OVERVIEW OF CAPACITORS - What is capacitor? - Classification of capacitors

Capacitors

The capacitor is a device that is used to store energy in electrostatic fields. In a practical capacitor, the device consists of two parallel metallic plates separated by an insulating material called the dielectric. The metal plates can be made of any conducting material like copper. The dielectric material may be ceramic, mica, etc.

Classification of capacitors

The capacitors can be classified into two major types :

Fixed capacitors

Variable capacitors

Fixed capacitors

The fixed capacitors can be classified under physical construction, dielectric usage, voltage rating, and polarization. Fixed capacitors are those whose capacitance values cannot be varied mechanically. The fixed capacitors are classified depending on their dielectric materials.

Paper capacitors

Paper capacitors make use of tissue or cellulose paper called Kraft paper. They are usually high-voltage capacitors and are mechanically very strong. The capacitance value of paper capacitors is usually between 0.002 µF and 0.05µF. Paper capacitors are very cheap but they have poor high-frequency characteristics.

Ceramic capacitors

Ceramic capacitors make use of ceramic as the dielectric material. Basic ceramic materials are formed from titanium dioxide. Some of the dielectric materials used are barium titanate and zirconium titanate. The range of ceramic capacitance between pF and nF. The ceramic capacitors are light in weight and they are suitable for use in hybrid integrated circuits. However, very high-voltage ceramic capacitors are not available. They are used in hybrid ICs and coupling capacitors in low-voltage circuits.

Mica capacitors 

   

Mica is a complex silicate of aluminium with K, Mg, Fe etc. Mica capacitors have good mechanical strength, they can be operated to temperatures as high as 900⁰C. But mica capacitors are silver migration occurs at high DC voltages, high temperatures, and high humidity.

Plastic capacitors

The plastic capacitors are made of polycarbonate, polyethene and polyvinyl chloride. Plastic capacitors have high dielectric strength and large permittivity. They can withstand temperatures up to 250⁰C. Plastic capacitors have high capacitance in small volumes. Plastic capacitors have comparatively high capacity with good high voltage and low leakage properties. They have low leakage but capacitance of more than a few micro-farads is not available at present.

Electrolytic capacitors

Due to an electrolysis chemical reaction, a thin oxide film is formed around one of the electrodes, and it is this film that acts as the dielectric material for the electrolytic capacitor. These types of capacitors are available with 1µF to thousands of micro-farads.

Variable capacitors

For some applications, we need the capacitance used in the circuit to be variable. For instance radio and television tuning sets. We also have air-cored and plastic-cored capacitors. Air-cored capacitors were used in the olden days. Plastic capacitors are widely used. Plastic-cored capacitors are made of polyvinyl chloride. Most of the modern radios use transistors and ICs; we use the PVC type of variable capacitors.