Module 1 − Introduction to Matter, Energy, and Direct Current
Figure 3-28. - Reference points in a series circuit.
When point B is used as the reference, as in figure 3-29, point D would be positive
50 volts in respect to the new reference point. The former reference point, A, is
25 volts negative in respect to point B.
Figure 3-29. - Determining potentials with respect to a reference point.
As in the previous circuit illustration, the reference point a circuit is always
considered to be at zero potential. Since the earth (ground) is said to be at a
zero potential, the term Ground is used to denote a common electrical point zero
potential. In figure 3-30, point a is the zero reference, or ground, and the symbol
for ground is shown connected to point A. Point C is 75 volts positive in respect
Figure 3-30. - use ground symbols.
In most electrical equipment, the metal chassis is the common ground for the
many electrical circuits. When each electrical circuit is completed, common points
a circuit at zero potential are connected directly to the metal chassis, thereby
eliminating a large amount connecting wire. The electrons pass through the metal
chassis (a conductor) to reach other points the circuit. An example a chassis grounded
circuit is illustrated in figure 3-31.
Figure 3-31. - Ground used as a conductor.
Most voltage measurements used to check proper circuit operation in electrical
equipment are taken in respect to ground. One meter lead is attached to a grounded
point and the other meter lead is moved to various test points. Circuit measurement
is explained in more detail in NEETS Module 3.
A circuit is said to be OPEN when a break exists in a complete conducting pathway.
Although an open occurs when a switch is used to deenergize a circuit, an open may
also develop accidentally. To restore a circuit to proper operation, the open must
be located, its cause determined, and repairs made.
Sometimes an open can be located visually by a close inspection the circuit components.
Defective components, such as burned out resistors, can usually be discovered by
this method. Others, such as a break in wire covered by insulation or the melted
element an enclosed fuse, are not visible to the eye. Under such conditions, the
understanding the effect an open has on circuit conditions enables a technician
to make use test equipment to locate the open component.
In figure 3-32, the series circuit consists two resistors and a fuse. Notice
the effects on circuit conditions when the fuse opens.
Figure 3-32. - Normal and open circuit conditions. (A) Normal current; (B) Excessive
Current ceases to flow; therefore, there is no longer a voltage drop across the
resistors. Each end the open conducting path becomes an extension the battery terminals
and the voltage felt across the open is equal to the applied voltage (EA).
An open circuit has INFINITE resistance. INFINITY represents a quantity so large
it cannot be measured. The symbol for infinity is ∞. In an open circuit, RT
A short circuit is an accidental path low resistance which passes an abnormally
high amount current. a short circuit exists whenever the resistance a circuit or
the resistance a part a circuit drops in value to almost zero ohms. a short ten
occurs as a result improper wiring or broken insulation.
In figure 3-33, a short is caused by improper wiring. Note the effect on current
flow. Since the resistor has in effect been replaced with a piece wire, practically
all the current flows through the short and very little current flows through the
resistor. Electrons flow through the short (a path almost zero resistance) and the
remainder the circuit by passing through the 10-ohm resistor and the battery. The
amount current flow increases greatly because its resistive path has decreased from
10,010 ohms to 10 ohms. Due to the excessive current flow the 10-ohm resistor becomes
heated. As it attempts to dissipate this heat, the resistor will probably be destroyed.
Figure 3-34 shows a pictorial wiring diagram, rather than a schematic diagram, to
indicate how broken insulation might cause a short circuit.
Figure 3-33. - Normal and short circuit conditions.
Figure 3-34. - Short due to broken insulation.
A meter connected across the terminals a good 1.5-volt battery reads about 1.5
volts. When the same battery is inserted into a complete circuit, the meter reading
decreases to something less than 1.5 volts. This difference in terminal voltage
is caused by the INTERNAL Resistance the battery (the opposition to current offered
by the electrolyte in the battery). All sources electromotive force have some form
internal resistance which causes a drop in terminal voltage as current flows through
This principle is illustrated in figure 3-35, where the internal resistance a
battery is shown as R1. In the schematic, the internal resistance is indicated by
an additional resistor in series with the battery. The battery, with its internal
resistance, is enclosed within the dotted lines the schematic diagram. With the
switch open, the voltage across the battery terminals reads 15 volts. When the switch
is closed, current flow causes voltage drops around the circuit. The circuit current
2 amperes causes a voltage drop 2 volts across R1. The 1-ohm internal battery resistance
thereby drops the battery terminal voltage to 13 volts. Internal resistance cannot
be measured directly with a meter. An attempt to do this would damage the meter.
Figure 3-35. - Effect of internal resistance.
The effect the source resistance on the power output a dc source may be shown
by an analysis the circuit in figure 3-36. When the variable load resistor (RL)
is set at the zero-ohm position (equivalent to a short circuit), current (I) is
calculated using the following formula:
This is the maximum current that may be drawn from the source. The terminal voltage
across the short circuit is zero volts and all the voltage is across the resistance
within the source.
Figure 3-36. - Effect source resistance on power output.
