Capacitors are used in almost every electronic circuit today. Capacitors are manufactured in the millions every day, but there are many different capacitor types available.
Each type of capacitor has its own advantages and disadvantages and can be used in different applications – in various types of electronic circuit design, RF design, etc. Accordingly, it is essential to know a little about each capacitor type in order to choose the right one for any use, application, electronic circuit design, etc.
It is important not only to determine the correct capacitor value but also which capacitor type is appropriate for the particular circuit design. Each type of capacitor has its own properties and some may be suitable for low-frequency circuit designs, others for power supplies, and some for RF designs.
Types of Capacitors Based on Construction
- Parallel Plate Capacitor
- Spherical capacitor
- Cylindrical capacitor
Parallel Plate Capacitor
Parallel plate capacitors are formed by an arrangement of electrodes and an insulating material or dielectric. A parallel plate capacitor can store only a limited amount of energy before the dielectric breaks down.
When two parallel plates are connected to a battery, the plates are charged and an electric field is established between them, and this setup is known as a parallel plate capacitor.
A parallel plate capacitor consists of two metal plates of area, A, and separated by a distance d. The top plate is given a charge of +Q and the bottom plate is given a charge of -Q. A potential difference of V is developed between the plates.
The separation is too small compared to the dimensions of the plate to ignore the effect of outward bending of the electric field lines at the edges and the non-uniformity of the surface charge density at the edges.
The magnitude of the charge density on each plate of a parallel plate capacitor is σ It happens.
σ = Q/A
By Gauss’ law, E = Q/ε0A
Also, E = V/d
Now taking the area outside the capacitor, due to surface charges,
This result is valid for the vacuum between the capacitor plates. For other mediums, then the capacitance will be
where k is the dielectric constant of the medium,
If there is a vacuum between the plates, k=1.
A spherical capacitor consists of a solid or hollow circular conductor surrounded by another hollow concentric sphere of a different radius.
A spherical capacitor consists of two concentric spherical spheres. Let the radius of the inner sphere, Rin = a, and the radius of the outer sphere, Rout = b. The inner shell is given a positive charge +Q and the outer shell is given -Q.
The potential difference,
The cylindrical capacitor consists of a hollow or a solid cylindrical conductor surrounded by a concentric hollow circular cylinder. Capacitors are widely used in electric motors, flour mills, electric juicers, and other electrical equipment.
Consider a solid cylinder of radius r. surrounded by a cylindrical shell, b. The length of the cylinder is l and is large enough from a–b to avoid edge effects. The capacitors are charged so that the charge on the inner cylinder is +Q and that on the outer cylinder is -Q.
By Gauss’ law,
Where λ = Q/l, linear charge density
The potential difference of a cylindrical capacitor is given by,
where we have chosen the integration path to be in the direction of the electric field lines. As expected, an external conductor with a negative charge has a lower potential. that gives
Once again, we see that the capacitance C depends only on the geometries L, A, and B.
Types of Capacitors Based on Structure
- Fixed Capacitors
- Variable Capacitor
A fixed capacitor is a capacitor that stores a certain amount of electric charge (capacitance) and is not adjustable at any instance. Their prices are mostly fixed during construction. A stationary capacitor helps to maintain a constant charge and energy output in electrical equipment or devices.
Generally, fixed capacitors are described in terms of the type of dielectric material used to make them. Some of them include paper capacitors, plastic capacitors or plastic film capacitors, ceramic capacitors, mica capacitors, electrolytic capacitors, and supercapacitors.
Fixed capacitors are classified into two main types
- Polarized Capacitors
- Non-polarized Capacitors.
Polarized capacitors are those that have specific positive and negative polarities. When using these capacitors in circuits, it should always be kept in mind that they are connected in perfect poles.
Electrolytic capacitors are those capacitors that indicate by the name that some electrolyte has been used in it. They are polarized capacitors that have anode + and cathode – with particular polarity.
A metal on which an insulating oxide layer is formed by anodizing is called the anode. A solid or non-solid electrolyte that covers the surface of the oxide layer acts as the cathode. Electrolytic capacitors have much higher capacitance-voltage CV values than others, because of their larger anode surface and thinner dielectric oxide layer.
Aluminum Electrolytic Capacitors
Aluminum electrolytic capacitors are the most common type of electrolytic capacitors. In these, pure aluminum foil with an etched surface serves as the anode. A thin layer of metal, with a thickness of a few micrometers, is placed between the two metals to electrically isolate them, acting as a diffusion barrier. Hence the diffusion barrier acts as a dielectric. The electrolyte acts as a cathode covering the rough surface of the oxide layer.
There are three types of aluminum electrolytic capacitors based on the electrolyte. they are –
- Wet Aluminum Electrolytic capacitors.
