A capacitor is a passive component that has the ability to store energy in the form of a potential difference between its plates. It resists sudden changes in voltage. The charge is stored as the potential difference between two plates, which are positive and negative depending on the direction of charge storage. Capacitors play an important role in many electrical and electronic circuits. The capacitor is also known as a condenser.
A dielectric field exists between these two plates which are called dielectric. This dielectric can be a vacuum, air, mica, paper, ceramic, aluminum, etc. The name of the capacitor is given by the dielectric used.
Following are some of the uses of capacitors:
- Capacitors are the main element of the filter.
- DC Blocking Capacitors – These types of capacitors block DC and only allow AC to go through certain parts of the circuit.
- Capacitors have the ability to connect one part of a circuit to another.
- It is used as a sensing device.
- It is used in the vehicle’s audio system.
Circuit Symbol, Unit of Capacitance, and Dimensional Formula
Unit of Capacitance : The standard unit of capacitance is the farad(F). Typically, the available capacitor values will be in the order of micro-farad(µF), pico-farad(pF), and nano-farad(nF).
Dimensional Formula: M-1L-2I2T4
commonly used scales:
- µF = = 10-6F
- nF = 10-9F
- pF = 10-12F
The capacitance of a capacitor is proportional to the distance between the plates and inversely proportional to the area of the plates. Also, the higher the permeability of a material, the higher the capacitance. The permittivity of a medium tells how much electric current is being produced per unit charge in that medium. The following image shows some practical capacitors.
When two plates of equal area A and equal width are placed parallel to each other with a separation of distance d, and if some energy is applied to the plates, then the capacitance of that parallel plate capacitor can be said to be –
Where,
- C = Capacitance of a capacitor
- ε0 = permittivity of free space
- εr = permittivity of the dielectric medium
- d = distance between the plates
- A = area of the two conducting plates
When some voltage is applied, the charge accumulates on the two parallel plates of the capacitor. This charge deposition occurs gradually and when the voltage across the capacitor equals the applied voltage, charging stops, as the entering voltage is equal to the drop voltage.
The rate of charging depends on the value of capacitance. The higher the value of capacitance, the slower the rate of change of voltage across the plates.
Working of Capacitor
A capacitor can be understood as a two-terminal passive component that stores electrical energy. This electrical energy is stored in the electrostatic field.
Initially, the negative and positive charges on the two plates of the capacitor are in equilibrium. Capacitors have no tendency to charge or discharge. The negative charge is created by the accumulation of electrons, whereas a positive charge is created by the loss of electrons. As it happens without any external charge, this state is the electrostatic state. The figure below shows a capacitor with constant charges.
The accumulation and reduction of electrons can be understood as “current flow” according to the different positive and negative cycles of the AC supply. This is called displacement current. The direction of flow of this current keeps changing because it is AC.
Charge of Capacitor
When an external voltage is given, the electric charge is converted into an electrostatic charge. This happens when the capacitor is charging. The positive potential of the supply attracts electrons from the positive plate of the capacitor, making it more positive. Whereas the negative potential of the supply forces electrons onto the negative plate of the capacitor, making it more negative.
During this process of charging, electrons move through the DC supply, but not through the dielectric, which is an insulator. This displacement is large when the capacitor starts to charge but decreases when charged. When the voltage across the capacitor becomes equal to the supply voltage, the capacitor stops charging.
Dielectric Behavior
As charges accumulate on the plates of the capacitor, an electrostatic field is created. The strength of this electrostatic field depends on the magnitude of the charge on the plate and the permittivity of the dielectric material. Permeability is a measure of the dielectric and how far it allows electrostatic lines to pass through it.
The dielectric is actually an insulator. It consists of electrons in the outermost orbitals of atoms. When there is no charge on the plates, the electrons in dielectrics move in a circular orbit.
When charge deposition occurs, the electrons move towards the positively charged plate, but still, they continue to move.
If the charge increases further, the orbits expand further. But if it still rises, the dielectric breaks down to short the capacitor. When the capacitor is fully charged, it is ready to be discharged. If we give them a way to go from the negative plate to the positive plate, that’s enough. Electrons flow without any external supply because there are too many electrons on one side and hardly any electrons on the other. This imbalance is adjusted by discharging the capacitor.
