150 Engineering Fundamentals
Diodes
Standard diodes are designed to allow
current fl ow in only one direction. They can be
used as rectifi ers to change alternating current to
direct current. Diodes have two electrodes called
the anode and the cathode. The anode is made of a
positive (P-type) semiconductor material and the
cathode is made of a negative (N-type) semicon-
ductor material. For the diode to allow current to
fl ow, the correct polarity must be applied. Polarity
is the condition of being positive or negative with
respect to ground. The negative power supply
terminal must be connected to the cathode (–)
and the positive terminal must be connected to
the anode (+). This is called forward bias. Current
will fl ow in forward bias. If the negative power
supply terminal were connected to the anode and
the positive to the cathode, a reverse bias condi-
tion would exist. Current cannot fl ow in reverse
bias. See diodes being used in a bridge rectifi er in
Figure 8-18. Bridge rectifi ers change ac to dc.
Zener Diodes
Zener diodes conduct electricity in reverse
bias. They block current until the voltage reaches
a certain level. Once this level is reached, zener
diodes conduct and help to keep the voltage at a
constant level. Because of this characteristic, they
are often used as voltage regulators, which help
to smooth out variations in voltage.
Transistors
Transistors can be used as solid-state
switches and as amplifi ers. They are called solid-
state switches because they perform a switching
function with no moving parts. Bipolar transis-
tors have three junction points: emitter, base,
and collector. Current fl owing between the emit-
ter and collector can be controlled by a current
delivered to the base. Transistors can also be used
as amplifi ers. Amplifi ers increase the power of a
signal, most commonly an audio signal. A small
amount of current applied to the base can create
a gain in collector/emitter current. Figure 8-19
shows a PNP and an NPN transistor.
Capacitors
Capacitors have the ability to store and
discharge electricity very quickly. They can store
much less electricity by volume than a battery,
but they can discharge and recharge much more
quickly. Capacitors can smooth out (fi lter) varia-
tions in voltage and can block continuous dc fl ow
while allowing pulses to pass. Capacitors can
smooth out variations in voltage by absorbing the
spikes and fi lling in the valleys. For example, the
internal workings of computers require dc, but
wall outlets supply ac. Inside of computers, ac is
rectifi ed, or changed, to dc. This dc voltage can
vary a great deal. Capacitors are used to smooth
out or fi lter the voltage.
Goodheart-Willcox PublisherhbliP G dh t Will
Figure 8-18.
Schematic of a bridge
rectifier, which converts
ac to dc.