There are different types of capacitors, which classified according to form, dielectric medium and electrode structure. Generally the classification of capacitors according to form it determined by whether the capacitor is variable or fixed.
Variable capacitors: air, mica, ceramic
Fixed capacitors: mica, ceramic paper etc.
According to dielectric medium used capacitors may be classified as: mica capacitor, glass capacitor, paper capacitor, paper capacitor, electrolytic capacitors.
Polar (electrolytic): Aluminium and Tantalum electrolytic capacitor.
Non-Polar (electrostatic): mica, ceramic capacitors etc.
It is necessary to vary the value of a capacitor when it remains connected in a circuit. For example may be desired to tune the circuit of an oscillator. Variable capacitors are available to fulfill such application requirements.
These are available in two types, continuous variation of the capacitance and limited adjustable of a capacitance. The former types of capacitors are used for varying frequency of a tuned circuit in which it is connected. Air capacitors are used to this application in transmitters. Capacitprs designed for limited adjustment of capacitance are named as Trimmer capacitors.
Mica is used as dielectric medium in mica capacitor. Mica is prepared in thin sheets. These sheets without any imperfection such as cracks, notes etc., are selected. Its dielectric constant lies in between 3 to 8 and a dielectric strength of 600 – 1500 volts/mil. Mica capacitors are made in two methods, such as stacked mica and silver mica capacitor.
Mica capacitors are made in two methods, such as stacked mica and silvered mica capacitors.
Stacked Mica Capacitor
These capacitors are constructed by thin metal sheets or tin foils, arranged one over another and separated by thin mica sheets placed between the metal sheets. These metal sheets serves as the capacitor plates and mica acts as dielectric. By combining alternate metal sheets, two sets of metal plates are formed. An electrode is attached to each set of plates and a terminal is connected to each electrode. The entire unit is then cased in a container of insulating electrode.
Capacitors with mica dielectric are noted for their excellent characteristics such as : temperature performance, low loss at all frequencies, high dielectric strenght and stability over time. Due to this they are recommended for use in filtering circuits, delay line circuits, oscillators
Silvered mica Capacitors
In this type of capacitors silver coated nica sheets are stacked alternately. A thin film or silver coated mica sheets are connected in parallel to obtain the required capacitance value. Later the unit is sealed in wax moulded case. These capacitors are small in size when compared to stacked mica Capacitors.
Ceramic Capacitor uses ceramic dielectric material. These are made in many shapes and sizes such as disc, tubular monolithic and barrier type. All these types used ceramic materials of different dialectical constants. There are two categories of dielectric constants i.e., low permittivity. Steatite, MgO, ; TiO2 mixture and TiO2 are the materials having their dielectric constants these materials having their dielectric constants these materials has better tolerance and temperature coefficients.
A ceramic disc is covered on two sides with a metal, for example, copper or silver. These coatings goes about as two plates of a capacitor. During the manufacture of the capacitor, tinned wire leads are likewise joined to each one plate. At that point the whole unit is covered with plastic checked with its capacitance esteem. These capacitors are having hub heads, which are adjusted off to dodge crown (ionization of air). Plates are made of either low primitive sort or high primitive sort running from few pico farads to 20,000 pf.
Electrolytic capacitors use solid dielectric media on which an oxide film, produced electrochemically. This oxide film posses high dielectric constant. These capacitors has large capacitance in a small volume. Electrolytic capacitors should be connected in a circuit with the proper polarities. Hence, these capacitors have polarized nature.
To these capacitors tantalum is used as an electrolytic instead of aluminium. These have excellent capacitance to zero ratio. The special features of this type compared to aluminium:
- High Dielectric Constant
- Low leakage current
- Longer shelf life
- Good Stability
- Better impedance
- Higher operating range of temperature
Wet type Tantalum Capacitors
In manufacturing of these capacitors different types of electrolytes are used such as sulfuric acid for low voltages and boric acid for high voltages.
A silver cup is used as anodes which contains electrolytinc solution acts a anode. The anode and cathode are separated by a Teflon seal. On the application of DC voltage electrolysis take place. It results information of a thin film and it acts as dielectric medium.