Capacitor

A Capacitor is a passive component that has the ability to store the energy in the form of potential difference between its plates. It resists a sudden change in voltage. The charge is stored in the form of potential difference between two plates, which form to be positive and negative depending upon the direction of charge storage.

A non-conducting region is present between these two plates which is called as dielectric. This dielectric can be vacuum, air, mica, paper, ceramic, aluminum etc. The name of the capacitor is given by the dielectric used.

Symbol and Units

The standard units for capacitance is Farads. Generally, the values of capacitors available will be in the order of micro-farads, pico-farads and nano-farads. The symbol of a capacitor is as shown below.

The Capacitance of a capacitor is proportional to the distance between the plates and is inversely proportional to the area of the plates. Also, the higher the permittivity of a material, the higher will be the capacitance. The permittivity of a medium describes how much electric flux is being generated per unit charge in that medium. The following image shows some practical capacitors.

When two plates having same 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 termed as −

C=ε0εrdAC=ε0εrdA

Where

C = Capacitance of a capacitor

ε0ε0 = permittivity of free space

εrεr = permittivity of dielectric medium

d = distance between the plates

A = area of the two conducting plates

With some voltage applied, the charge deposits on the two parallel plates of the capacitor. This charge deposition occurs slowly and when the voltage across the capacitor equals the voltage applied, the charging stops, as the voltage entering equals the voltage leaving.

The rate of charging depends upon the value of capacitance. The greater the value of capacitance, the slower the rate of change of voltage in the plates.

Working of a Capacitor

A Capacitor can be understood as a two-terminal passive component which stores electrical energy. This electrical energy is stored in electrostatic field.

Initially, the negative and positive charges on two plates of the capacitor are in equilibrium. There is no tendency for a capacitor to get charged or discharged. The negative charge is formed by the accumulation of electrons, while the positive charge is formed by the depletion of electrons. As this happens without any external charge given, this state is electrostatic condition. The figure below shows the capacitor with static charges.

The accumulation and depletion of electrons according to the varying positive and negative cycles of the AC supply, can be understood as “current flow”. This is called as Displacement Current. The direction of this current flow keeps on changing as this is AC.

Charging of a Capacitor

When an external voltage is given, the electric charge gets converted into electrostatic charge. This happens while the capacitor is charging. The positive potential of the supply, attracts the electrons from the positive plate of the capacitor, making it more positive. While the negative potential of the supply, forces the electrons to the negative plate of the capacitor, making it more negative. The figure below explains this.

During this process of charging, the 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 reduces as it charges. The capacitor stops charging when the voltage across capacitor equals the supply voltage.

Let us see what happens to the dielectric when the capacitor begins to charge.

Dielectric behavior

As the charges deposit on the plates of the capacitor, an electrostatic field is formed. The strength of this electrostatic field depends upon the magnitude of charge on the plate and the permittivity of the dielectric material. Permittivity is the measure of dielectric whether how far it allows the electrostatic lines to pass through it.

The dielectric is actually an insulator. It has electrons in the outer most orbit of the atoms. Let us observe how they get affected. When there is no charge on the plates, the electrons in the dielectric move in circular orbit. This is as shown in the figure below.

If the charge increases further, the orbits expand more. But if it still increases, the dielectric breaks down shorting the capacitor. Now, the capacitor being fully charged, it’s ready to get discharged. It is enough if we provide a path for them to travel from negative to positive plate. The electrons flow without any external supply as there are too many number of electrons on one side and barely any electrons on the other. This imbalance is adjusted by the discharge of the capacitor.

Also, when a discharge path is found, the atoms in the dielectric material tend to get to their normal circular orbit and hence forces the electrons to get discharged. This kind of discharge enables capacitors to deliver high currents in a short period of time, just as in a camera flash.

Color Coding

To know the value of a capacitor, it is usually labelled as below −

n35 = 0.35nF or 3n5 = 3.5nF or 35n = 35nF and so on.

Sometimes the markings will be like 100K which means, k = 1000pF. Then the value will be 100 × 1000pF = 100nF.

