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Most electronic devices employ capacitors that are an invaluable part for electronic products. Capacitors are very popular among many applications like electronic circuits, power circuits, and power supply units.
The capacitor is referred to as Big Three Passive Components together with resistance and coil, which are the basics of electronic circuits. Passive components are an electronic part which receives the power to consume, store, and supply.

Unlike integrated circuits (IC), it has no active operation where low power is amplified to constantly output the power. You might as well regard capacitor as a simple part to receive and supply the electricity. However, more importantly, such passive components are indispensable parts to accurately perform active components.

The Three Passive Components are also called as LCR which stands for Coil, Capacitor, and Resistance.

Consisting of two metallic plates and insulator, Basic model of capacitor

[Fig.1] Basic structure of a capacitor
image for capacitor construction

Capacitor is basically formed from an insulator and two metal plates that are attached on the both sides of the insulator. Insulators do not carry current. The insulator used for capacitors is specifically called as dielectrics. While the electricity is flowing, the positive and negative charges are transferred within the conductor.

Charged with the electricity, the flow of the charge is started, but it is blocked since there is an insulator between metal plates. Then, the charges are built up on only one of the two metal plates. Meanwhile, another metal plate attached to the insulator has opposing charge.

Thus, capacitors have a structure to store the electricity between the two metal plates. For materials of insulator, gases, oils, ceramics, and resin are used. As for shapes of the metal plates, there are a wide variety of types with parallel plates, foil wrapping, multi-layers, and so on. The amounts of stored charges as well as the supported frequencies are different depending on the types of insulators or the structure of capacitors. So, it is necessary to select a suitable capacitor to meet your requirements.

Importance of Capacitors

In principle of capacitors, there are two important parts.

  1. Storing the electric charge (electricity)
  2. AC flowing but not DC

For details regarding storing electricity, please refer to the above-mentioned Basic structure of a capacitor.
As the electric charge is stored between the metal plates, the electric charge transfer is stopped, making DC stop flowing. However, in other words, until capacitors are fully charged, even DC can flow for a short period of time. In the case of AC, the current direction is switched with a certain interval, and then, a capacitor is charged and discharged. Therefore, the electricity looks like passing through the capacitor.
Accordingly, the higher the AC frequency is, the easier the passing is through capacitors. Thus, capacitors play the three following important roles in the electronic circuit.

1) Charging and discharging electric charges

Capacitors can charge and discharge because of the structure. Featured by the electric charge and discharge, capacitors also can be used as power supply. Camera flashes utilize this feature of capacitors.
In order to have strong light emitting, a high voltage must be instantly applied to it. Meanwhile, such the high voltage is not required in the circuit to operate the camera. Then, there is a suitable structure of a capacitor where such high light emission is provided by instantly discharging the electric charge stored in the capacitor.

2) Keeping the voltage at the same level

Apart from the above feature, capacitors have also functions to keep the voltage at a certain level. Capacitors are useful to reduce the voltage pulsation. When the high voltage is applied to the parallel circuit, the capacitor is charged, and on the other hand, it is discharged with the low voltage.
While electricity flowing out is alternating current, most of electronic circuits work with direct current. Therefore, AC is converted to DC via a rectifier circuit which changes AC to DC, but the converted DC is an unstable current with ripples at this stage. To deal with this, a capacitor is used to correct the ripples and keep the voltage constantly.

3) Removing noise

In terms of noise reduction, the feature in a capacitor of flowing AC but DC is useful for removing noise. In general, as the noise in DC is an AC component with high frequency, it has a tendency to easily go through the capacitor.
By inserting a branch circuit between the input and output, the ground is formed to connect to the capacitor. Following this, the AC component only goes through the capacitor, and then, DC flows in the output circuit.

Types of Capacitors

Aluminum Electrolytic Capacitor
The capacitor is made from aluminum and another metal. As oxide film blocks the electricity, it is used as a dielectric material by forming on the surface of the aluminum. This type of capacitors offers the large capacitance at an affordable price. Therefore, it has widely been used as a large-capacity capacitor. However, it has some weak points like bad frequency characteristics, larger in size, the loss of dielectric due to liquid leakage.
Tantalum Capacitor
The capacitor uses tantalum for the anode and tantalum pentoxide for the dielectric material. It has a relatively large capacitance while it is smaller in size than an aluminum electrolytic capacitor. Furthermore, the capacitor is superior to the aluminum capacitor in terms of leakage current characteristics, frequency property, capacitors, and temperature characteristics.
Electric Double Layer Capacitor
Electric double layer capacitors feature an extremely large capacitance. It is more than 1,000 times to 10,000 times greater than aluminum electrolytic capacitors, and it can be used repeatedly for a long period without limitations such as the number of charge/discharge cycles. Taking advantage of the unique feature, the capacitor can be used repeatedly. Electric double layer capacitors have electric charges oriented at the boundary of electrolyte and electrode which is called "electric double-layer" with the size of a single molecule. The layer is used as the dielectric material of double layer capacitors. The price of electric double layer capacitors is relatively high compared to the other ones.
Ceramic Capacitor
The capacitor is largely divided into three types based on the types of ceramics used as dielectric materials: low dielectric type, high dielectric type, and semiconductor type. As a main feature of the capacitor, the increasing voltage changes its capacitance. The small capacitor is heat-resistant although it is fragile and could be chipped or broken.
Film Capacitor
In this type, films such as polyester and polyethylene are used as the dielectric material. Polyester, polypropylene and other films are sandwiched between the electrode foils on both sides, and they are wound into a cylindrical shape. The non-polar capacitor that is larger than ceramic capacitor has high insulation resistance and no electric loss. It also provides high reliability with excellent properties in frequency and temperature.
Mica Capacitor
The capacitor uses mica, which is a natural mineral, as the dielectric material. Mica is ideal for the capacitor as it has the high dielectric property and can be peeled off. Mica capacitors have excellent characteristics such as insulation resistance, dielectric loss tangent, frequency, and temperature characteristics although there are some disadvantages in that they are expensive and a large-sized unit.
For more information about types of capacitors, please refer to the link below.
Types of capacitors. Basic component knowledge