A capacitor is a device for storing electric charge. In effect, it is a reservoir into which charge can be deposited and then later extracted.
A capacitor consists, basically, of two conductors separated by a dielectric. The operation of a capacitor in a circuit is dependent upon its ability to charge and discharge. When a capacitor charges, an excess of electrons is accumulated on one plate, and a deﬁciency of electrons is created on the other plate. Capacitance is determined by the size of the conductive material (the plates) and their separation (determined by the type and thickness of the dielectric material).
Capacitors can be classified as fixed and variable capacitors.
Film capacitors can be classified as paper and plastic film capacitors.
Paper film capacitors
In the early days of radio, capacitors were commonly made by placing paper, soaked with mineral oil, between two strips of foil, rolling the assembly up, attaching wire leads to the two pieces of foil, and enclosing the rolled-up foil and paper in a cylindrical case.
These capacitors can still sometimes be found in electronic equipment. They have values ranging from about 0.001 µF to 0.1 µF, and can handle low to moderate voltages, usually up to about 1000 V.
Various different plastics make good dielectrics for the manufacture of capacitors. Polyester, polyethylene, and polystyrene are commonly used. The substance called mylar that you might have seen used to tint windows makes a good dielectric for capacitors.
The method of manufacture is similar to that for paper capacitors when the plastic is flexible. Stacking methods can be used if the plastic is more rigid. The geometries can vary, and these capacitors are therefore found in several different shapes.
Capacitance values for plastic-film units range from about 50 pF to several tens of microfarads. Most often they are in the range of 0.001 µF to 10 µF. Plastic capacitors are employed at audio and radio frequencies, and at low to moderate voltages. The efficiency is good, although not as high as that for mica-dielectric or air-dielectric units.
Porcelain is another material that works well as a dielectric. Sheets of metal are stacked alternately with wafers of ceramic to make these capacitors. Ceramic, like mica, has quite low loss, and therefore allows for high efficiency.
For low values of capacitance, just one layer of ceramic is needed, and two metal plates can be glued to the disk-shaped porcelain, one on each side. This type of component is known as a disk-ceramic capacitor. Alternatively, a tube or cylinder of ceramic can be employed, and metal ink applied to the inside and outside of the tube. Such units are called tubular capacitors.
Ceramic capacitors have values ranging from a few picofarads to about 0.5 µF. Their voltage ratings are comparable to those of paper capacitors. Since ceramic capacitors are relatively brittle, they may be damaged by shock and vibration.
All of the above-mentioned types of capacitors provide relatively small values of capacitance. They are also nonpolarized, meaning that they can be hooked up in a circuit in either direction. An electrolytic capacitor provides considerably greater capacitance than any of the above types, but it must be connected in the proper direction in a circuit to work right. Therefore, an electrolytic capacitor is a polarized component. Electrolytic capacitors can be classified as aluminum, tantalum, and niobium capacitors.
Aluminum electrolytic capacitors
Aluminum electrolytic capacitors assume a special position among the various types of capacitors since their principle of operation relies, in part, on electrochemical processes. The advantages of aluminum electrolytic capacitors that have led to their wide application range are their high volumetric efficiency (i.e. capacitance per unit volume), which enables the production of capacitors with up to one Farad capacitance, and the fact that an aluminum electrolytic capacitor provides a high ripple current capability together with high reliability and an excellent price/performance ratio.
As is the case with all capacitors, an aluminum electrolytic capacitor comprises two electrically conductive material layers that are separated by a dielectric layer. One electrode (the anode) is formed by an aluminum foil with an enlarged surface area. The oxide layer (Al2O3) that is built upon this is used as the dielectric. In contrast to other capacitors, the counter electrode (the cathode) of aluminum electrolytic capacitors is a conductive liquid, the operating electrolyte. A second aluminum foil, the so-called cathode foil, serves as a large-surfaced contact area for passing current to the operating electrolyte.
Tantalum electrolytic capacitors
Another type of electrolytic capacitor uses tantalum rather than aluminum. The tantalum can be foil, as is the aluminum in a conventional electrolytic capacitor. It might also take the form of a porous pellet, the irregular surface of which provides a large area in a small volume. An extremely thin oxide layer forms on the tantalum. Tantalum capacitors have high reliability and excellent efficiency. They are often used in military applications because they do not fail often and they can perform well in low temperatures. They can be used in audio-frequency and digital circuits in place of aluminum electrolytic.
A niobium capacitor is a polar capacitor that makes a dielectric the pent-oxidization niobium formed in the sintering body surface side of niobium metal powder. It has a small and large capacitance and flame-retardant features. Moreover, it is an environmental-friendly component that meets lead-free soldering.
Supercapacitors can be classified as electrochemical double layer, pseudo, and hybrid capacitors.
Electrochemical double layer capacitors
Electrochemical double-layer capacitors (EDLCs) are constructed from two carbon-based electrodes, an electrolyte, and a separator. Like conventional capacitors, EDLCs store charge electrostatically, or non-Faradaically, and there is no transfer of charge between electrode and electrolyte. EDLCs utilize an electrochemical double-layer of charge to store energy. As voltage is applied, charge accumulates on the electrode surfaces. Following the natural attraction of unlike charges, ions in the electrolyte solution diffuse across the separator into the pores of the electrode of opposite charge. However, the electrodes are engineered to prevent the recombination of the ions. Thus, a double-layer of charge is produced at each electrode. These double-layers, coupled with an increase in surface area and a decrease in the distance between electrodes, allow EDLCs to achieve higher energy densities than conventional capacitors.