Though these number markings are being used now-a-days, an International color coding scheme was developed long ago, to understand the values of capacitors. The color coding indications are just as given below.

These indications were used to identify the value of capacitors.

In these five band capacitors, the first two bands represent digits, third one indicates multiplier, fourth for tolerance and the fifth represents voltage. Let us look at an example to understand the color coding process.

Example 1 − Determine the value of a capacitor with a color code yellow, violet, orange, white and red.

Solution − The value of yellow is 4, violet is 7, orange is 3 which represents multiplier. White is ±10 which is the tolerance value. Red represents the voltage. But to know the voltage rating, we have got another table, from which the particular band to which this capacitor belongs, has to be known.

Hence the value of the capacitor is 47nF, 10% 250v voltageforVbandvoltageforVband

The following table shows how voltage is determined depending upon the bands the capacitors belong to.

With the help of this table, the voltage rating for each band of capacitors is known according to the color given. The type of voltage ratings also indicates the type of capacitors. For example, TYPE J ones are Dipped Tantalum Capacitors, TYPE K ones are Mica Capacitors, TYPE L ones are Polystyrene Capacitors, TYPE M ones are Electrolytic Band 4 Capacitors and TYPE N ones are Electrolytic Band 3 Capacitors. These days, the color coding has been replaced by simple printing of value of the capacitors as mentioned previously.

Capacitive Reactance

This is an important term. Capacitive Reactance is the opposition offered by a capacitor to the alternating current flow, or simply AC current. A capacitor resists the change in the flow of current and hence it shows some opposition which can be termed as reactance, as the frequency of the input current should also be considered along with the resistance it offers.

Symbol: X_C

In a purely capacitive circuit, the current I_C 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 a capacitor that might occur is understood as the temperature coefficient of capacitors.

All the capacitors are usually manufactured considering a reference temperature of 25°C. Hence the temperature coefficient of capacitors is considered for the values of temperatures that are above and below this value.

Here is a table with the most common prefixes for capacitors:

Types of Capacitor

The types of capacitors available range from very small delicate trimming capacitors using in oscillator or radio circuits, up to large power metal-can type capacitors used in high voltage power correction and smoothing circuits.

The comparisons between the the different types of capacitor is generally made with regards to the dielectric used between the plates. Like resistors, there are also variable types of capacitors which allow us to vary their capacitance value for use in radio or “frequency tuning” type circuits.

Commercial types of capacitors are made from metallic foil interlaced with thin sheets of either paraffin-impregnated paper or Mylar as the dielectric material. Some capacitors look like tubes, this is because the metal foil plates are rolled up into a cylinder to form a small package with the insulating dielectric material sandwiched in between them.

Small capacitors are often constructed from ceramic materials and then dipped into an epoxy resin to seal them. Either way, capacitors play an important part in electronic circuits so here are a few of the more “common” types of capacitor available.

Dielectric Capacitor

Dielectric Capacitors are usually of the variable type were a continuous variation of capacitance is required for tuning transmitters, receivers and transistor radios. Variable dielectric capacitors are multi-plate air-spaced types that have a set of fixed plates (the stator vanes) and a set of movable plates (the rotor vanes) which move in between the fixed plates.

The position of the moving plates with respect to the fixed plates determines the overall capacitance value. The capacitance is generally at maximum when the two sets of plates are fully meshed together. High voltage type tuning capacitors have relatively large spacings or air-gaps between the plates with breakdown voltages reaching many thousands of volts.

Variable Capacitor Symbol

As well as the continuously variable types, preset type variable capacitors are also available called Trimmers. These are generally small devices that can be adjusted or “pre-set” to a particular capacitance value with the aid of a small screwdriver and are available in very small capacitance’s of 500pF or less and are non-polarized.

