1

Electrical Engineering & Telecommunications

Lecturer:Ted Spooner

Lecture -Intro to Circuit Basics

Elec1111Elec1111

Rm 124A EE email: e.spooner@unsw.edu.au

How is the Electrical system of an Aircraft Powered?

One of 4 variable frequency 150kVA generators on the new A380 Airbus Ram Air Turbine

Auxiliary Power Unit (APU)

How do we connect it all?

• How do we go about designing and evaluating the design?– Have to analyse circuits– How do we do that?

Possible Circuit solutions to a simplified aircraft electrical system.

Generator 1 Generator

2

LOAD 1 LOAD 2

LOAD 3

LOAD 4

LOAD 5

Possible Circuit solutions to a simplified aircraft electrical system.

Generator 1

Generator 2

LOAD 1LOAD 2

LOAD 5

LOAD 4

LOAD 3

How do we analyze a circuit like this?

• Audio Power Amplifier

2

Electric Circuit VariablesCharge

Current

Voltage

Power

Energy

Need to Understand Concepts

How do we calculate current in each element of this circuit?

10V

1000 10

87 20 3V

currentconstant: direct current (dc)

time-varying: many formse.g. sinusoidal, also called

alternating current (ac)

t (s)

i(A)

0

sinusoid (ac)

rampexponential

Time-varying current

t (s)

i(A)

0

Direct current (dc)

t (s)

i(A)

0

sinusoid (ac)

rampexponential

Time-varying current

t (s)

i(A)

0

Direct current (dc)

VOLTAGE• When charge flows through an element

then energy is transferred either to or from the element

a bi

3

• Voltage (potential difference) = measure of the amount of energy transferred per unit of charge.

• Unit: volt, V

• Notation: V, v(t) or v

energy

chargedqdwv =

Voltage

• As charge flows through an element then energy is transferred either to or from the element

a b

vi

+ _

Must have a sense of direction.If i and v are positive then: energy is being transferred to the element.

Voltage

• Element absorbing energy

a bi + _

iv

a bi +_

• Element delivering energy

iv

If i and v are positive.

Voltage is analogous to pressure

PUMP

Resistance to flow

Voltage is like pressure HV transmission lines: up to 500kV

Household supply: 230V

Car battery: 12V – 13V

Flashlight battery: 1.5V

Electroencephalography (EEG):10μV

4

Voltage Sign conventions

• Voltage is a measure of potential difference between two points.

• ie Vab is the difference in potential between nodes a and b.

• Vab is the voltage of node a with respect to node b.

a

b

+

_

abvWhen we say voltage of node When we say voltage of node aarelative to node relative to node bb we write it like:we write it like:

Voltage Sign conventionsVoltage is a measure of

potential difference between two points.

Reference Ground

va

+

_

vb

+

_

vab = va - vb

a

b

+

_

bavWhen we say voltage of node When we say voltage of node bbrelative to node relative to node aa we write it like:we write it like:

Now voltage of b with respect to a

Reference Ground

vb

+

_

va+

_

vba = vb - va

a b

+ _

bav

abv

+_

a b

Then :Then :

IfIf 5abv V=

5bav V= −

a b

+ _abv

5abv V=

COM V

+5V Reading

Measuring Vab

Sources = devices that supply energy to circuit

VoltageVoltagesourcessources

CurrentCurrentsourcessources

SourcesSources

2 types

Sources can be independentindependent or dependentdependent.

5

Independent Voltage source

DCvoltagesource

V

ACvoltagesource

v(t)

Idealvoltagesource

v(t)

Battery

V

positiveterminal Independent current sourceIndependent current source

i(t)