Chapter 3 Introduction to Basic Electrical Circuit Materials 61
economics. If a low number of boards are to be produced,
hand assembly is less costly. If thousands of boards are to
be produced and assembled, automated systems are used.
Chassis
In the early years of electronics, components were
mounted on metallic surfaces called chassis. The chassis
itself served as part of the circuitry. Use of a chassis as a
conductor is not as common today, but it is still in great
use. One of the most visible uses of the chassis as a con-
ductor is in the automobile. The metal frame and body
parts serve as a chassis providing a conductor path for the
negative side (ground) of the circuit. See Figure 3-13.
The positive line leaves the positive battery terminal
and connects to a device such as the car’s headlamps. A
fuse or circuit breaker is in series with the lamps for pro-
tection. A switch is also in series for turning the lamps on
and off. The return to the negative side of the battery is
through the frame or car chassis itself. The negative
conductor for the lamp simply terminates at the lamp
location, connecting to the metallic shell of the car. The
electrons return to the battery through the metallic frame
of the car.
4. Connection of devices to a PCB is usually made by
__________.
5. What other types of transportation could use the
frame for part of the electrical circuit?
3.3 COMMON CIRCUIT DEVICES
There are several common circuit devices that are
present in most electrical and electronic circuits. They
provide a means of controlling electron flow through the
conductor paths and provide for safe operation of circuits.
The three important items discussed here are switches,
connectors, and circuit protection devices.
Switches
Switches are installed in circuits to control the flow
of electrons through the circuit. They can be categorized
by their actuator and electrical switching path. The
actuator is the mechanical device that causes the circuit
to open and close. Various types of switches are illus-
trated in Figure 3-14. The schematic symbol associated
with each type of switch is also included.
Some of the most common actuators are the slide,
toggle, rotary, and push button. As you can see, the name
of the switch indicates the type of actuator used to turn
the circuit on and off. The electrical circuit inside the
switch is described in terms of poles and throws. The
simplest type of switch is the single-pole single-throw
switch, which is abbreviated SPST. The term single-pole
means that the switch provides one path for the electron
flow and that it can be turned on or off. The term
single-throw means that the switch controls only one
circuit.
A single-pole double-throw switch (SPDT) has one
common connection point and can complete a circuit path
to two different circuits. However, only one circuit can be
completed at a time. There are many possibilities and
combinations for a switch of this type.
A useful application of the single-pole double-throw
switch is its ability to control a load, such as a lamp, from
two different locations. In residential wiring systems, the
single-pole double-throw switch is referred to as a
three-way switch. In Figure 3-15, two single-pole
double-throw switches are connected to a lamp. Either
switch is capable of turning the lamp on or off.
A double-pole double-throw switch (DPDT) has two
common connection points and can provide two circuit
paths simultaneously. The DPDT switch is like having
two SPST switches connected in parallel.
Negative side of the
car battery is connected
to the chassis.
The taillight
circuit is completed
by connecting the
return wire to the
chassis.
The break pedal acts as a switch
to complete the circuit between
the battery and taillight.
Figure 3-13. The frame of the car is used as the return
path for the taillight circuit. This type of wiring is referred to
as chassis wiring.
Review Questions for Section 3.2
1. A temporary circuit is usually wired using a
__________.
2. The most economical way to mass-produce an elec-
tronic circuit is using __________.
3. The automobile is an example of __________
wiring.