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Transcript of 1 Today’s Agenda More on potentiometers Introduction to AC signals 1.
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1
Today’s Agenda
More on potentiometers
Introduction to AC signals
1
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EGR 101 2
What Potentiometers Look Like:
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EGR 101 3
Trimmer potentiometers.
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EGR 101 4
Potentiometer construction.
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EGR 101 5
The effect of turning the control shaft on the component resistances.
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EGR 101 6
R1
Key = A 1kOhm
50%
XMM1
Potentiometer in Multisim
Found in Basic category
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EGR 101 7
R1
Key = A 1kOhm
70%
XMM1
•Pressing the ‘a’ key increases percentage•Pressing ‘Shift a’ decreases percentage
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EGR 101 8
Note, in Multisim, if you place the potentiometer with the arrow angled down, the resistance will change in the opposite direction
R1
1K_LINKey = A
70%
XMM1
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EGR 101 9
1220
600
VI mA
R
R1
Key = A 1kOhm
60%V112 V
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EGR 101 10
R1
Key = A 1kOhm
60%V112 V
XMM1
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EGR 101 11
Information on Lamps in Multisim
The lamp you’ll need for the prelab can be found under Indicators/Virtual_Lamp
11
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1212
Chapter 9 Alternating Current
DC & AC Circuits
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1313
DC: Direct CurrentA DC current is a current that does not change direction in time.
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EGR 101 1414
Example of DC circuits:
Portable flashlight circuit. Internal circuit inside a DMM to
measure resistance. Circuit to control a dimmer light
using a potentiometer, as you will on Thursday.
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1515
AC: Alternating CurrentAn AC current is a current that changes direction in time.
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EGR 101 1616
Imagine you take a battery with one polarity between times t0 and t1 (top schematic).
Flip polarity between t1 & t2 (bottom schematic).
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EGR 101 1717
Flipping batteries is unrealistic. However, you can built an AC circuit using two batteries and a SPDT switch, as shown below.
When the switch is flipped to the right you get +15 V. When the switch is flipped to the left you get -15 V.
V1
15 V
V2
15 V
J1
Key = Space
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EGR 101 1818
In-Class Activity 1 Working in pairs, simulate the circuit below in Multisim. The single pole, double throw
(SPDT) switch can be found in the Basic, switch category. Flip the SPDT switch using the space key and watch how the meter reading alternates
between +15 V and -15 V.
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EGR 101 1919
Oscilloscope – piece of equipment that provides a visual representation of a voltage waveform
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EGR 101 2020
In-Class Activity 2
Repeat simulation using an oscilloscope (4th instrument down on right column). Watch on the scope how fast the voltage alternates:
click the space bar slowly. (see slide 10) click the space bar fast. (see slide 11)
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EGR 101 2121
Slow Clicks (Low Frequency AC)
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EGR 101 2222
Fast Clicks (High Frequency AC)
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EGR 101 2323
AC using 555 timer
Although you can easily create the previous AC circuit using two batteries and SPDT, you can automate the circuit using a 555 timer, as you are going to do in your semester projects, as shown in the next slide.
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EGR 101 2424
Switching between two batteries automated using 555 timer
555 timerSPDTreplacedwith Relay
twobatteries
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EGR 101 2525
How do we characterize the differences in the waveforms we generated?
Insert Figure 9.39
For periodic rectangular waves:
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EGR 101 2626
Rectangular Waves- Terminology and Time Measurements
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EGR 101 2727
Rectangular Waves- Duty Cycle – ratio of pulse width to cycle time
where PW = the pulse width of the circuit input T = the cycle time of the circuit input
100(%)cycleduty XT
PW
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EGR 101 28
In-Class Activity 3
a)
For the following waveforms, specify the pulse width, space width and period:
2 ms 5 ms 10 ms
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EGR 101 2929
One of the most important AC signals is the periodic sinusoid, as shown below. Power generation power plants. Design of radios and radio stations.
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EGR 101 3030
Difference between AC & DC
Demo. Loss of DC power over long distance. AC power transport is more efficient. http://www.pbs.org/wgbh/amex/edison/sfeature/acdc.html
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EGR 101 3131
Generating a Sine Wave
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EGR 101 3232
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EGR 101 3333
Insert Figure 9.3
Alternations and Cycles Alternations – the
positive and negative transitions
Cycle – the complete transition through one positive alternation and one negative alternation
Half-Cycle – one alternation
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EGR 101 3434
Cycle Time (Period) – The time required to complete one cycle of a signal
ms20div
ms5div4 T
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EGR 101 3535
In-Class Activity 4
Calculate T in ms Calculate T in ms
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EGR 101 3636
Frequency the rate at which the cycles repeat themselves Unit of Measure – Hertz (Hz) = cycles/second
T = 200 ms = 0.2sf = 1/0.2 = 5 cps = 5 Hz
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EGR 101 3737
Relation between Cycle Time (Period) and Frequency
where T = the cycle time (period) of the waveform in seconds
fT
Tf
1or
1
Another way to describe periodicity of the wave is through theangular frequency defined as
2 f where
= angular velocity, in radians per second2 = the number of radians in one cycle f = the number of cycles per second (frequency)
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EGR 101 3838
Note that is not the
same thing as
What is a radian?
Hzs
f 1
][
s
rad][
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EGR 101 3939
1 Radian – the angle formed within a circle by two radii separated by an arc of length equal to the radii
1
1 arradr
when ra = r1
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EGR 101 4040
where Vpk is the magnitude of the voltage.
In terms of f,
Instantaneous Value
– the value of a sinusoidal voltage or current at a specified point in time can be expressed as:
)sin()( tVtv pk
)2sin()( ftVtv pk
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EGR 101 4141
In-Class Activity 5
An AC voltage in volts is given by
what is the unit of the number 10? what is the unit of the number 377? what is the angular frequency in rad/s? what is the frequency in Hz or cps? what is the period in ms?
)377sin(10)( ttv
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EGR 101 4242
In-Class Activity 5
A device emits a sinusoidal signal that has a magnitude of 1 volt and a frequency of 690 kHz.
what is the angular frequency in rad/s? what is the period in ms? Express this signal as (i.e. fill in the
values for A and )
Express this signal as
)sin()( tAtv
)2sin()( ftAtv