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INDEX

Introduction ------------------------------------------------Page 2

General introduction on capacitor --------------------Page 3

Principle of functioning ----------------------------------Page 4

Composition of capacitor ----------------------------------Page 6

Different types of capacitors ---------------------------Page 8

Variable Capacitor -----------------------------------------Page 11

Behaviour of capacitor ----------------------------------Page 13

Application of capacitor ----------------------------------Page 14

Part II -----------------------------------------------Page 15

Conclusion ----------------------------------------Page 17

Reference ----------------------------------------Page 18

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Introduction

Ever since Ewald Georg von Kleist invented capacitor, it has become inevitable in mankind’s life. It is hard to find an electronic component without a capacitor in it. Without them this so called information age itself wouldn’t itself exist. Imagine a world without proper computers, music systems, fans etc. It is an important component in computers as well, it is found on the outside of the chip package or surround the chip on the motherboard. In signal processing, a capacitor and resistor smooth the spikes and sharp edges from a signal. Therefore capacitor plays a crucial and vital role in our day to day life.

Through this assignment, we are being asked to research and find out of what capacitors is, its application, various types of capacitor, its effect in a circuit.

In Part I of assignment ask to describe capacitor, its functioning and main effects on a circuit. The composition of a capacitor and different types of capacitor found in today market has to be explained. A section of part I require to explain variable capacitor and behavior of capacitor in a circuit looking into factors such as energy, voltage and process of charging and discharging of a capacitor. At last not least an application of capacitor should mention in detail.

Part II is to find right combination of capacitors in order to get a desired total output capacitance. Four capacitors are given and have to be arranged in such a way that each capacitor is only used once and all capacitors has to be used to get the desired output

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Capacitor

A capacitor is an electronic device which consists of two electrically conductive plates separated by an insulator or a dielectric. In short, capacitor stores electrical charges. Therefore it is used in wide range of circuits due its ability to store charge. It is very hard to find an electronic component hardly without any capacitor in it.

The above diagram shows various kinds of capacitors available in market today.

The capacitor’s capacitance is obtained by the amount of charge (Q) stored on each plate for a given potential difference or voltage (V) which is present between the plates

C=Q /V

The unit of capacitance is Farad, named after Michael Faraday and symbol is denoted by F. Therefore 1 farad capacitor can store one coulomb of charge at 1 volt. Since the farad is very large unit, values are usually expressed in microfarads (mF), nano farads (nF) or pico farads (pF).

However electrical symbol for capacitor is ---||---

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Functioning of a capacitor

The way capacitor works is by having two metallic plates separated by a dielectric. When the capacitor is place in a circuit with an active current, the electrons from the negative side build up on the closest plate (the negative flows to the positive that is why the negative is the active lead, although many capacitors are not polarized). The charge is stored at the surface of the plates, at the boundary with the dielectric. Once the plate can no longer hold them, they are forced past the dielectric and onto the other plate, thus displacing the electrons back into the circuit.

Factors affecting capacitance

Electric Field

When electric charge accumulates on the plates, an electric field is created in the region between the plates that is proportional to the amount of accumulated charge. This electric field creates a potential difference, V=Ed , between the plates of parallel plate capacitor.

Electric Flux Density

An electric field is a property that describes the space that surrounds electrically charged particles or that which is in the presence of a time-varying magnetic field. This electric field exerts a force on other electrically charged objects.

Electric Potential

The electric potential (also called the electrostatic potential) at a point in space is electrical potential energy divided by charge that is associated with electric field. 

Permittivity

Permittivity is the measure of how much resistance is encountered when forming an electric field in a vacuum. In other words, permittivity is a measure of how an electric field affects a dielectric medium. Permittivity is determined by the ability of a material to polarize in response to the field, and thereby reduce the total electric field inside the material. Thus, permittivity relates to a material's ability to transmit or "permit" an electric field.

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Relative Permittivity

The relative static permittivity also called as static relative permittivity of a material under given conditions is a measure of the extent to which it concentrates electrostatic lines of flux. It is the ratio of the amount of stored electrical energy when a voltage is applied, relative to the permittivity of a vacuum. The relative static permittivity is the same as the relative permittivity evaluated for a frequency of zero.

