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Tuesday, 26 March 2019

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 Ɛr.

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



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