A Capacitor And Different Types

A capacitor is a passive component, which stores electrical charge or  electrical energy. Capacitors are more commonly used components in  an   electronic circuit. it is used for storing electrical energy. Capacitors vary by their voltage, dielectric material, capacitance, tolerance etc.

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Structure of a Capacitor

The  capacitor consists of parallel conductive plates that do not touch each  other and are separated by a dielectric material. It has two terminals,  which are connected to the parallel  conductive plates inside the  capacitor. The conductive plates of the capacitors are separated with an  insulating material, which is called the dielectric medium. The  conductors of the capacitor come in various shapes and sizes depending  on the  voltage and the amount of charge it stores. Dielectric of the  capacitor also varies depending on the voltage, capacitance, and  frequency of operation. Examples of dielectric media are glass,  air, paper, vacuum, metal oxide, polystyrene film, polyester film,  mica, semiconductor etc. The conductors and the dielectric medium is  enclosed in the body of the capacitor.

 

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Capacitance  is the ratio of electric charge stored on each conductor to the  potential difference between them. The capacitance of a capacitor  depends on the surface area of the conductors, the distance between each  plates of the capacitor, and the dielectric constant of the material. The greater is the capacitance when  there is greater area of the conductors, closer distance between the  plates of the capacitor, and greater the dielectric constant. The capacitance is measured in Farad, which has the symbol (F). One  Farad is the capacitance when 1 coulomb of electrical charge is stored  in the conductor on the application of 1 volt of potential difference.

The charge stored by the capacitor is denoted by

Q = CV

Where,  Q is the charge stored by the capacitor, C is the capacitance value of  the capacitor, and V is the voltage applied across the capacitor.

Normal value of  capacitance is smaller and it is measured in microfarad, nanofarad,  and picofarad. A 3-digit code is used to indicate the value of the  capacitor. The capacitance value is written in letters and numbers and   also just in numbers according to the manufacturers description.  Capacitors are also characterized by the voltage  specification. The maximum voltage is the voltage  that can be applied  to a capacitor without breaking down the capacitor.  It is usually  written on the capacitor along with the capacitor value.

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Operation of a Capacitor

A   capacitor is isolated when there is no electrical charge across the   conductors. In an isolated capacitor, the conductors hold equal and   opposite charges on the conductor surface. When a DC voltage is applied   to the terminals of a capacitor, the capacitor gets  charged. The current begins to flow across the terminals and an  electrostatic charge develops across the conductive  plates  of the  capacitor. Energy as electrons are stored in the electrostatic field  across  the plates. The positive charge gets collected to one  plate and  the negative charge to the other. As the charges get accumulated on the  plates there is current flowing through  the terminals into the capacitor. The capacitor continues to charge  until its voltage equalizes with the voltage that is applied.  When the  capacitor is fully charged, the current flow is reduced and becomes  zero.  The capacitor is discharged gradually  over time when the voltage source is removed. It can also be discharged  when the terminals are made to contact with each other. When a higher charge is stored in the capacitor, it can give a spark while it is discharged.

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The breakdown voltage is the voltage that can be given to a capacitor  which  when exceeded will cause the dielectric to breakdown and become  a  conductor. The dielectric breaks down and becomes a conductor causing  a  large current to flow through the capacitor. It depends on the  capacitor  and the voltage specification that is being used. The failure  can be a  with an explosion with the capacitor exploding and spreading  the  contents around. When selecting a capacitor for use, the breakdown voltage rating must be twice or thrice the value of the operating voltage. Some capacitors such as the electrolytic capacitors show polarity that has +ive and -ive terminals. When connecting these capacitors the polarity needs to be correct, otherwise the capacitor might get damaged.

Capacitor Applications

Capacitors provide very large resistance to DC and small resistance for AC. Capacitors are used for a variety of applications such as blocking DC and bypassing AC currents, store charge in a flash camera, filtering the  power supplies from ripples and spikes, resonant tuned circuits and frequency  tuning in radios, coupling of stages, tone control in audio circuit,  timing circuits, phase alteration, stabilizing voltage and power flows  etc. Applications such as storing very high charges as required in a  battery are being gradually developed.

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Types of Capacitors

There are many different types of capacitors, based on the type of conductive electrodes and the dielectric used in making them. There are 2 major types of capacitors, fixed capacitors and variable capacitors.

