Kranthi Kiran's

What is a capacitor and how does it work? When you go to a showroom and watch some plasma panels maybe you do not know that you are watching some capacitors. Yes, a plasma panel can be considered a capacitor.
The capacitor is a device able to store electric energy. Practically every time two conductor materials (called "plates") will be near and separate from a non conductor material we will have a capacitor. In a PDP (Plasma Display Panel) the plates are the two glasses (front and rear panel) and the non conductor material is the dielectric material that is between them. If we apply a voltage to a capacitor, it will charge at the same power supply potential. In a capacitor the process of storing energy is called "charging" and it involves electric charges of equal magnitude but opposite polarity. Initially, at flat capacitor, the plates are electrically neutral since we have the same numbers of electrons and protons on them.

If we connect the capacitor to a continuous voltage generator, it happens that the generator positive pole catches the electrons from the connected plate while the other plate catches electrons from the generator negative pole (see below). Little by little the capacitor increases its charge, the potential of the plate connected to the positive pole increases and comes close to the generator potential. So the potential difference, at the two resistance ends, decreases and the current intensity decreases also. Therefore the capacitor charges quickly initially, then more and more slowly. Once the capacitor is charged (at the same generator voltage value) we will have a positive potential on the plate A (positive charges predominance) and a negative potential on the plate B (negative charges predominance). The voltage (charge) remains even if we disconnect the capacitor from the generator (no losses, in the case of ideal capacitor of course).
Below there are the voltage and current progress graph and their formulas. The product RC will be explained later.
 
If we connect the plates through a resistance we will have the capacitor discharge, that is we will have an electric charge equilibration and the voltage decreases to zero value (the energy stored in the capacitor dissipates in the resistance). During the discharge process the current direction (that is the electrons movement direction) is opposite to the charge process one.
For the charge, during the discharge process, we will have the following equation:
Q = Q₀ e^–t/RC
where Q is the capacitor charge (Coulomb), Q0 is the charge at the start, "e" is the exponential number (Euler's number =2.718..), t is the time (Seconds), C is the capacitance (Farad), R the resistance (Ohm).
For voltage and current the equation becomes:
Below there is the current progress graph and its equation.
Let's explain briefly the product RC.
The capacitor charge or discharge happens in a time depending from the resistance value (in a series to the capacitor) and from the capacitance value of the capacitor. Laboratory tests have shown that the needed time to charge the capacitor at 63% of the applied voltage is equal to the product result between resistance and capacitance. The product result is called time constant (t), so
t = R * C,
where t is expressed in Seconds, R in Ohm and C in Farad.
Moreover it has been demonstrated that the capacitor is charged in a time T = 5 t because after the first t it charges 63% of the applied voltage and after every other t it charges a further 63%, but of the remaining difference.
The aptitude at the electric energy storing is called capacitance: it is directly proportional to the one plate surface (A) and inversely proportional to their distance (d) and depends, in directly proportional manner, from the relative static permittivity value of the used insulator εr. The formula is
C = ε_r * ε₀ * A/d
where ε0 is the vacuum permittivity, the measure unit is the Farad (F).
The insulator placed between the plates is called dielectric and it can be liquid, solid or gaseous. The dielectric type allows a first capacitors classification. The most used capacitors, in the electronic area, are the ones with air or solid dielectric. The most used types of solid dielectric are: mica, ceramic, plastic film, paper. The capacitance value of a capacitor is clearly showed on the capacitor body (for the big ones) or codified by different codes (colours or alphanumeric). Now let's have a look to some capacitor types, at their features and application areas.
Electrolytic capacitors
The electrolytic capacitors are formed from two metallic sheets, cylindrically wrapped, that are separated by a thin oxide layer (got through an electrolytic process). The very thin layer thickness (approx. 0,001 µm) and its relative static permittivity value, relatively high, allow to get huge capacitances values (until 1.000.000 of µFarad in the aluminium electrolytic capacitors) even if they can suffer a potential difference of a few ten volts only. Due to their structure they are polarized, that is they must observe a polarity verse: one plate must be always positive, the other one must be always negative. Changing the polarity direction is very dangerous: the capacitor could explode.
As we said above they have big capacities, so they can accumulate a large energy quantity. For this reason they are used, mainly, in the power supply units, for the voltage levelling and for the ripple reduction.


Ceramic capacitors
The ceramic capacitors are constituted from a sandwich of conductor sheets alternated with ceramic material. In these capacitors the dielectric material is a ceramic agglomerate whose relative static permittivity value can be changed from 10 to 10.000 by dedicated compositions. The ceramic capacitors, with low relative static permittivity value, have a stable capacitive value and very low losses, so they are preferred in the floating and high precision circuits. The ones with high relative static permittivity value allow to get high capacities occupying a small space. Generally the ceramic capacitors have small dimensions and they are preferred in the high frequencies area. The most used ceramic capacitor shape is the disc one, that is a little ceramic disc metalized on both sides and with the extremities welded on them. Typically they have very small capacities, from some pF to some nF, and they can suffer big potential differences.


Paper capacitors
In the paper capacitors the dielectric material is constituted from a special paper saturated with a fluid or viscous substance. To increase the insulation, in these capacitors, often two or more layers are coupled. The finished envelopment is again saturated under vacuum with insulating oil or is dipped in the resin. Generally they are used as filter capacitors.

Plastic film capacitors
The membranes in plastic film can be produced with lower thickness than the saturated paper and are more uniform. So there are capacitors that use these membranes as dielectric material (a few µm of thickness only) and they can suffer high voltages. The plastic film capacitors are mainly used in the transistor circuits. In the polyester capacitors a metallic sheet is used as electro-conductor layer or the metal can be deposited directly on the film by under vacuum vaporization, with a layer thickness of 0,02 - 0,05 µm. The capacitance of these capacitor can reach some µF. They are used in the low frequency circuits mainly.

Tantalum capacitors
The tantalum capacitors, as the electrolytic ones, are polarized, but they have the tantalum pentoxide as dielectric material. Compared to the electrolytic ones, they are better both the temperature stability and high frequencies, but they cannot suffer over-voltage peaks and can be damaged, sometimes exploding with violence. On the other hand they are more expensive and they have much lower capacity.

Niobium capacitors
The tantalum capacitors have two drawbacks: the tantalum cost due to this material rarity and its susceptibility to certain low level ppm of thermal runaway failures. Because of the increasing demand for tantalum capacitors a new technology has been developed and the niobium capacitors have been launched into the market. With at least 100 times more deposits than tantalum, the niobium guarantees good availability and lower price. So the niobium capacitors are very similar to the tantalum ones, but they have low cost, surge robustness and it is raising the conviction they can have better performances in other fields like voltage range, ESR and miniaturisation.