A capacitor is an element which stores charges. It is a passive element as it stores energy. Michael Faraday invented capacitor. The unit of capacitance is Farad was the name of honor of Michael Faraday. In electronics, communication, power systems, computer etc field capacitors are extensively used. In circuit capacitor in an important circuit element, its role can not be described in words.
Two conducting plates separated by an insulator or dielectric in a capacitor. The construction of capacitor is shown in figure 1. Practically the plates may be aluminum foil and the dielectric may be paper, ceramic, air, mica etc.
If a voltage source is connected across a capacitor where positive terminal of a voltage source is connected to capacitors positive terminal and negative is connected to negative terminal of capacitor. Then the capacitor stores q charge which is directly proportional to the applied voltage v. Mathematically
q = Cv,
Where C is proportionality constant, which is familiar as capacitance of the capacitor. Farad is the unit capacitance of capacitor. According to equation 1 Farad = coulomb/ volt. From the equation we concept that capacitance C depends on charge q and voltage v, but this is not true. Capacitance depends on the dimensions of the plates and dielectric between two plates.
C = ε A / d
The capacitance depends on
i) the surface area A of each plate,
ii) the distance d between two plates and
iii) the permittivity of the dielectric material between the plates.
Figure 2 shows parallel plate capacitor where figure 2a is small in area and figure 2b big size in area. So we can tell that figure 2b plate type capacitor can store more charge than figure 2a capacitor. Suppose figure 2a
area, A = 2m2 and figure 2b area, A = 4m2 where distance between two plates d and permittivity ε are constant or same for both areas. Then we get
For figure 2a capacitance, C = 2 ε/d Farad and for figure 2b capacitance, C = 4 ε/d Farad. Where figure 2b result is greater than figure 2a area. It proves if area increases the capacitance of a capacitor increases.
The capacitance depends on the distance between two plates also. If the distance between two plates increases, the capacitance decreases if the distance decreases then the capacitance increases. The relation between capacitance and distance is inversely proportional. Here in figure 3a and 3b shows the distance d between two parallel plates of a capacitor where area A is same for two figures. But the distance is different. Figure 3a distance, d = 1m and figure 3b distance, d = 3b where area A and permittivity ε are same or constant. Putting the values in formula,
For figure 3a the capacitance, C = 1 ε A Farad
And for figure 3b the capacitance, C = ε A/5 = 0.2 ε A Farad which is smaller than figure 3a value. So it proves that if the distance between two plates decreases capacitance increases and if distance increases capacitance decrease.
The capacitance depend another fact that is permittivity ε of the material used between two plates in parallel plate capacitor. Ceramic, mica, paper etc dielectric material gives more capacitance and efficiency than air although air is also used as dielectric material. The value of permittivity depends upon the dielectric material.
Figure 4a, 4b shows the dielectric material symbol.
In market capacitors are different types with different values commercially. Normally capacitors have picofarad (pF) to microfarad (µF) value range in market.
According to polarity there are two types of capacitor viz i) polar & ii) non polar. Polar has two poles positive and negative. Non polar has no pole. Figure 5a shows polar and 5b shows non polar capacitor.
Capacitor value can be fixed or variable. The diagram of fixed value and variable capacitor are different figure 6 shows them.
Capacitor varies from construction of it. It varies according to material such the construction of plates, dielectric and application differ for it packaging. There are many types of capacitor are ceramic capacitors, electrolytic capacitors, film capacitors, tantalum capacitors etc.
Ceramic capacitors are made with porcelain or ceramic. Ceramic capacitors are small in size, cheap and have high frequency. The capacitance of it varies with voltage. Although they are small size but they have high dielectric constant. They are non polarized capacitors. Mainly three types of ceramic capacitors are available include leaded disc ceramic capacitors, multilayer surface mount chip ceramic capacitors and specialist microwave bare leadless disc ceramic capacitors.
Electrolytic capacitors provide high capacitance in a small component. Where large capacitance needed electrolytic capacitors are used generally. They have wide tolerance with low expenses. For their high capacitance, small physical state electrolytic capacitors are used in DC power supply also. They are also called supercapacitors or ultracapacitors. The application of electrolytic capacitors similar as rechargeable batteries.
Film capacitors are most common and available capacitors. They have many different shapes like oval, rectangular, round so many. The shape of film capacitors are various due to dielectric materials. Film capacitors have big family.
Tantalum capacitors provide very high capacity density. They are similar as electrolytic capacitors. They are polarized also. Tantalum capacitors are made with tantalum metal; here tantalum metal is used as anode. They are small in size but expensive. So tantalum capacitors are used where small size and better performance needed.