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Capacitor Types: Fixed & Variable Capacitors
Capacitors can be classified depending upon their fixed or variable capacitance as follows −
Those capacitors whose value of capacitance is fixed during the manufacturing and cannot be changed later are known as fixed capacitors. The symbol of the fixed capacitor is shown in figure.
The fixed capacitors are classified into two categories as −
Those capacitors which are having the specified positive and negative polarities, are called as polarized capacitors. When these capacitors are used in the circuits, they should be connected in perfect polarities.
Polarized capacitors are further classified as −
Those capacitors in which some electrolyte is used as a dielectric medium are called as electrolytic capacitors. These are the polarised capacitors i.e. marked with specific polarities as anode (+) and cathode (-). There are mainly three types of electrolytic capacitors −
Aluminium Electrolytic Capacitor
In these capacitors, the anode (+) terminal is made of a pure aluminium foil with an etched surface. The aluminium forms a thin insulating layer of aluminium oxide that acts as the dielectric of the capacitor. A non-solid electrolyte covers the rough surface of the oxide layer and acts as the cathode (-) terminal of the capacitor.
Tantalum Electrolytic Capacitor
It consists of a pellet of porous tantalum metal that acts as the anode of the capacitor. The tantalum metal is covered by an insulating oxide layer that forms the dielectric and surrounded by a solid or liquid electrolyte that acts as cathode terminal.
Niobium / Niobium Oxide Electrolytic Capacitor
In a niobium capacitor, the anode terminal is made of passivated niobium metal on which an insulating niobium pent-oxide layer acts as the dielectric. A solid electrolyte on the surface of the oxide layer acts as the cathode of the capacitor.
These are the types of capacitors whose capacitance value is much greater and they can store and deliver charge much faster than the other types of capacitors. As these capacitors are also polarized capacitors, therefore they are also having specific positive and negative polarities.
The types of super capacitors are as follows:
Double Layer Capacitor
It is an electrostatic capacitor. The principle of this capacitor is based on the double-layer capacitance. In an electrical double layer which appears at the interface between a conductive electrode and a liquid electrolyte. Two layers of charge with opposite polarity form at this interface, one at the surface of electrode and one in the electrolyte. These two layers are separated by a single layer of solvent molecules that stick to the surface of electrode and act as dielectric in the capacitor.
The charge in the pseudo capacitor is stored faradically by the transfer of charge between electrode and electrolyte. The pseudo capacitor is a part of an electrochemical capacitor.
These capacitors are the combination of double layer capacitor and pseudo capacitor. In these super capacitors, the anode is made of pre doped carbon and the cathode is of activated carbon. The lithium-ion capacitor is an example of hybrid capacitor.
The non-polarised capacitors are the ones that does not have specific positive and negative polarities i.e. they can be connected in the circuits without considering that which lead is connected to the positive and which one to the negative. There are various types of nonpolarized capacitors as follows −
When the ceramic material is used as a dielectric medium in the capacitor, the capacitor is called as ceramic capacitor.
These types of capacitors have a film substance as a dielectric medium. They are of following types −
The dielectric medium used in paper capacitors is a waxed or oiled paper. The paper is sandwiched in between two thin tin foils sheets and these sheets are rolled into a cylindrical shape and encapsulated in a plastic enclosure.
Metal Film Capacitor
In these capacitors, a paper coated with metallic film is used as a dielectric medium. The metal (aluminium) coated sheets are rolled in the form of cylinder and encapsulated in a plastic enclosure.
There are some more capacitors which are named according to dielectric material used. Some of them are −
These capacitors consist of a thin mica sheet as dielectric. In these capacitors, a mica sheet is sandwiched in between thin metal sheets, leads are taken out and this whole assembly is enclosed in a plastic enclosure.
In these capacitors, the glass is used as the dielectric medium. These are noise free capacitors with the low temperature coefficient.
When air is used as the dielectric medium in the capacitor, the capacitor is known as an air capacitor.
In these capacitors, vacuum is used as a dielectric medium.
The variable capacitors are the ones whose capacitance value can altered either electrically or mechanically. The variable capacitors consist of sets of stationary and movable plates. A shaft is attached with the movable plates and by rotating the shaft, the capacitance value can be altered.
There are mainly two types of variable capacitors −
The tuning capacitors are type of variable capacitor consisting of a set of semi-circular stationary (stator) and movable (rotor) plates, a shaft is attached to the movable plates, and this whole assembly is supported by a frame. The movable plates when moved into the space between the stator plates, they come close to form plates a capacitor. When the movable plates sit completely in between the stator plates, the capacitance value being maximum and when the movable plates are completely out of stator, the capacitance value is minimum.
The capacitance of a trimmer capacitor is varied by using a screwdriver. These capacitors consist of two plates: one is stationary and the other is movable and these plates are arranged parallel to each other with a dielectric material in middle.
These capacitors have three terminal leads: one is connected to stationary plate; second is connected to movable plate and the third lead is common. The variation of capacitance value can be observed with the movement of semi-circular shaped movable disc. Trimmer capacitors are frequently arranged on the printed circuit boards so that the user does not have the right of entry to change them.
Depending upon the type of dielectric material used, these types of capacitors are classified into two types −
Air trimmer capacitor
Ceramic trimmer capacitor
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