Understanding the Types of Capacitors: A Comprehensive Guide

Engr Fahad

Table of Contents

Types of Capacitors

There are two types of capacitors according to their operation;

Fixed capacitors
Variable capacitors

Fixed Capacitor

If a capacitor is designed in such a way that its different components cannot be moved from their original place, then such a capacitor is called fixed capacitor. Or, the capacitors having a constant capacitance value, i.e., values that cannot be changed in any way, are called fixed or permanent capacitors. If the capacitance cannot be deliberately controlled, a capacitor is called fixed or those capacitors whose capacitance cannot be controlled are called fixed capacitors.

Remember that just like other circuit components, the capacitance of fixed capacitors can be changed only through changing the environment or atmosphere, under which they are operating, e.g., temperature. There are following two types of fixed capacitors;

(i). Electrostatic

(ii). Electrolytic

(i). Electrostatic Capacitors

These types of capacitors are constructed through two metal plates by placing dielectric material (e.g., paper, ceramic, mica, etc.) between them. These capacitors can be fixed in the circuit in any direction. That’s while installing these capacitors, the concept of polarity is not taken into consideration. An electrostatic capacitor has the following types according to the dielectric material.

(a). Mica Capacitors

(b). Paper Capacitors

©. Ceramic Capacitors

(d). Poly Carbonate capacitors or, Plastic Capacitors

(e). Oil Filled Capacitors

The details of these capacitors are as follows;

(a). Mica Capacitors

These types of capacitors are constructed by placing a sheath of mica between metal plates. There are further two types of mica capacitors. The first type is called stacked foil and the second type is called silver mica. The basic structure of a stacked foil-type capacitor has been illustrated in figure 6.14. These capacitors are composed of stacked metal foils, the number of which depends on the required capacitance of a capacitor. A thin mica foil is placed between every two metal sheets, which functions as a dielectric material. Every metal foil works as a plate. As such, alternate metal foils are combined together to increase the plate area. That’s the greater the number of metal foils connected together; the larger will be the plate area. And with an increase in plate area, the conductor’s capacitance also increases. By pressing mica and foil assembly or stack, they are encapsulated in an insulated material box, etc., as has been illustrated in figure (b).

Figure 6.14; Typical mica capacitor

The silver mica capacitor is also manufactured in exactly the same manner. The only difference is that a silver electrode material is used in them. that’s a silver coating is done above every mica sheet, which functions as a capacitor’s plate. Remember that the connecting ends of a mica processor are dragged out in such a way that every substitute metal sheet is combined with other metal sheets, and one of the ends is dragged out. That’s the first plate of every set that is connected from one end and other plates from the other end, as has been shown in figure (a). Mica capacitors are tiny in size. Mica has a dielectric 5% higher than the air and does not absorb moisture, that’s why these capacitors have the ability to function permanently. These capacitors are available in the range of one pekoe farad (1 PF) to 0.2 microfarads and their voltage rating range tends to be between 100 to 2500 volts. As a result of their high price, these types of capacitors are used in high-frequency circuits (where dielectric loss has to be minimized). As a consequence of the high dielectric constant, they cannot withstand high voltages. Moreover, such capacitors have the feature of performing excellently even at variable temperatures. Apart from fixed values, mica capacitors are also available in variable values. In figure 6.15, mica processors have been illustrated.

Figure 6.15; Mica capacitors

(b). Paper Capacitors

It is that form of a fixed type capacitor, which is most vastly used. The shape of paper capacitors is cylindrical and they are prepared from two thin aluminum sheets, in which an ordinary tissue paper, oil, or wax impregnated paper functions as a dielectric. These aluminum plates are known as metal sheets. As a result of wrapping paper around the conducting plates, this capacitor occupies a small space. Paper is inserted between metal foils and wrapped in the form of a cylinder. Outward connections are carried out through metal sheets, and the capacitor’s entire assembly is enclosed in cardboard or plastic boxes.

Figure 6.16; (a). Paper capacitor construction (b). A fixed value paper capacitor encapsulated in plastic.

As the paper is porous, more than one dielectric layer is used, so that moisture vapors cannot penetrate it. In figure 6.16, a paper capacitor has been shown.

Paper capacitors are vastly used for high voltage high discharge. These types of capacitors are available within a range of 0.001 μF to 1.0 μF, and they can operate maximum from 1600 to 2000 voltage. The basic function of these capacitors is to couple or decouple the circuits. Their application in radio receivers’ high side frequency is quite common. At the time of installing these capacitors, no attention is paid to the matter of polarity, and they can be installed in a circuit at one’s own will.

