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SR217A104DAA Datasheet(PDF) 4 Page  AVX Corporation 

SR217A104DAA Datasheet(HTML) 4 Page  AVX Corporation 
4 / 72 page 2 GENERAL INFORMATION A capacitor is a component which is capable of storing electrical energy. It consists of two conductive plates (electrodes) separated by insulating material which is called the dielectric. A typical formula for determining capacitance is: C = .224 KA t C = capacitance (picofarads) K = dielectric constant (Vacuum = 1) A = area in square inches t = separation between the plates in inches (thickness of dielectric) .224 = conversion constant (.0884 for metric system in cm) Capacitance – The standard unit of capacitance is the farad. A capacitor has a capacitance of 1 farad when 1 coulomb charges it to 1 volt. One farad is a very large unit and most capacitors have values in the micro (106), nano (109) or pico (1012) farad level. Dielectric Constant – In the formula for capacitance given above the dielectric constant of a vacuum is arbitrarily chosen as the number 1. Dielectric constants of other materials are then compared to the dielectric constant of a vacuum. Dielectric Thickness – Capacitance is indirectly propor tional to the separation between electrodes. Lower volt age requirements mean thinner dielectrics and greater capacitance per volume. Area – Capacitance is directly proportional to the area of the electrodes. Since the other variables in the equation are usually set by the performance desired, area is the easiest parameter to modify to obtain a specific capaci tance within a material group. Energy Stored – The energy which can be stored in a capacitor is given by the formula: E = 1⁄2CV 2 E = energy in joules (wattssec) V = applied voltage C = capacitance in farads Potential Change – A capacitor is a reactive component which reacts against a change in potential across it. This is shown by the equation for the linear charge of a capacitor: I ideal = C dV dt where I = Current C = Capacitance dV/dt = Slope of voltage transition across capacitor Thus an infinite current would be required to instantly change the potential across a capacitor. The amount of current a capacitor can “sink” is determined by the above equation. Equivalent Circuit – A capacitor, as a practical device, exhibits not only capacitance but also resistance and inductance. A simplified schematic for the equivalent circuit is: C = Capacitance L = Inductance R s = Series Resistance R p = Parallel Resistance Reactance – Since the insulation resistance (Rp) is normally very high, the total impedance of a capacitor is: Z = R 2 S + (XC  XL ) 2 where Z = Total Impedance R s = Series Resistance X C = Capacitive Reactance = 1 2 π fC X L = Inductive Reactance = 2 π fL The variation of a capacitor’s impedance with frequency determines its effectiveness in many applications. Phase Angle – Power Factor and Dissipation Factor are often confused since they are both measures of the loss in a capacitor under AC application and are often almost identical in value. In a “perfect” capacitor the current in the capacitor will lead the voltage by 90°. The Capacitor R L R C P S 
