concepts of engineering and technology basic electricity and electronics: dc circuits copyright ©...
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Concepts of Engineering and Technology
Basic Electricity and Electronics:DC Circuits
Copyright © Texas Education Agency, 2012. All rights reserved.
1
Common Circuit Elements
Copyright © Texas Education Agency, 2012. All rights reserved.
2
Battery DC Voltage source
– Voltage amount is typically given, as in 12 VDC
Consists of one or more cells– Cells are in series– Cell voltage adds to give total
source voltage The long side is positive The short side is negative The negative side is usually
grounded or called ground
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3
Ground
Also called common– Meaning shared– A return path for every circuit branch– Connected together
Usually the most negative point in the circuit
By definition, ground voltage is zero volts– Why?
Ground is not a device
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4
Fuse One of the major protection elements
– Another is called a circuit breaker– A ground fault device (GFCI) is a type of circuit
breaker Protects the circuit, not the person!
– A GFCI protects people– Turns off the circuit, like a switch
• A switch is a control element, not a protection element
Replaced once it is blown– With the same amperage rating!
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5
Resistor
One of the most common circuit elements Used to limit or restrict current Resistance amount indicated with a color
code– A circle or band around the resistor
The physical size indicates the power rating– Not the resistance value!
The smaller the resistance value, the larger the amount of current
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6
Voltmeter
Measures across a device– Red lead to positive, black to negative
Measures a voltage difference Has a high amount of internal resistance
– So no current flows from the circuit into the meter– Does not change circuit voltage, current, or
resistance Voltage range is usually a meter setting
– Selector switch or button
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7
Ammeter Measures amperage
– Amp meter Must be placed in the circuit
– Current must flow into (through) the meter to be measured
An ammeter has low (zero) internal resistance– So it doesn’t reduce circuit current– Leads must be changed from volts/ohms– Dangerous to the meter!
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8
An Electrical Circuit Must have a voltage source Must have a load
– A resistor or other device to limit current– Can be a circuit component, like a light
Must have a complete path from one side of the voltage source to the other– Usually a wire– A path to earth ground is dangerous and
unwanted Should have a protection device and a
control device
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9
Open switch, no current– Resistance is infinite– Voltage is dropped across the switch
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Closed switch, current flowsCurrent flows from negative to positiveAmount of current determined by Ohm’s
Law
Voltage is dropped across the light
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11
The Simplest Circuit
VS R
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12
The Same Circuit
VS R
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13
Definitions and Units
The unit of charge is the COULOMB– Charge has the symbol Q, abbreviation C
The unit of voltage is the VOLT– The symbol is V, the abbreviation is V
The unit of current is the AMP– Current uses the symbol I, abbreviation A
The unit of resistance is the OHM– Resistance has the symbol W, abbreviation R
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Definitions
Voltage is caused by a buildup of charge Current is a flow of charge
– One amp equals one coulomb per second Ohm’s Law shows the relationship
between current, voltage, and resistance
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15
The Ohm’s Law Circle
•
V
I R =
=I
I
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Circuit Calculations
VS = R =
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Circuit Calculations
VS = R =
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I = 5.9 mA, R = 2.2 kΩ What is V?
V = I x R = .0059 A x 2200 Ω = 13 V
Circuit Calculations
VS = R =
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19
A Series CircuitR1 R2
R3VS
1. What is the formula for total resistance? RT = R1 + R2 + R3 (Resistance Adds)
2. What is the formula for current? IT = I1 = I2 = I3
(current is the same everywhere)3. What is the formula for voltage across R1?
V1 = I1 x R1 VS = V1 + V2 + V3
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Problem 1R1 R2
R3VS
VS = 12 V, R1 = 250 Ω, R2 = 150 Ω, R3 = 500 Ω
1. What is total resistance? 2. What is total current? 3. What is V1?
Solution 1
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21
Problem 2R1 R2
R3VS
VS = 20 V, V1 = 6 V, V2 = 4 V, R3 = 600 Ω
What is R1?
20 V
V1 = 6 V V2 = 4 V
600 Ω
Solution 2
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22
Problem 3R1 R2
R3VS
VS = 18 V, V1 = 3 V, R3 = 600 Ω, IT = 6 mA
What is R2?
18 V
V1 = 3 V
IT = 6 mA
1500 Ω
Solution 3
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23
A Parallel Circuit
The voltage across each branch is the sameVS = V1 = V2 = V3
The current from each branch addsIT = I1 + I2 + I3
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24
A Parallel Circuit
The voltage across each branch is the sameVS = V1 = V2 = V3
The current from each branch addsIT = I1 + I2 + I3
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25
Parallel Example
12 V 300 Ω 500 Ω 750 Ω
VS = 12 V, R1 = 300 Ω, R2 = 500 Ω, R3 = 750 ΩSolve for RT
300 500 750 150 Ω
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Alternate Method
12 V 300 Ω 500 Ω 750 Ω
IT = I1 + I2 + I3
IT = .04 + .024 + .016 = .08 A
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Problem 4
250 Ω 400 Ω
I1 = .064 A
RT = 130 Ω
R1 = 250 Ω, R2 = 400 Ω, RT = 130 Ω, I1 = .064 A
Solve for I3 Solution 4
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