Capacitors

Capacitors

A capacitor (originally known as a condenser) is a passive 2 terminal componenet used to store energy electrostatically in anelectric feild. The forms of practical capacitors vary widely, but all contain at least two electric conductors separated by a dielectric(insulator) for example, one common construction consists of metal foils separated by a thin layer of insulating film. Capacitors are widely used as parts of electric circuits in many common electrical devices

Source-from wikipedia

A capacitor (originally known as a condenser) is a passive 2 terminal componenet used to store energy electrostatically in anelectric feild. The forms of practical capacitors vary widely, but all contain at least two electric conductors separated by a dielectric(insulator) for example, one common construction consists of metal foils separated by a thin layer of insulating film. Capacitors are widely used as parts of electric circuits in many common electrical devices

Source-from wikipedia

Defination

Generally capacitors can be called as electric charge storage units only difference between a battery and capacitor is battery released electric charge slowly where as capacitor releases as fast as it can.

Units to measure

Standard Units of Capacitance

• Microfarad  (μF)   1μF = 1/1,000,000 = 0.000001 = 10-6 F
•  
• Nanofarad  (nF)   1nF = 1/1,000,000,000 = 0.000000001 = 10-9 F
•  
• Picofarad  (pF)   1pF = 1/1,000,000,000,000 = 0.000000000001 = 10-12 F

Then using the information above we can construct a simple table to help us convert between pico-Farad (pF), to nano-Farad (nF), to micro-Farad (uF) and to Farads (F) as shown.

Pico-Farad	Nano-Farad	Micro-Farad	Farads
1,000	1.0	0.001
10,000	10.0	0.01
1,000,000	1,000	1.0
	10,000	10.0
	100,000	100
	1,000,000	1,000	0.001
		10,000	0.01
		100,000	0.1
		1,000,000	1.0

 

Capacitance of a Parallel Plate Capacitor

The capacitance of a parallel plate capacitor is proportional to the area, A of the plates and inversely proportional to their distance or separation, d (i.e. the dielectric thickness) giving us a value for capacitance of C = k( A/d ) where in a vacuum the value of the constant k is 8.84 x 10-12 F/m or 1/4.π.9 x 109, which is the permittivity of free space. Generally, the conductive plates of a capacitor are separated by air or some kind of insulating material or gel rather than the vacuum of free space.

The Dielectric of a Capacitor

As well as the overall size of the conductive plates and their distance or spacing apart from each other, another factor which affects the overall capacitance of the device is the type of dielectric material being used. In other words the "Permittivity" (ε) of the dielectric. The conductive plates are generally made of a metal foil or a metal film but the dielectric material is an insulator.

The various insulating materials used as the dielectric in a capacitor differ in their ability to block or pass an electrical charge. This dielectric material can be made from a number of insulating materials or combinations of these materials with the most common types used being: air, paper, polyester, polypropylene, Mylar, ceramic, glass, oil, or a variety of other materials.

The factor by which the dielectric material, or insulator, increases the capacitance of the capacitor compared to air is known as the Dielectric Constant, k and a dielectric material with a high dielectric constant is a better insulator than a dielectric material with a lower dielectric constant. Dielectric constant is a dimensionless quantity since it is relative to free space. The actual permittivity or "complex permittivity" of the dielectric material between the plates is then the product of the permittivity of free space (εo) and the relative permittivity (εr) of the material being used as the dielectric and is given as:

Complex Permittivity

As the permittivity of free space, εo is equal to one, the value of the complex permittivity will always be equal to the relative permittivity. Typical units of dielectric permittivity, ε or dielectric constant for common materials are: Pure Vacuum = 1.0000, Air = 1.0005, Paper = 2.5 to 3.5, Glass = 3 to 10, Mica = 5 to 7, Wood = 3 to 8 and Metal Oxide Powders = 6 to 20 etc.

This then gives us a final equation for the capacitance of a capacitor as:

One method used to increase the overall capacitance of a capacitor is to "interleave" more plates together within a single capacitor body. Instead of just one set of parallel plates, a capacitor can have many individual plates connected together thereby increasing the area, A of the plate. For example, a capacitor with 10 interleaved plates would produce 9 (10 - 1) mini capacitors with an overall capacitance nine times that of a single parallel plate.

Modern capacitors can be classified according to the characteristics and properties of their insulating dielectric:

• Low Loss, High Stability such as Mica, Low-K Ceramic, Polystyrene.
•  
• Medium Loss, Medium Stability such as Paper, Plastic Film, High-K Ceramic.
•  
• Polarized Capacitors such as Electrolytic's, Tantalum's.

Voltage Rating of a Capacitor

All capacitors have a maximum voltage rating and when selecting a capacitor consideration must be given to the amount of voltage to be applied across the capacitor. The maximum amount of voltage that can be applied to the capacitor without damage to its dielectric material is generally given in the data sheets as: WV, (working voltage) or as WV DC, (DC working voltage). If the voltage applied across the capacitor becomes too great, the dielectric will break down (known as electrical breakdown) and arcing will occur between the capacitor plates resulting in a short-circuit. The working voltage of the capacitor depends on the type of dielectric material being used and its thickness.

The DC working voltage of a capacitor is just that, the maximum DC voltage and NOT the maximum AC voltage as a capacitor with a DC voltage rating of 100 volts DC cannot be safely subjected to an alternating voltage of 100 volts. Since an alternating voltage has an r.m.s. value of 100 volts but a peak value of over 141 volts!. Then a capacitor which is required to operate at 100 volts AC should have a working voltage of at least 200 volts. In practice, a capacitor should be selected so that its working voltage either DC or AC should be at least 50 percent greater than the highest effective voltage to be applied to it.

Another factor which affects the operation of a capacitor is Dielectric Leakage. Dielectric leakage occurs in a capacitor as the result of an unwanted leakage current which flows through the dielectric material. Generally, it is assumed that the resistance of the dielectric is extremely high and a good insulator blocking the flow of DC current through the capacitor (as in a perfect capacitor) from one plate to the other.

However, if the dielectric material becomes damaged due excessive voltage or over temperature, the leakage current through the dielectric will become extremely high resulting in a rapid loss of charge on the plates and an overheating of the capacitor eventually resulting in premature failure of the capacitor. Then never use a capacitor in a circuit with higher voltages than the capacitor is rated for otherwise it may become hot and explode.