- Manganese dioxide Aluminum Electrolytic capacitors.
- Polymer Aluminum Electrolytic capacitors.
The main advantage of these aluminum electrolytic capacitors is that they have low impedance values even at the mains frequency and are cheap. They are mostly used in power supply circuits, SMPS switched mode power supplies, and DC-DC converters.
Tantalum Electrolytic capacitors
These are another type of electrolytic capacitor whose anode is made of tantalum on which a very thin insulating oxide layer is formed. This layer acts as a dielectric and the electrolyte acts as a cathode covering the surface of the oxide layer.
Tantalum provides a high permeability dielectric layer. Tantalum has high capacitance and low weight per volume. But due to the frequent unavailability of tantalum, these are costlier than aluminum electrolytic capacitors.
Niobium Electrolytic Capacitors
A niobium electrolytic capacitor is another type of electrolytic capacitor in which an inert niobium metal or niobium monoxide is treated as the anode and an insulating niobium pentoxide layer is added to the anode so that it acts as a dielectric. A solid electrolyte is placed on the surface of the oxide layer which acts as the cathode.
Niobium Capacitors are commonly available as SMD SurfaceMountDevices chip capacitors. These easily fit into a PCB. These capacitors must be operated in full polarity. Any type of reverse voltage or ripple current greater than specified will eventually destroy the dielectric and capacitor as well.
High capacitance electrochemical capacitors with very high capacitance values as compared to other capacitors are called super capacitors. These can be classified as a group that lies between electrolytic capacitors and rechargeable batteries. These are also called ultra capacitors.
These capacitors have many advantages such as –
- They have a high capacitance value.
- They can collect and disburse charges very fast.
- They can handle more charge and discharge cycles.
Double-layered capacitors are electrostatic capacitors. The charge deposition in these capacitors is done according to the principle of the double layer.
- When all solids disperse in a liquid, the surface layer has a negative charge.
- This is due to the high dielectric coefficient of the liquid.
- All the positive ions come near the surface of the solid to form the skin.
- The deposition of positive ions near the solid material becomes looser with distance.
- The charge created due to the deposition of ions and cations on this surface leads to some capacitance value.
This double-layer phenomenon is also called the Helmholtz double layer.
These capacitors are simply called Electric Double Layered Capacitor EDLC. They use carbon electrodes to achieve the separation of charge between the conductive electrode and the surface of the electrolyte. Carbon acts as dielectric and the other two as anode and cathode. The separation of charge is much smaller than in a conventional capacitor.
These capacitors follow the electrochemical process for the deposition of charge. It is also called the Faradaic process. At an electrode, when a chemical substance is reduced or oxidized, some current is generated. During such a process, these capacitors store electric charge by electron transfer between the electrode and the electrolyte. This is the working principle of the pseudo capacitor.
They charge very fast and store as much charge as a battery does. Their operation is fast. These are used in conjunction with the battery to improve life. These are used in grid applications to handle power fluctuations.
A hybrid capacitor is a combination of an EDLC and a pseudo capacitor. In hybrid capacitors, activated carbon is used as the cathode and the pre-doped carbon material acts as the anode. Li-ion capacitors are a common example of this type.
They have a high tolerance for a wide range of temperature variations from -55 °C to 200 °C. Hybrid capacitors are also used in aerial applications. Although the cost is high, these capacitors are highly reliable and compact. These are rugged and can tolerate extreme shocks, vibrations, and pressure from the environment. Hybrid capacitors have higher energy density and higher specific power than any electrolytic capacitor.
These are capacitors that have no specific polarity, meaning they can be connected in a circuit without bothering about the placement of the right lead and left lead in any way. These capacitors are also called non-electrolytic capacitors.
Ceramic capacitors are fixed capacitors that have a ceramic material as a dielectric.
These ceramic capacitors are classified as Class 1 and Class 2 based on their applications. For example, 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 a silver compound on both walls so that both sides act as two capacitor plates and the ceramic acts as a dielectric. Is. Lead is drawn from these two surfaces and this entire assembly is covered in a moisture-proof coating.
The most commonly 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 1ηF to 100µF.
Film capacitors are those that contain a film material as a dielectric material. Based on 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 waxed paper whereas a metallic film capacitor uses metalized paper.
Paper capacitors use paper as a dielectric material. Two thin tin foil sheets are taken and placed between sheets of thin waxed or oiled paper. This paper acts as a dielectric. Nowadays paper has been replaced by plastic.
These sheets are sandwiched and rolled into a cylindrical shape and placed in a plastic enclosure. Leads are removed.
