A capacitor is an electrical device that stores energy in an electric field. The effect of a capacitor, a passive component with two terminals, is known as capacitance. Capacitance is the ratio of change in electric charge of a system to the corresponding change in its electric potential. There are many different types of capacitors, ranging from very small, delicate trimming capacitors used in oscillators of radio circuits, to large metal-can type capacitors found in high voltage power correction circuits. Generally speaking, there are three categories of capacitors: dielectric, ceramic, and electrolytic. This blog will explain each of them, their subtypes, and what makes them unique.
Dielectric Capacitors are variable capacitors used when a variation of capacitance is required for tuning transmitters, receivers, and transistor radios. Variable capacitors feature a set of fixed plates and a set of movable plates which move in between the stationary plates. The position of the moving plates relative to the fixed plates determines the capacitance value. Within the dielectric capacitor category, there are three types of capacitors: film capacitors, radial lead capacitors, and axial lead capacitors.
Film capacitors are the most commonly available type of capacitors, comprising many capacitors with their only differences being in their dielectric properties. Film capacitors also come in many shapes and styles. The other types of dielectric capacitors are radial lead type and axial lead type capacitors. Radial lead capacitors consist of a rectangular outer case with a dielectric layer, metal-film electrodes, and wire leads that connect to the electrodes. Axial lead types feature dielectric material sandwiched by strips of metal foil which is then wound into a tight roll and sealed to a wire.
Ceramic capacitors, also known as disc capacitors, are made by coating two sides of a small porcelain or ceramic disc with silver and subsequently stacking them together. This type of capacitor has a very high dielectric constant and is configured such that a high capacitance can be obtained by a relatively small capacitor. Ceramic capacitors exhibit large, non-linear changes in capacitance relative to temperature. This, combined with the fact that they are non-polarized devices, make them popular for use as decoupling or bypass capacitors. While ceramic capacitors have values ranging from very few picofarads to one or two microfarads, their voltage ratings are generally low.
This type of capacitor has a 3-digit code printed onto their body to signify their capacitance value expressed in picofarads. The first two digits indicate the capacitors value while the final digit represents the number of zeros to be added. So, if a ceramic capacitor has ‘154’ printed on it, it would mean the capacitor has a value of 150,000 picofarads, or 150 nanofarads. Additionally, some capacitors may feature a letter code to symbolize their tolerance values. For example, J = 5% resistance, K = 10% resistance, M = 20%, etc.
Electrolytic capacitors are used when very large capacitance values are required. In electrolytic capacitors, a semi-liquid electrolyte solution serves as the second electrode, rather than a very thin metallic film layer. The dielectric is a very thin layer of oxide which is grown electro-chemically with the thickness of the film being less than ten microns. This insulating layer is so thin that even the smallest of capacitors can have large capacitance values due to the distance between the plates.
Most types of electrolytic capacitors are polarized, meaning the direct current voltage applied to the capacitor must be a compatible polarity. All polarized electrolytic capacitors are clearly marked to denote their polarity. Electrolytic capacitors are commonly used in direct current power supply circuits due to their large capacitance and small size, which reduces the ripple voltage in both coupling and decoupling applications.
There are two main types of electrolytic capacitors: aluminum and tantalum. Aluminum electrolytic capacitors are capacitors whose anode electrode is made from aluminum foil with an etched surface. The aluminum creates a thin layer of insulation that acts as the dielectric of the capacitor. Tantalum capacitors consist of a pellet of porous tantalum metal as an anode, covered by an oxide layer surrounded by liquid or solid electrolyte, forming the dielectric. Because of its very thin and high permittivity dielectric layer, the tantalum capacitor distinguishes itself from other conventional capacitors through its high capacitance and low weight.
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