Chapter 3

Chapter 3

Chapter 3Summary

Voltage is

Review of V, I, and R

the amount of energy per charge available tomove electrons from one point to another in a circuit.

Current is the rate of charge flow and is measured in amperes.

Resistance is the opposition to current and is measured in ohms.

Chapter 3

The most important fundamental law in electronics is Ohm’s law, which relates voltage, current, and resistance.

VIR

What is the current in from a 12 V source if the resistance is 10 ? 1.2 A

It takes one volt to push one amp through one ohm.

In a DC circuit, the current is directly proportional to the voltage and inversely proportional to the resistance.

Chapter 3

Ohm’s law

If you need to solve for voltage, Ohm’s law is:

What is the voltage across a 680 resistor if the current is 26.5 mA? 18 V

V IR VIR

Chapter 3

Ohm’s law

If you need to solve for resistance, Ohm’s law is:VRI

115 V

V

1 s

1 s

40 mA

10 A

COM

RangeAutorangeTouch/Hold

Fused

OFF V

V

Hz

mV

A

What is the (hot) resistance of the bulb? 132

VIR

Chapter 3Summary

Ohm’s law

Chapter 3

Graph of Current versus Voltage

I (m

A)

V (V)00 2 4

4

6 8

8

10

12

14

16

A student takes data for a resistor and fits the straight line shown to the data. What is the resistance and the conductance of the resistor?

The slope represents the conductance.

14.8 mA - 0 mA 1.48 mS10.0 V - 0 V

G

The reciprocal of the conductance is the resistance:

1 1 676 Ω1.48 mS

RG

Chapter 3

Graph of Current versus Voltage

Voltage (V)

Cur

rent

(mA

)

0 10 20 300

2.0

4.0

6.0

8.0

10Notice that the plot of current versus voltage for a fixed resistor is a line with a positive slope. What is the resistance indicated by the graph?

2.7 k

What is its conductance? 0.37 mS

mAmAVV

010027

Chapter 3

Graph of Current versus Resistance

If resistance is varied for a constant voltage, the current verses resistance curve plots a hyperbola.

Determine the voltage source for the curve?

Resistance (k )

Cur

rent

(mA

)

0 1.0 2.0 3.00

2.0

4.0

6.0

8.0

10

(8.0mA)(375Ω) = 3V(2.4mA)(1.25kΩ) = 3V

Chapter 3

0 1 2 3 4 5 6 7 8 9

1

2

3

4

Volts

Am

pere

s

2 +

_0 to 9 Volts

2 1

4

The smaller the resistor, the steeper the slope.

A linear resistance has a constant value of ohms. Its R does not change with the applied voltage, so V and I are directly proportional.

Linear Resistance

Carbon-film and metal-film resistors are examples of linear resistors.

Chapter 3Nonlinear Resistance

– In a nonlinear resistance, increasing the applied Vproduces more current, but I does not increase in the same proportion as the increase in V.

– Example of a Nonlinear Volt–Ampere Relationship:

• As the tungsten filament in a light bulb gets hot, its resistance increases.

Chapter 3

– Another nonlinear resistance is a thermistor. – A thermistor is a resistor whose resistance value

changes with its operating temperature. – As an NTC (negative temperature coefficient)

thermistor gets hot, its resistance decreases.

VoltsA

mpe

res

Thermistor

VoltsA

mpe

res

Thermistor

Nonlinear Resistance

Chapter 3

Application of Ohm’s law

The resistor is green-blue brown-gold. What should the ammeter read?

15V/560Ω = 26.8 mA

Power Supply

+15 V- + - +5 V 2A

DC Ammeter+

-

Gnd

V A

Chapter 3

?

20 V 4 I = 20 V4

= 5 A

1 A

? 12 V = 1A × 12 = 12 V

3 A

6 V ? R = 6 V3 A

= 2

Applying Ohm’s Law VI R

Application of Ohm’s law

Chapter 3Open-Circuit

An open circuit has zero current flow.

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Chapter 3

A short circuit has excessive current flow.

As R approaches 0, I approaches .

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Short-Circuit

Chapter 3Prototyping Board

Example of how components are Inserted in the protoboard

Chapter 3Summary

TroubleshootingAPM – Analysis, Planning, Measuring

? 1. Analyze clues or symptoms of the failure.

- Is the power properly applied to circuit.- Has the circuit ever worked. Conditions of failure.- Check fuses or circuit-breakers, if applicable.- Any components burnt, solder splashes, etc.?

2. Planning – Formulate a plan of attack and working knowledge of the circuit.

- Schematics- Instruction manuals

3. Measuring – Narrow the possible failures by isolating sections of the circuit and making carefully thought out measurements.

Chapter 3

Linear

Ohm’s law

Troubleshooting

Characterized by a straight-line relationship.

