electronics fundamentals
circuits, devices, and applications
THOMAS L. FLOYD
DAVID M. BUCHLA
chapter 3
Chapter 3
Review of V, I, and R
Voltage is the amount of energy per charge available to
move electrons from one point to another in a circuit
and is measured in volts.
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
Ohm’s law
• Most important fundamental law in electronics
• Relates voltage, current, and resistance.
• Georg Simon Ohm formulated the equation
I
V
R
What is the current in a circuit with a 12 V source if
the resistance is 10 ? 1.2 A
Chapter 3
If you need to solve for voltage,
Ohm’s law is: V IR
Changes
What is the voltage across a 680
current is 26.5 mA? 18 V
resistor if the
Chapter 3
If you need to solve for resistance,
Ohm’s law is: R V
I
What is the resistance of
the bulb? 132
OFF
V
Hz
115 V
V
mV
A
Range
Autorange
Touc h/Hold
1s
1s
10 A
V
40 mA
COM
Fused
Chapter 3
Knowns:
1. Constant Resistance
2. Voltage Changes
3.
V
I
R
Chapter 3
Knowns:
1. Resistance Changes
2. Voltage Constant
3. V IR
Chapter 3
What is the System Current?
I
V
R
Chapter 3
Create a Graph showing the
relationship of Voltage to Current
for the circuit show.
Chapter 3
Current for varying voltage and a constant
resistance
2
1.8
1.6
Current - Amps
1.4
1.2
1
Current
0.8
0.6
0.4
0.2
0
0
20
40
60
Volts
80
100
Chapter 3
1
G= R
A student takes data for a resistor and fits the
straight line shown to the data. What is the
conductance and the resistance of the resistor?
16
14
G
14.8 mA - 0 mA
1.48 mS
10.0 V - 0 V
The reciprocal of the
conductance is the
resistance:
R
1
G
1
1.48 mS
I (mA)
The slope represents the
conductance.
12
8
4
0
676 Ω
0
2
4
6
V (V)
8
10
Chapter 3
Graphic aid for the Ohm’s law
Chapter 3
What is the system current
State your knowns and value required!!
100
I
5.0
V
R
100
I=
I = 20*10-3 amp or “X” mAmp
3
5 *10
Chapter 3
What is the System Voltage?
56k
Vs = IR Vs = 5 x 10-3 amp * 56 x 103 ohms Vs = 280 volts
Chapter 3
What is the Resistance of the
Photocell?
9.0
R = 5696.2 ohms (5.67 kΩ)
Chapter 3
Application of Ohm’s law
26.8 mA
The resistor is green-blue
brown-gold. What should the
ammeter read?
Power Supply
V
A
+15 V
Gnd
5 V 2A
- +
- +
+
m
DC Am
eter -
Chapter 3
Energy and Power
When a constant force is applied to move an object over
a distance, the WORK (W) is the force times the
distance.
The force must be measured in the same direction as
the distance. The unit for work is the newton-meter
(N-m) or joule (J).
Distance
Force
Chapter 3
Energy and Power
One joule is the work done when a force of
one newton is applied through a distance of
one meter. A joule is a small amount of work
approximately equal to the work done in
raising an apple over a distance of 1 m.
The symbol for energy, W, represents
work, but should not be confused with the
unit for power, the watt, W.
1n
1m
Chapter 3
Energy and Power
• Energy is closely related to work.
• Energy is the ability to do work.
1. Measured in the same units as work,
2. Newton-meter (N-m) (same as a joule (J)).
What amount of energy is converted to heat in
sliding a box along a floor for 5 meters if the
force to move it is 400 n?
W = Fd = (400 N)(5 m) = 2000 N-m = 2000 J
Chapter 3
Energy and Power
Power is the rate of doing work.
• Rate means a time unit is required.
• Joule per second (J/s) = a watt (W).
W
P
t
What power is developed if the box in the previous
example is moved in 10 s?
P
W
t
2000 J
10 s
200 W
Chapter 3
Energy and Power
The kilowatt-hour (kWh) is a much larger unit of
energy than the joule.
• 3.6 x 106 J in a kWh.
• kWh is convenient for electrical appliances.
