DC CIRCUIT TERMINOLOGY Basic DC Theory

DC CIRCUIT TERMINOLOGY

Before operations with DC circuits can be studied, an understanding of the types

of circuits and common circuit terminology associated with circuits is essential.

EO 1.4 Given a diagram, IDENTIFY it as one of the following

types:
a. Schematic diagram
b. One-line diagram
c. Block diagram
d. Wiring diagram

EO 1.5 DEFINE the following terms:

a. Resistivity
b. Temperature coefficient of resistance
c. Closed circuit
d. Open circuit
e. Short circuit
f. Series circuit
g. Parallel circuit
h. Equivalent resistance

EO 1.6 Given a circuit, DETERMINE whether the circuit is an

open circuit or a closed circuit.

Schematic Diagram

Schematic diagrams are the standard means by which we communicate information in electrical
and electronics circuits. On schematic diagrams, the component parts are represented by graphic
symbols, some of which were presented earlier in Module 1. Because graphic symbols are small,
it is possible to have diagrams in a compact form. The symbols and associated lines show how
circuit components are connected and the relationship of those components with one another.

As an example, let us look at a schematic diagram of a two-transistor radio circuit (Figure 9).
This diagram, from left to right, shows the components in the order they are used to convert
radio waves into sound energy. By using this diagram it is possible to trace the operation of the
circuit from beginning to end. Due to this important feature of schematic diagrams, they are
widely used in construction, maintenance, and servicing of all types of electronic circuits.

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Figure 9 Schematic Diagram

One-Line Diagram

The one-line, or single-line, diagram shows the components of a circuit by means of single lines
and the appropriate graphic symbols. One-line diagrams show two or more conductors that are
connected between components in the actual circuit. The one-line diagram shows all pertinent
information about the sequence of the circuit, but does not give as much detail as a schematic
diagram. Normally, the one-line diagram is used to show highly complex systems without
showing the actual physical connections between components and individual conductors.

As an example, Figure 10 shows a typical one-line diagram of an electrical substation.

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Figure 10 One-Line Diagram

Block Diagram

A block diagram is used to show the relationship between component groups, or stages in a
circuit. In block form, it shows the path through a circuit from input to output (Figure 11). The
blocks are drawn in the form of squares or rectangles connected by single lines with arrowheads
at the terminal end, showing the direction of the signal path from input to output. Normally, the
necessary information to describe the stages of components is contained in the blocks.

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Figure 11 Block Diagram

Wiring Diagram

A wiring diagram is a very simple way to show wiring connections in an easy-to-follow manner.
These types of diagrams are normally found with home appliances and automobile electrical
systems (Figure 12). Wiring diagrams show the component parts in pictorial form, and the
components are identified by name. Most wiring diagrams also show the relative location of
component parts and color coding of conductors or leads.

Figure 12 Wiring Diagram

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DC CIRCUIT TERMINOLOGY Basic DC Theory

Resistivity

Resistivity is defined as the measure of the resistance a material imposes on current flow. The
resistance of a given length of conductor depends upon the resistivity of that material, the length
of the conductor, and the cross-sectional area of the conductor, according to Equation (2-1).

L
R = ρ (2-1)
A

where

R = resistance of conductor, Ω

ρ = specific resistance or resistivity cm-Ω/ft

L = length of conductor, ft

A = cross-sectional area of conductor, cm

The resistivity ρ (rho) allows different materials to be compared for resistance, according to their
nature, without regard to length or area. The higher the value of ρ, the higher the resistance.

Table 1 gives resistivity values for metals having the standard wire size of one foot in length and
a cross-sectional area of 1 cm.

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TABLE 1
Properties of Conducting Materials

ρ = Resistivity
Material at 20°C-cm-Ω/ft (a)
Aluminum 17
Carbon (b)
Constantan 295
Copper 10.4
Gold 14
Iron 58
Nichrome 676
Nickel 52
Silver 9.8
Tungsten 33.8

(a) Precise values depend on exact composition of material.