A law stating that current is directly proportional to voltage and inversely proportional to current.

A systematic process of isolating, identifying, and correcting a fault in a circuit or system.

Selected Key Terms

Chapter 3Quiz

1. Holding the voltage constant, and plotting the current against the resistance as resistance is varied will form a

a. straight line with a positive slope

b. straight line with a negative slope

c. parabola

d. hyperbola

Chapter 3Quiz

1. Holding the voltage constant, and plotting the current against the resistance as resistance is varied will form a

a. straight line with a positive slope

b. straight line with a negative slope

c. parabola

d. hyperbola

Chapter 3Quiz

2. When the current is plotted against the voltage for a fixed resistor, the plot is a

a. straight line with a positive slope

b. straight line with a negative slope

c. parabola

d. hyperbola

Chapter 3Quiz

2. When the current is plotted against the voltage for a fixed resistor, the plot is a

a. straight line with a positive slope

b. straight line with a negative slope

c. parabola

d. hyperbola

Chapter 3Quiz

3. For constant voltage in a circuit, doubling the resistance means

a. doubling the current

b. halving the current

c. there is no change in the current

d. depends on the amount of voltage

Chapter 3Quiz

3. For constant voltage in a circuit, doubling the resistance means

a. doubling the current

b. halving the current

c. there is no change in the current

d. depends on the amount of voltage

VIR

Chapter 3Quiz

4. A four-color resistor has the color-code red-violet-orange-gold. If it is placed across a 12 V source, the expected current is

a. 0.12 mA

b. 0.44 mA

c. 1.25 mA

d. 4.44 mA

Chapter 3Quiz

4. A four-color resistor has the color-code red-violet-orange-gold. If it is placed across a 12 V source, the expected current is

a. 0.12 mA

b. 0.44 mA

c. 1.25 mA

d. 4.44 mA

kV

2712

Chapter 3Quiz

5. If the current in a 330 resistor is 15 mA, the applied voltage is approximately

a. 5.0 V

b. 22 V

c. 46 V

d. 60 V

Chapter 3Quiz

5. If the current in a 330 resistor is 15 mA, the applied voltage is approximately

a. 5.0 V

b. 22 V

c. 46 V

d. 60 V

)330)(15( mA

Chapter 3Quiz

6. The current in a certain 4-band resistor is 22 mA when the voltage is 18 V. The color bands on the resistor are

a. blue-gray-red-gold

b. red-red-brown gold

c. gray-red-brown-gold

d. white-brown-red-gold

Chapter 3Quiz

6. The current in a certain 4-band resistor is 22 mA when the voltage is 18 V. The color bands on the resistor are

a. blue-gray-red-gold

b. red-red-brown gold

c. gray-red-brown-gold

d. white-brown-red-gold

8202.8182218

mAV

Chapter 3Quiz

7. The circuit with the largest current is

a. (a)

b. (b)

c. (c)

d. (d)

R 10 k

+12 V R 15 k

+15 V R 22 k

+18 V R 27 k

+24 V

(a) (b) (c) (d)

Chapter 3Quiz

7. The circuit with the largest current is

a. (a)

b. (b)

c. (c)

d. (d)

R 10 k

+12 V R 15 k

+15 V R 22 k

+18 V R 27 k

+24 V

(a) (b) (c) (d)mAkV 2.1

1012

mAkV 0.1

1515

mAkV 82.0

2218

mAkV 89.0

2724

Chapter 3Quiz

8. The circuit with the smallest current is

a. (a)

b. (b)

c. (c)

d. (d)

R 10 k

+12 V R 15 k

+15 V R 22 k

+18 V R 27 k

+24 V

(a) (b) (c) (d)

Chapter 3Quiz

8. The circuit with the smallest current is

a. (a)

b. (b)

c. (c)

d. (d)

R 10 k

+12 V R 15 k

+15 V R 22 k

+18 V R 27 k

+24 V

(a) (b) (c) (d)mAkV 2.1

1012

mAkV 0.1

1515

mAkV 82.0

2218

mAkV 89.0

2724

Chapter 3Quiz

9. Before troubleshooting a faulty circuit you should find out

a. If the circuit ever worked

b. The conditions that existed when it failed

c. The symptoms of the failure

d. All of the above

Chapter 3Quiz

9. Before troubleshooting a faulty circuit you should find out

a. If the circuit ever worked

b. The conditions that existed when it failed

c. The symptoms of the failure

d. All of the above

Chapter 3Quiz

10. A troubleshooting method that starts in the middle and works toward a fault is

a. short-circuit testing

b. comparison testing

c. half-splitting

d. resistance testing

Chapter 3Quiz

10. A troubleshooting method that starts in the middle and works toward a fault is

a. short-circuit testing

b. comparison testing

c. half-splitting

d. resistance testing