Electric Circuits

Electricity for Refrigeration, Heating and Air Conditioning 7th

Edition

Chapter 3 Electric Circuits

Electric CircuitsUpon completion of this chapter the student will be

able to:• Explain the concepts of a basic electric circuits.• Explain the characteristics of a series circuit.• Explain the characteristics of a parallel circuit.• Describe how series circuits are utilized as control circuits in the air conditioning

industry.• Describe how parallel circuits are utilized as power circuits in the air-

conditioning industry.• Explain the relationship and characteristics of the current, resistance and

electromotive force in a parallel circuit.Calculate the current resistance, and electromotive force in a series circuit.

• Calculate the current, resistance and electromotive force in a parallel circuit.

Electrical Safety

Upon completion of this chapter the student will be able to:

• Explain the characteristics of the series-parallel circuit.• Describe how series-parallel circuits are utilized in the air-

conditioning industry.

Key Terms

• Closed• Control Circuit• Electric Circuit• Open• Parallel Circuit• Power Circuit• Series Circuit• Series-Parallel Circuit• Voltage Drop

Basic Concepts of Electric Circuits

• An electric circuit is the complete path of an electric current, along with any necessary elements, such as power source and a load.

• When the circuit is complete so that the current can flow, it is termed closed.

• When the path of the current flow is interrupted, the circuit is termed open.

• All electric circuits must have a complete path for electrons to flow through, a source of electrons, and some electric device (load) that requires electric energy for its operation.

Basic Concepts of Electric Circuits

Series Circuits

• Switches and controls are commonly wired in series with each other to control one or more loads.

• The simplest and easiest electric circuits to understand is the series circuits.

• The series circuit allows only one path of current flow through the circuit.

• An example of a series circuit is Christmas Lights

Series Circuits

Characteristics of a Series Circuit and Calculations for Current, Resistance and Voltage

• The current drawn in a series circuit is the same throughout the entire circuit because there is only one path for the current to flow.

• The total resistance in a series circuit is the sum of all the resistances in the circuit

• Rt = R1 + R2 + R3 + R4 + R….

Characteristics of a Series Circuit and Calculations for Current, Resistance and Voltage

• The voltage in a series circuit is completely used by all the loads in the circuits.

• The voltage of a series circuit changes through each load.• This change is called voltage drop.• The voltage drop is the amount of voltage (electrical pressure) used

or lost through any load or conductor in the process of moving the current (electron flow) through that part od the circuit.

• The sum of the voltage drop of any part of a series circuit is equal to the voltage being applied to the circuit.

Series Circuit Example

In the circuit below find the total resistance, the voltage drop for each element, and the current for the circuit.

Series Circuit Example

1. Find the total resistance of the circuit.

The total resistance of the circuit can be found by using the following formula.

Rt = R1 + R2 + R3 + R4 + R….

Rt = 5 ohms + 20 ohms + 35 ohms + 50 ohms

Rt = 110 ohms

Series Circuit Example

2. Find the amperage draw of the circuit.

The amperage draw of the circuit can be determined using Ohms law.

I = E/R

I = 120 Volts / 110 Ohms

I = 1.09 Amps

Series Circuit Example3. Find the voltage drop across each element in the

circuit.

The voltage drop across each element in the circuit can be determined by rewriting ohms law.

E = IR

For the 5 ohm resistor the voltage drop would be:

E = IR

E = 1.09 amps * 5 ohms

E = 5.45 volts

Series Circuit Example

For the 20 ohm resistor the voltage drop would be:

E = IR

E = 1.09 amps * 20 ohms

E = 21.8 volts

Series Circuit Example

For the 50 ohm resistor the voltage drop would be:

E = IR

E = 1.09 amps * 50 ohms

E = 54.5 volts

Series Circuit Example

For the 50 ohm resistor the voltage drop would be:

E = IR

E = 1.09 amps * 35 ohms

E = 38.15 volts

Parallel Circuits

• The Parallel circuit has more than one path for the electrons to flow.

• Electric devices are arranged in the circuit so that each is connected the supply voltage conductors.

• Parallel circuits are the most common in industry because almost all devices operate at the line voltage (120 volts to 240 volts).

• Example of a parallel circuit is standard wiring in a residence.

Parallel Circuits

Characteristics of a Parallel Circuit and Calculations for Current, Resistance, and Voltage

• The current draw in a parallel circuit is determined for each part of the circuit, depending on the resistance of that portion of the circuit.

• The total current draw of the entire parallel circuit is the sum of the currents in the individual sections of the parallel circuit.

• The resistance of a parallel circuit gets smaller as more resistances are added to the circuit.

• The total resistance of a parallel circuit can not be obtained by taking the sum of all the resistances.

Characteristics of a Parallel Circuit and Calculations for Current, Resistance, and Voltage

• The resistance for two resistors in a parallel circuit can be obtained using the following formula.

• Rt = (R1 * R2)/(R1 + R2)• The resistance for three of more resistor in a parallel circuit can be

obtained using the following formula.• 1/Rt = 1/R1 + 1/R2 + 1/R3 + 1/R4 + …..• The voltage drop in a parallel circuit is the line voltage being

supplied to the load.

Parallel Circuit Example

In the following parallel circuit calculate the total resistance of the circuit, the total current draw of the circuit and the current draw for each resistor.

Parallel Circuit Example

1. Calculate the current draw for each individual load.

The current for each individual resistor can be determined using Ohms law.

I = E/R

For the 35.7 Ohm resistor the current draw would be:

I = 120 volts / 35.7 ohms

I = 3.36 Amps

Parallel Circuit Example

For the 15.5 Ohm resistor the current draw would be:

I = 120 volts / 15.5 ohms

I = 7.74 Amps

Parallel Circuit Example

2. Determine the total resistance for the circuit.

The total resistance of the parallel circuit can be obtained using the formula:

Rt = (R1 * R2)/(R1 + R2)

Rt = (15.5 * 35.7)/(15.5 + 35.7)

Rt = (553.35)/(51.2)

Rt = 10.80 Ohms

Parallel Circuit Example

3. Determine the total current draw for the circuit.

The total current draw for the circuit can be determined using Ohms Law

I = E/R

I = 120 volts / 10.80 Ohms

I = 11.11 Amps

Complex Circuits• Complex circuits are also know as Series-Parallel Circuits.• A Series-Parallel circuit is a combination circuit containing loads in

both series and parallel.• Complex circuits are the most widely used circuits in industry today.

Solving a Complex Circuit

• Start with a small simple chunk of the circuit that you know how to work with that’s either all series or all parallel.

• Then keep redrawing circuit at each step as it simplifies.