From these factors we can arrive to conclusion that:-

C=QV= ε0 εr

Ad

Where;

C is capacitance

Q is charge present

V is potential difference

ε 0 is absolute permittivity

εr is relative permittivity

A is area between two plates

D is length of dielectric distance

Composition of a capacitor

There are different types of capacitor at present such as:-

Electrolytic Capacitors

The major raw material for this capacitor is Aluminum. They are used as electrodes as a thin oxidation membranes. It is constructed from two conducting aluminum foils, one of

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which is coated with an insulating oxide layer, and a paper spacer soaked in electrolyte. The foil insulated by the oxide layer is the anode while the liquid electrolyte and the second foil acts as the cathode. This stack is then rolled up, fitted with pin connectors and placed in a cylindrical aluminum casing.

Ceramic Capacitors

This type of capacitor is made by metal and ceramic. It is a capacitor constructed of alternating layers of metal and ceramic, with the ceramic material acting as the dielectric. It has two terminal ends as well

Silver Mica Capacitors

This type of capacitor consists of sheets of mica and copper foil sandwiched together

and clamped. However this method is outdated in these days. Now, instead of being

clamped with foils these use sheets of mica coated on both sides with deposited metal.

The assembly is dipped in epoxy

Types of Capacitors

There are a very large variety of different types of Capacitors available in the market place and each one has its own set of characteristics and applications from small delicate trimming capacitors up to large power metal can type capacitors used in high voltage power correction and smoothing circuits. Some of its types are:

1. Electric double-layer capacitor

They also known as super capacitors and is a quite a wonder. It can produce a capacitance of 0.47F. However the physical size of it rather small compared to its capacitance. It is being used in variety of commercial application such as energy smoothing and momentary-load devices even in home solar systems where extremely fast charging is required. In this type of capacitor do not have a conventional dielectric. Rather than two separate plates separated by an intervening substance, these capacitors use "plates" that are in fact two layers of the same substrate, and their electrical properties, the so-called "electrical double layer", result in the effective separation of charge despite the vanishingly thin physical separation of the layers. The lack of need for a bulky layer of dielectric permits

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the packing of "plates" with much larger surface area into a given size, resulting in extraordinarily high capacitances in practical-sized packages.

They are quite conductive in nature, have high energy density and long life. It has low cost per cycle. They have high output power and extremely low internal resistance. Also it has the highest dielectric absorption of any type of capacitor.

.

2.   Film Capacitors

Film Capacitors are the most commonly available of all types of capacitors, consisting of a relatively large family of capacitors with the difference being in their dielectric properties. These include polyester (Mylar), polystyrene, polypropylene, polycarbonate, metalized paper, Teflon etc. Film type capacitors are available in capacitance ranges from 5pF to 100uF depending upon the actual type of capacitor and its voltage rating.

Examples of film capacitors are the rectangular metalized film and cylindrical film & foil types as shown below.

Radial Lead Type

Axial Lead Type

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This type of capacitor is not used in high frequency circuits, because they are constructed like a coil inside. However they are widely used filter circuits or timing circuits which run at several hundred kHz or less

3.   Ceramic Capacitors

Ceramic Capacitors or Disc Capacitors as they are generally called are made by coating two sides of a small porcelain or ceramic disc with silver and are then stacked together to make a capacitor. For a very low capacitance values a single ceramic disc of about 3-6mm is used. Ceramic capacitors have a high dielectric constant (High-K) and are available so that relatively high capacitances can be obtained in a small physical size. They exhibit large non-linear changes in capacitance against temperature and as a result are used as de-coupling or by-pass capacitors as they are also non-polarized devices.

They are non- polarized. Titanium and barium are used as dielectric. They are used in circuits which bypass high frequency signals to ground.

