146 Engineering Fundamentals
Adding more branches to a parallel circuit causes
the total resistance to decrease. Resistance in a
parallel circuit can be expressed as:
Rt =
1
1
+
1
+
1
+…+
1
R1 R2 R3 RN
Series-Parallel Combination Circuits
Series-parallel combination circuits are not
simply series or parallel. They are a combination
of both types of circuits. Therefore, the rules and
formulas for solving a series or parallel circuit
cannot be used. In order to study a series-parallel
circuit, the parallel parts must be broken down
and studied as if they were series elements.
Figure 8-13 shows a series-parallel circuit with
three lights where the fi rst light is in series and
the other two lights are in parallel. The light in
series would be much brighter because it receives
roughly twice the current of each of the other two.
Circuit Components
Electrical engineers must fully understand
each component that can be used in a circuit.
Electrical engineers often design, build, and trou-
bleshoot electrical circuits. They must know how
each component of the circuit works so they can
select the best parts to solve a given design prob-
lem or troubleshoot a circuit that is not working
properly.
Conductors
Conductors provide a path for current to
fl ow to the parts that will control and use it.
Materials with very low resistance are conduc-
tors. They easily share electrons from one atom to
the next when a charge is applied. Copper is the
most common conductor. Aluminum is not quite
as good as copper, but is lighter and less expen-
sive. Aluminum is often used in thick service
entrance cables that bring electricity into houses.
Silver and gold are better conductors than copper
and aluminum, but are only used in very specifi c
applications due to their cost.
Conductors come in a wide variety of confi g-
urations. They can be solid wire, stranded wire,
ribbon, and bar shapes based on their intended use.
For example, solid copper wire is commonly used in
residential wiring because it does not need to move
once it is installed. If a solid conductor is bent back
and forth too many times, it will break like a paper
clip that is bent repeatedly. Stranded copper wire is
used in extension cords because it bends easily and
will not break over time.
Conductors are sized based on their cross-
sectional area. The size of round conductors
is determined using the American Wire Gage
(AWG) system. In the AWG system, smaller
numbers represent larger wire. For instance,
12-gage wire is larger than 30-gage wire. 14-gage
wire is commonly used in houses for general
lighting and receptacle circuits. Figure 8-14
shows common wire types.
3 V
This schematic shows a series-parallel lighting circuit.
Figure 8-13.
Goodheart-Willcox Publisher
Common solid, stranded, one-conductor, two-conductor,
and three-conductor wire types are shown.
Figure 8-14.
Goodheart-Willcox Publisher