Fundamental And Types Of Capacitors

Introduction To Capacitors

A capacitor is a body which can store an electrical charge. It consists of 2 conducting plates facing each other and separated by an insulating material. This insulating material is also called dielectric material. When a charge is stored in one plate, an equal and opposite charge is inducted on the other plate and thus a potential difference is set up between the plates.

The unit of measurement for capacitance is Farad but this unit is much too large for practical work. It is usually measured in microfarads(uF) or picofarads(pF). The formula of calculating capacitance is

C= [(0.224 KA)(n-1)]/d where

C = capacitance in pF
K = dielectric constant of material between plates
A = area of one side of the plates square inches
d = separation of plate in inches
n = number of plates

The potential difference V developed when a charge Q is stored depends directly on the value of Q and inversely with the capacitance C of the cap.

V = Q/C

They are used in timing circuits as it takes time for a cap. to be charged up. They are used to smooth varying DC power supplies by acting as a reservoir of charge. They are also used in filter circuits because they easily pass AC signals but they block DC signals.

DC Voltage Rating

The DC working voltage of a cap. is the maximum voltage which may be applied continuously on the electrodes of the cap. at the upper limits of the working temperature range. The peak value of an alternating voltage should not exceed this rating and have to be derated according the the FMEA as recommended in FMEA.

Leakage Resistance

The dielectric of a practical cap. introduces power losses which can be represented by a smallresistance connected in series with the cap. The insulating resistance is often greater than3,000 Mohm.

Types Of Capacitors

There are many different types of cap. that are used for different types of applications. Theyare electrolytic cap., ceramic cap., tantalum cap., polyester cap., polystyrene cap. and safety cap.(namely X and Y types of cap.).

Electrolytic Type

Electrolytic cap. have leads that are marked with + or - signs. They have polarity and must be connected with the correct polarity. The values of the capacitance and voltage rating are are printed with on its body. The voltage rating can range from 5V up to 440V DC. Generally this type of capacitor is usedas smoothing cap. in power supply regulation. The bigger the value of the cap. is, the lessripple the DC supply that has been rectified will be.

Ceramic Type

This type of capacitor is most commonly used and both through hole and surface mount types are available.Its application is mainly used in general digital circuit, as power supply bypass capacitor, converters(both for input and output), Smoothing capacitors, DC-DC converters, Switching power supplies (secondary side).

Figures below show the typical characteristics of this type of capacitor. It shows the construction and thetemperature range and codes of the cap. The impedance vs frequency change is also shown.

Tantalum type have low voltage ratings and they are expensive but very small. Usually they are used in application where a large capacitance is needed in a small size.

Metallized Polyester Film Type

Metallized Polyester Film is used for general purpose applications. Some of the features are its Self-healing property and flame retardant epoxy resin coating. Its voltage ranges from 100V DC to 1250V DC.

This type is classified into classes like X and Y types. It is usually used on the live parts of the circuity. One of the applications is in the area of reducing the harmonics on inverter motor drives type ofapplication. As safety is a main concern over here, usually this type of cap. has UL, VDE, SEMKO and othertypes of marking which certifies that it conform to the safety standard of a particular country.

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