©. Ceramic Capacitors

Ceramic dielectrics have a high dielectric constant (i.e., Ԑr= 1200), due to which a relatively high capacitance value can be gained from a very tiny size. As ceramic capacitors are made from extremely high relative permittivity ceramic dielectric, that’s why they are used for high capacitance in high capacitance circuits.

Ceramic capacitors are available in both discs as well as tabular form. In figure 6.17 (a), a ceramic disc capacitor, and in figure 6.17 (b), a ceramic tabular capacitor has been displayed. These capacitors are also available in multi-layers structure, as has been shown in figure 6.18. In figure 6.19, ceramic capacitors of different sizes and different shapes have been shown.

Figure 6.17 (a); A ceramic disk capacitor and its basic construction

Figure 6.17 (b); Ceramic tabular capacitor

Figure 6.18; Multi-layer, radial–lead ceramic capacitor

Figure 6.19; A group of disk ceramic capacitors

In these capacitors, different ceramic materials or different types of silicates are used as a dielectric. Normally, Titanium oxide, Barium Titanate, or different types of such silicates are used as ceramic materials, which have a very high Ԑr. A thin layer of silver compound is coated on both sides of the dielectric disc, which functions as a capacitor plate. To make tabular ceramic circuits, a silver or copper coating is carried out in and outside the hollow ceramic tube, which functions as two plates, as has been illustrated in the figure. Leads are connected to the electrodes through plates. After this, a ceramic insulating coating is done on plates and dielectric. Some variable disc types capacitors made from ceramics are also available, the value of which can be changed through a screwdriver. In order to protect ceramics capacitors from humidity, a wax coating is occasionally carried out on their outer surface, because entering a capacitor, moisture,, etc., can cause damage to it. See figure 6.20

Figure 6.20; Ceramic dielectric capacitors (a). ceramic disc capacitor (b). ceramic tube capacitor

Ceramic capacitors are cheap from an economic point of view and have high capacitance in spite of a very tiny size. They can be used in both AC and DC circuits. The capacitance range of these capacitors tends to be from 1Pϝ to 2.2 𝜇 ϝ and they can be used up to 6Kv. As compared to uniform capacity mica capacitors, these capacitors have relatively low inductance. That’s the reason they are used on high frequencies. Their performance up to 200 MHZ tends to be superb and they function excellently in a reliable manner. Their performance can be affected as well as they can be damaged as a result of heat. Besides, excessive leakage resistance, and variations in their capacitance as a result of a change in applied voltage, are its drawbacks. Moreover, due care needs to be exercised at the time of their installation.

(d). Plastic Capacitors

Nowadays, plastic capacitors of different types are manufactured. Polycarbonate, Perylene, Polyester, Polystyrene, Polypropylene, Mylar, etc., are some dielectric materials, from which plastic capacitors are made. The capacitors manufactured from such materials may be up to 100 𝜇 ϝ.

Polycarbonate is the most important material following the recent amazing development in the plastic field. The Ԑr of this material is about 2.8. the dielectric loss of this material is low and its resistivity is very high. The operating temperature of this material tends to be around 125℃ to 150℃. As a result of these advantages, it is considered one of the best dielectrics for making a capacitor. In figure 6.21, the structure of a plastic dielectric capacitor has been illustrated. This capacitor is manufactured by employing a sheet of plastic dielectric between two thin metal foils (which work as plates).

Figure 6.21; Basic construction of tabular plastic dielectric capacitors

One of the leads is connected to the inner plate while the other lead is connected to an outer plate, as has been depicted in the figure. Its foil is swathed to give it a shape of a cylinder, above which a molded sheath is coated. As such, a very large plate area capacitor is manufactured from a very small size. The object behind this is to attain maximum capacitance. These capacitors are manufactured normally from 0.001 to 1.0 microfarad. It must be remembered that paper capacitors and plastic capacitors are nearly the same from the construction point of view. The only difference is that plastic is used instead of paper to cover up its deficiencies. In this way, a capacitor can work on high voltages without developing any fault in its dielectric, and their sizes are also reduced to a substantial extent. In some of the capacitors, paper is wrapped up on mylars layers, and it is sealed in a plastic casing. Such capacitors remain protected against weather effects. In figure 6.22, a polyester capacitor has been shown.

