Depending on the basis of the dielectric used, following types of capacitors are used in electronic circuits: 1. Air Insulated and Vacuum Capacitors 2. Paper Insulated Capacitors 3. Mica Capacitors 4. Plastic Film Capacitors 5. Ceramic Capacitors 6. Electrolytic and Tantalum Capacitors.
These are described in brief as follows:
Type # 1. Air Insulated and Vacuum Capacitors:
These capacitors consist of a number of metallic plates insulated from each other and separated by air. A typical air insulated capacitor is used for the tuning or radio receivers. The capacitor is of the variable type that is a set of moving plates meshing with a set of fixed plates.
Two parallel plates of a small capacitor of air insulated (variable) type can also be mounted, insulated from each other, inside an evacuated and sealed glass envelope so that the dielectric is vacuum.
Type # 2. Paper Insulated Capacitors:
ADVERTISEMENTS:
In these types of capacitors, the dielectric consists of one or more layers of extremely thin kraft paper which looks very much like cigarette paper. The metal plates consist of aluminium foil and the whole “sandwich” is rolled into an element. The connection to the metal plates (foils) is made through metallic strips emerging at the side of the capacitor.
Type # 3. Mica Capacitors:
Mica, a mineral that can be readily sliced into thin chips, makes an excellent dielectric and is used in capacitors incorporated in circuits requiring stable and accurate capacitors with low losses and high leakage resistance. The capacitors consists either of alternate layers of metallic foil, with mica forming a stack which is then hermetically sealed in some hard plastic or, alternatively, the mica can be silvered, stacked and enclosed in a plastic moulding.
Type # 4. Plastic Film Capacitors:
These are similar in construction to paper insulated capacitors, except that transparent plastic foil is used as a dielectric instead of paper.
Type # 5. Ceramic Capacitors:
As the name indicates, the dielectric of this type of capacitors is a ceramic. The basis of the ceramic material is mainly barium titanate or a similar material, but other ceramic substances including hydrous silicate of magnesia or tale are also used. The electrodes are applied in the form of silver which is either sprayed or plated on to the opposite faces of a thin tube, wafer or disc made from the ceramic material. Connecting wires are then soldered to this deposit and the whole capacitor dipped in for a suitable coating.
Type # 6. Electrolytic and Tantalum Capacitors:
ADVERTISEMENTS:
In Electrolytic and Tantalum capacitors type of capacitors the dielectric consists of an extremely thin film of aluminium oxide formed on one of its aluminium foil plates. Intimate contact with the other plate is achieved by impregnating the paper between the foils with an electrolyte in the form of viscous substance, such as ammonium borate. The sandwich is then rolled into a cylindrical element and housed in either metallic cardboard, plastic or ceramic protective tube.
In the tantalum capacitor, the aluminium foil is replaced by a thin sheet of tantalum, the dielectric thus being tantalum. While such capacitors are physically smaller, they are limited to lower voltages.