4.   Electrolytic Capacitors (Electrochemical type capacitor)

Electrolytic Capacitors are generally used when very large capacitance values are required. Here instead of using a very thin metallic film layer for one of the electrodes, a semi-liquid electrolyte solution in the form of a jelly or paste is used which serves as the second electrode (usually the cathode). The dielectric is a very thin layer of oxide which is grown electro-chemically in production with the thickness of the film being less than ten microns. This insulating layer is so thin that it is possible to make large value capacitors of a small size. The majority of electrolytic types of capacitors are polarized, that is the voltage applied to the capacitor terminals must be of the correct polarity as an incorrect polarization will break down the insulating oxide layer and permanent damage may result.

Electrolytic Capacitors are made up of aluminum have polarity and are generally used in DC power supply circuits to help reduce the ripple voltage or for coupling and decoupling applications. They are used to have large value of capacitance.

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Variable Capacitor

A variable capacitor is a capacitor whose capacitance may be intentionally and repeatedly changed mechanically or electronically. Variable capacitors are often used in L/C circuits to set the resonance frequency, e.g. to tune a radio (therefore they are sometimes called tuning capacitors), or as a variable reactance, e.g. for impedance matching in antenna tuners.

How is it made?

Variable capacitors are manufactured in various shapes and sizes, but common feature for all of them is a set of immobile, interconnected aluminum plates called stator, and another set of plates, connected to a common axis, called rotor. In axis rotating, rotor plates get in between stator plates, thus increasing capacity of the device. Naturally, these capacitors are constructed in such a way that rotor and stator plates are placed consecutively. Insulator (dielectric) between the plates is a thin layers of air, hence the name variable capacitor with air dielectric. When setting these capacitors, special attention should be paid not to band metal plates, in order to prevent short-circuiting of rotor and stator and ruining the capacitor.

A variable capacitor made from single crystal silicon fracture surface pairs.

Use of variable capacitors to filter out frequencies that are not part of the main signal. These components generally have metal plates to hold different charges. A variable capacitor is a perfect tool in situations such as radio tuning,

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where applications need adjustment in capacitance.

Behaviour of Capacitor in a Circuit

In Series

Connected in series, the schematic diagram reveals that the separation distance, not the plate area, adds up. The capacitors each store instantaneous charge build-up equal to that of every other capacitor in the series. The total voltage difference from end to end is apportioned to each capacitor according to the inverse of its capacitance. The entire series acts as a capacitor smaller than any of its components.

Where n is number of capacitors present

In Parallel

Capacitors in a parallel configuration each have the same applied voltage. Their capacitances add up. Charge is apportioned among them by size. Using the schematic diagram to visualize parallel plates, it is apparent that each capacitor contributes to the total surface area.

Where n is number of capacitors present

Relation with Energy

As opposite charge accumulate on the plates of a capacitor due to the separation of charge, a voltage develops across the capacitor owing to the electric field of these charges. Ever increasing work must be done against this ever increasing electric field as more charge is separated. The energy measured in joules, stored in a capacitor is equal to the amount of work required to establish the voltage across the capacitor, and therefore the electric field, the energy stored is given by

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Capacitors in AC and DC Circuit

When capacitors are connected across a direct current DC supply voltage they become charged to the value of the applied voltage, acting like temporary storage devices and maintain or hold this charge indefinitely as long as the supply voltage is present. During this charging process, a charging current, I will flow into the capacitor opposing any changes to the voltage at a rate that is equal to the rate of change of the electrical charge on the plates. This charging current can be defined as: I = CdV/dt. Once the capacitor is "fully-charged" the capacitor blocks the flow of any more electrons onto its plates as they have become saturated.

However, if we apply an alternating current or AC supply, the capacitor will alternately charge and discharge at a rate determined by the frequency of the supply. Then capacitors in AC circuits are constantly charging and discharging. A capacitor allows only ac signals to pass thru it n does not allow dc to pass through it.

Charging & Discharging a Capacitor

Consider the following circuit.

Assume that the capacitor is fully discharged and the switch connected to the capacitor has just been moved to position A. The voltage across the 100uf capacitor is zero at this point and a charging current I begins to flow charging up the capacitor until the voltage across the plates is equal to the 12v supply voltage. The charging current stops flowing and the capacitor is said to be "fully-charged". Then, Vc = Vs = 12v.

