The basic structure of a capacitor consists of two metal plates separated by a layer of dielectric.
Capacitors can be fixed capacitors or variable capacitors.
Electrolytic capacitors, otherwise called polarized capacitors, are the most frequently used capacitor type.
Capacitors are the most frequently used electronic component after resistors. A capacitor is a passive component that is used to store electric energy for a short period of time. The ability of the capacitor to hold electric charge is called capacitance and is measured in Farads. Like resistors, capacitors can be arranged in series or parallel combinations, and thus effective capacitance can be varied. There are several types of capacitors that have been developed for use in electronic circuits. Let’s take a look at these types and their functions.
Capacitors are a commonly-used element in all branches of electrical and electronics engineering. The basic structure of a capacitor consists of two metal plates separated by a layer of dielectric. The capacitor terminals are taken out from the metal plates for external connections. The capacitance of the structure can be given by the following equation:
Factors Influencing Capacitance
The capacitance of a capacitor can be varied by changing the following parameters:
- Area of the plate: The capacitance increases by increasing the area of the plate.
- Overlap of the plates: As the parallel plate overlap area increases, the capacitance value increases.
- Distance between the plates: The closer the proximity of the parallel plates, the greater the capacitance value.
- The nature of the dielectric: Choosing a dielectric material of high permittivity increases the capacitance value.
Usually, thin plastic films are used as the dielectric between the thin metal foil plates, which brings the plates closer and increases the capacitance. The whole arrangement is encapsulated in plastic to make the structure rigid.
There are several types of capacitors; capacitors can be fixed or variable capacitors. In variable capacitors, the capacitance value can be intentionally changed using electronic or mechanical means. Variable capacitors are often used for radio tuning. Compared to variable capacitors, the capacitors whose value cannot be changed by any means are called fixed capacitors. Fixed capacitors can be further classified into the following types.
Types of Fixed Capacitors
- Paper Capacitor: In paper capacitors, the metal foil layers are interleaved with paper impregnated with oil or wax. These types of capacitors are widely used in power circuits, especially in home appliances.
- Mica Capacitor: The dielectric losses associated with mica capacitors are very low, which is why these capacitors are used in high-frequency circuits. The cost of mica capacitors is high compared to other capacitor types. There are two arrangements available for mica capacitors:
- Mica and metal foils are arranged in alternate layers and clamped tightly together.
- On the ends of a mica sheet, thin films of silver are sputtered.
- Polyester Capacitor: Two thin polyester films that are metalized at one end are rolled together to form a structure similar to a paper capacitor. The terminal connected to the outer metal foil electrode is marked by a black band and should be connected to a lower working potential. The polyester capacitor uses color-coding similar to resistors. These capacitors are suitable for high voltage applications.
- Ceramic Capacitor: A ceramic capacitor is made by coating a metal at either end of a thin ceramic dielectric material. Several of these ceramic layers are stacked together and are separated from each layer using more ceramic. The layers are connected by metal electrodes and are taken out as terminal leads. The ceramic capacitor value varies from 1pF to about 1µF, with a working voltage rating of up to a few thousand volts. These capacitors are suitable for high-temperature applications. Ceramic capacitors are classified into:
- Class 1 Ceramic Capacitor: This type of ceramic capacitor uses ceramic materials that are not sensitive to temperature changes. Typically, the capacitance value is less with high stability and low losses regardless of the temperature. These types of ceramic capacitors are commonly used in high-frequency circuits in TV and radio tuners, oscillators, and filters.
- Class 2 Ceramic Capacitor: Ceramic materials derived from barium titanate (with permittivity equal to 6000+) that are temperature sensitive are used in class 2 ceramic capacitors. These capacitors offer larger capacitance with small surface-mount packages. They are suitable for coupling, bypass, and buffer applications.
- Class 3 Capacitor: Ceramic capacitors offer higher volumetric efficiency than class 2 ceramic capacitors. However, class 3 ceramic capacitors offer poor temperature stability, accuracy, and aging over time compared to their counterparts.
- Polycarbonate Capacitor: A polycarbonate capacitor with a very stable dielectric material offers wide tolerance. It can operate from -55°C to +125°C and offers good insulation resistance and dissipation factors. This capacitor is a part of thermoplastic polymer capacitors.
- Electrolytic Capacitor: Electrolytic capacitors, otherwise called polarized capacitors, are the most frequently used capacitor type in electrical and electronics engineering applications. Electrolytic capacitors are polarity sensitive. They consist of an anode with an insulating oxide layer forming the dielectric of the capacitor. The thickness of oxide layer formation is so thin that the value of ‘d’ in the above equation becomes small, making the capacitance value high. Electrolytic capacitors are available at a high voltage rating and are typically used for coupling and decoupling applications. Depending on the dielectric material, electrolytic capacitors can be categorized as:
- Tantalum electrolytic capacitors with tantalum pentoxide as a dielectric.
- Aluminum electrolytic capacitors with aluminum oxide as a dielectric.
- Niobium electrolytic capacitors with niobium pentoxide as a dielectric.
The list of capacitor types is even longer with other variants such as polystyrene film capacitors, PTFE film capacitors, silicon capacitors, and supercapacitors. When choosing any of these capacitor types for a given application, you need to look into factors such as stability, cost, precision, leakage, temperature, and size.
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