. Incorrect connection can lead to catastrophic failure under reverse-biased conditions, as described below.

Electrolytic Capacitor Failure: What Happens When It’s Wrong?

When an electrolytic capacitor becomes reverse biased, the dielectric layer is damaged. This results in a short circuit between the capacitor’s terminals.

This short circuit allows a large amount of current to flow through the capacitor, causing it to overheat and potentially leak or vaporize. In some cases, the capacitor’s enclosure can even burst.

This event is referred to as electrolytic capacitor failure. This type of failure can typically occur with reverse voltages of about 1 volt or higher.

To mitigate the risks of failure, a safety valve is often incorporated into the capacitor’s enclosure.