Transcript Electronics Technology Fundamentals

Today’s Agenda
More on potentiometers
Introduction to AC signals
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What Potentiometers Look Like:
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Trimmer potentiometers.
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Potentiometer construction.
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The effect of turning the control shaft on the component
resistances.
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Potentiometer in Multisim
Found in Basic category
XMM1
R1
Key = A
1kOhm
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50%
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R1
XMM1
Key = A
1kOhm
70%
•Pressing the ‘a’ key increases percentage
•Pressing ‘Shift a’ decreases percentage
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Note, in Multisim, if you place the potentiometer with the arrow
angled down, the resistance will change in the opposite direction
XMM1
R1
1K _LIN
Key = A
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70%
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R1
Key = A
1kOhm
V1
12 V
60%
V
12
I 
 20mA
R 600
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XMM1
R1
Key = A
1kOhm
V1
12 V
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60%
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Information on Lamps in Multisim

The lamp you’ll need for the prelab can be found under
Indicators/Virtual_Lamp
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11
Chapter 9 Alternating Current
DC & AC Circuits
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DC: Direct Current
A DC current is a current that
does not change direction in time.
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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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AC: Alternating Current
An AC current is a current that
changes direction in time.
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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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
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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
V2
V1
V.
15 V
15 V
J1
Key = Space
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In-Class Activity 1
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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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
Oscilloscope – piece of equipment that provides
a visual representation of a voltage waveform
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In-Class Activity 2
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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 21)
 click the space bar fast. (see slide 22)
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Slow Clicks (Low Frequency AC)
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Fast Clicks (High Frequency AC)
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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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Switching between two batteries automated
using 555 timer
555 timer
SPDT
replaced
with Relay
two
batteries
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
How do we characterize the differences in
the waveforms we generated?
For periodic rectangular waves:
Insert Figure 9.39
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Rectangular Waves
- Terminology and Time Measurements
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Rectangular Waves
- Duty Cycle – ratio of pulse width to cycle time
PW
duty cycle (%) 
X 100
T
where
PW = the pulse width of the circuit input
T = the cycle time of the circuit input
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In-Class Activity 3
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For the following waveforms, specify the pulse width, space
width and period:
a)
2 ms
5 ms
10 ms
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One of the most important AC signals is the periodic
sinusoid, as shown below.
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Power generation power plants.
Design of radios and radio stations.
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Difference between AC & DC
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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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Generating a Sine Wave
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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
Insert Figure 9.3
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
Cycle Time (Period)

– The time required to complete one cycle of a signal
ms
T  4 div  5
 20 ms
div
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In-Class Activity 4
Calculate T in ms
Calculate T in ms
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
Frequency

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the rate at which the cycles repeat themselves
Unit of Measure – Hertz (Hz) = cycles/second
T = 200 ms = 0.2s
f = 1/0.2 = 5 cps
= 5 Hz
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Relation between Cycle Time (Period) and
Frequency
1
f 
T
or
1
T 
f
where T = the cycle time (period) of the waveform in seconds
Another way to describe periodicity of the wave is through the
angular frequency defined as
  2 f
where
 = angular velocity, in radians per second
2 = the number of radians in one cycle
f = the number of cycles per second (frequency)
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
1
Note that [ f ]   Hz is not the
s
rad
same thing as [ ] 
s
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What is a radian?
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
1 Radian – the angle formed within a circle by two radii
separated by an arc of length equal to the radii
ra
1 rad 
r1
when ra = r1
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Instantaneous Value
– the value of a sinusoidal voltage or current at a
specified point in time can be expressed as:
v(t)  Vpk sin( t)
where Vpk is the
magnitude
of the voltage.
In terms of f,
v(t)  Vpk sin( 2ft)
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In-Class Activity 5
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An AC voltage in volts is given by
v(t )  10 sin( 377t )
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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?
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In-Class Activity 5
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A device emits a sinusoidal signal that has a magnitude of
1 volt and a frequency of 690 kHz.
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what is the angular frequency in rad/s?
what is the period in ms?
Express this signal as v(t )  A sin( t ) (i.e. fill in the values
for A and )
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Express this signal as
v(t )  A sin( 2ft)
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