What is Capacitor?

Definition of Capacitor - Capacitor was originally known as the condenser. Capacitor is a passive electronic component. It consists of two metal plates that are separated from each other by dielectric or air. A capacitor is used to store electrical energy. It stores energy into electrostatic field. It contains two terminals; positive and negative terminal and both must be connected in a circuit in the correct polarity. Non-polarized capacitors do not have a positive or negative terminal and can be connected to a circuit with any terminal. The two electrodes of non-polarized capacitors can be randomly inserted into the circuit.

Different types of capacitors commonly find in different circuits.

These types of capacitors are mainly used in coupling, decoupling, feedback, compensation, and oscillation circuits. Capacitor is different from battery. Battery stores electric energy in chemical form and converts it into electric energy again when required by load.

But a capacitor stores electric energy into electrostatic field. A battery can store a good amount of energy. But a capacitor can store a low amount of energy. A capacitor can charge very fast. And it gets discharged very quickly as well when connected to the load. It is a very useful feature of a capacitor. This makes them more suitable for applications where a high burst of power is needed. In general capacitor is a very useful component that is why we find a lot of capacitors in our circuit boards very often. It is used in almost every circuit.

Construction of a Capacitor

The construction of most of the capacitors is similar. A capacitor consists of two metal plates parallel to each other. Both plates do not touch each other.

Parallel plate capacitor.svg

These two plates are made of good conductive material to easily pass the electric current through them. These conductive plates are normally made of materials such as copper, brass or aluminum. The plates are separated from each other by a small distance(d) between them or a non-conductive material is placed between plates known as dielectric. The small distance between the plates can be filled with air or vacuum, glass etc. Dielectric between the plates is used to stop the flow of charged particles or electrons flowing from one plate to another plate.

How does a Capacitor Work?

To learn how a capacitor works one must know how a capacitor charges and discharges.

To learn how a capacitor works one must know how a capacitor charges and discharges. When there is no voltage source across the capacitor the total number of charge particles (electrons and protons) on the left side plate is equal. Means the total number of electrons is equal to the total numbers of protons. So, the left plate is electrically neutral. And the right-side plate is also electrically neutral.

Charging of capacitor-When a capacitor is forward biased (when positive terminal of the battery is connected to the positive terminal of capacitor and negative terminal of battery is connected to the negative terminal of capacitor) with a voltage that does not exceed the rated voltage on the capacitor, capacitor starts to charge. Due to voltage source electrons of the negative terminal stars to move towards positive terminal. As shown in the below image.

But between the positive and negative terminal there is capacitor. And electrons get stuck in the right plate of capacitor because after plate there is dielectric which does not allow the electron flow through it because it is an insulator. Thus, many electrons are trapped on the right plate of the capacitor. And the right plate becomes negatively charged. Electrons from the left side plates get attracted by the positive terminal of the battery. When electrons leave the left plate there is a lack of electrons or the creation of holes. Now the positive charge or holes in the left plate is in the majority so the left plate becomes positively charged. Now both plates of the capacitor are charged. Because the numbers of electrons on the right plate is higher than on the left plate it causes a voltage or potential difference created between the plates of the capacitor. The potential difference of capacitor which is created between the plates is increasing continuously until that becomes equal to the potential difference of battery. If the battery has a potential difference of 12 volts, then the capacitor will charge until it gets a potential of 12v. After that capacitor stops charging. Because the voltage created in the capacitor opposes the voltage of the battery.

Discharging of Capacitor-To understand the discharging of the capacitor there is an image of a simple circuit below.

There is a bulb, a fully charged capacitor, a switch which is open and a battery.

When the circuit is open, or the external voltage source is removed from the circuit, current still flows in the circuit and bulb is still glowing because charge stored in capacitor. Now the capacitor starts to discharge. When capacitor is connected to bulb the excess amount of charge or electrons stored on the right plate moves towards the left plate through the bulb. At that time the glow of bulb is maximum because capacitor is fully charged now. But when the majority of electrons transferred to the left plate from the right plate bulb is glowing very dim and continuously dimming gradually. When potential difference is neutralized, bulb stops glowing. And Capacitor is discharged now.

Capacitance- If a capacitor has high capacitance, it can store large amounts of electric charge in the form of electrostatic field. If it has small capacitance, it can store less electric charge.

Capacitance is commonly represented by capital letter C.

Capacitance is the ratio of amount of charge q on conductor plates to the potential difference V between conductor plates.

Mathematically,         C = q/V.

Different Symbols of Capacitor

The SI unit of capacitance is Farad, represented by the symbol F. SI unit Farad is named after the English physicist Michael Faraday.

A capacitor has a capacitance of 1 farad when it is charged with 1 coulomb of electric charge and has 1 volt of potential difference between its plates. .

Factors Affecting Resistance

The capacitance of a capacitor depends upon three factors:

Area of Plates- Area of the both plates is directly proportional to the capacitance of the capacitor. If the capacitor plates have more area, then it can store more electric charge inside of it.

Distance between plates- If the distance between the plates is less then capacitance of the capacitor is increased. And if the distance between the plates is increased then capacitance of the capacitor is increased.

Dielectric between plates- Dielectric is the non-conducting material placed between the plates such as mica, glass or it could be simply just air or vacuum. Dielectric material which has greater dielectric constant supports the greater electrostatic force.

Capacitors in Series

Capacitors are said to be connected in series when –ve terminal of the first capacitors is connected to the +ve of the second capacitor and the –ve of second capacitor is connected to the +ve terminal of the third capacitor and soon on.
The equivalent capacitance (Ceq) of capacitors connected in series is always less than the smallest individual capacitor present in the circuit.

