DC and AC Circuits

Electronics - Devices

Oct 5, 2013 (4 years and 9 months ago)

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Today’s Agenda

More on potentiometers

Introduction to AC signals

1

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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

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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

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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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12

12

Chapter 9 Alternating Current

DC & AC Circuits

13

13

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.

15

15

AC: Alternating Current

An AC current is a current that
changes direction in time.

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16

Imagine you take a battery with one polarity between
times t
0
and t
1

(top schematic).

Flip polarity between t
1

& t
2

(bottom schematic).

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17

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.

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18

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
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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20

In
-
Class Activity 2

Repeat simulation using an oscilloscope (4
th

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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21

Slow Clicks (Low Frequency AC)

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22

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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24

Switching between two batteries automated
using 555 timer

555 timer

SPDT

replaced

with Relay

two

batteries

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25

How do we characterize the differences in
the waveforms we generated?

Insert Figure 9.39

For periodic rectangular waves:

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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

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

a)

For the following waveforms, specify the pulse width, space
width and period:

2 ms

5 ms

10 ms

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29

One of the most important AC signals is the periodic
sinusoid, as shown below.

Power generation power plants.

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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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Generating a Sine Wave

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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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Cycle Time (Period)

The time required to complete one cycle of a signal

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In
-
Class Activity 4

Calculate
T

in
ms

Calculate
T

in
ms

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Frequency

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

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

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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Note that is not the

same thing as

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the angle formed within a circle by two radii
separated by an arc of length equal to the radii

when
r
a

=
r
1

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where
V
pk

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:

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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
/
s
?

what is the frequency in
Hz

or
cps
?

what is the period in
ms
?

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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
/
s
?

what is the period in
ms
?

Express this signal as

(i.e. fill in the values
for
A

and

)

Express this signal as