What are Electrolytic Capacitors?

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Editorial Team - everything PE

Aug 29, 2022

Electrolytic capacitors are capacitors that use an electrolyte as their dielectric material. An electrolyte is a high ion concentration liquid or a gel. The capacitance of a capacitor is directly proportional to the surface area of the electrodes and inversely proportional to the thickness of the dielectric. An electrolytic capacitor has a large surface area of electrodes and a very thin dielectric which results in higher capacitance. Electrolytic capacitors have higher capacitance-voltage (CV) product per unit volume than ceramic or film capacitors which makes them perfect for large capacitance applications.

Electrolytic Capacitors are used in filter circuits to minimize the amount of voltage ripple. They are used as a low-pass filter to remove faint AC components from DC signals. These capacitors are also used widely in power supplies to filter out noise (decoupling purpose) and in flashlamps to store energy.

Structure of Electrolytic Capacitors

An electrolytic capacitor achieves a significantly higher capacitance per unit volume by using an electrolyte—which can be solid, liquid, or gel—as the cathode or negative plate. A metal plate acts as an anode (positive terminal). This metal plate undergoes anodization and forms a thin oxide layer. This oxide layer acts as the dielectric. Anodization is an electrolytic process that is used to increase the natural oxide layer thickness on a metal surface.

Types of Electrolytic Capacitors

Based on the type of dielectric and materials used for construction, Electrolytic capacitors are classified into three groups:

  • Aluminium Electrolytic Capacitors: These capacitors use a high-purity etched aluminium foil with aluminium oxide as the dielectric. These are commonly used in automotive airbags, power factor correction, camera flashes, etc. Aluminium is used as electrodes in electrolytic capacitors as they are readily available, cheap and provide similar physical properties when compared to other conductors. It easily forms a thin oxide layer with a high dielectric constant (K), meaning it can store more charge. The aluminium oxide layer can withstand high voltages greater than 400 V whereas Tantalum and Niobium are only capable of withstanding low voltages of under 40 V.

  • Tantalum Electrolytic Capacitors: These capacitors use a slug(sintered pellet) of high purity tantalum powder as the anode, tantalum oxide as dielectric and solid manganese dioxide electrolyte as the cathode. They have low-leakage current and high capacitance and are usually used in power supply decoupling targets along with ceramic and film capacitors. These capacitors also have a self-healing mechanism to reduce MnO2 electrolytes into insulating Mn2O3. As a result of the "self-healing" property, any flaws in the capacitor's working process can be fixed, restoring the insulation capacity the capacitor should have and preventing avalanche breakdown of the dielectric.

  • Niobium Electrolytic Capacitors: Niobium electrolytic capacitors use a slug of high purity niobium or niobium oxide powder with niobium pentoxide as the dielectric. They have thicker dielectric and low breakdown voltage than tantalum electrolytic capacitors. Along with a self-healing mechanism, they also have a self-arresting mechanism that protects the chip from short-circuiting due to local breakdown. These are used in consumer, aircraft fields, automotive applications etc.

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