If the load resistance (RL) were increased (the internal resistance remaining
the same), the current drawn from the source would decrease. Consequently, the voltage
drop across the internal resistance would decrease. At the same time, the terminal
voltage applied across the load would increase and approach a maximum as the current
approaches zero amps.
Power Transfer and Efficiency
Maximum power is transferred from the source to the load when the resistance
the load is equal to the internal resistance the source. This theory is illustrated
in the table and the graph figure 3-36. When the load resistance is 5 ohms, matching
the source resistance, the maximum power 500 watts is developed in the load.
The efficiency power transfer (ratio output power to input power) from the source
to the load increases as the load resistance is increased. The efficiency approaches
100 percent as the load resistance approaches a relatively large value compared
with that the source, since less power is lost in the source. The efficiency power
transfer is only 50 percent at the maximum power transfer point (when the load resistance
equals the internal resistance the source). The efficiency power transfer approaches
zero efficiency when the load resistance is relatively small compared with the internal
resistance the source. This is also shown on the chart figure 3-36.
The problem a desire for both high efficiency and maximum power transfer is resolved
by a compromise between maximum power transfer and high efficiency. Where the amounts
power involved are large and the efficiency is important, the load resistance is
made large relative to the source resistance so that the losses are kept small.
In this case, the efficiency is high. Where the problem matching a source to a load
is important, as in communications circuits, a strong signal may be more important
than a high percentage efficiency. In such cases, the efficiency power transfer
only about 50 percent; however, the power transfer would be the maximum
which the source is capable supplying.
You should now understand the basic concepts series circuits. The principles
which have been presented are lasting importance. Once equipped with a firm understanding
series circuits, you hold the key to an understanding the parallel circuits to be
Q25. a circuit has a source voltage 100 volts and two 50-ohm resistors
connected in series. If the reference point for this circuit is placed between the
two resistors, what would be the voltage at the reference point?
Q26. If the reference point in question 25 were connected to ground, what
would be the voltage level the reference point?
Q27. What is an open circuit?
Q28. What is a short circuit?
Q29. Why will a meter indicate more voltage at the battery terminal when
the battery is out a circuit than when the battery is in a circuit?
Q30. What condition gives maximum power transfer from the source to the
Q31. What is the efficiency power transfer in question 30?
Q32. a circuit has a source voltage 25 volts. The source resistance is
1 ohm and the load resistance is 49 ohms. What is the efficiency power transfer?
PARALLEL DC Circuits
The discussion electrical circuits presented up to this point has been concerned
with series circuits in which there is only one path for current. There is another
basic type circuit known as the PARALLEL Circuit with which you must become familiar.
Where the series circuit has only one path for current, the parallel circuit has
more than one path for current.
Ohm's law and Kirchhoff's law apply to all electrical circuits, but the characteristics
a parallel dc circuit are different than those a series dc circuit.
PARALLEL Circuit Characteristics
A PARALLEL Circuit is defined as one having more than one current
path connected to a common voltage source. Parallel circuits, therefore, must contain
two or more resistances which are not connected in series. An example a basic parallel
circuit is shown in figure 3-37.
Figure 3-37. - Example a basic parallel circuit.
Start at the voltage source (Es) and trace counterclockwise around the circuit.
Two complete and separate paths can be identified in which current can flow. One
path is traced from the source, through resistance R1, and back to the source. The
other path is from the source, through resistance R2, and back to the source.
Voltage in a Parallel Circuit
You have seen that the source voltage in a series circuit divides proportionately
across each resistor in the circuit. IN a PARALLEL Circuit, The SAME Voltage Is
PRESENT IN EACH BRANCH. (A branch is a section a circuit that has a complete path
for current.) In figure 3-37 this voltage is equal to the applied voltage (Es).
This can be expressed in equation form as:
ES = ER1 = ER2
Voltage measurements taken across the resistors a parallel circuit, as illustrated
by figure 3-38 verify this equation. Each meter indicates the same amount voltage.
Notice that the voltage across each resistor is the same as the applied voltage.
Figure 3-38. - Voltage comparison in a parallel circuit.
Example: Assume that the current through a resistor a parallel circuit is known
to be 4.5 milliamperes (4.5 mA) and the value the resistor is 30,000 ohms (30 k
). Determine the source voltage. The circuit is shown in figure 3-39.
Figure 3-39. - Example problem parallel circuit.
and Direct Current
|Alternating Current and Transformers
|Circuit Protection, Control, and Measurement
|Electrical Conductors, Wiring Techniques,
and Schematic Reading
|Generators and Motors
|Electronic Emission, Tubes, and Power Supplies
|Solid-State Devices and Power Supplies
|Wave-Generation and Wave-Shaping Circuits
|Wave Propagation, Transmission Lines, and
|Introduction to Number Systems and Logic Circuits
|- Introduction to Microelectronics
|Principles of Synchros, Servos, and Gyros
|Introduction to Test Equipment
|Radio-Frequency Communications Principles
|The Technician's Handbook, Master Glossary
|Test Methods and Practices
|Introduction to Digital Computers
|Introduction to Fiber Optics
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