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ENGG 1203 Tutorial

Electrical Circuit

28 Feb

Learning Objectives

Analysis circuits through circuit laws (Ohm’s Law, KCL

and KVL)

News

HW2

Ack.: HKU ELEC1008, MIT 6.111, MIT 6.01

Q1

Determine the indicated parameters for

each of the following circuits.

2

Q2

Determine the indicated

parameters for each of

the following circuits.

3 4

Q3

If VAB = 4V, determine R1, R2, R3 and R4.

For the circuit in the figure, determine i1 to i5.

5

Q4

6

R1 = 80Ω, R2 = 10Ω, R3 = 20Ω,R4 = 90Ω, R5 = 100Ω

Battery: V1 = 12V, V2 = 24V, V3 = 36V

Resistor: I1, I2, …, I5 = ? P1, P2, …, P5 = ?

Step 1, Step 2

Q5

Q6

You have connected the lamp, with Vcc = 12V. The datasheet of the lamp states that it only turns on when VL > 8V.The lamp has an internalresistance of 1k ohm.

What is the range of R

that would allow thecircuit to functioncorrectly with all inputcombinations.

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Q7a

A force sensitive resistor (FSR) is a resistor with its resistance changed according to the force applied to it.

For simplicity sake, your partnerhas wired up the FSR using asimple potential divider circuit

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Calculate the following quantities

when Rref = 10k ohm:

Voltage across the FSR;

Voltage at Vout;

Current owing through the FSR.

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Q7b

The output Vout is used to detect the presence of a ball.

Due to its light weight, the ball produces only 0.5N when

it is located on top of the sensor. The rest of the system

requires that VIL = 2V and VIH = 10V

VIL is the maximum voltage that the system regards as logical

LOW and VIH is the minimum voltage that the system regards as

logical HIGH.

Determine the range of value that Rref may take for

correct functioning of the circuit.

It should output a logical HIGH when a ball is presence and a

logical LOW otherwise.

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Q7c

Your partner suggests that it may be possible to use 2

FSRs connected to perform a logical OR operation:

When the ball rolls over either one of the 2 FSRs, the

output Vout is HIGH, and is LOW otherwise.

What is the output voltage Vout?

one of the FSRs is under pressure of 0.5N,

both FSRs are under a pressure of 0.5N each,

none of the FSRs is under pressure.

Assume Rref is 100k ohm.

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Q7d

Recall that VIL is 2V and VIH is 10V,

is the circuit functioning correctly as

a 2-input OR?

If there are 3 FSRs connected in

parallel, assumer Rref remains at

100k, will the circuit behave as a 3-

input OR?

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(Appendix) Q8

Find V2 using single loop analysis

Without simplifying the circuit

Simplifying the circuit

1 2 3 1 2 32 , 2 , 2 , 1 , 2 , 4

s s sV v V v V v R R R= = = = Ω = Ω = Ω

R1

Vs1

Vs3

Vs2

R3

-

R2

+

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(Appendix) Q9

Find Req and io in the circuit of the figure.

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(Appendix) Q10

Assume all resistors have the same resistance, R. Determine the voltage vAB.

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(Appendix) Q11

Find V2 using single loop analysis

Without simplifying the circuit

Simplifying the circuit

1 2 3 1 2 32 , 2 , 2 , 1 , 2 , 4

s s sV v V v V v R R R= = = = Ω = Ω = Ω

R1

Vs1

Vs3

Vs2

R3

-

R2

+

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(Appendix) Rules Governing Currents

and Voltages Rule 1: Currents flow in loops

The same amount of current flows into the bulb (top path) and out of the bulb (bottom path)

Rule 2: Like the flow of water, the flow of electrical

current (charged particles) is incompressible

Kirchoff’s Current Law (KCL): the sum of the currents into a node is zero

Rule 3: Voltages accumulate in loops

Kirchoff’s Voltage Law (KVL): the sum of the voltages around a closed loop is zero

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Assign node voltage variables to every node except ground (whose voltage is arbitrarily taken as zero)

Assign component current variables to every component in the circuit

Write one constructive relation for each component in terms of the component current variable and the component voltage

Express KCL at each node except ground in terms of the component currents

Solve the resulting equations

Power = IV = I2R = V2/R

(Appendix) Analyzing Circuits

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(Appendix) Parallel/Series

Combinations of Resistance To simplify the circuit for analysis

1 2

1 2

s

s

v R i R i

v R i

R R R

= +

=

∴ = +

1 2

1 2

1 2

1 2

1

1 1

//

p

R RR

R RR R

R R

= =++

=

Series

Parallel

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(Appendix) Voltage/Current Divider

1 2

1

1 1

1 2

2

2 2

1 2

VI

R R

RV R I V

R R

RV R I V

R R

=+

= =+

= =+

( )1 2

1 2 2

1

1 1 1 2

1

2

1 2

//

//

V R R I

R R RVI I I

R R R R

RI I

R R

=

= = =+

=+

Voltage Divider

Current Divider