Figure 6.22; polyester film capacitor

(e). Oil Filled Capacitors

As its name implies, an oil-impregnated paper is used as dielectric in such capacitors. These capacitors are mostly used in those industries, where apart from withstanding high currents and high backup voltages on power lines frequencies, there is also an urgency to provide a very high capacitance. Apart from starting AC motors, these types of capacitors are also used for phase splitting, voltage regulation, and improving the power factor. Some of the capacitors are such, in which a paper polyester film is used as a dielectric. They are called DC oil-filled capacitors. These capacitors are used in DC circuits for filtering, bypassing, arc suppression, coupling, voltage multiplication, etc. Moreover, oil dielectric capacitors are also commonly used in wireless, radar transmitters, and module circuits. An oil-filled circuit has been illustrated in figure 6.23 below;

Figure 6.23; the oil-filled capacitor

(ii). Electrolyte Capacitors

The electrolyte capacitors are polarized. One of their plates tends to be negative while the other plate is positive. These capacitors are used for high capacitance (i.e., up to 200,000 μϝ. A chemical solution known as an electrolyte is used in them as a dielectric. As a thin layer of this dielectric is used, thus excessively high capacitance capacitors can be built from very low sizes. While installing electrolyte capacitors, polarity has particularly to be taken into consideration, because they bear plus and minus marks quite plainly. The process of polarization (forming of a hydrogen layer on a plate through some chemical operation) can be undertaken only through the selection of a capacitor having the right polarity, owing to which a dielectric layer forms.

Construction

The basic structure of an electrolytic capacitor has been shown in figure 6.24 (a). An electrolytic capacitor basically consists of two foils or sheets, or two aluminum or tantalum foils, between which a gauze of paper or gauze strip or insulator material is used, which is saturated through an electrolyte (e.g., aluminum borate). During the process of manufacturing, a layer of aluminum oxide or tantalum oxide is coated on the interior surface of the positive plate through an electrochemical reaction. This layer of oxide functions as dielectric, whereas aluminum works as a positive plate. Another aluminum layer without having oxide is stacked above it, which functions as a negative plate. Sometimes, the negative plate is connected directly to the aluminum container. All these three sheets are wrapped together and enclosed in a container. During the process of manufacturing, DC voltages are supplied between both the foils, as a consequence of which, resulting current flows and a thin oxide layer coats on a foil sheet. It means that the capacitor has been polarized, and whenever it is required to be used, polarity does not need to be changed. These capacitors have mainly been designed to operate on DC voltages, however, there are capacitors, which can be used on AC circuits for the starting of a motor. The dielectric material of an electrolytic capacitor consists of a very thin oxide film, which forms as a result of the electrolytic operation. As a result of this thin film of dielectric, attainment of an extremely high capacitance is possible from a very small space. Capacitance up to thousands of micro- farads can be achieved through electrolytic capacitors of reasonable size and costs. In figure 6.24 (b), some specific electrolytic capacitors have been illustrated. Their working voltage (voltage on which a capacitor can work for long hours without developing any sort of fault) tends to be around 500 volts.

Figure 6.24 (a); Electrolytic capacitors Fig. 6.24 (b)

When an electrolytic capacitor becomes polarized, then its positive plate should always be connected to the positive side of the circuit (that’s their polarity must particularly be heeded upon while installing them in the circuit, and the negative should be connected to the negative and positive with the positive polarity). Its reason is that the capacitor can function properly so long as voltages are supplied in just one direction. When voltage polarity changes (owing to a wrong polarity), then the oxide layer breaks down and the capacitor starts passing on the current like a low-value resistor. If an electrolytic capacitor is fitted in the circuit in the wrong way, a very huge quantity of current will flow through it. As a result, it will become useless. In figure 6.25,

the construction of a tantalum capacitor has been demonstrated.

Figure 6.25; hermetically sealed electrolytic capacitor

Disadvantages

Low breakdown voltage
Excessive leakage magnitude
Unsuitable for application in AC circuits
An extremely high inner inductance
Very low capability to sustain temperature
An impending risk of exploding owing to a very low withstanding capacity
Capacitance reduces as a result of remaining idle for a long time
A risk of installing on the wrong polarities
A very low performance on high frequencies
A very low insulation resistance

Uses

Due to being cheap, these capacitors are most profusely used in power supply filtering, DC coupling circuits, and in all those places where the provision of a high capacitance is necessary. Further, they are also used to improve power factors within very large circuits.

Note; Apart from the afore-mentioned capacitors, the usage of chip capacitors (or monolithic chip capacitors) is increasing very rapidly nowadays. These are very small in size, and ceramic material is being used in them as a dielectric. Their capacitance value ranges between 10 Pϝ to 4 μϝ. They are designed from ceramic and alternate layers of conducting material.

