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Capacitors

Posted on : Sat , 01 2014 by : virusi
Capacitors, in particular, are essential in nearly every circuit application. They are used for waveform generation, filtering, and blocking and bypass applications.

Some historical data :


JarsCapacitor In October 1745, Ewald Georg von Kleist of Pomerania in Germany found that charge could be stored by connecting a high-voltage electrostatic generator by a wire to a volume of water in a hand-held glass jar. Von Kleist’s hand and the water acted as conductors, and the jar as a dielectric (although details of the mechanism were incorrectly identified at the time). Von Kleist found that touching the wire resulted in a powerful spark, much more painful than that obtained from an electrostatic machine. The following year, the Dutch physicist Pieter van Musschenbroek invented a similar capacitor, which was named the Leyden jar, after the University of Leiden where he worked He also was impressed by the power of the shock he received, writing, “I would not take a second shock for the kingdom of France.”
Daniel Gralath was the first to combine several jars in parallel into a “battery” to increase the charge storage capacity. Benjamin Franklin investigated the Leyden jar and came to the conclusion that the charge was stored on the glass, not in the water as others had assumed. He also adopted the term “battery”, (denoting the increasing of power with a row of similar units as in a battery of cannon), subsequently applied to clusters of electrochemical cells. Leyden jars were later made by coating the inside and outside of jars with metal foil, leaving a space at the mouth to prevent arcing between the foils. The earliest unit of capacitance was the jar, equivalent to about 1 nanofarad.

Theory of operation :


Before proceeding let’s take a look at the capacitors symbol and how capacitors look like.
CapacitorSymbols
Fig.1 Capacitor Symbol
RealCapacitor
Fig.2 How capacitor look like
As you have understood from the history chapter a capacitor is an electronic component that stores electric charge. The capacitor is made of 2 close conductors (plates) that are separated by a dielectric metal. The plates accumulate electric charge when connected to power source. One plate accumulates positive charge and the other plate accumulates negative charge.
As you can see from the fig.2 they are a large variety of capacitors. In general ceramic and Mylar types are used for most noncritical circuit applications; tantalum capacitors are used where greater capacitance is needed and electrolytic are used for power-supply filtering. Below you can check the module of the capacitor.
CapacitorModel
Fig.1 Model of Capacitor.
The capacitance of a capacitor is a ratio of the amount of charge that will be present in the capacitor when a given potential (voltage) exists between its leads. The unit of capacitance is the farad (F) which is equal to one coulomb per volt. Typical capacitors have values on the order of nanofarads (1nanofarad =
10^{-9}
F), picofarads(1picofarad =
10^{-12}
F) or microfarads(1microfard =
10^{-6}
F).Below you can check the types of capacitors and capacitors symbols.

An ideal capacitor is wholly characterized by a constant capacitance C :
\large C = \frac{Q}{V}
(1)
Sometimes charge build-up affects the capacitor mechanically, causing its capacitance to vary. In this case, capacitance is defined in terms of incremental changes :
\large C = \frac{dQ}{dV}
(2)
Where C is the capacitance in farads, V is the potential in volts and Q is the charge measured in coulombs. From the formula (1) we can calculate the voltage across the capacitor.
\large V = \frac{Q}{C}
(3)
Because capacitors are varying in time voltage and current also are varying in time. Below you check voltage and current equation:
\large V = \frac{1}{C}*\int I*dt
(4)
\large I = C*\frac{dV}{dt}
(5)
So a capacitor is more complicated than a resistor the current is not simply proportional to voltage but rather to the rate of change of voltage. Unlike resistors, the power (V times I) associated with capacitive current is not turned into heat, but is stored as energy in the capacitor’s internal electric field. You get all that energy back when you discharge the capacitor. Capacitors are only efficient in AC applications but not in DC applications.

Practical Example :


If you don’t see the example below than you should follow this steps:

– In your browser allow Java SE 7.

– Lower you java security settings (Go to Control Panel >> Java >> Security and set the security level to medium) .

– Edit Site List (Go to Control Panel >> Java >> Security and click on Edit Site List… and add eagerlearning.org in the list).

Sorry, you need a Java-enabled browser to see the simulation. This is a very simple circuit with 2 resistors, 1 capacitors and 2 switches. If you want to charge the capacitor than the 1 switch should be closed and the 2 switch should be opened. If you want to discharge the capacitor than you should close the 2 switch and open the 1 switch. The charge time depends on the R1 resistance and the discharge time depends on the resistance R2 so if you want your capacitor to charge/discharge faster than you should put a smaller resistance or vice versa if you want your capacitor to charger/discharge slower. The diagram is showing the current and voltage that is flowing through the capacitor. If you want to change the value of a component just double click on it and insert the desired value.
Last updated on Mon , 03 2014
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