Introduction to Alternating Current
(AC)
Online Resource for ETCH 213
Faculty: B. Allen
The right-angle triangle
C
B
A
Pythagorean Theorem
C 2 = A2 + B 2
Online Resource for ETCH 213
Faculty: B. Allen
C = A +B
2
A= C −B
2
B= C −A
2
2
2
2
Online Resource for ETCH 213
Faculty: B. Allen
Sin = Opposite side / hypotenuse
Cos
= Adjacent side / hypotenuse
Tan
= Opposite side / adjacent side
Online Resource for ETCH 213
Faculty: B. Allen
Direct Current (dc) – Current flow in only one
direction
Alternating Current (ac) – Electrical current that rises
from zero to a maximum in one direction, falls to
zero, then rises to a maximum in the opposite
direction and repeats another cycle, the positive and
negative alternations being equal.
Online Resource for ETCH 213
Faculty: B. Allen
Why alternating current?
Power Transfer – To supply electrical power
for lighting, heating, cooling, appliances, and
machinery in both home and industry.
Information Transfer – To communicate or
carry information, such as radio, music, and
TV between two points.
Online Resource for ETCH 213
Faculty: B. Allen
Generator and transformer
Generator – A device used to convert a mechanical
energy input into an electrical energy output.
Transformer – A device consisting of two or more
coils that are used to couple electric energy from one
circuit to another, yet maintain electrical isolation
between the two.
Online Resource for ETCH 213
Faculty: B. Allen
AC power distribution
Online Resource for ETCH 213
Faculty: B. Allen
Information transfer
Communications – Transmission of information
between two points.
Sound wave – Traveling wave propagated in an elastic
medium that travels at a speed of approximately 1133
ft/sec.
Transducer – Any device that converts energy from one
form to another.
Electrical wave – Traveling wave propagated in a
conductive medium that is a variation in voltage or
current and travels slightly slower than the speed of
light.
Online Resource for ETCH 213
Faculty: B. Allen
Wireless Communications – Term describing
radio communications that require no wires
between two communications points.
Electromagnetic (Radio) Waves – A wave that
consists of both an electric and a magnetic
variation, and travels at the speed of light.
Online Resource for ETCH 213
Faculty: B. Allen
Information transfer
Online Resource for ETCH 213
Faculty: B. Allen
AC wave shapes
Sine Wave – A wave whose amplitude is the sine of a linear
function of time. It is drawn on a graph that plots amplitude
against time or radial degrees relative to the angular rotation
of an alternator.
Online Resource for ETCH 213
Faculty: B. Allen
AC waves
Amplitude – The magnitude or size an alternation
varies from zero.
Vector – A quantity that has both magnitude and
direction. It is normally represented as a straight
line. The length of the line indicates magnitude; the
arrowhead on one end indicates direction.
Online Resource for ETCH 213
Faculty: B. Allen
Peak-to-peak value of a sine wave
The difference between the maximum positive
and minimum
negative values
of the sine wave.
Vpp = 2 × Vpeak
Online Resource for ETCH 213
Faculty: B. Allen
Root-Mean-Square
( RMS ) value of a sine wave
rms = 0.707 × peak
RMS value of an ac voltage, current, or power
waveform is equal to 0.707 times the peak
value. The rms value is the effective or dc
value equivalent of the ac wave.
Online Resource for ETCH 213
Faculty: B. Allen
Average value of a sine wave
Average Value – The mean value found when
the area of a wave above a line is equal to the
area of the wave below the line.
Average value = 0.637 × peak
Online Resource for ETCH 213
Faculty: B. Allen
Period and Frequency
Period (t) – The time taken for one complete
cycle of a periodic or repeating waveform.
Frequency (f) – The rate of recurrences of a
periodic wave normally within a unit of one
second, measured in hertz (cycles per second).
t = 1/f
f = 1/t
Online Resource for ETCH 213
Faculty: B. Allen
Wavelength of a sine wave
Wavelength ( ) – The distance between
two points of corresponding phase equal to
the waveform velocity or speed divided by
the frequency.
= Velocity/Frequency
Online Resource for ETCH 213
Faculty: B. Allen
Phase relationship
Phase – The angular relationship between two
waves, normally between current and voltage
in an ac circuit.