Film Capacitor Type

Film Capacitors are the most commonly available of all types of capacitors, consisting of a relatively large family of capacitors with the difference being in their dielectric properties. These include polyester (Mylar), polystyrene, polypropylene, polycarbonate, metalised paper, Teflon etc. Film type capacitors are available in capacitance ranges from as small as 5pF to as large as 100uF depending upon the actual type of capacitor and its voltage rating. Film capacitors also come in an assortment of shapes and case styles which include:

• Wrap & Fill (Oval & Round)–  where the capacitor is wrapped in a tight plastic tape and have the ends filled with epoxy to seal them.
• Epoxy Case (Rectangular & Round)–  where the capacitor is encased in a moulded plastic shell which is then filled with epoxy.
• Metal Hermetically Sealed (Rectangular & Round)–  where the capacitor is encased in a metal tube or can and again sealed with epoxy.

with all the above case styles available in both Axial and Radial Leads.

Film Capacitors which use polystyrene, polycarbonate or Teflon as their dielectrics are sometimes called “Plastic capacitors”. The construction of plastic film capacitors is similar to that for paper film capacitors but use a plastic film instead of paper. The main advantage of plastic film capacitors compared to impregnated-paper types is that they operate well under conditions of high temperature, have smaller tolerances, a very long service life and high reliability. Examples of film capacitors are the rectangular metalised film and cylindrical film & foil types as shown below.

Radial Lead Type

Axial Lead Type

The film and foil types of capacitors are made from long thin strips of thin metal foil with the dielectric material sandwiched together which are wound into a tight roll and then sealed in paper or metal tubes.

These film types require a much thicker dielectric film to reduce the risk of tears or punctures in the film, and is therefore more suited to lower capacitance values and larger case sizes.

Metalised foil capacitors have the conductive film metalised sprayed directly onto each side of the dielectric which gives the capacitor self-healing properties and can therefore use much thinner dielectric films. This allows for higher capacitance values and smaller case sizes for a given capacitance. Film and foil capacitors are generally used for higher power and more precise applications.

Ceramic Capacitors

Ceramic Capacitors or Disc Capacitors as they are generally called, are made by coating two sides of a small porcelain or ceramic disc with silver and are then stacked together to make a capacitor. For very low capacitance values a single ceramic disc of about 3-6mm is used. Ceramic capacitors have a high dielectric constant (High-K) and are available so that relatively high capacitance’s can be obtained in a small physical size.

Ceramic Capacitor

They exhibit large non-linear changes in capacitance against temperature and as a result are used as de-coupling or by-pass capacitors as they are also non-polarized devices. Ceramic capacitors have values ranging from a few picofarads to one or two microfarads, ( μF ) but their voltage ratings are generally quite low.

Ceramic types of capacitors generally have a 3-digit code printed onto their body to identify their capacitance value in pico-farads. Generally the first two digits indicate the capacitors value and the third digit indicates the number of zero’s to be added. For example, a ceramic disc capacitor with the markings 103 would indicate 10 and 3 zero’s in pico-farads which is equivalent to 10,000 pF or 10nF.

Likewise, the digits 104 would indicate 10 and 4 zero’s in pico-farads which is equivalent to 100,000 pF or 100nF and so on. So on the image of the ceramic capacitor above the numbers 154 indicate 15 and 4 zero’s in pico-farads which is equivalent to 150,000 pF or 150nF or 0.15μF. Letter codes are sometimes used to indicate their tolerance value such as: J = 5%, K = 10% or M = 20% etc.

Electrolytic Capacitors

Electrolytic Capacitors are generally used when very large capacitance values are required. Here instead of using a very thin metallic film layer for one of the electrodes, a semi-liquid electrolyte solution in the form of a jelly or paste is used which serves as the second electrode (usually the cathode).

The dielectric is a very thin layer of oxide which is grown electro-chemically in production with the thickness of the film being less than ten microns. This insulating layer is so thin that it is possible to make capacitors with a large value of capacitance for a small physical size as the distance between the plates, d is very small.

Electrolytic Capacitor

The majority of electrolytic types of capacitors are Polarised, that is the DC voltage applied to the capacitor terminals must be of the correct polarity, i.e. positive to the positive terminal and negative to the negative terminal as an incorrect polarisation will break down the insulating oxide layer and permanent damage may result.

All polarised electrolytic capacitors have their polarity clearly marked with a negative sign to indicate the negative terminal and this polarity must be followed.

