Electrical Circuits

Introduction

An electrical circuit is a path or group of interconnecting paths, capable of carrying electrical current.

Current flows from one end of a circuit to the other end when the ends are connected to opposite charges.

These ends usually called power and ground. Current flows only in a closed or complete

circuit.

Circuit components

Every electrical circuit should contain the following components:– Power source.– Protection device.– Control device.– Load.

Types of electrical circuits

Three basic types of electrical circuits:– Series circuits.– Parallel circuits.– Series-parallel circuits.

Series circuit

A series circuit is the simplest kind of circuits. In a series circuit, each electrical device is

connected to other electrical devices in such away that there is only one path for the current to flow.

Series circuit rules

At any given point in the circuit the current value is the same.

The total circuit resistance is equal to the sum of all individual resistances.

The voltage drop across all circuit loads are equal to the applied voltage.

Parallel circuit

There is more than one path for current to flow.

Each current path is called a branch. All branches have the same voltage. The current flow in each branch depend on

resistance.

Parallel circuit rules

The voltage is the same in each parallel branch.

The total current is the sum of each individual branch currents.

Equivalent resistance is smaller than the smallest resistance of any individual branch.

The following equation is used to calculate the equivalent resistance:

Series-parallel circuits

A series-parallel circuit is composed of series section and parallel section.

All the rules of series & parallel are applicable.

Notes for Series-parallel circuits

Examine the circuit and determine the paths that current may flow through circuit before return to the source.

Redraw a complex circuit to simplify its appearance.

A correctly redrawn circuit contain only one series resistor.

Electrical circuits laws

The most important laws for electrical circuits:– ohm's law.– Kirchoff's current law (KCL).– Kirchoff's voltage law (KVL).

Ohm's law

Ohm's law is a fundamental law of electricity that relates the quantities of voltage, current and resistance in a circuit.

Ohm's law states that current flow in a circuit is directly proportional to circuit voltage and inversely proportional to circuit resistance.

Notice

Most Caterpillar electrical circuits use 12 or 24 volt, so the amount of current will be determined by how much resistance is present in the circuit.

Remember, the current flow does the work. Voltage is only the pressure that moves the current and resistance is opposition to current flow.

Ohm's law

Assuming the resistance doesn't change:– As voltage increases, current increases.– As voltage decreases, current decreases.

Assuming the voltage doesn't change:– As resistance increases, current decreases.– As resistance decreases, current increases.

Ohm's law equation

Ohm's law can be expressed as an algebraic equation in which:

– “E” stands for electromotive force (voltage).– “I” stands for intensity (amperage).– “R” stands for resistance (ohms).

E = I X R

To determine voltage, multiply current times resistance.

To determine current, divide voltage by resistance.

To determine resistance, divide voltage by current.

Ohm's law solving circle

The ohm's law solving circle is an easy way to remember how to solve any part of the equation.

To use the solving circle cover any letter that you don't know.

Voltage unknown

The value of the source voltage is unknown, so the voltage is current times resistance.

Resistance unknown

In this circuit the value of resistance is unknown, so the resistance is voltage divided by current.

Current unknown

In this circuit the current is unknown, so to calculate the value of current, voltage is divided by resistance.

Power

Power is a measure of the rate at which energy is produced or consumed.

In engine, the output power is a measure of its ability to do mechanical work.

In electric, the power is a measure of the rate at which electrical energy is converted into heat.

The unit of measurements for power is watt. One watt equals one volt times one ampere. The basic equation for the power formula is:

Kirchoff's current law (KCL)

Kirckoff's current law (KLC) states that: The algebraic sum of the currents at any junction in an electrical circuit is equal to zero.

Simply stated, all the current that enters a junction is equal to all the current that leaves the junction.

Kirchoff's voltage law (KVL)

Kirchoff`s voltage law (KVL) states that : The algebraic sum of the electromotive forces and voltage drops around any closed electrical loop is zero.

Examples for applying ohm's law

In the following circuit many components connected in series:

Two values of the circuit (voltage & resistance) are known, so to calculate the current the total resistance should be calculated.

Rt = R1 + R2 + R3 or Rt = 3+3+6= 12 ohm

Since E = I x R

then I = E/R I = 24/12 = 2 ampere

Exercise

Use ohm's law and rules for series circuits to calculate the unknown value.

Ohm's law in parallel circuit

The following circuit shows two resistors connected in parallel:

The following equation is used to solve for total resistance:

The value of current can be calculated from ohm's law as follow:

I = E/Rt then I = 24/2 =12 ampere

Example

Two ways to calculate the total current (A) for the following circuit:

Exercise

For the following circuit:– What is the voltage drop across R1?– What is total circuit resistance?– How much current is following through resistor R3?– What is total circuit current?

SI

The International System of units (SI) is a measuring system used through the world.

The most common units in the study of basic electrical theory are Mega (millions), Kilo (thousands) , Milli (thousandths) and Micro (millionths) .

The following table lists some of the more common prefixes and their standard abbreviations and power of 10:

Any Questions?