Types of capacitor

Capacitors are commercially available in different values and types. Typically, capacitors have values in the picofarad (pF) to microfarad (µF) range. They are described by the dielectric material they are made of and by whether they are of fixed or variable type. [Fig. 1] shows the circuit symbols for fixed and variable capacitors. Note that according to the passive sign convention, current is considered to flow into the positive terminal of the capacitor when the capacitor is being charged, and out of the positive terminal when the capacitor is discharging. Like resistors, all capacitors can be included under either of two general headings: fixed or variable. The curved line represents the plate that is usually connected to the point of lower potential.

Fixed Capacitors

Many types of fixed capacitors are available today. Some of the most common are the mica, ceramic, electrolytic, tantalum, and polyester film capacitors.

mica capacitor

: The typical flat mica capacitor consists basically of mica sheets separated by sheets of metal foil. The plates are connected to two electrodes, as shown in [Fig. 2]. The total area is the area of one sheet times the number of dielectric sheets. The entire system is encased in a plastic insulating material as shown for the two central units of [Fig. 3]. The mica capacitor exhibits excellent characteristics under stress of temperature variations and high voltage applications (its dielectric strength is 5000 V/mil). Its leakage current is also very small (1000 MΩ). Mica capacitors are typically between a few picofarads and $0.2 \mu F$, with voltages of 100 V or more.

Ceramic Capacitor

: The ceramic capacitor is made in many shapes and sizes, two of which are shown in [Fig. 4]. A ceramic base is coated on two sides with a metal, such as copper or silver, to act as the two plates. The leads are then attached through electrodes to the plates. An insulating coating of ceramic or plastic is then applied over the plates and dielectric. Ceramic capacitors also have a very low leakage current ( 1000 MΩ) and can be used in both dc and ac networks. They can be found in values ranging from a few picofarads to perhaps $2 \mu F$, with very high working voltages such as $5000 V$ or more.

Electrolytic Capacitor

: The electrolytic capacitor is used most commonly in situations where capacitances of the order of one to several thousand microfarads are required. They are designed primarily for use in networks where only dc voltages will be applied across the capacitor because they have good insulating characteristics (high leakage current) between the plates in one direction but take on the characteristics of a conductor in the other direction. Electrolytic capacitors are available that can be used in ac circuits (for starting motors) and in cases where the polarity of the dc voltage will reverse across the capacitor for short periods of time. The basic construction of the electrolytic capacitor consists of a roll of aluminum foil coated on one side with an aluminum oxide, the aluminum being the positive plate and the oxide the dielectric. A layer of paper or gauze saturated with an electrolyte is placed over the aluminum oxide on the positive plate. Another layer of aluminum without the oxide coating is then placed over this layer to assume the role of the negative plate. In most cases the negative plate is connected directly to the aluminum container, which then serves as the negative terminal for external connections. Because of the size of the roll of aluminum foil, the overall area of this capacitor is large; and due to the use of an oxide as the dielectric, the distance between the plates is extremely small. The negative terminal of the electrolytic capacitor is usually the one with no visible identification on the casing. The positive is usually indicated by such designs as +, △ ,▯, and so on. There are fundamentally two types of tantalum capacitors: the solid and the wet-slug. In each case, tantalum powder of high purity is pressed into a rectangular or cylindrical shape, as shown in [Fig. 6]. Next the anode (+) connection is simply pressed into the resulting structures, as shown in the figure. The resulting unit is then sintered (baked) in a vacuum at very high temperatures to establish a very porous material. The result is a structure with a very large surface area in a limited volume. Through immersion in an acid solution, a very thin manganese dioxide ($MnO_2$) coating is established on the large, porous surface area. An electrolyte is then added to establish contact between the surface area and the cathode, producing a solid tantalum capacitor. If an appropriate "wet" acid is introduced, it is called a wet-slug tantalum capacitor.

Polyester-film capacitor

: The last type of fixed capacitor to be introduced is the polyester-film capacitor, the basic construction of which is shown in [Fig. 7]. It consists simply of two metal foils separated by a strip of polyester material such as Mylar. The outside layer of polyester is applied to act as an insulating jacket. Each metal foil is connected to a lead that extends either axially or radially from the capacitor. The rolled construction results in a large surface area, and the use of the plastic dielectric results in a very thin layer between the conducting surfaces. Data such as capacitance and working voltage are printed on the outer wrapping if the polyester capacitor is large enough. Color coding is used on smaller devices. A band (usually black) is sometimes printed near the lead that is connected to the outer metal foil. The lead nearest this band should always be connected to the point of lower potential. This capacitor can be used for both dc and ac networks. Its leakage resistance is of the order of $100 MΩ$. An axial lead and radial lead polyester-film capacitor appear in [Fig. 8]. The axial lead variety is available with capacitance levels of $0.1 \mu F$ to $18 \mu F$, with working voltages extending to $630 V$. The radial lead variety has a capacitance range of $0.01 \mu F$ to $10 \mu F$, with working voltages extending to $1000 V$.

Variable Capacitors

The most common of the variable-type capacitors is shown in [Fig. 8]. The dielectric for each capacitor is air. The capacitance in [Fig. 8(a)] is changed by turning the shaft at one end to vary the common area of the movable and fixed plates. The greater the common area, the larger the capacitance. The capacitance of the trimmer capacitor in [Fig. 8(b)] is changed by turning the screw, which will vary the distance between the plates (the common area is fixed) and thereby the capacitance.