ac and oscilloscope
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Alternating Voltage – Chapter 6
Alternating Voltage
• Alternating voltage is produced by
generators like alternators, dynamos, or low
frequency generators LFG.
• Such generators have terminals
that reverse (switch, alternate)
their signs quickly and regularly.
Alternating Voltage Generators
• Thus the polarity (+ and -) of the terminals of
such generators does not matter
• A possible symbol of AC source is
Alternating Voltage
* An alternating variable voltage is
displayed on an oscilloscope screen as a
set of repeating cycles.
* In each cycle the voltage varies from
a maximum to a minimum value and
alternates its sign + then –
* The same cycle then repeats within
the same duration or time needed for a
cycle, called the period of a cycle.
or sinusoidal form
Sinusoidal Alternating Voltage
A common form of AC Voltage.
The Voltage supplied to homes and establishments
by EDL (Electricite du Liban) is sinusoidal alternating
voltage.
Sinusoidal Alternating Voltage
This voltage has four quantitative characteristics:
1- Maximum voltage
2- Effective voltage (or root mean square value, rms value of the
voltage)
3- Period of the voltage
4- Frequency of the voltage.
Sinusoidal Alternating Voltage1 / 4 characteristics:
First : along the vertical axis, voltage axis:
1- Maximum voltage :
U max = Sv x y
Where y is the amplitude of the voltage or the number
of divisions between the base line and the highest or
lowest point.
2- Effective voltage
U eff = U max / 2 or U eff = U max / 1.4
• Effective voltage is the value of the DC voltage that
has the same effect in an identical load.
• It is also known as the root mean square value, rms
value of the voltage.
• Its value can be measured directly by a voltmeter set
on VAC mode.
• Its value can be calculated from U max, as shown
above for an alternating sinusoidal voltage.
• EDL supplies us with sinusoidal alternating voltage
of U eff = 220V
Sinusoidal Alternating Voltage2 / 4 characteristics:
Sinusoidal Alternating Voltage3 / 4 characteristics:
Second: along the horizontal, time axis
3- Period of the voltage
T = X x S h
• Where X is the number of divisions along the x axis
corresponding to one cycle.
• And S h is the horizontal sensitivity in seconds per
division, s/div (also called sweeping speed)
• Then T the period of the voltage is expressed in
the SI unit of seconds
Sinusoidal Alternating Voltage
3 / 4 characteristics:
4- Frequency of the voltage
f = 1 / T
• After calculating the period T of a voltage, we can
determine the frequency of the voltage.
•The frequency is the number of cycles in one
second, it is expressed in units of Hertz or Hz.
• The sinusoidal alternating voltage in Lebanon has a
frequency of 50 Hz.
S h = 10 ms /
div
S v = 5 V /
div
Name the type and determine the four characteristics of the shown
voltage.
Type of voltage is:
Characteristics:
1- Maximum voltage:
2- Effective voltage:
3- Period of the voltage:
4- Frequency of the
voltage:
Y = 3 div
S v = 5 V / div S h = 10 ms / div
S h = 10 ms / divS v = 5 V / div
X = 4 div
Type of voltage is:
Characteristics:
1- Maximum voltage:
2- Effective voltage:
3- Period of the voltage:
4- Frequency of the
voltage:
Alternating sinusoidal voltage
Having the following characteristics:
U max = S v x y = 5V/div x 3 div
= 15V
U eff = U max / 2 = 15V / 1.4
= 10.71V
T = S h x X = 10ms / div x 4 div
= 40ms or 0.04s
f = 1 / T = 1 / 0.04s = 25 cycles per s
f = 25 Hz
Determine the value of x
and of y in the following
three oscillogram displays:
Oscillogram 1
Oscillogram 2
Oscillogram 3
Use the same settings of sensitivities as in the previous slide and
determine:
* Effective voltage in display 1
* The frequency of the voltage in display 3
Thank you
Next: Book exercises
starting on page 79
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