What is the energy used in operating a
1200 W heater for 20 minutes?
1200 W = 1.2
? kW
1.2 kW X 20minutes/60 minutes/hr = 0.396 kWh
Chapter 3
Electrical Energy to Thermal Energy
Chapter 3
Energy and Power
In electrical work, the rate energy is dissipated
can be determined from the power formula.
Given: P = I*V
and
V = I*R
Then: P I 2 R
or
V2
P
R
Together, the three forms are called Watt’s law.
Chapter 3
Energy and Power
What power is dissipated in a 27
current is 0.135 A?
resistor if the
Given that you know the resistance and current,
substitute the values into P =I 2R.
P
I 2R
(0.135 A) 2 27
0.49 W
Chapter 3
Energy and Power
What power is dissipated by a heater that draws 12 A
of current from a 120 V supply?
The most direct solution is to use P = IV.
P
IV
12 A 120 V
1440 W
Chapter 3
Energy and Power
What power is dissipated in a 100
across it?
resistor with 5 V
The most direct solution is to use
V2
P
R
5V
100
2
0.25 W
P
V2
R
It is useful to keep in mind that
small resistors operating in low
voltage systems need to be sized
for the anticipated power.
Chapter 3
Power rating of a resistor
Amount of power a resistor can dissipate without being
damaged by the heat generated
It is useful to keep in mind that
small resistors operating in low
voltage systems need to be sized
for the anticipated power.
Chapter 3
Metal film resistors come in
standard power ratings
1/8 to 1 watt
Chapter 3
A loss of energy (voltage) by electrons (charge) as they
flow through a resistance creates a voltage drop
because voltage equals energy divided by charge.
Chapter 3
Troubleshooting
Some questions to ask before starting any
troubleshooting are:
1. Has the circuit ever worked?
2. If the circuit once worked, under what
conditions did it fail?
3. What are the symptoms of the failure?
4. What are the possible causes of the failure?
?
Chapter 3
Troubleshooting
?
Plan the troubleshooting by reviewing
pertinent information:
1. Schematics
2. Instruction manuals
3. Review when and how the failure occurred
4. Talk with the operator or anyone else having
knowledge of the failure
Chapter 3
Troubleshooting
You may decide to start at the
middle of a circuit and work in
toward the failure. This approach is
called half-splitting.
Based on the plan of attack, look over
the circuit carefully and make
measurements as needed to localize the
problem. Modify the plan if necessary
as you proceed.
?
After solving the problem, it is useful to ask, “How can I
prevent this failure in the future?”
Chapter 3
Half-splitting method of
troubleshooting
Chapter 3
Ampere-hour Rating of Batteries
• Expected battery life is given as the ampere-hours
specification.
• Factors affecting battery life:
1. rate of current withdrawal (load),
2. age of battery,
3. temperature
How many hours can you expect to have a
battery deliver 0.5 A if it is rated at 10 Ah?
20 h
Battery
Chapter 3
Chapter 3
Chapter 3
Selected Key Terms
Ohm’s law A law stating that current is directly
proportional to voltage and inversely
proportional to current.
Linear Characterized by a straight-line relationship.
Energy The ability to do work. The unit is the joule (J).
Power The rate of energy usage.
Joule The SI unit of energy.
Chapter 3
Selected Key Terms
Watt The unit of power. One watt is the power
when 1 J of energy is used in 1 s.
Kilowatt-hour A common unit of energy used mainly by
utility companies.
Ampere-hour A number determined by multiplying the
rating current (A) times the length of time (h) that a
battery can deliver that current to a load.
Efficiency The ratio of output power to input power of a
circuit, usually expressed as a percent.
Chapter 3
1. 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 3
2. 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 3
3. A four-color resistor has the color-code red-violetorange-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 3
4. If the current in a 330 resistor is 15 mA, the voltage
across it is approximately
a. 5.0 V
b. 22 V
c. 46 V
d. 60 V
Chapter 3
5. A unit of power is the
a. joule
b. kilowatt-hour
c. both of the above
d. none of the above
Chapter 3
6. The SI unit of energy is the
a. volt
b. watt
c. joule
d. kilowatt-hour
Chapter 3
7. If the voltage in a resistive circuit is doubled, the power
will be
a. halved
b. unchanged
c. doubled
d. quadrupled
Chapter 3
8. The approximate power dissipated by a 330
with 9 V across it is
a. ¼ W
b. ½ W
c. 1 W
d. 2 W
resistor
Chapter 3
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