Components 123voltage from a battery, cannot be passed through a capacitor, but an AC signal, say it’s
coming from transistor radio’s earpiece socket, will pass through a capacitor. A resistor,
by comparison, will pass both AC and DC signals by the same amount.
In practical circuit situations there are many instances in which the AC signal has to be
passed but the DC component needs to be blocked. One such instance is when a power
amplifi er’s signal is fed to a speaker. You’ll always see a capacitor feeding the signal to
the speaker. Another area in which you’ll always notice the presence of capacitors is at the
input and output of AC amplifi ers. Capacitors are measured in units of farads, but because
these are very large units, the much smaller units of pico-, nano-, and microfarads are
most often used. A picofarad is 10 ^12 farads, a nanofarad is 10 ^9 farads, and a microfarad
is 10 ^6 farads. The conversion between the units is such that 1 pF equals 10 ^6 μ F.
Remember the simple LED circuit we discussed, the one with the resistor acting as the
current limiting device? If the resistor were replaced by a capacitor, the LED would not
function because no DC current would be allowed to pass through. Capacitors have a
property that is equivalent to DC resistance; they have AC resistance or reactance. The
capacitor’s reactance is calculated in ohms (like that of the resistor), and it is a function
of the frequency of the signal under consideration. The capacitive reactance is inversely
proportional to frequency; that is, as the frequency increases, the reactance decreases.
Capacitors can be broken into two basic categories based on their physical structure: the
simple nonpolarized type, which is also small in size and small in electrical value (i.e.,
capacitance), and the larger polarized type, with higher associated capacitance values.
Figure 4.3 shows the two basic types. Figure 4.4 shows the series, parallel combinations.
Figure 4.5 shows the axial, radial types.
104
100 uF100 uF electrolytic
capacitor0.1 uF disc
ceramic capacitor
Figure 4.3 : Disc and electrolytic capacitors.