jb What is a Capacitor ?

A capacitor is a small device that can be charged up with electrical energy, store it and then release it. Just like a rechargeable battery. But unlike a battery, it does not use a chemical reaction and it can only hold a very small charge. A very large capacitor can only light up an LED for a few seconds. They come in many shapes and sizes and a few are shown below. The bigger the capacitor, the more charge it will hold. Capacitor is made from two metal plates or metal foils separated by an insulator called a Dielectric material.

The Dielectric materials can be made from Ceramic, Mica, Polypropylene, Polyester, Electrolytic, Tantalum and even air.

The larger capacitors look like tubes, this is because the metal foil plates are rolled up with an insulating dielectric material sandwiched in between.

The value of capacitance is determined by

1.The size of the plates,

2.The distance between them,

3.The type of dilectric material used.

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Glass capacitor construction

The construction of glass dielectric capacitors is relatively straightforward to understand. The capacitor consists of three basic elements: the glass dielectric, aluminium electrodes and the encapsulation. However the assembly of the glass capacitors is undertaken in a manner that ensures the required performance is obtained.

As the capacitance between two plates is not always sufficient to provide the required level of performance, the majority of capacitors use a multiplayer construction to provide several layers of plates with interspersed dielectric to give the required capacitance.

Although the glass plates are always flat, and tubular forms of construction are not applicable, the glass capacitors are usually available with leads emanating in either a radial or axial form. Essentially the leads either exit the encapsulation at the side or the end.http://www.jbcapacitors.com/

Half-life of a Capacitor

Symbol:T
Unit:(s)
When a capacitor discharges its charge throught a resistor, the charge of the capacitor decreases with time. The decreasing charge follows an exponential decay curve.

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jb Summary of Capacitor Types

The table below gives and overview of the main characteristics of the various types of capacitor.

Capacitor types Capacitance range Accuracy Temperature stability Leakage Comments
Electrolytic 0.1 µF - ~1 F V poor V poor Poor Polarised capacitor - widely used in power supplies for smoothing, and bypass where accuracy, etc is not required.
Ceramic 10 pF - 1 µF Variable Variable Average Exact performanceof capacitor depends to a large extent on the ceramic used.
Tantalum 0.1 µF - 500 µF Poor Poor Poor Polarised capacitor - very high capacitance density.
Silver mica 1 pF - 3000 pF Good Good Good Rather expensive and large - not widely used these days except when small value accurate capacitors are needed.
Polyester (Mylar) 0.001 µF - 50 µF Good Poor Good Inexpensive, and popular for non-demanding applications.
Polystyrene 10 pF - 1 µF V good Good V good High quality, often used in filters and the like where accuracy is needed.
Polycarbonate 100 pF - 20 µF V good V good Good Used in many high tolerance and hash environmental conditions. Supply now restricted.
Polypropylene 100pF - 50 µF V good Good V good High performance and low dielectric absorption.
Teflon 100 pF - 1 µF V good V v good V v good High performance - lowest dielectric absorption.
Glass 10 pF - 1000 pF Good Good V good Excellent for very harsh environments while offering good stability. Very expensive.
Porcelain 100 pF - 0.1 µF Good Good Good Good long term stability
Vacuum and air 1 pF - 10 000 pF       Often used as variable capacitors in transmitters as a result of their very high voltage capability.

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