In contrast to EDLCs, which store charge electrostatically, pseudocapacitors store charge Faradaically through the transfer of charge between electrode and electrolyte. This is accomplished through electrosorption, reduction-oxidation reactions, and intercalation processes. These Faradaic processes may allow pseudo capacitors to achieve greater capacitances and energy densities than EDLCs. There are two electrode materials that are used to store charge in pseudocapacitors, conducting polymers and metal oxides.
Hybrid capacitors attempt to exploit the relative advantages and mitigate the relative disadvantages of EDLCs and pseudocapacitors to realize better performance characteristics. Utilizing both Faradaic and non-Faradaic processes to store charge, hybrid capacitors have achieved energy and power densities greater than EDLCs without the sacrifices in cycling stability and affordability that have limited the success of pseudocapacitors. Research has focused on three different types of hybrid capacitors, distinguished by their electrode configuration: composite, asymmetric, and battery-type respectively.
Mica capacitors are very old in the electronics field and have reached a fairly stable state of development. This material makes an excellent dielectric for capacitors.
Mica capacitors can be made by alternately stacking metal sheets and layers of mica, or by applying silver ink to the sheets of mica. The metal sheets are wired together into two meshed sets, forming the two terminals of the capacitor.
Mica capacitors have low loss; that is, they waste very little power as heat, provided their voltage rating is not exceeded. Voltage ratings can be up to several thousand volts if thick sheets of mica are used. But mica capacitors tend to be large physically in proportion to their capacitance. The main application for mica capacitors is in radio receivers and transmitters. Their capacitances are a little lower than those of paper capacitors, ranging from a few tens of picofarads up to about 0.05 µF.
Glass dielectric capacitors have been the capacitors of choice for extreme long-term stability and reliability for almost fifty years. They are available in glass or glass composition. You will occasionally find glass capacitors in RF and transmitting equipment due to their low loss. They cannot be sintered like ceramic so this limits the capacitance that can be obtained. They have a 5% tolerance.
Feedthrough capacitors are used for EMI suppression of all electrical installations and equipment. They are also outstandingly well suited for telephone exchanges and base stations. Thanks to broadband EMI suppression up to the GHz range, they prevent external interference pulses from being transmitted to the inside of the equipment via the supply network and vice versa.
In feedthrough capacitors, the conductor carrying the operating current is connected concentrically to one electrode and is run centrally through the capacitor. The other electrode is contacted concentrically to the capacitor case. Feedthrough capacitors are dimensioned so that they perform well from low frequencies to far above 300 MHz.
Semiconductors. These materials, in their many different forms, have revolutionized electrical and electronic circuit design in the past several decades.
These materials can be employed to make capacitors. A semiconductor diode conducts current in one direction and refuses to conduct in the other direction. When a voltage source is connected across a diode so that it does not conduct, the diode acts as a capacitor. The capacitance varies depending on how much of this reverse voltage is applied to the diode. The greater the reverse voltage, the smaller the capacitance. This makes the diode a variable capacitor. Some diodes are manufactured to serve this function. Their capacitances fluctuate rapidly along with pulsating dc. These components are called varactor diodes or simply varactors.
Capacitors can be formed in the semiconductor materials of an integrated circuit (IC) in much the same way. Sometimes, IC diodes are fabricated to serve as varactors. Another way to make a capacitor in an IC is to sandwich an oxide layer into the semiconductor material, between two layers that conduct well.
You have probably seen ICs in electronic equipment; almost any personal computer has dozens of them. They look like little boxes with many prongs.
Semiconductor capacitors usually have small values of capacitance. They are physically tiny and can handle only low voltages. The advantages are miniaturization, and an ability, in the case of the varactor, to change in value at a rapid rate.
Capacitors can be varied in value by adjusting the mutual surface area between the plates, or by changing the spacing between the plates. The two most common types of variable capacitors (besides varactors) are the air variable and the trimmer. You might also encounter coaxial capacitors.
By connecting two sets of metal plates so that they mesh, and by affixing one set to a rotatable shaft, a variable capacitor is made. The rotatable set of plates is called the rotor, and the fixed set is called the stator. This is the type of component you might have seen in older radio receivers, used to tune the frequency. Such capacitors are still used in transmitter output tuning networks.
Air variables have a maximum capacitance that depends on the number of plates in each set, and also on the spacing between the plates. Common maximum values are 50 pF to about 1,000 pF; minimum values are a few picofarads. The voltage-handling capability depends on the spacing between the plates; some air variables can handle many kilovolts.
Air variables are used primarily at radio frequencies. They are highly efficient, and are nonpolarized, although the rotor is usually connected to common ground (the chassis or circuit board).
When it is not necessary to change the value of a capacitor very often, a trimmer might be used. It consists of two plates, mounted on a ceramic base and separated by a sheet of mylar, mica, or some other dielectric. The plates are “springy” and can be squashed together more or less by means of a screw.
Sometimes two sets of several plates are interleaved to increase the capacitance. Trimmers can be connected in parallel with an air variable so that the range of the air variable can be adjusted. Some air-variable capacitors have trimmers built-in. Typical maximum values for trimmers range from a few picofarads up to about 200 pF. They handle low to moderate voltages and are highly efficient. They are nonpolarized.
Recall from the previous chapter that sections of transmission lines can work as inductors. They can act as capacitors too.
If a section of a transmission line is less than 1/4 wavelength long and is left open at the far end (rather than shorted out), it will act as a capacitor. The capacitance will increase with length.
The most common transmission-line capacitor uses two telescoping sections of tubing. This is called a coaxial capacitor and works because there is a certain effective surface area between the inner and the outer tubing sections. A sleeve of plastic dielectric is placed between the sections of tubing. This allows the capacitance to be adjusted by sliding the inner section in or out of the outer section.
Coaxial capacitors are used in radio-frequency applications, particularly in antenna systems. Their values are generally from a few picofarads up to about 100 pF.