Specifications and Uses of Capacitors

Specifications of Capacitors:

The specifications of capacitors are:

1. Capacitor Value:

It provides the value of a capacitor C as farads moreover printed or colour coded over the body of the capacitor. It's units are Farads. Practical capacitors are available from 1pf to 1000 mF.

2. Tolerance:

The variation in capacitance value from the indicated value.

3. Dielectric Constant:

It is defined as the ratio of capacitance of a capacitor containing the dielectric material, to the capacitance of the same capacitor with air or vaccum as the dielectric. It is denoted by the letter K. It may be expressed as

K = Capacitance of the capacitor with dielectric/ Capacitance of the capacitor with air dielectric.

The dielectric constant is also known as relative permittivity Ɛ_r.

The relative permittivities of some dielectric media are listed in the Table.

Material	Dielectric constant or relative permittivity (Ɛr)
Air or vacuum	1
Bakelite	4.5 – 5.5
Ceramic	50 - 300
Fibre	5 – 8
Glass	7 – 8
Mica	3 – 6
Paper (dry)	2 – 2.5
Paper (waxed)	3 – 5
Polystyrene	2.6
Porcelain	5 – 6
Rubber	2 – 3.5

4. Dielectric Strength:

 It is very important specification of a capacitor (insulator or dielectric medium), which gives the maximum voltage gradient that a unit thickness of the medium can withstand without breaking down. It’s unit is Volt/metre (V/m) although it is usually expressed in KV/m.m.

Factors affecting the Dielectric Strength:

a. It decreases with increase in the thickness of dielectric material.

b. It decreases with increase in frequency.

c. It decreases with humidity and temperature

d. It decreases with increasing time of application of electric current.

5. Power Factor:

It gives the fraction of input power dissipated as heat loss in the capacitor. The quality of a capacitor in terms of minimum loss is often indicated by its power factor. The lower the numerical value of the power factor the better is the quality of the capacitor.

There are some other specifications that are to be considered while selecting a capacitor. They are.

6. Temperature Coefficient: Variation of capacitance value with the temperature.

7. Voltage Rating: Maximum voltage that can be applied across a capacitor.

8. Leakage Resistance/Leakage Current: Reciprocal terms that denote leakage property of capacitor.

Uses of Capacitors:

No	Type	Max Working Voltage in Volts	Typical Uses/ Applications
1	Tubular Rolled paper	1000	Coupling and decoupling AF circuit
2	Metallised paper	400	Coupling and decoupling AF circuit
3	Polystyrene	125 – 500	Resonant circuits, coupling measuring circuit
4	Polyester	500	Coupling Decoupling and Smoothing
5	Stacked Mica	2000	RF Coupling Bypassing circuit
6	Silvered Mica	1000	RF Resonant circuits and Measuring circuits.
7	Silvered Ceramic	1000	RF amplifiers, RF Bypass Circuits
8	Ceramic with high dielectric constant	350
9	Aluminium Electrolytic	600	Rectifier filters and smoothing
10	Tantalum Electrolytic	6 – 150	Coupling, decoupling Bypassing in transistor amplifiers and PCB’s
11	Air/ Polystyrene variable capacitance	1000	Tuning circuits in receivers and transmitters
12	Mica Trimmers and padders	500	Tracking and alignment of receivers
13	Air Trimmers	500	Tracking and alignment of receivers