Concepts of Engineering and Technology Basic Electricity and Electronics: DC Circuits Copyright ©...

Concepts of Engineering and Technology

Basic Electricity and Electronics:DC Circuits

Copyright © Texas Education Agency, 2012. All rights reserved.

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Common Circuit Elements


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http://upload.wikimedia.org/wikipedia/commons/6/69/SimpleCircuit.SVG

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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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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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!


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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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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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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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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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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The Simplest Circuit

VS R


12

The Same Circuit

VS R


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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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The Ohm’s Law Circle

•

V

I R =

=I

I


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Circuit Calculations

VS = R =


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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


VS = R =


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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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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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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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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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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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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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