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386 ⏐⏐⏐ CAPACITORS
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FIG. 10.15
Electrolytic capacitors: (a) Radial lead with
extended endurance rating of 2000 h at 85°C.
Capacitance range: 0.1–15,000 mF with a
voltage range of 6.3 to 250 WV dc (Courtesy
of Illinois Capacitor, Inc.). (b) Solid aluminum
electrolytic capacitors available in axial,
resin-dipped, and surface-mount configurations
to withstand harsh environmental
conditions (Courtesy of Philips Components,
Inc.).
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. 10.16.
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.
Cathode ( – )
Solder
MnO2 coat
Carbon
Solder
Tantalum
FIG. 10.16
Tantalum capacitor. (Courtesy of Union Carbide Corp.)
The last type of fixed capacitor to be introduced is the polyester-film
capacitor, the basic construction of which is shown in Fig. 10.17. 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.
Metal foils
Tantalum wire
Polyester (plastic) film
FIG. 10.17
Polyester-film capacitor.
Lead wire
Anode ( + )
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 (see Appendix D). 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