Once the capacitor is "fully-charged" in theory it will maintain its state of voltage charge even when the supply voltage has been disconnected as they act as a sort of temporary storage device. However, while this may be true of an "ideal" capacitor, a real capacitor will slowly discharge itself over a long period of time due to the internal leakage currents

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flowing through the dielectric. This is an important point to remember as large value capacitors connected across high voltage supplies can still maintain a significant amount of charge even when the supply voltage is switched OFF.

If the switch was disconnected at this point, the capacitor would maintain its charge indefinitely, but due to internal leakage currents flowing across its dielectric the capacitor would very slowly begin to discharge itself as the electrons passed through the dielectric. The time taken for the capacitor to discharge down to 37% of its supply voltage is known as its Time Constant.

If the switch is now moved from position A to position B, the fully charged capacitor would start to discharge through the lamp now connected across it, illuminating the lamp until the capacitor was fully discharged as the element of the lamp has a resistive value. The brightness of the lamp and the duration of illumination would ultimately depend upon the capacitance value of the capacitor and the resistance of the lamp (t = CxR). The larger the value of the capacitor the brighter and longer will be the illumination of the lamp as it could store more charge.

Applications

Capacitors have many uses in electronic and electrical systems. They are so common that it is a rare electrical product that does not include at least one for some purpose. Here in this case we are going to discuss about motor starters in detail.

Motor starters (Electrostatic Motors)An electrostatic motor or capacitor motor is a type of electric motor based on the attraction and repulsion of electric charge. Usually, electrostatic motors are the dual of conventional coil-based motors. They typically require a high voltage power supply, although very small motors employ lower voltages. Conventional electric motors instead employ magnetic attraction and repulsion, and require high current at low voltages. Today the electrostatic motor finds frequent use in micro-mechanical (MEMS) systems where their drive voltages are below 100 volts, and where moving, charged plates are far easier to fabricate than coils and iron cores. Also, the molecular

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machinery which runs living cells is often based on linear and rotary electrostatic motors.

In single phase squirrel cage motors, the primary winding within the motor housing is not capable of starting a rotational motion on the rotor, but is capable of sustaining one. To start the motor, a secondary winding is used in series with a non-polarized starting capacitor to introduce a lag in the sinusoidal current through the starting winding. When the secondary winding is placed at an angle with respect to the primary winding, a rotating electric field is created. The force of the rotational field is not constant, but is sufficient to start the rotor spinning. When the rotor comes close to operating speed, a centrifugal switch (or current-sensitive relay in series with the main winding) disconnects the capacitor. The start capacitor is typically mounted to the side of the motor housing. These are called capacitor-start motors, that have relatively high starting torque.

There are also capacitor-run induction motors which have a permanently-connected phase-shifting capacitor in series with a second winding. The motor is much like a two-phase induction motor.

Motor-starting capacitors are typically non-polarized electrolytic types, while running capacitors are conventional paper or plastic film dielectric types.

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END OF PART I

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Part IIA set of capacitors are given, which has to be assembled in such a way that a desired output of

37.6nF should be achieved. The given capacitors are as follows

C1=15nF

15nF

C2=40nF

40nF

3=65nF

65nF

C4=90nF

90nF

TOTAL OUTPUT REQUIRED

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Total Capacitance Ctotal=37.6nF

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Solving

Taking C1 and C4 in series

15nF 90nF

1C14

= 1C1

+ 1C 4

C14=15∗9015+90=12.85nF

Taking C2 and C3 in series

40nF 65nF

1C23

= 1C2

+ 1C3

C23=40∗6540+65=24.76nF

Taking C14 and C23 in parallel

Ctotal= C14+C23

=12.85nF+24.67nF

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=37.62nF

15nF 90nF

40nF 65nF

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END OF PART II

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Conclusion

Through this assignment we were able understand all property

and characteristics of a capacitor and its role in day to day life.

It enlightened us with different types of capacitors available today

and how each differs with each other.

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References

www.wikipedia.org

www.scribb.com

http://www.howstuffworks.com/capacitor.htm

http://www.tpub.com/neets/book2/3b.htm

http://dictionary.reference.com/browse/electric+flux+density

http://hyperphysics.phy-astr.gsu.edu/hbase/electric/elepe.html

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