Types of Capacitors

Capacitors are generally classified according to the kind of dielectric used in them. They may be divided into the following four groups:

• Capacitors that use vacuum, air, or other gases.
• Capacitors in which the dielectric is mineral oil or castor oil.
• Capacitors in which the dielectric is a combination of solid and liquid dielectrics such as paper, films of synthetic materials, glass, mica, etc., and mineral oil, castor oil, silicone, nitrobenzene, chlorinated diphenyl, etc.
• Capacitors with strictly solid dielectric such as glass, mica, titanium oxide, etc.

The first group of capacitors are used in applications where values of capacitance required do not have to be large, but where the energy loss in the dielectric must be very small. Energy losses in dielectrics are discussed in Section 2-22. The dielectric losses in vacuum, air, or other gases, are negligible. Some applications for these capacitors are in radio-frequency circuits and in low-frequency measuring circuits where great precision is required. A precision variable air capacitor is shown in Fig. 2-20.

Oil-insulated capacitors are used in applications where larger values of capacitance are required than can be obtained readily with the first group and where a small amount of dielectric power loss can be tolerated. Paper capacitors with liquid impregnation are used in applications where precise values of capacitance are unimportant but where large amounts of capacitance can be obtained in a relatively small volume. In these applications dielectric strength and long life are also factors. One application of this type of capacitor is correcting power factor in electric power distribution systems as discussed in Section 2-17.

Mica has excellent properties as a dielectric: it has a high dielectric constant and high insulation resistance, and is affected but little by time and temperature. The dielectric losses are relatively low for a solid material and there is very little drift in the dielectric constant. Capacitors with mica as a dielectric are extensively used in laboratories as standards because of their stability and the relatively high values of capacitance for a given volume.

Figure 2-20. Precision variable air capacitor, type 1422. (Courtesy of General Radio Company.)

There are other types of capacitors not included in the above groups. One of these is the electrolytic capacitor, which has an oxide film formed on aluminum. This film is the dielectric and is exceedingly thin, making for very large values of capacitance in relatively small volumes. This film may have a thickness as small as 10^-5 cm. The aluminum serves as one of the plates and the electrolyte as the other plate. The electrolyte employed in wet electrolytic capacitors is an aqueous solution of boric acid. Dry electrolytic capacitors have electrolytes consisting of solutions in which the water content is small. Solutions of polyhydric alcohols such as ethylene glycol, boric acid, and small quantities of ammonium are electrolytes generally used in the dry electrolytic capacitors.

Electrolytic capacitors have certain limitations. The aluminum must be the positive electrode and the electrolyte the negative electrode. If the polarity is reversed the film becomes conducting. This arrangement is suitable for service where steady or pulsating unidirectional potentials are applied. However, dry electrolytic capacitors can be made to operate on alternating current. Such capacitors consist of two anode foils immersed in an electrolyte that is common to both. This is equivalent to operating two electrolyte capacitors in series back to back with the negative terminals connected together.

Capacitors that use ceramics for dielectrics have been classified by the Radio-Electronics-Television Manufacturers Association into Class I and Class II dielectrics as shown in Table 2-3.

Table 2-3. Classification of ceramic dielectrics for capacitors RETMA Standard REC-107-A, Ceramic Dielectric Capacitors, MIL-C-11015A.
	Class I	Class II
K range (k' at room temperature)	6 to 500	500 to 10000
Temperature coefficient of capacitance	P120 to N5600
Power factor	0.04 to 0.4%	0.4 to 3%
Insulation resistance	10000 to 100000 MΩ	10000 to 100000 MΩ
Minimum capacitance decrease (-55 to +85°C)	0 to 40%	15 to 80%
Maximum capacitance increase (-55 to + 85°C)	0 to 40%	0 to 25%
Aging Characteristics of k		Approximately 4 % per decade time

Mineral rutile TiO₂ and combinations of titanium oxide with other oxides are the ceramic materials generally used in these types of capacitors. The Class I dielectrics are used in resonant circuits and other applications where a low dissipation factor, which is a measure of the dielectric energy loss as explained in Section 2-22, or a high Q, which is the reciprocal of the dissipation factor, and good stability are required. These materials have dielectric constants that are highly sensitive to temperature and can be used as temperature compensators in electronic circuits.

The Class II dielectrics, because of their extremely high values of dielectric constant, have found their greatest application where general purpose capacitors of very small dimensions are needed. The applications, however, must be such that changes in capacitance with temperature are not too critical.