Page 422 - Robot Builder's Bonanza

CAPACITORS 391

Polarity 104Z
marking - - - 10 μF
Electrical charge
between plates

Typical electrolytic Typical disc
capacitor capacitor
Figure 31- 13 Component outline and
or or
schematic symbols for two popular styles of
capacitors. Capacitors may be polarized
or nonpolarized. When using polarized
capacitors, there will be a polarity marking
Polarized Non-polarized
on the body of the component. Be sure to
Capacitor schematic symbols properly orient the capacitor in the circuit.

Capacitors come in many more sizes, shapes, and varieties than resistors, though only a
small handful are truly common. However, most all capacitors are made of the same basic
stuff: two or more conductive elements are separated by an insulating material called the
dielectric (see Figure 31- 13).
This dielectric can be composed of many materials, including air, paper foil, epoxy, plastic,
even oil. When you select a capacitor for a particular job, you must generally also specify the
dielectric. The most common are summarized in Table 31- 3, later in this chapter, along with
their common uses.

HOW CAPACITORS ARE RATED
Capacitors have two important ratings:

• Capacitance. Capacitance is the ability of the component to hold a charge. The larger the
capacitance, the longer the charge is retained.
• Dielectric breakdown voltage. At higher voltages the dielectric becomes partially or com-
pletely electrically conductive and the capacitor no longer functions as it should. The
capacitor must be used below this voltage.

Capacitance is measured in farads. The farad is a large unit of measurement, so the bulk
of capacitors available today are rated in microfarads; one microfarad is a millionth of a
farad.
When the capacitor is under 1 microfarad, its value may be shown as a decimal point
number— for example, 0.1 for one- tenth of a microfarad. Or it may be shown as a nanofarad.
A nanofarad is a thousandth of a microfarad— that 0.1 microfarad capacitor is instead listed
as 100 nanofarad. Same value, different way of expressing it. An even smaller unit of measure
is the picofarad, or a millionth of a microfarad.
The “micro- ” in the term microfarad is most often represented by the Greek “mu” ( )
character, as in 10 F, or 10 microfarads. Keeping up with the shorthand, the nanofarad is
nF, and the picofarad is pF.

31-chapter-31.indd 391 4/21/11 11:56 AM