Furthermore, when a discharge path is found, the atoms in the dielectric material revert to their normal circular orbit and therefore force the electrons to be discharged. This type of discharge enables the capacitor to deliver high currents in a short amount of time, such as in a camera flash.
What is Capacitance and How to calculate it?
The charge on the capacitor (Q) is proportional to the potential difference (V) between the plates ie.
Q ∝ V
Q = CV
The constant of proportionality (C) is called the capacitance of the capacitor.
C = Q / V
Where,
- Q is the electric charge measured in the coulomb.
- C is the capacitance measured in farads.
- V is the voltage across the plates measured in volts.
Capacitor Color Code
To calculate capacitor value based on color code and value, it is first necessary to know what is μF(microfarad), nF(nanofarad), and pF(picofarad).
- 1 μF (microfarad, one millionth (10−6) of a farad) = 0.000 001 F = 1000 nF = 1000000 pF.
- 1 nF (nanofarad, one billionth (10−9) of a farad) = 0.000 000 001 F = 0.001 μF = 1000 pF.
- 1 pF (picofarad, one trillionth (10−12) of a farad) = 0.000 000 000 001 F = 0.001 nF = 0.000001 μF.
Two common ways to find the capacitive value of a capacitor are by measuring it using a digital multimeter, or by reading the capacitor color code printed on it. These colored bands represent the capacitance value as per the color code, including the voltage rating and tolerance.
Sometimes the actual values of capacitance, voltage, or tolerance are marked on the body of the capacitor in the form of alphanumeric characters. However, problems arise with the notation of the “decimal point” when the capacitance value is of a decimal value because it cannot be easily seen resulting in a misinterpretation of the actual capacitance value.
Instead, letters such as p (Pico) or n (nano) are used instead of the decimal point to identify its position and the weight of the number. For example, a capacitor may be labeled as n47 = 0.47nF, 4n7 = 4.7nF or 47n = 47nF, etc.
Also, capacitors are sometimes marked with a capital letter K to denote a value of one thousand pico-farads, so for example, a capacitor with markings of 100K would be 100 x 1000pF or 100nF.
An international color coding scheme was developed many years ago as a simple way of identifying capacitor values and tolerances. It consists of colored bands (in spectral order) commonly known as the capacitor color code system.
Capacitor Voltage Reference
- Type J – Dipped Tantalum Capacitors.
- Type K – Mica Capacitors.
- Type L – Polyester/Polystyrene Capacitors.
- Type M – Electrolytic 4 Band Capacitors.
- Type N – Electrolytic 3 Band Capacitors.
Example 1 :
The value of yellow is 4, purple is 7, and orange is 3 which represents the multiplier. White is ±10 which is the tolerance value. Red represents voltage. But to know the voltage rating, we have got another table, of which this capacitor belongs to which particular band, we need to know
Hence the value of a capacitor is 47nF, 10% 250v. Voltage Form Voltage Rating V
Example 2 :
There is a ceramic disc-type capacitor with code 473J printed on its body. Then 4 = 1 point, 7 = 2 points, 3 is the multiplier in pico-farads, pF and the letter J is the tolerance and translates to: 47pF * 1,000 (3 zeros) = 47,000 pF, 47nF or 0.047 μF. J Indicates tolerance of ±5%
Capacitive Reactance
This is an important word. Capacitive reactance is the opposition given by a capacitor to alternating current flow, or simply AC current. A capacitor resists changes in the flow of current and therefore shows some resistance which can be called reactance, as the frequency of the input current must also be considered along with the resistance.
Symbol: XC
In a purely capacitive circuit, the current IC leads the applied voltage by 90°.
Temperature Coefficient of Capacitors
The maximum change in capacitance of a capacitor, over a specified temperature range, can be known by the temperature coefficient of a capacitor. It states that when the temperature exceeds a certain point, the change in capacitance of the capacitor can be understood as the temperature coefficient of the capacitor.
All capacitors are generally manufactured considering a reference temperature of 25 °C. Therefore the temperature coefficient of the capacitor is assumed to have temperature values above and below this value.