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Lesson 5

EET 150

Ac Sources

Learning Objectives

In this lesson you will:

learn how magnetic dipoles interact.

see how electromagnets are constructed

observe changes in magnetic fields for dc and ac currents

see how generators create ac voltage

see how transformers operate

learn schematic symbols for ac generators and transformers

study a block diagram for a typical laboratory dc power supply

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Magnetic fields exert force on objects

S N S N Opposite

Poles

Attract Force Force

S N S N Like Poles

Repel

Force Force

Magnetic

flux

Currents flowing in wires produce magnetic flux

Voltage

Source, V

Current

I

X1 I into

page

X1 X1 Y1 Y1

Y1 I into

page

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Coiling wire focuses flux

Dc current produces

constant flux

I

Magnetic

Field

Ac current produces alternating

flux

I

I

N

S

N

S

S

N

S N

A

B

C

D

Conductor

Loop

V

Time

Vo

lta

ge

, V

A

B

C

D

Rotating a conductor through a magnetic field produces an Ac voltage

generators (Alternators)

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Schematic Symbol for Ac Generators (Alternators)

AC 100 V

60 Hz

Symbol represents both power

generators and small signal

devices used in electronics

Must specify voltage and frequency values

Two coils wound on a common magnetic core form a transformer

Number of turns in coils determine characteristics

Used to increase and decrease voltage levels in ac systems

Will not work with dc

Air Core Magnetic Core

Schematic Symbols Iron core transformer 120/12.6 V

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Industrial Power Supply

Bench-top Isolation Transformer

120V

60 HzV V

Magnetic field produced in primary coil induces voltage in secondary coil

Vp Vs

Primary

coil Secondary

coil

Vp=120 V Vs=60 V

Np = turns in primary coil

Ns = turns in secondary coil

Vp = primary voltage applied

Vs = secondary voltage induced

Vs < Vp Step-down

Vs > Vp Step-up

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120V

60 HzV VNp Ns

Induced voltage is proportional to number of turns in coils

s

p

s

p

V

V

N

N

Equation Example: Given Np =100, Ns=50 and Vp=120 find

Vs

s

p

s

p

V

V

N

N

sV

120

50

100

)120(50V100 s

60)120(100

50Vs

Dc Power Supplies – Instruments that produce controllable

dc voltages and currents from 120 V Ac outlet voltages

Replace batteries in lab experiments and electronics design

Typical Voltages

0 -20 V dc

0-30 V dc

5 V dc fixed

Supplies can have single output or

Multiple outputs

Current limits prevent damage

to supply or designed circuit

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Step Down

Transformer

Rectifiers

(Ac to Dc

Conversion)

Filters

(Smooth

Pulsing Dc)

Voltage &

Current

Regulators

(Maintain &

Control V, I)

Ac Power

Source 120 V

Fixed

Dc Outputs

Variable

Dc Outputs

Schematic Symbol

12 Vdc

Power Switch

Voltmeter

Voltmeter

Controls

Output Terminals

Output

Tracking

Voltage

Adjust

Ammeter

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End Lesson 5 EET 150

Coming Next: Simple Circuit Analysis

Using Ohm’s Law