Capacitor

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 = ε₀ε_rA / d

Where

C = Capacitance of a capacitor

ε₀ = permittivity of free space

ε_r = 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 1.17 below explains this.

Figure showing the electrons from positive plate to deposit on negative plate of a capacitor.

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 behavioral

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 the plates, the electrons in the dielectric move in circular orbit. This is as shown in the figure below.

When charge deposition takes place, the electrons tend to move towards the positive charged plate, but still they keep on revolving as shown in the figure.

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

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

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, sword 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 47 nF, 10% 250 V voltage for V band voltage for V band.

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: 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 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.

Types of Capacitors

There are many types of capacitors depending upon their function, the dielectric material used, their shape etc. The main classification is done according to fixed and variable capacitors. The classification is as shown below.

• Fixed Capacitors: Non‒ Polarized, Polarized

• Variable Capacitors: Tuning Capacitors, Trimmer Capacitors

The main classification is just like the above one. The fixed capacitors are the ones whose value is fixed at the time of manufacturing itself and the variable ones provide us with an option to vary the value of capacitance.

1. Fixed Capacitor

The Capacitors whose value is fixed while manufacturing and cannot be altered later are called as Fixed Capacitors. The main classification of fixed capacitors is done as polarized and non‒polarized. Let us have a look at Non‒polarized capacitors.

Non‒Polarized Capacitors

These are the capacitors that have no specific polarities, which means that they can be connected in a circuit, either way without bothering about the placement of right lead and left lead. These capacitors are also called as Non‒Electrolytic Capacitors.

The main classification of Non‒Polarized capacitors is done as shown below.

1.1 Ceramic Capacitors

The common capacitors used among fixed type are Ceramic Capacitors. The Ceramic capacitors are fixed capacitors that have ceramic material as a dielectric.

These ceramic capacitors are further classified as class1 and class2 depending upon their applications. For instance, Class1 has high stability and works best for resonant circuit applications, while class2 has high efficiency and gives its best for coupling applications.

A hollow tubular or plate like ceramic material such as titanium dioxide and barium titanate is coated with a deposition of silver compound on both walls, so that both sides act as two capacitor plates and ceramic acts as a dielectric. Leads are drawn from these two surfaces and this whole assembly is encapsulated in a moisture‒proof coating.

The most often used modern ceramic capacitors are Multi‒Layer Chip Capacitors (MLCC). These capacitors are made in surface mounted technology and are mostly used due to their small size. These are available in the order of 1cF to 100%F.

1.2 Film Capacitors

The Film Capacitors are the ones which have a film substance as a dielectric material. Depending upon the type of film used, these are classified as Paper and Metal film capacitors.

These film capacitors are both paper dielectric capacitors whereas a paper capacitor uses a waxed paper while a metallic film capacitor uses a metallized paper. The arrangement is almost same as shown below.

1.3 Paper Capacitors

Paper capacitors use Paper as a dielectric material. Two thin tin foil sheets are taken and placed between thin waxed or oiled paper sheets. This paper acts as a dielectric. Now‒a‒days paper is being replaced by plastic.

These sheets are sandwiched and are rolled into a cylindrical shape and encapsulated in a plastic enclosure. Leads are drawn out. The following figure 1.18 shows an example of Paper Capacitors.

Paper capacitors are available in the order of 0.001%F to 2% F and the voltage rating can be as high as 2000volts. These capacitors are useful in high voltage and current applications.

1.4 Metal Film Capacitors

Metal Film capacitors are another type of film capacitors. These are also called as Metal Foil Capacitors or Metallized Paper Capacitors as the dielectric used here is a paper coated with metallic film.

Unlike in paper capacitors, a film of Aluminum or Zinc is coated on a paper to form a dielectric in this metallic film capacitors. Instead of Aluminum sheets placing between papers, the paper itself is directly coated here. This reduces the size of the capacitor.

The Aluminum coating is preferred over zinc coating so as to avoid destruction of capacitor due to chemical reduction. The Aluminum coated sheets are rolled in the form of a cylinder and leads are taken. This whole thing is encapsulated with wax or plastic resin to protect the capacitor. These capacitors are useful in high voltage and current applications.

Other Capacitors

These are the miscellaneous capacitors that are named after the dielectric materials used. This group includes Mica Capacitors, Air Capacitors, Vacuum Capacitors and Glass Capacitors

1.5 Mica Capacitors

The Mica Capacitors are made by using thin Mica sheets as dielectric materials. Just like paper capacitors, thin metal sheets are sandwiched with mica sheets in between. Finally the layers of metal sheets are connected at both ends and two leads are formed.

