A diode (fig. 1-8) is a device that will allow current
to pass through itself in only one direction. A diode can
be thought of as an electrical check valve. Diodes are
constructed by joining N-type material and P-type
material together. The negative electrical terminal is
located on the N-type material and the positive
terminal is located on the P-type material.
When a diode is placed in a circuit, the N-material is
connected to the negative side of the circuit and the
positive side of the circuit is connected to the P-material.
In this configuration, which is known as forward bias,
the diode is a good conductor. This is because the
positively charged holes in the P-type material move
toward the junction and fill these holes using them to
move across the P-material. If the connections to the
diodes are reversed, current flow will be blocked. This
design is known as reverse bias. When the diode is
connected backwards, the positively charged holes are
attracted away from the junction to the negative
terminal and the free electrons in the N-material are
attracted away from the junction to the positive
terminal. Without the presence of holes at the junction,
the electrons are not able to cross it.
A zener diode (fig. 1-9) is a special type of diode
that conducts current in the reverse direction as long as
the voltage is above a predetermined value that is built
into the device during manufacturing. For instance, a
certain zener diode may not conduct current if the
reverse bias voltage is below 6 volts. As the voltage
Figure 1-8.Diode operation.
increases to 6 volts or more, the diode suddenly will
begin to conduct reverse bias current. This device is
used in control circuits, such as voltage regulators.
A transistor (fig. 1-10) is an electrical device that is
used in circuits to control the flow of current. It
operates by either allowing current to flow or not
allowing it to flow. Transistors operate electronically
and have no moving parts to perform their function.
This design allows for a longer operating life of the
component. The major automotive applications of
transistors are for electronic ignition systems and
The PNP transistor (fig. 1-11) is the most common
design in automotive applications. It is manufactured
by sandwiching an N-type semiconductor element
between two P-type semiconductor elements. A
positive charge is applied to one of the P-type
elements. This element is called the emitter. The other
P-type element connects to the electrical component.
This element is called the collector. The third element,
which is in the middle, is made of N-type material and
is called the base. The application of low current
negative charge to the base will allow a heavy current
to flow between the emitter and the collector.
Whenever the current to the base is switched off, the
current flow from the emitter to the collector is
The NPN transistor (fig. 1-11) is similar to the PNP
transistor. The difference is that it is used in the
negative side of the circuit. As the term NPN implies,