Electrolytic Capacitors are generally used in DC power supply circuits due to their large capacitance’s and small size to help reduce the ripple voltage or for coupling and decoupling applications. One main disadvantage of electrolytic capacitors is their relatively low voltage rating and due to the polarisation of electrolytic capacitors, it follows then that they must not be used on AC supplies. Electrolytic’s generally come in two basic forms; Aluminium Electrolytic Capacitors and Tantalum Electrolytic Capacitors.

Electrolytic Capacitor

1. Aluminium Electrolytic Capacitors

There are basically two types of Aluminium Electrolytic Capacitor, the plain foil type and the etched foil type. The thickness of the aluminium oxide film and high breakdown voltage give these capacitors very high capacitance values for their size.

The foil plates of the capacitor are anodized with a DC current. This anodizing process sets up the polarity of the plate material and determines which side of the plate is positive and which side is negative.

The etched foil type differs from the plain foil type in that the aluminium oxide on the anode and cathode foils has been chemically etched to increase its surface area and permittivity. This gives a smaller sized capacitor than a plain foil type of equivalent value but has the disadvantage of not being able to withstand high DC currents compared to the plain type. Also their tolerance range is quite large at up to 20%. Typical values of capacitance for an aluminium electrolytic capacitor range from 1uF up to 47,000uF.

Etched foil electrolytic’s are best used in coupling, DC blocking and by-pass circuits while plain foil types are better suited as smoothing capacitors in power supplies. But aluminium electrolytic’s are “polarised” devices so reversing the applied voltage on the leads will cause the insulating layer within the capacitor to become destroyed along with the capacitor. However, the electrolyte used within the capacitor helps heal a damaged plate if the damage is small.

Since the electrolyte has the properties to self-heal a damaged plate, it also has the ability to re-anodize the foil plate. As the anodizing process can be reversed, the electrolyte has the ability to remove the oxide coating from the foil as would happen if the capacitor was connected with a reverse polarity. Since the electrolyte has the ability to conduct electricity, if the aluminium oxide layer was removed or destroyed, the capacitor would allow current to pass from one plate to the other destroying the capacitor, “so be aware”.

2. Tantalum Electrolytic Capacitors

Tantalum Electrolytic Capacitors and Tantalum Beads, are available in both wet (foil) and dry (solid) electrolytic types with the dry or solid tantalum being the most common. Solid tantalum capacitors use manganese dioxide as their second terminal and are physically smaller than the equivalent aluminium capacitors.

The dielectric properties of tantalum oxide is also much better than those of aluminium oxide giving a lower leakage currents and better capacitance stability which makes them suitable for use in blocking, by-passing, decoupling, filtering and timing applications.

Also, Tantalum Capacitors although polarised, can tolerate being connected to a reverse voltage much more easily than the aluminium types but are rated at much lower working voltages. Solid tantalum capacitors are usually used in circuits where the AC voltage is small compared to the DC voltage.

However, some tantalum capacitor types contain two capacitors in-one, connected negative-to-negative to form a “non-polarised” capacitor for use in low voltage AC circuits as a non-polarised device. Generally, the positive lead is identified on the capacitor body by a polarity mark, with the body of a tantalum bead capacitor being an oval geometrical shape. Typical values of capacitance range from 47nF to 470uF.

Aluminium & Tantalum Electrolytic Capacitor

Electrolytic’s are widely used capacitors due to their low cost and small size but there are three easy ways to destroy an electrolytic capacitor:

• Over-voltage–  excessive voltage will cause current to leak through the dielectric resulting in a short circuit condition.
• Reversed Polarity–  reverse voltage will cause self-destruction of the oxide layer and failure.
• Over Temperature–  excessive heat dries out the electrolytic and shortens the life of an electrolytic capacitor

Reference:

https://www.electronics-tutorials.ws/capacitor/cap_1.html

https://en.wikipedia.org/wiki/Capacitor

https://www.electronics-notes.com/articles/basic_concepts/capacitance/capacitor-formulas-equations.php

https://www.illinoiscapacitor.com/tech-center/formulas.aspx