Then the whole assembly is enclosed in plastic Bakelite capsule. The following figure 1.19 shows how a Mica capacitor looks like.

Mica Capacitors are available in the range of 50pF to 500pF. The Mica capacitors have high working voltage up to 500volts. These are most commonly used capacitors for electronic circuits such as ripple filters, Resonant circuits, Coupling circuits and high power, high current RF broadcast transmitters.

1.6 Air Capacitors

The Air Capacitors are the ones with air as dielectric. The simplest air capacitors are the ones with conducting plates having air in between. This construction is exactly the same as the variable tuning capacitor discussed above. These capacitors can be fixed and variable also but fixed are very rarely used as there are others with superior characteristics.

1.7 Vacuum Capacitors

The Vacuum Capacitors uses high vacuum as dielectric instead of air or some other material. These are also available. in fixed and variable modes. The construction of these

capacitors is similar to vacuum tubes. They are mostly seen in the form of a glass cylinder which contain inter‒meshed concentric cylinders.

The following figure 1.20 shows a variable vacuum and fixed vacuum capacitor.

Variable vacuum capacitors are available at a range of 12 pF to 5000 pF and they are used for high voltage applications such as 5 kV to 60 kV. They are used in main equipment such as high power broadcast transmitters, RF amplifiers and large antenna tuners.

1.8 Glass Capacitors

Glass capacitors are very exclusive ones with many advantages and applications. As all of the above types, here glass is the dielectric substance. Along with glass dielectric, Aluminum electrodes are also present in these capacitors. Plastic encapsulation is done after taking out the leads. The leads can be axial leads or tubular leads,

There are many advantages of a glass capacitor such as ‒

• The temperature coefficient is low.

• These are Noise‒free capacitors.

• They produce high quality output with low loss.

• They have the capability of handling high operating temperatures.

• These capacitors can handle large RF currents.

There are many applications for these glass capacitors such as

• Used in circuits that need to be at high temperature zones.

• Used in circuits that need high Q.

• Used in high power handling circuits.

• Used for circuits that need high tolerances.

2. Variable Capacitors

Let us know something about the variable capacitors whose value alters when you vary, either electrically or mechanically. Variable capacitors in general consists of interwoven sets of metallic plates in which one is fixed and the other is variable. These capacitors provide the capacitance values so as to vary between 10 to 500 pF.

The ganged capacitor shown here is a combination of two capacitors connected together. A single shaft is used to rotate the variable ends of these capacitors which are combined as one. The dotted line indicates that they are connected internally.

There are many uses of these variable resistors such as for tuning in LC circuits of radio receivers, for impedance matching in antennas etc. The main types of variable capacitors are Tuning capacitors and Trimmer capacitors.

2.1 Tuning Capacitors

Tuning capacitors are popular type of variable capacitors. They contain a stator, a rotor, a frame to support the stator and a mica capacitor. The constructional details of a tuning capacitor are shown in the following figure 1.22.

The stator is a stationary part and rotor rotates by the movement of a movable shaft. The rotor plates when moved into the slots of stator, they, come close to form plates of a capacitor. When the rotor plates sit completely in the slots of the stator then the capacitance value is maximum and when they don't, the capacitance value is minimum.

The above figure 1.22 shows a ganged tuning capacitor having two tuning capacitors connected in a gang. This is how a tuning capacitor works. These capacitors generally have capacitance values from few Pico Farads to few tens of Pico Farads. These are mostly used in LC circuits in radio receivers. These are also called as Tuning Condensers.

2.2 Trimmer Capacitors

Trimmer capacitors are varied using a screwdriver. Trimmer capacitors are usually fixed in such a place where there is no need to change the value of capacitance, once fixed.

There are three leads of a trimmer capacitor, one connected to stationary plate, one to rotary and the other one is common. The movable disc is a semi‒circular shaped one. A trimmer capacitor would look like the ones in the following figure 1.23.

There are two parallel conducting plates present with a dielectric in the middle. Depending upon this dielectric used, there are air trimmer capacitors and ceramic trimmer capacitors. The constructional details of a trimmer capacitor are as shown fig. 1.23.

One of the two plates is movable, while the other is fixed. The dielectric material is fixed. When the movable plate is moved, opposite to the area between movable and fixed electrode, then the capacitance can be changed. The capacitance will be higher if the opposite area gets bigger, as both the electrodes act as two plates of a capacitor.

The Trimmer Capacitors are easily fixed on a PCB (Printed Circuit Board) and they are mostly used for calibration of equipment.