CH15 basics of electrical systems STUDENT VERSION rev

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Chapter 15

Basics of Electrical Systems

Flow of Electricity (cont’d)

Flow of Electricity (cont’d)

• To have continuous flow of electricity:

– Must be an excess of electrons in one place – Must be a lack of electrons in another place – Must be a path between the two places

Flow of Electricity (cont’d)

Automobiles use two power sources

• Battery

– Uses chemical energy to provide energy

– Produces direct current electricity

• AC Generator

– Moves magnetic field passed a conductor – Produces alternating current

Amperage or Current

• DC current – flows in one direction only

• AC current – electrons change direction at a fixed rate

• Automobiles typically use DC

• Some components generate or use AC

• Voltage does not flow.

• Also called electromotive force (EMF)

Voltage

• When any substance flows, it meets resistance

• Resistance to electron flow can be measured

• Resistance produces heat

• Loads are good resistance

• Too low or too high resistance can cause circuit faults

Circuit Terminology

• Complete circuits provide a path that

connects the positive and negative

• A complete circuit is called a closed circuit

• An incomplete circuit is an open circuit

• Electrical flow can be controlled and

perform work in a closed circuit

Ohm’s Law Formula

• 1 volt can push 1 ampere through 1 ohm of resistance

• The formula can be expressed as:

– E (volts) = I (amps) x R (resistance)

– R (resistance) = E (volts) / I (amps)

– I (amps) = E (volts) / R (resistance)

Using Ohm’s Law

Using Ohm’s Law (cont’d)

• Since E = I · R

12 = 4 · 3

3 = 12 / 4

4 = 12 / 3

Voltage Drops

• The amount of voltage required to cause

current flow through a load

• Energy (voltage) is changed to another form

as it flows through a load

• The voltage leaving the load is less than the voltage supplied – the difference is the

voltage drop

• Dependent upon current and resistance

Measuring Voltage Drop

Voltage Drops (cont’d)

Characteristics of a

Series Circuit

• The current is the same at all points of the

circuit

• The voltage drop across each resistor will be

different if the resistor values are different

• The sum of the voltage drops across each

resistor equals the source voltage

• The total resistance is equal to the sum of all the resistances

Series Circuit

Characteristics of a

Parallel Circuit

• The total resistance will always be less than the smallest resistor lowest resistance leg

• The voltage drop across each parallel leg

will be the same

• The voltage applied to each leg is the same

Parallel Circuit

• Each leg has 12V supply.

• 12V dropped by each load/leg

Using Ohm’s Law

• Resistances not added in parallel circuits.

• For a two leg circuits:

– R total = R1 x R2/R1 + R2

• For multiple leg circuits:

Characteristics of a Series-Parallel Circuit

• The total resistance is the sum of the

resistance value of the parallel portion and the series resistance

• The voltage drop over the parallel branch resistance is determined by the resistance value of the series resistor

Characteristics of a Series-Parallel Circuit (cont’d)

• The total amperage is the sum of the

current flow through each parallel branch

• The total amperage of each parallel

branch is determined by the resistance in the branch

Series-Parallel Circuit

Circuit Components

Power Sources

• Modern vehicles use 12-volt systems

• Battery voltage is 12.6 volts

• Charging system output 14 – 15 volts

• Increasing voltage reduces amperage for the same resistance

– A higher voltage supply requires smaller wire due to decreased current flow

High Voltage (HV) Systems

• Battery used to power electric motors and some accessories, such as A/C

• May have a low voltage starter motor and battery

• Hybrids typically run from 42 to 600 volts

• Higher voltage reduces amperage

Types of Circuit Protection

Fuse Types

Types of Circuit Protection

– Take the place of fusible links.

– Are used to protect several circuits.

Types of Circuit Protection

(cont’d)

• Circuit Breaker

– Is designed to open when circuit current

exceeds a given point.

– Most will reset when current flow stops.

• PTC Circuit Protector

– Its resistance increases when current rises above a specific level.

• Can have several inputs and/or several

outputs (ganged switch)

Relay Characteristics

• Relay

– Uses a small current circuit to control a larger current circuit.

– Uses an electromagnetic switch to open or

close contacts that control the larger current – Can be normally open or normally closed.

Typical Relay

Conductors and Insulators

• Conductors have low resistance to current flow

• Four or fewer electrons in outer shell or ring

• Most metals are conductors

• Insulators resist current flow

• More than four electrons in outer shell or ring

• Thermal plastics most common

Conductors and Insulators (cont’d)

• Conductor resistance depends on:

– Length of wire

– Diameter of wire

– Temperature of wire

• Double wire length – double the resistance

• Double wire diameter – halve the resistance

• Increase wire temperature – increase resistance

Closed Circuit

Open Circuit