1. Fixed capacitors

A. Film capacitors such as paper capacitor, metalized paper capacitor, glass capacitor,  mica capacitor, silver mica capacitor, ceramic capacitor, polyester capacitor, polystyrene capacitor, metalized polyester capacitor, polycarbonate capacitor, polypropylene capacitor, Teflon capacitor, porcelain capacitor, etc.

B. Electrolytic capacitors such as aluminum electrolyte capacitor, tantalum electrolyte capacitor, etc.

2. Variable capacitors

The variable capacitors include air variable capacitor, polyester film capacitor, etc.

Electrolytic Capacitors

The  electrolytic capacitors have a positive and a  negative electrode.  Aluminum  is used as the electrode in an electrolytic capacitor. A thin  oxide  layer act as the dielectric material in this capacitor. These  capacitors  can vary in capacitance from 1 micro Farad to thousands of  micro  Farads. It is used in varying DC voltage   conditions for power supply applications. Electrolytic capacitors are   used  as ripple filters in power supply units and also as a  filter for   bypassing low frequency signals in audio amplifiers etc. There is   polarity written on an electrolytic capacitor either + or - sign, which   indicates the terminals when connecting to the voltage.   Reversing the terminals of the electrolytic capacitor can cause damage   to the capacitor and also sometimes the capacitor may explode with  some  gases and the contents inside.

Tantalum Capacitors

Tantalum   capacitors are electrolytic capacitors and they use tantalum as the   electrode plates. They are superior than the aluminum electrolytic   capacitors in temperature and in frequency. They have high stability  and  are used in circuits which demand high stability in capacitance  values.  They are used in analog signal  circuits. It is a little bit more  expensive than the electrolytic  capacitors. Tantalum capacitors comes in  values such as 0.33  micro Farads, 0.47 micro Farads, 10 micro Farads etc.

Ceramic Capacitors

Ceramic capacitors are made of materials such as titanium and barium oxide used as the dielectric. They are usually seen as disc shaped. The  capacitance values are considerably smaller. They have no polarity.  They  are suitable for high frequencies. They are mostly used to bypass high frequency signals to the ground in circuits. Usual values of ceramic capacitors are 10 pico Farad, 100 pico Farad, 0.01 micro Farad, etc.

Polystyrene Film Capacitors

These  capacitors have the polystyrene film as the dielectric. Copper is used  as the electrode  material They are used in filter circuits, and in  timing circuits. 

Electric Double Layer Capacitors

These   capacitors are also called as super capacitors. Super capacitors store large capacitance and have values in farads, which is a large value of capacitance. They have values such as 0.47 Farad.

Polyester Film Capacitors

These   capacitors use polyester film as the dielectric material. They do not   have high tolerance level and have reasonable tolerance of +/-5 to   +/-10. They are very cheap. They comes in values ranging from 0.001  micro Farads, 0.1 micro Farads, 0.22 micro Farads etc.

Polypropylene Capacitors

Polypropylene film is used as the dielectric. These capacitors are used where higher tolerance is necessary.

Mica Capacitors

These capacitors use Mica for the dielectric. Mica capacitors have good stability as their temperature coefficient is small. They have good insulation. They have high frequency tolerance and so they are used in resonant tuned circuits and high frequency filters etc. They also have  high voltage tolerance. Usual values for Mica capacitors are 47 pico Farads, 220 pico Farads, 1000 pico Farads, etc.

Metallized Polyester Film Capacitors

These are a type of polyester film capacitor. Their electrodes are thin and so they are small in size. They are rugged and they have no polarity.

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Variable Capacitors

Tuning Capacitors

This is a large variable capacitor used for tuning the radio. It is also called as a gang condenser. It consists  of  2 sets of plates, which can be varied by a spindle or a tuning  control.  There may be rotation control, spindle, or screws that helps to vary the capacitance. Air or polyester film is used as the dielectric. When an air is used as a dielectric usual values that can be varied is 5 pico Farads to 40 pico Farads and in polyester film capacitor, 12 pico Farad to 150 pico Farad can be varied.

Trimmer Capacitors

These are capacitors, which can vary their capacitance values. The value of the capacitance is varied by the use of a control. They have moving plates fixed one above the other in sets. One set of plates can be moved while the other set may be fixed to the frame. The movable plates are rotated to vary the capacitance range. The capacitor is varied by turning a screw. Mostly air or polyester film is used as the dielectric. They are used in fine tuning and adjusting the resonant circuits in a radio.