Paper capacitors are available in the order of 0.001μF to 2μF and voltage ratings can be as high as 2000 volts. These capacitors are useful in high voltage and current applications.
Metal Film Capacitors
Metal film capacitors are another type of film capacitor. These are also called metal foil capacitors or metalized paper capacitors because the dielectric used here is paper coated with a metal film.
Unlike paper capacitors, these metal film capacitors have a film of aluminum or zinc coated on the paper to form a dielectric. Instead of the aluminum sheet placed between the papers, the coating is applied directly on the paper itself. This reduces the size of the capacitor.
The aluminum coating is preferred over zinc coating to avoid the destruction of capacitors due to chemical reduction. The aluminum-coated sheet is rolled into a cylinder and lead is taken. This whole thing is covered with wax or plastic resin to protect the capacitor. These capacitors are useful in high voltage and current applications.
These are various capacitors named after the dielectric material used. This group includes mica capacitors, air capacitors, vacuum capacitors, glass capacitors, etc.
Mica capacitors are made by using thin mica sheets as the dielectric material. Like paper capacitors, thin metal sheets are sandwiched in between sheets of mica. Finally, layers of metal sheets are joined at both ends and two leads are formed. The entire assembly is then encased in a plastic Bakelite capsule.
Mica capacitors are available in the range of 50pF to 500pF. Mica capacitors have a high working voltage of up to 500 volts. These are the most commonly used capacitors for electronic circuits such as ripple filters, resonant circuits, coupling circuits, and high power, high current RF broadcast transmitters.
Air capacitors are those in which the air is dielectric. The simplest air capacitors are those with an air plate in between. This construction is exactly the same as the variable tuning capacitor discussed above. These capacitors can be fixed and variable as well but fixed are rarely used as there are others with better characteristics.
Vacuum capacitors use high vacuum as the dielectric instead of air or any other material. These are also available in fixed and variable modes. The construction of these capacitors is similar to that of a vacuum tube. They are mostly seen as glass cylinders with inter-lattice concentric cylinders.
Variable vacuum capacitors are available in the range of 12pF to 5000pF and are used for high voltage applications such as 5kV to 60kV. They are used in core equipment such as high-power broadcast transmitters, RF amplifiers, and large antenna tuners.
Glass capacitors are very specialized with many advantages and applications. Like all the above types, here glass is a dielectric material. Glass dielectrics, as well as aluminum electrodes, are present in these capacitors. The encapsulation of the plastic is done after the lead has been removed. Leads can be axial leads or tubular leads.
Glass capacitors have many advantages such as −
- The temperature coefficient is low.
- These are noise-free capacitors.
- They produce high-quality output with low losses.
- They have the ability to handle 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 in high-temperature areas.
- Used in circuits that require high Q.
- Used in high power handling circuits.
- Used for circuits that require high tolerances.
Variable capacitors whose value changes when you change them, either electrically or mechanically. Variable capacitors in general consist of interwoven sets of metal plates with one constant and the other variable. These capacitors provide capacitance values so as to vary between 10 and 500pF.
A ganged capacitor 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 intrinsically linked.
These variable resistors have many uses such as tuning in LC circuits of radio receivers, impedance matching in antennas, etc.
Variable capacitors are classified into two main types
- Tuning Capacitors.
- Trimmer Capacitors.
Tuning capacitors are a popular type of variable capacitor. They consist of a stator, a rotor, a frame to support the stator, and a mica capacitor.
The stator is a stationary part and the rotor rotates at the speed of a moving shaft. When the rotor plates are moved into the stator slots, they come close to forming the capacitor plates. When the rotor plates are fully seated in the slot of the stator the capacitance values are maximum and when they are not, the capacitance values are minimum.
A gang tuning capacitor consists of two tuning capacitors connected in a gang. This is how a tuning capacitor works. These capacitors typically have capacitances ranging from a few pico farads to a few tens of pico farads. These are mostly used in LC circuits in radio receivers. These are also called tuning condensers.
Trimmer capacitors are varied using a screwdriver. Trimmer capacitors are usually installed where the capacitance value does not need to be changed once fixed.
A trimmer capacitor has three leads, one connected to the stationary plate, one to the rotary, and the other to the common. The movable disc is of a semi-circular shape.
There are two parallel conducting plates with a dielectric in between. Depending on this dielectric used, there are air trimmer capacitors and ceramic trimmer capacitors.
One of the two plates is movable, while the other is stationary. The dielectric material is fixed. When the movable plate is moved against the area between the movable and stationary electrodes, the capacitance can be changed. If the opposite field becomes larger the capacitance will be greater, as both the electrodes act as two plates of a capacitor.
Trimmer capacitors are easily fixed on a PCB Printed Circuit Board and are mostly used for calibration of equipment.