Phase Shift or Angle – The change in phase of
a waveform between two points, given in
degrees of lead or lag. Phase difference
between two waves is normally expressed in
degrees.
Online Resource for ETCH 213
Faculty: B. Allen
Square wave
Square Wave – A wave that alternates
between two fixed values for an equal amount
of time.
Online Resource for ETCH 213
Faculty: B. Allen
Duty cycle
A term used to describe the amount of ON
time versus OFF time. ON time is usually
expressed as a percentage.
Duty cycle (%) =
(Pulse Width (PW))/(Period (t)) × 100%
Online Resource for ETCH 213
Faculty: B. Allen
Square wave
Rise Time – The time it takes a positive edge
of a pulse to rise from 10% to 90% of its peak
value.
Fall Time – The time it takes a negative edge
of a pulse to fall from 90% to 10% of its peak
value.
Online Resource for ETCH 213
Faculty: B. Allen
Square wave rise and fall times.
(a) ideal. (b) actual.
Online Resource for ETCH 213
Faculty: B. Allen
Time-domain and frequency-domain analysis
Time-Domain Analysis – A method of representing a
waveform by plotting its amplitude versus time.
Frequency-Domain Analysis – A method of representing a
waveform by plotting its amplitude versus frequency.
Online Resource for ETCH 213
Faculty: B. Allen
Fundamental and Harmonic
frequencies
Fundamental Frequency – This sine wave is always
the lowest frequency and the largest amplitude
component of any waveform shape and is used as a
reference.
Harmonic Frequency – A sine wave that is smaller in
amplitude and is some multiple of the fundamental
frequency.
Online Resource for ETCH 213
Faculty: B. Allen
Pulses
Rectangular (Pulse) Wave – A repeating wave that alternates
only between two levels of values and remains at one of these
values for a small amount of time relative to the other.
Pulse Repetition Frequency – The number of times per second
that a pulse is transmitted.
Pulse Repetition Time – The time interval between the start of
two consecutive pulses.
Pulse Width, Pulse Length, or Pulse Duration – The time
interval between the leading edge and trailing edge of a pulse
at which the amplitude reaches 50% of the peak pulse
amplitude.
Online Resource for ETCH 213
Faculty: B. Allen
Triangular wave
A repeating wave that has equal positive and negative ramps
that have linear rates of change with time.
Linear - Relationship between input and output in which the
output varies in direct proportion to the time.
Online Resource for ETCH 213
Faculty: B. Allen
Sawtooth wave
A repeating waveform that rises from zero to a maximum
value linearly, and then falls to zero and repeats.
Online Resource for ETCH 213
Faculty: B. Allen
Other waveforms
Online Resource for ETCH 213
Faculty: B. Allen
ac meter
Online Resource for ETCH 213
Faculty: B. Allen
Current Clamp – A device used in conjunction with an ac
ammeter containing a magnetic core in the form of hinged
jaws that can be snapped around the current-carrying wire
to measure current.
Radio-Frequency Probe – A probe used in conjunction
with an ac meter to measure high-frequency (RF) signals.
High-Voltage Probe – An accessory to the voltmeter that
has added multiplier resistors within the probe to divide
up the large potential being measured by the probe.
Online Resource for ETCH 213
Faculty: B. Allen
Oscilloscope
Online Resource for ETCH 213
Faculty: B. Allen
Oscilloscope grid
Online Resource for ETCH 213
Faculty: B. Allen
Function Generator – A signal generator that
can function as a sine, square, rectangular,
triangular, or sawtooth wave form generator.
Frequency Counter – A meter used to measure
the frequency or cycles per second of a
periodic wave.
Online Resource for ETCH 213
Faculty: B. Allen
Frequency counter
Online Resource for ETCH 213
Faculty: B. Allen
Scopemeter
A hand-held, battery-operated instrument
that combines a multimeter, oscilloscope,
frequency counter, and signal generator in
one.
Online Resource for ETCH 213
Faculty: B. Allen
End of
Alternating Current (AC)
Online Resource for ETCH 213
Faculty: B. Allen