Like resistors, capacitors can be obtained in the older wire-connected style or more commonly now, as SMD components. Small-value capacitors can be made using thin plates of insulating material (a dielectric) metallized on each side to form the conductors. Thin plates can be stacked and interconnected (Figure-c), to form larger capacitance values up to 1000 pF or more.
Silvered mica (also called silver mica) types were formerly used where high stability of value is important, as in oscillators, but are now quite rare, having been replaced by porcelain types or for some purposes by the C0G/NP0 types. Porcelain has come into use because; unlike mica (a natural material whose specifications can vary wildly) the materials can be manufactured to a tight specification. Porcelain capacitors are found mainly in SMD form, and are used extensively in RF and microwave circuits.
Ceramics are used generally for less critical applications such as RF coupling and decoupling. Ceramic tubular capacitors make use of small ceramic tubes that are silvered inside and outside. Ceramic capacitors have, typically, values that range from 1 pF to 0.22 μF for ceramic disks, and up to 10 μF for multi-layer types (ceramic chips). The scale of values usually follows the E12 values of 1.0, 1.2, 1.5, 1.8, 2.2, 2.7, 3.3, 3.9, 4.7, 5.6, 6.8, 8.2.
Ceramic capacitors are graded into types referred to as C0G, X7R, and Y5V. The letter-number-letter references are used to identify temperature characteristics, using codes that depend on the classification of the dielectric. There are four classes, and the lower the class number the better the performance of the capacitor; the Class 4 types are virtually obsolete. The code for Class 1 dielectrics uses the first symbol to indicate the significant figures of the temperature coefficient in ppm/◦C, the second figure is the multiplier, and the third is the tolerance in ppm/◦C. Typical of this class is the C0G type; the complete coding is shown in Table.
Table Letter-Number Coding Of Ceramic Capacitors
For Class 2 and 3 dielectrics (which include the popular X7R, X5R, Z5U and Y5V types), the code is different (Table 2.3). The first symbol indicates the lower limit of the operating temperature range, the second indicates the upper limit of the operating temperature range, and the third indicates the maximum capacitance change allowed over the operating temperature range.
Table Class 2 and 3 Coding System
The popular C0G types have zero temperature coefficient (usually ±30 ppm/◦C) and have the highest stability and lowest loss of all the ceramic types. The X7R ceramics have higher losses, but are small and cheap, and are obtainable as multilayer types (particularly in SM form). Ceramic chip capacitors use ceramic dielectric materials which have been formed into thin layers with metal film electrodes alternately exposed on opposite edges of the set of laminates. This assembly is then fired at high temperature in absence of oxygen to produce a single block of ceramic, to which metal connections can be made at the opposite edges. The film chip type can be made in high values (up to 4.7 μF), intended particularly for power supply filtering applications where a low effective series resistance (ESR, see later) is desirable.
Rolled capacitors use strips of insulating material as their dielectric. Paper was formerly used, but because the characteristics of paper are so variable, it is much more common to use polyethylene (polythene), polyester, polycarbonate, polypropylene or other plastics films which are metallized and then rolled up (Figure), with another insulating strip to prevent the metallizing on one side shorting against the metallizing on the other side.
Using this construction, quite large capacitance values can be achieved in a small volume and values of up to several μF are common.
Electrolytic capacitors are used when very large capacitance values are needed; the more common type is the aluminum electrolytic. One plate is of aluminum foil in contact with an aluminum perborate solution in the form of a jelly or paste; the other plate is an aluminum container. The insulator is a film of aluminum oxide which forms on the positive plate when a voltage, called the forming voltage, is applied during manufacture.
Because the film of oxide can be very thin, only a few molecules thick, and the surface area of the aluminum foil can be very large, especially if the surface is roughened, very large capacitance values (up to several farads) can be achieved.
The disadvantages of aluminum electrolytic include leakage current (which is high compared to other capacitor types), the need for polarization (the + and − markings must be observed and DC applied) and comparatively low-voltage operation (less significant in transistor and IC circuits, but ruling out the use of electrolytic in high-voltage transmitter circuits). Incorrect polarization can cause the oxide layer to break down and if large currents then flow, as is likely if the capacitor is used as the reservoirin a power supply unit, the capacitor will explode, showering its surroundings with corrosive jelly. Tantalum electrolytic use a solid dielectric and can be used unpolarized (but not necessarily reverse polarized) and have much lower leakage currents than aluminum types, making them more suitable for some applications.
One factor that is quoted for electrolytic to a greater extent than for other types is the ESR, effective series resistance. The ESR is the pure resistance of a capacitor to an AC signal. The significance of this is that if the reactance of a capacitor is very low, its capability for carrying current is reduced, and the heating caused by current is much greater. High ESR values can cause many problems with power supplies, particularly switching power supplies, and can also present problems with time constants and circuit loading. Ultra-low ESR electrolytic are quoted as having ESR of 0.025 W or less, but some electrolytic types can have values of more than 1 W, some even more than 10 W. Many modern applications call for low-ESR capacitors to be used.
The ESR is related to the loss factor for the capacitor. The loss angle (d) for a capacitor is defined as the phase angle between signal current and signal voltage, and the loss factor is the (trigonometric) tangent of the angle, tan d. The relationship between d and ESR is:
Low-impedance electrolytic can be specified for critical tasks, and the choice is between the Sanyo OS-CON aluminum type, which uses an organic semiconducting electrolyte, and the low-ESR type of tantalum electrolytic.
In Figure are shown the shapes of a variety of capacitor types.