1/Ceq = 1/C1 + 1/C2 + 1/C3 + ... + 1/Cn

Capacitors in parallel

Capacitors are said to be connected in parallel when –ve terminal of first capacitor is connected to –ve of the second capacitor and –ve of the second capacitor is connected to the –ve terminal of the third capacitor and so on. And the same connections are made with all positive terminals.
When capacitors are connected in parallel the equivalent capacitance (Ceq) is the sum of individual capacitance of each capacitor.

Ceq = C1 + C2 + C3 + ... + Cn
Capacitors connected in a parallel configuration have the same applied voltage across each of them.

Types of Capacitors

1. Ceramic Capacitor
2. Electrolytic capacitor
3. Film capacitor
4. Power film capacitor
5. Variable capacitor
6. Paper capacitors
7. Super Capacitors

1. Ceramic Capacitor- It is a non-polarized capacitor which means it does not have positive and negative terminal. Both the terminals can be connected in any direction in a circuit. The size of ceramic capacitor is small so it can be used for compact circuits. Ceramic acts as a dielectric material which is placed between the metal electrodes. Ceramic capacitors have a 3-digit number printed on the protective layer. The first two digits indicate the capacitor’s value, and third digit indicates the numbers of zeros to be added to know the actual value of capacitor in pico-farad. Below image shows a ceramic capacitor with digit 221 written on it.

In that 22 indicates 22 and 1 indicates numbers of zeros which is equivalent to 220 pF. 220 pF ceramic capacitor (code 221).

2. Electrolytic Capacitor- It is a polarized capacitor and has positive and negative terminals. Positive terminal must be connected to the positive and negative terminal must be connected to negative otherwise in incorrect polarization it will cause break down of the insulating oxide layer and permanent damage of capacitor. Electrolytic capacitor provides large capacitance value. The oxide layer is used as dielectric.

The anode and cathode layers are rolled into a cylindrical shape with a separator between the anode and cathode. Electrolytic capacitor is used in noise filtering, decoupling and energy storage in flashlamps. Electrolytic capacitors do have relatively low voltage ratings. Electrolyte capacitors has different types such as Aluminum electrolytic capacitors with aluminum oxide as dielectric Tantalum electrolytic capacitors with tantalum pentoxide as dielectric Niobium electrolytic capacitors with niobium pentoxide as dielectric.

3. Film Capacitor- It is a non-polarized capacitor, so the terminals are interchangeable. A thin plastic film is used as a dielectric. Then the plastic film is metalized (coated with a thin layer of metal). Terminals are added in it. Then the plastic film is rolled into a cylindrical shape. This assembly is then covered with a plastic case to protect it from environmental damage. This capacitor provides a high capacitance value.

These capacitors are relatively cheap, stable over time and have low self-inductance. Non polarization of these capacitors makes them suitable for AC circuits and signal and power supplies. Film capacitors are used in voltage regulators, power electronics devices, filter circuits, analogue to digital, resonant circuits converters etc.

4. Power film capacitor- it is a non-polarized capacitor and suitable for AC signals and power use. This capacitor comes with high and very high-power ratings. This capacitor can withstand reactive power exceeding 200 volt-amperes. Power film capacitors are used in power systems, electrical installations, power electronics, phase shifters, X-ray flashes etc. It comes with high reliability and life expectancy. The working frequency of power film capacitors ranges from 5 kHz to 1 MHz. Power film capacitors use polypropylene film as dielectric material. Power ratings of power film capacitor ranges from 150 to 1000 KVAR (Kilovolt-Ampere Reactive).

5. Variable Capacitor- A variable capacitor is a capacitor whose capacitance can be adjusted within a certain range by mechanical motion. Variable capacitors have two plates like any other capacitors, but one plate is movable, and another is fixed.

Movable plate can be adjusted or rotated. The mechanical motion allows the capacitor to change the distance between plates which results in the changed capacitance value. The variable capacitors mainly divided in two categories: A.) Tuning Capacitor B.) Trimmer Capacitor Variable capacitors are used in antenna tunning, RF filters, frequency generators etc.

6. Paper Capacitors- Paper capacitor is also known as fixed capacitor and stores a fixed amount of electric charge. Paper capacitor use paper as dielectric between plates. Aluminum foil is used as capacitor plates and electrodes are added to it.

Then both paper and aluminum are rolled up into a cylindrical shape. And then it is encapsulated to prevent environmental damage. Paper capacitor is used in coupling, noise filtering, high voltage and high current applications.

7. Super Capacitor- It is also known as ultra capacitor because it provides higher capacitance value than other capacitors. A super conductor is a polarized capacitor. Do not reverse the polarity to prevent permanent damage of capacitor. A super capacitor contains the properties of both capacitor and battery.

A super capacitor provides the highest available capacitance values per unit volume and the greatest energy density among all capacitors.

Super capacitors have a larger area to store charge than other capacitors. A super capacitor can charge within seconds and has a huge tolerance for numerous charge/discharge cycles. The construction of super capacitors is somewhat like electrolyte capacitor. Two electrodes are connected to the metal plates and separated by a separator. Electrolyte solution is used as dielectric. Solid electrolyte is preferred than liquid electrolyte because of higher terminal voltage. The solid electrolyte is generally a solvent mixed with conductive salts.

Super capacitors are divided into three categories.

A.) Double-layer capacitors
B.) Pseudo-capacitors
C.) Hybrid capacitors