(b) Carbon has 2500-7500 times the resistance of copper.

Temperature Coefficient of Resistance

Temperature coefficient of resistance, α (alpha), is defined as the amount of change of the

resistance of a material for a given change in temperature. A positive value of α indicates that
R increases with temperature; a negative value of α indicates R decreases; and zero α indicates
that R is constant. Typical values are listed in Table 2.

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TABLE 2
Temperature Coefficients for Various Materials

Temperature
Material Coefficient, Ω per °C

Aluminum 0.004
Carbon -0.0003
Constantan 0 (avg)
Copper 0.004
Gold 0.004
Iron 0.006
Nichrome 0.0002
Nickel 0.005

For a given material, α may vary with temperature; therefore, charts are often used to describe
how resistance of a material varies with temperature.

An increase in resistance can be approximated from equation (2-2).

Rt Ro Ro(α∆T) (2-2)

where

Rt = higher resistance at higher temperatures

Ro = resistance at 20oC

α = temperature coefficient

∆T = temperature rise above 20oC

Electric Circuit

Each electrical circuit has at least four basic parts: (1) a source of electromotive force,
(2) conductors, (3) load or loads, and (4) some means of control. In Figure 13, the source of
EMF is the battery; the conductors are wires which connect the various component parts; the
resistor is the load; and a switch is used as the circuit control device.

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Figure 13 Closed Circuit

A closed circuit (Figure 13) is an uninterrupted, or unbroken, path for current from the source
(EMF), through the load, and back to the source.

An open circuit, or incomplete circuit, (Figure 14) exists if a break in the circuit occurs; this
prevents a complete path for current flow.

Figure 14 Open Circuit

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A short circuit is a circuit

which offers very little
resistance to current flow
and can cause dangerously
high current flow through a
circuit (Figure 15). Short
circuits are usually caused
by an inadvertent connection
between two points in a
circuit which offers little or
no resistance to current flow.
Shorting resistor R in Figure
15 will probably cause the
fuse to blow.

Figure 15 Short Circuit

Series Circuit

A series circuit is a circuit where there is only one path for current flow. In a series circuit
(Figure 16), the current will be the same throughout the circuit. This means that the current flow
through R1 is the same as the current flow through R2 and R3.

Figure 16 Series Circuit

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Parallel Circuit

Parallel circuits are those circuits which have two or more components connected across the
same voltage source (Figure 17). Resistors R1, R2, and R3 are in parallel with each other and the
source. Each parallel path is a branch with its own individual current. When the current leaves
the source V, part I1 of IT will flow through R1; part I2 will flow through R2; and part I3 will flow
through R3. Current through each branch can be different; however, voltage throughout the
circuit will be equal.

V = V1 = V2 = V3.

Figure 17 Parallel Circuit

Equivalent Resistance

In a parallel circuit, the total resistance of the resistors in parallel is referred to as equivalent
resistance. This can be described as the total circuit resistance as seen by the voltage source.
In all cases, the equivalent resistance will be less than any of the individual parallel circuit
resistors. Using Ohm’s Law, equivalent resistance (REQ) can be found by dividing the source
voltage (V) by the total circuit current (IT), as shown in Figure 17.

V
REQ
It

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Summary

The important information concerning basic DC circuits is summarized below.

DC Circuit Terminology Summary

There are four types of circuit diagrams.

- Schematic diagram
- One-line diagram
- Block diagram
- Wiring diagram

Resistivity is defined as the measure of the resistance a material imposes on

current flow.

Temperature coefficient of resistance, α (alpha), is defined as the amount of

change of the resistance of a material for a given change in temperature.

A closed circuit is one that has a complete path for current flow.

An open circuit is one that does not have a complete path for current flow.

A short circuit is a circuit with a path that has little or no resistance to current
flow.

A series circuit is one where there is only one path for current flow.

A parallel circuit is one which has two or more components connected across
the same voltage source.

Equivalent resistance is the total resistance of the resistors in parallel.

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