What is a capacitor and what is its function?

Capacitors are reactive elements that react to the passage of current through the accumulation of electrical charges, that is, the capacitor is capable of storing electrostatic energy. The most common capacitors are built by two conductive plates (metallic), separated by a dielectric material (insulating material).

The working principle of a capacitor occurs when an electrical voltage is applied between its conductive plates, known as “armatures”. One side of the conductive armature stores positive charges, the other side stores negative charges. The loads are accumulated in the same way, balanced, both negative and positive charges have the same value in module.

The dielectric material used to insulate the plates usually gives the name to the capacitor (ceramic, polyester, mica, etc.). We can say that the main function of a capacitor is to accumulate electrical charges in a circuit to later discharge these same charges.

The charge period of a capacitor is called a transient regime, after it is fully charged, that is, stable it passes to a regime called permanent.

Capacitance concept

The storage capacity of electrical charges is called capacitance and is symbolized by the letter C, the unit of measurement of this storage capacity is given in farads, it is the unit of measurement for the amount of capacitor charge storage, as shown in the image below :

Capacitance formula, given in Farads (F).

Main behaviors of a capacitor in DC circuits.

When studying capacitors in an RC circuit, a capacitive resistive circuit, it behaves in two ways, during the capacitor charge and during discharge, let us see a little of the behavior when charging a capacitor:

  • During the period of charge of a capacitor the current in the charge begins to decrease, this is because the instant the capacitor receives the voltage from the source it absorbs the electrical voltage until the capacitor is fully charged to the mathematical formula that models this behavior in the cargo is in the image below:

Formula to find the tension over the load.

  • The time constant is important to understand how long the capacitor will reach full load, we consider that a capacitor is fully charged when it reaches a 5T instant, when a capacitor is under electrical voltage during this period it is considered to be maximum load. The “T” represents the Greek letter “Tau” and it is the unit of measurement of this quantity. To reach the value of “Tau” we use the following mathematical model, given by multiplying the capacitance by the total resistance of the circuit:

T = C .R

  • The mathematical model for the calculation of the total current of the circuit proves that the current of the circuit decreases during the period of 5T, exactly because the capacitor is absorbing the load of the source:

Formula to find the total current flowing in the circuit.

  • The voltage in the capacitor increases during its charge, this behavior happens until the capacitor is completely saturated, reaching its maximum capacitance limit, observe the mathematical model that regulates this behavior:

Formula to find the voltage over the capacitor.

Capacitor Association

To perform the association of capacitors in series, we need to add the inverse of the capacitances in the circuit:

Formula to find the total capacitance of the circuit.

When associating capacitors in parallel, just add their capacitance to reach the total capacitance value of the circuit:

CT = C1 + C2

CT = total capacitance of circuit
C1 = Capacitor 1
C2 = Capacitor 2

Capacitors application.

There are of different sizes and applications, usually varying according to the amount of charge that you want to store in the circuit. Capacitors are widely used in electronic circuits, generally with the function of stabilizing the voltage in the circuit, they are also applied in power electronics where large capacitors work to sustain the voltage, by switching the voltage across the thyristors and IGBTS.

A very common application in the industry is the use of a mega capacitor to perform the power factor correction using reactive energy to correct the inductive energy present in the circuit.

The use of capacitors in fans is very common, since single-phase motors do not have an angular lag between the supply phases, so without the help of the capacitors they are unable to start. The capacitors are used to make an angular lag in the voltage in the stator, generating a rotating magnetic field that makes the motor start to rotate.

Another example are cameras that need a flash to generate a better quality image, batteries that perform the function of charging the capacitor with energy for a few seconds, however, when taking the photo, only one capacitor can do the discharge of the entire charge in the flash bulb instantly.

Main types of capacitors

How many types of capacitors are there? Have you ever stopped to think about it? We can highlight four different types:

  1. Electrolytic capacitors (polarized, non-polarized and tantalum).
  2. Polyester condensers (metallic and non-metallic)
  3. Ceramic capacitors (disc and plate)
  4. SMD Capacitors

Understand what capacitors are, where they are used and their main characteristics.

Ceramic capacitors:

Ceramic capacitors are generally of two different types. Ceramic discs are the most common and have a very simple shape: it is a disc of dielectric material made of ceramic with high metallic insulating capacity on both sides.

On the metallized sides, two terminals are welded, the capacitor receives a bath and epoxy paint at the end of manufacture, to cover the disk and part of the terminals. This type of capacitor is supplied with capacities from 2.2 pF to 0.1F with relatively low voltages of 63V. There are also high voltage ceramic disk capacitors for special applications that reach values ​​of 2 KV.

Capacitors with plastic dielectric:

They are usually made of two thin sheets of polyester that are rolled together with two very thin sheets of aluminum to form the capacitor plates. It is normally used in circuits that require a high current circulating through them, since the presence of metal plates helps to extract internal heat and the size of the capacitor helps to dissipate heat by exchanging heat with the outside environment. The type of dielectric used is used to build high voltage insulation capacitors that are standardized at 250V, 400V and 630V. They can be found on the market with a capacitance variation of 1,000 pF and reaches 0.47 uF (usually 0.47 uF) or 1 uF.

Electrolytic capacitors:

Electrolytic capacitors are very important, they are one of the most used in the market, their capacitance usually starts at 0.47 uF and reaches up to 10 mF. An electrolytic capacitor is constructed by wrapping two sheets of aluminum and two sheets of paper soaked in acidic water called an electrolyte. The electrolyte is a relatively low resistance path, that is, immediately after being manufactured, we do not have a capacitor, but an unfinished device that is called a protocapacitor.

The protocapacitor is connected to a current source so that the acid oxidizes to one of the aluminum plates. As the oxide is an insulator, a time later a polarized electrolytic capacitor is formed where the positive plate is the oxidized one.

The value of capacity and voltage does not only depend on the geometric characteristics of the plates, but also strongly depends on this interesting treatment process, which is not permanent, as electrolytic capacitors need regular use or may lose their capacity.

We can say that many of the advances in modern electronics came through the increasing development of capacitors, today we can find capacitors the size of a grain of rice and even nano capacitors used in tiny electronic circuits. Thanks to the advances made in the development of capacitors, we have managed to achieve the current advancement of electronic circuits including the major advances in power electronics.