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Capacitor and Capacitance

2014-11-16 14:22  
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Capacitors are electronic passive components designed by placing two conductive plates (usually metal) in close proximity with each other, separated using a dielectric . There are many different types of capacitor construction and many different types of capacitors with particular ratings and purposes.

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For example , a  small value capacitors can be constructed using two circular plates sandwiching an insulating material will suffice and for larger capacitor values, the ”plates” may be strips of metal foil sandwiched around a flexible insulating medium and rolled up for compactness.

In both case the general idea is the same: two conductor separated by an insulator (dielectric material ) .

The schematic symbol for a capacitor is two short parallel lines (representing the plates) separated by a gap ( dielectric ). At that two parallel lines( plates ) are attached two pins for connection to other components.

When a voltage is applied across the two plates of a capacitor  a concentrated field flux is created between them allowing a significant difference of free electrons (a charge) to develop between the two plates .

The capacitors has a main feature that permit to store energy in a form of an electric field.

The capacitor’s ability to accumulate( store ) energy (voltage ) (potential difference between the two leads) results in a tendency to try to maintain voltage at a constant level at its leads .

When voltage across a capacitor is increased the capacitor store more energy and the voltage across it is increased.

As you can see in this formula C=Q/U , the capacitance is defined by the Q (electric charge ) an the U voltage applied at the capacitors leads .

The measurement unit of the capacitance is F – Farad but 1 F is a huge value, so in practice you will find sub-multiples 1uf=10^-6 ;1nF=10^-9; 1pf=10^-12F .

The main parameters of the capacitors are:

- Nominal capacity Cn [F] is the condenser capacity value to be realized through the manufacturing process and is marked on it body.
- Tolerance t is expressed as a percentage and represents the maximum deviation of the real value of capacity compared to the nominal value.
- Rated voltage Un [V] is the maximum continuous voltage or maximum voltage effective alternative that can be applied continuously to capacitor terminals in working temperature range
- Riz insulation resistance is the ratio of voltage applied to a capacitor and continuous current that crosses it at one minutes after is applied a voltage
- Dielectric strength is the maximum continuous voltage that a capacitor must withstand a minimum time (usually one minute) to emerge without breakthrough
- Working temperature range (Tmin-Tmax) represents the temperature limits between which the capacitor operates over a long time.

Capacitors can be classified by several criteria: the nature of the dielectric, field frequency, type of constructive field used.
Classification after construction type :
- Fixed value capacitor, maintain a constant value during the whole period of operating
- Adjustable value capacitor , characterized by the fact that capacity value can be changed in small range
- Variables value capacitor , whose value can and be changed frequently between relatively wide limits (ex. Condensers for radio tuning frequency)
Depending on the nature of dielectric capacitors can be:
- Gas dielectric type (air, vacuum)
- Fluid dielectric type(mineral oil or transformer oil , often made and used)
- Organic or inorganic solid dielectric types , are dielectric materials glass, mica, ceramics and organic solid dielectric paper, film synthetic non-polar (polystyrene, Teflon, polypropylene) and polar synthetic films ( polycarbonate, polyamide resin)
- oxides film dielectric  have a dielectric oxide films (Al2O3, Ta2O5, TiO2) are most use aluminum and tantalum oxides