Concepts of Engineering and Technology Basic Electricity and Electronics: DC Circuits Copyright © Texas Education Agency, All rights reserved. 1

Common Circuit Elements Copyright © Texas Education Agency, 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 Copyright © Texas Education Agency, All rights reserved. 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 Copyright © Texas Education Agency, All rights reserved. 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! Copyright © Texas Education Agency, All rights reserved. 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 Copyright © Texas Education Agency, All rights reserved. 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 Copyright © Texas Education Agency, All rights reserved. 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! Copyright © Texas Education Agency, All rights reserved. 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 Copyright © Texas Education Agency, All rights reserved. 9

 Open switch, no current –Resistance is infinite –Voltage is dropped across the switch Copyright © Texas Education Agency, All rights reserved. 10

 Closed switch, current flows  Current flows from negative to positive  Amount of current determined by Ohm’s Law Voltage is dropped across the light Copyright © Texas Education Agency, All rights reserved. 11

The Simplest Circuit VSVS R Copyright © Texas Education Agency, All rights reserved. 12

The Same Circuit VSVS R Copyright © Texas Education Agency, All rights reserved. 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 , abbreviation R Copyright © Texas Education Agency, All rights reserved. 14

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 Copyright © Texas Education Agency, All rights reserved. 15

The Ohm’s Law Circle V I R = = I I Copyright © Texas Education Agency, All rights reserved. 16

 Circuit Calculations V S = R = Copyright © Texas Education Agency, All rights reserved. 17

 Circuit Calculations V S = R = Copyright © Texas Education Agency, All rights reserved. 18

 I = 5.9 mA, R = 2.2 kΩ What is V?  V = I x R =.0059 A x 2200 Ω = 13 V Circuit Calculations V S = R = Copyright © Texas Education Agency, All rights reserved. 19

A Series Circuit R1R1 R2R2 R3R3 VSVS 1. What is the formula for total resistance? R T = R 1 + R 2 + R 3 (Resistance Adds) 2. What is the formula for current? I T = I 1 = I 2 = I 3 (current is the same everywhere) 3. What is the formula for voltage across R 1 ? V 1 = I 1 x R 1 V S = V 1 + V 2 + V 3 Copyright © Texas Education Agency, All rights reserved. 20

Problem 1 R1R1 R2R2 R3R3 VSVS V S = 12 V, R 1 = 250 Ω, R 2 = 150 Ω, R 3 = 500 Ω 1.What is total resistance? 2.What is total current? 3.What is V 1 ? Copyright © Texas Education Agency, All rights reserved. 21

Problem 2 R1R1 R2R2 R3R3 VSVS V S = 20 V, V 1 = 6 V, V 2 = 4 V, R 3 = 600 Ω What is R 1 ? 20 V V 1 = 6 VV 2 = 4 V 600 Ω Copyright © Texas Education Agency, All rights reserved. 22

Problem 3 R1R1 R2R2 R3R3 VSVS V S = 18 V, V 1 = 3 V, R 3 = 600 Ω, I T = 6 mA What is R 2 ? 18 V V 1 = 3 V I T = 6 mA 1500 Ω Copyright © Texas Education Agency, All rights reserved. 23

A Parallel Circuit The voltage across each branch is the same V S = V 1 = V 2 = V 3 The current from each branch adds I T = I 1 + I 2 + I 3 Copyright © Texas Education Agency, All rights reserved. 24

A Parallel Circuit The voltage across each branch is the same V S = V 1 = V 2 = V 3 The current from each branch adds I T = I 1 + I 2 + I 3 Copyright © Texas Education Agency, All rights reserved. 25

Parallel Example 12 V 300 Ω500 Ω750 Ω V S = 12 V, R 1 = 300 Ω, R 2 = 500 Ω, R 3 = 750 Ω Solve for R T Ω Copyright © Texas Education Agency, All rights reserved. 26

Alternate Method 12 V 300 Ω500 Ω750 Ω I T = I 1 + I 2 + I 3 I T = =.08 A Copyright © Texas Education Agency, All rights reserved. 27

Problem Ω400 Ω I 1 =.064 A R T = 130 Ω R 1 = 250 Ω, R 2 = 400 Ω, R T = 130 Ω, I 1 =.064 A Solve for I 3 Copyright © Texas Education Agency, All rights reserved. 28