In the previous tutorial we learned about the practical construction of a capacitor and the different technical aspects associated with a standard capacitor. Let us now discuss the different types of commercial capacitors and the practical technical specifications associated with them. This will help immensely in choosing the appropriate capacitor for a particular application. Capacitors are classified based on the dielectric material used by them as follows:
1) Paper
2) Mica
3) Plastic Film
4) Glass
5) Ceramics
6) Electrolytic
7) Semiconductor
8) Variable
Paper Capacitors
Paper capacitors are one of the earliest types of capacitors. They are made by placing paper soaked in mineral oil between two sheets of aluminum. The entire set is rolled up, the wires are attached to the aluminum sheets and the set is enclosed in a cylindrical cardboard box and sealed with wax.
Paper capacitors are bulky and have almost been replaced by plastic film capacitors. They can still be found in some electrical applications – most ceiling fans and air coolers still use paper capacitors. These capacitors have the following technical specifications:
Mica & Metallized Mica Capacitors
Mica capacitors use sheets of mica as a dielectric and are generally constructed as multi-plate capacitors. Various mica capacitors use silver-dyed mica sheets as the dielectric for better immunity to moisture and ionization. Mica capacitors are known for their low tolerance (up to 1%), low operating losses (dissipation factor of 0.001%), high quality factor and high frequency stability. However, these capacitors are quite bulky, proportional to their capacitance. Mica capacitors have the following technical specifications:
Plastic Film Capacitors
Film capacitors include many families of capacitors that use different plastics as dielectric materials. They have almost replaced paper capacitors in audio circuits, radio circuits, and circuits that operate at low to moderate voltages. Some of the plastics commonly used in film capacitors include polycarbonate, polyester (PET), polypropylene (PP), polystyrene, polysulfone, parylene, Kapton polyamide, Teflon (fluorocarbon PTFE), and metallized polyester (metallized plastic). These capacitors come in various geometries such as oval or round wrap and fill, rectangular epoxy case, round epoxy case, hermetically sealed metal rectangular or round case, and radial or axial conductors. Some of the popular families of plastic film capacitors are described below.
1) Polycarbonate Film (PC) – These capacitors are known for their long service life, low operating losses over a wide range of frequencies and have almost replaced polycarbonate capacitors. These capacitors have the following technical specifications:
2) Polyester Film (PET) – These capacitors are popular for their low cost and small size. They have mainly replaced metallized plastic and polystyrene capacitors in low-frequency DC and AC applications. These capacitors can be used with voltages up to 60,000 V, but are not recommended for high power applications due to significant operating losses at high temperatures and frequencies. These capacitors have the following technical specifications:
3) Polypropylene (PP) Film – These are the most popular plastic film capacitors and are known for their low temperature coefficient, low operating losses, low moisture absorption, and stability at high temperature and high power. However, these capacitors are subject to damage and breakdown due to transient overvoltages and voltage inversions. They have the following technical specifications:
4) Polystyrene Film (PS) – These capacitors are known for their narrow capacitance and high stability and are quickly being replaced by PET capacitors. These capacitors have the following technical specifications:
5) Polysulfone Film – Polysulfone capacitors are similar to polycarbonate capacitors and are known for their thermal stability. There are limited manufacturers of these capacitors, which are not readily available. These capacitors have the following technical specifications:
6) Kapton Polyimide Film – These capacitors have the highest dielectric strength and can have operating temperatures of up to 250°C. However, these high-cost capacitors are not suitable for RF applications. These capacitors have the following technical specifications:
7) Teflon Film (PTFE) – These bulky and expensive capacitors are best known for their low operating losses, wide temperature range, and excellent stability in critical applications. These capacitors have the following technical specifications:
8) Metallized Plastic – These capacitors are improved variations of PET, PP, PEN, PPS and Teflon capacitors. They are smaller in size compared to their non-metallic plastic versions. The only limitation of these capacitors is their low ripple current ratings.
9) Foil – These are metal film versions of PP, PET and Teflon capacitors. They are known for pulse stability and better ripple current ratings.
There are many other types and variations of plastic and plastic film capacitors, including film-based power capacitors. Popular film-based power capacitors include metallized paper capacitors, paper film/foil capacitors (Kraft paper), metallized polypropylene capacitors (single- or double-side metallized PP), PP film/foil capacitors, and MKV power (polypropylene as a dielectric between metallized papers as conductors) Capacitors.
