Capacitors

Discription :
A Capacitor is a passive two terminal component which sotres electric charge.This component consists of two conductors which are separated by a dielectric medium.The potential difference when applied across the conductors polarizes the dipole ions to store the charge in the dielectric medium.The circuit symbol of a capacitor is shown below :

The capacitance or the potential storage by the Capacitor is measured in farads which is symbolized as F .One Farad is the capacitance when one coulomb of electric charge is sotred in the conductor on the application of one volt potential difference.
The charge stored in a capacitor is given by

Q=CV
Where  Q= charge stored by the capacitor
            C= capacitance value of the capacitor
            V=Voltage applied across the capacitor
Note the other formula of current,     I=dQ/dt
dQ/dt=d(CV)/dt
Form the above statment,we can express the equation as
I=C9dV/dt)
As you turn on the power supply,the current begins to flow through the capacitor inducing the positive and negative potentials across its plates.The capacitor continues to charge until the capacitor voltage equalizes up to the supply voltage which is called as the charging phase of the capacitor.Once the capacitor is fully charged at the end of this phase,it gets open circuited for DC.It begins to discharge when the power of the capacitor is switched off.The charging and discharging of the capacitor is given by a time constant.

t=Rc the voltage across the capacitor is given by

Capacitors are widely used in a variety of applications of electronics circuits such as
store charges such as in a camera flash circuit
smoothing the output of power supply circuits
coupling of two stages of a circuit (coupling of an audio stage with a loud speaker)
filter networks (tone control of an audio system )
delay applications (as in 555 timer IC controlling the charging and discharging )
tuning radios to particular frequencies
phase alteration

The conductors offer a series resistance and if the capacitor is constructed using tubular structure then some inductance is also induced.The dielectric medium between the plates has an electric field strength limit and also passes a small amount of leakage current which results into a breakdown voltage .
The capacitor can be fixed or variable.They are categorized into  two groups,polarized or non-polarized.Electrolytic capacitors are polarized.Most of the low value capacitors are non-polarized.The symbol of capacitors from each group is shown below :

Construction and Types :
The capacitor consists of two conducting plates that are separated by an insulating medium know as the dielectric.The capacitance is dependent upon the surface area of the plates,the distance between the dielectric medium and the dielectric constant of the object.The greater the area of the plates,the closer they are together and greater the value of the dielectric constant the greater is the value of capacitance.High capacitance capacitors are now avialable in small size .This has been achieved employing a number of techniques like having several sets of plates,placing the plates very close to one another,having a thin layer of dielectric placed between them and developing special insulating dielectric materials.

The capacitance of a capacitor is also affected by the shape or structure of the capacitors.The capacitors are available in diffrent shapes like radial lead type which are rectangular or cubical or axial type which are tubular or cylindrical.
The variable type of capacitors can vary the capacitance by chanding the distance between the plates or the effective area of the capacitor.
The polarized type of capacitors should be connected as per thier polarity or else the capacitor can be damaged due to incorrect connection .
The low value capacitors are non-polarized and can be connected in any manner.They are not damaged by heat when soldering,except for the polystyrene type of capaciotr.They have high voltage ratings of at least 50V,usually 250V or so ,Many small value capacitors have thier value printed but without a multiplier,s you need to use experience to work out what the multiplier should be
For example :
0.1 means 0.1 uF=100nF
Sometimes the multiplier is used in place of the decimal point :
For example 4n7 means 4.7 nF

1. Variable Capacitors :

The various types of capacitors are given below :

1. Fixed capacitors :Flim Capacitors like glass capacitor,mica capacitors,silver mica capacitor,ceramic capacitor,papaer capacitor,metalized paper capacitor,polyester capacitor,polystrene capacitor,metalized polyester capacitor,polcarbonate capacitor,polypropylene capacitors,teflon capacitors,porcelain capacitor,electrolyte capacitors like aluminum electrolyte,tantalum electolyte,aluminum-tantalum
2. Variable Capacitors :

The variable type of capacitors can vary the capacitance by changing the distance between the plates or the effective area of the capacitor.
a.Air-gap capacitors :
These capacitors user air as the dielectric medium.The capacitance values iffered are high and can be used with high voltages.These are used for high frequency operation in communication systems.
b. Vacuum capacitors :
these capacitors have glass or cerramic encapsulation and vacuum as the dielectric.Their complex construction makes it very expensive .The oretically,it has less losses and are used in RF applications.

Capacitor color code :

A color code was used on polyester capacitors for many years.It is now obsolete,but of course there are many still around.The colors should be read like the resistor code.

• The top three color nabds give the value in pF
• The 4th band is for tolerance.
• The 5th band is for the voltage rating.

For example :
brown,black,orange,means 10000pF=10nF=0.01uF
Note : There are no gaps between the color bands,so 2 identical bands actually appear as a wide band.
Wide red,yellow means 220nF=0.22uF