Variable Capacitors

A variable capacitor is one the capacitance of which can be changed through rotating a shaft. In other words, if a capacitor is designed in such a way that its various components could move from their respective places, then such capacitors are called variable capacitors. The capacitance of these types of capacitors can be changed by changing the area of their plates or changing the mutual distance between these plates. The variable capacitors are mostly used in those circuits, the capacitance values of which need to be changed either manually or automatically, e.g., radio or TV tuners, etc. In these capacitors, air, mica, ceramic, etc. are used as a dielectric. The capacitors, in which air is used as dielectric, are known as tuning or ganged capacitors. And those capacitors, in which other dielectrics are used instead of air, are known as trimmers or padders. The important forms of variable or adjustable capacitors are as follows;

(i). Air Capacitor

(ii). Trimmers and Padders

(iii). Varactors

(i). Air Capacitors

The capacitors, in which air is used as a dielectric medium between two plates, are called air capacitors. In other words, these capacitors consist of two sets of metal plates having air between them. One of the sets of metal plates is static, which is called stater and it is insulated with the capacitor frame (on which it is installed). Another set of plates is fitted along with the shaft, and it can be rotated. That’s why it is called a router. With the assistance of a suitable knob fitted along with the shaft, router plates can be intruded into stater plates by rotating the router and can also be jutted out from the stater plates (because a very small space is available between stater and router plates). When router plates completely enter the stater plates, the capacitance tends to be maximum at that time. And when router plates are outside from the straighter plate, the capacitance at that time tends to be minimum.

As one set of plates in this capacitor is mobile as compared to the other set, thus capacitance changes as a result of changes in effective plate area (remember that fixed and moveable plates do not collide with each other). Mobile plates are linked together mechanically so that when the shaft is rotated (or touter is moved) all the plates move collectively. These capacitors are used as tuning capacitors for the selection of the desired frequency. If the number of total plates is “n” and the internal distance between any two connected plates is “d”, then

C = (n – 1) Ԑ0 A/ d

When two or more capacitors of these types are operated through a single shaft, then such a capacitor is called a ganged capacitor. These types of capacitors are most commonly used in radio receiver sets. In figure 6.26 (a), one ganged variable capacitor, in figure 6.26 (b), two ganged variable capacitors and in figure 6.26 (c) three ganged variable capacitors have been shown. Most often, a capacitor comprising three gangs is used in commercial receivers. Air capacitors are usually reliable, durable, and stable.

(ii). Trimmers and Padders

Trimmer is a very popular form of a variable capacitor. These adjustable capacitors (having a screw-driven adjustment) can be used in a circuit for a very splendid type of adjustment. That’s, they can basically be used for overall capacitance fine adjustment of any device. In these types of capacitors, mica and ceramic are used as a commonly used dielectric. The capacitances of these types of capacitors are normally changed by means of varying the internal distance between the two plates. And the internal distance between the plates can be changed by rotating a screw with the help of a screwdriver fitted above a capacitor. As the screw rotates in an inward direction, the internal distance between these plates minimizes, and thus capacitor’s capacitance increases.

Figure 6.26; Variable air capacitors

The trimmer’s capacitance can be changed between 5 Pϝ to 30 pekoe farads, in figure 6.28, a small adjustable trimmer capacitor has been shown.

Figure 6.28: A small adjustable trimmer capacitor

Trimmers and padders are normally similar in appearance. The only difference between these two is that padders are a bit larger in size, that’s the number of plates in padders is relatively large. In other words, when two or more variable capacitors (having the same capacitance) are used in such a way in the form of a gang that the capacitance of different tune circuits can be balanced or neutralized via them, or the capacitance of any circuit could be adjusted, such a variable capacitor is called a padder. The capacitance of a padder can be varied between 10 Pϝ to 500 Pϝ.

As has been discussed above, variable capacitors are mostly used as tuning capacitors in radio receivers. When we tune two different stations, we actually have been changing the capacitance by moving in or out the router plates from the stater plates. As a tuning capacitor has been linked to inductance, therefore, variable capacitance tunes the receiver according to every transmitting station into different resonant frequencies. As a result of which, one can listen to the stations of different frequencies.

Varactors

A varactor is a type of semiconductor device, in which capacitance can be altered by producing voltages parallel to its terminals. It is also known as a voltage variable capacitor (VVC). As the capacitance of a varactor tends to be in the pekoe farad (PF) range, therefore, it is used as an automatic frequency control device in high-frequency circuits. It must be remembered that the capacitance of a varactor changes when as a result of variation in its parallel voltage, the dielectric’s effective thickness, or the insulating layer existing within a diode, changes. Varactors are installed in modern TV sets for the selection of a channel, which functions as tuning. In figure 6.29, different symbols of a varactor have been shown.

Figure 6.29