Glass Capacitors
These expensive capacitors are best known for their robustness, stability, accuracy, and reliability in extreme environmental conditions. They are even resistant to nuclear radiation and are best suited for military-grade applications. These capacitors have the following technical specifications:
Ceramic Capacitors
Ceramic capacitors refer to a wide range of capacitors available such as disc capacitors, MLC (Multilayer Ceramic) capacitors and SMD capacitors. The composition of these capacitors varies depending on the manufacturers. Some commonly used materials in the construction of ceramic capacitors include strontium titanate, titanium oxide, barium titanate, etc. The Electronics Industries Alliance (EIA) has classified ceramic capacitors into the following three classes:
Class 1 Ceramic Capacitors – These are the best ceramic capacitors that offer tighter tolerance, thermal stability and low operational losses, and are best suited for high frequency applications. These capacitors are composed of titanium oxide with other additives such as Magnesium, Cobalt, Calcium, Zinc, Niobium, etc. These capacitors have their own EIA codes to specify their nominal capacitance and tolerance. These capacitors have a high dielectric constant ranging from 5 to 150. All the advantages of Class 1 ceramic capacitors are in the cost of their size. These capacitors have the following technical specifications:
Class 2 Ceramics – Class 2 ceramic capacitors are less stable and accurate than Class 1 capacitors. However, these capacitors are smaller compared to the latter. These also have their own EIA codes to specify their capacitance and tolerance. These capacitors have the following technical specifications:
Class 3 Ceramics – These capacitors are the least stable and accurate among ceramic capacitors, with other disadvantages such as non-linear temperature dependence, voltage dependence and high operational losses. They have a short lifespan and cannot withstand high voltages. However, they have high permittivity (up to 50,000) and are compact (dimensions of a few millimeters). Other types of capacitors have already replaced Class 3, and the EIA no longer standardizes them.
Electrolytic Capacitors
Electrolytic capacitors are polarized capacitors that offer high capacitance per unit volume. Since these capacitors are polarized, they must be connected in a circuit with the correct polarity. They have a terminal as an anode, a metal plate coated with metal oxide; a liquid or solid electrolytic serves as the cathode. When DC current flows through the electrolytic capacitor, the metal plate starts to oxidize due to an electrolytic. A thin insulating layer of metal oxide, which is a dielectric, is deposited on it. The metal oxide layer is extremely thin, so it offers very high capacitance per unit volume. Generally, these capacitors are designed to maximize the anode surface area.
When connecting these capacitors in reverse polarity, the electrolyte emits gas, which expands in the sealed capacitor body and can cause an explosion. These capacitors have a significant leakage current, which makes them unsuitable for many applications. Electrolytic capacitors are available in the following three families:
1) Aluminum Electrolytic – These capacitors use aluminum foil as the anode and aluminum oxide as the dielectric medium. They may have ethylene glycol, dimethylacetamide, gamma-butyrolactone or dimethylformamide as the liquid electrolyte, manganese dioxide or conductive polymer as the solid electrolyte. These capacitors have the following technical specifications:
2) Tantalum Electrolytic – These capacitors use Tantalum as the anode and Tantalum Oxide as the dielectric medium. They can have sulfuric acid, manganese dioxide or conductive polymer as the electrolytic and are highly reliable, efficient and smaller than aluminum electrolytic types. They are often used in military applications due to their stability and reliability. These capacitors have the following technical specifications:
3) Niobium Electrolytic Capacitors – These capacitors use niobium as the anode, niobium pentoxide as the dielectric medium, and manganese dioxide or conductive polymer as the electrolytic. They are available as SMD capacitors and have the following technical specifications:
Semiconductor Capacitors
Semiconductor capacitors are nanoscale capacitors manufactured into integrated circuits by forming patterns of oxide layers sandwiched between semiconductor materials. These capacitors are packaged as capacitor arrays and are mainly used in memory and processor ICs. Their tiny capacitances can handle low, pulsating DC voltages.
Variable Capacitors
The capacitance of variable capacitors can be varied by changing the distance between the conducting plates or by changing the mutual surface area between the overlapping plates. Common types of variable capacitors are as follows:
1) Air Variable (Air Gap Trimmer) – These variable capacitors have a rotating set of plates called a rotor and a fixed set of plates called a stator. Capacitance is varied by rotating a control shaft that varies the distance or surface area between the plates. These capacitors can have a capacitance of a few Picofarads to 000 Picofarads and voltage ratings of up to thousands of volts. These non-polarized capacitors were commonly used in RF and audio circuits. Varactor diodes have almost replaced these capacitors.
2) Ceramic Trimmer – Class 1 ceramic capacitors are designed to provide variable capacitance. They are designed to set capacitance that does not need to be changed very often. Its capacitance can vary from a few Picofarads to 200 Picofarads. Due to their non-polarization, they can handle low to moderate voltages and are commonly used together with air variable capacitors.
3) Coaxial – These are transmission line capacitors that provide capacitance between an internal and external tube of a coaxial transmission line. Its capacitance can be varied by sliding the inner tube into or out of the outer tube. These capacitors are used in radio frequency circuits and can have capacitance from a few Picofarads to 100 Picofarads.
4) Air gap adjustment – These are circular or logarithmic variable capacitors that use rotor and stator as conductors and air as a dielectric medium. These bulky and expensive variable capacitors are used in commercial electrical devices.
5) Vacuum adjustment – These are similar to air gap adjustment capacitors and use vacuum as a dielectric medium. These bulky and expensive variable capacitors are used in high power RF applications.
6) FC 6 Gas Filled Tuning – These capacitors are similar to air gap tuning capacitors and use SF 6 gas as the dielectric medium for better efficiency and low losses.
The next article discusses selecting a capacitor for a given application and reading capacitor packages.