1

Today’s Agenda

More on potentiometers

Introduction to AC signals

1

EGR 101 2

What Potentiometers Look Like:

EGR 101 3

Trimmer potentiometers.

EGR 101 4

Potentiometer construction.

EGR 101 5

The effect of turning the control shaft on the component resistances.

EGR 101 6

R1

Key = A 1kOhm

50%

XMM1

Potentiometer in Multisim

Found in Basic category

EGR 101 7

R1

Key = A 1kOhm

70%

XMM1

•Pressing the ‘a’ key increases percentage•Pressing ‘Shift a’ decreases percentage

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

EGR 101 9

1220

600

VI mA

R

R1

Key = A 1kOhm

60%V112 V

EGR 101 10

R1

Key = A 1kOhm

60%V112 V

XMM1

EGR 101 11

Information on Lamps in Multisim

The lamp you’ll need for the prelab can be found under Indicators/Virtual_Lamp

11

1212

Chapter 9 Alternating Current

DC & AC Circuits

1313

DC: Direct CurrentA DC current is a current that does not change direction in time.

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.

1515

AC: Alternating CurrentAn AC current is a current that changes direction in time.

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).

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

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.

EGR 101 1919

Oscilloscope – piece of equipment that provides a visual representation of a voltage waveform

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 21) click the space bar fast. (see slide 22)

EGR 101 2121

Slow Clicks (Low Frequency AC)

EGR 101 2222

Fast Clicks (High Frequency AC)

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.

EGR 101 2424

Switching between two batteries automated using 555 timer

555 timerSPDTreplacedwith Relay

twobatteries

EGR 101 2525

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

Insert Figure 9.39

For periodic rectangular waves:

EGR 101 2626

Rectangular Waves- Terminology and Time Measurements

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

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

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.

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

EGR 101 3131

Generating a Sine Wave

EGR 101 3232

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

EGR 101 3434

Cycle Time (Period) – The time required to complete one cycle of a signal

ms20div

ms5div4 T

EGR 101 3535

In-Class Activity 4

Calculate T in ms Calculate T in ms

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

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)

EGR 101 3838

Note that is not the

same thing as

What is a radian?

Hzs

f 1

][

s

rad][

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

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

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

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