Parallel Circuits

10:42 / Posted by tech data / comments (1)

In Parallel Circuits, the loads are connected across the power line to form branches. The loads operate independently of each other, and therefore a break in any one branch does not prevent the line voltage from being applied to the remaining branches. The result is that one path (branch) can be open with the load not receiving current without the other loads being affected, as in the newer strings of holiday lights.

Current has a number of paths to follow. If all paths are available, the current divides itself between the branches back to the source. If a path is open, the current divides between the remaining available paths and goes back to the source. Parallel circuits are used in the majority of industrial, commercial and residential applications of electricity.

The next two circuit illustrations show three resistors in parallel. The only difference between the two circuits is the resistor values. To use Ohm's Law to solve the equations, you need to know how resistance, current and voltage behave in parallel circuits. The total resistance (RT) of a parallel circuit decreases as more branches are added. The total resistance of a parallel circuit is always less than the resistance of any of its branches and is therefore less than the value of the lowest resistance in the circuit.

To determine total resistance (RT) two different formulas are used:

  • Resistors with equal values
  • Resistors with unequal values

Resistors with Equal Values: This RT is determined by dividing the value of one of the resistors by the total number of resistors in the circuit. Using this formula, the total resistance for the first circuit is calculated to be four ohms.

Resistors with Unequal Values: Calculating RT is more complicated and is shown below:

As a point of interest, the RT for this circuit is 1.09W.

To determine current, you need to find the total current, which is the sum of all currents in all the branches. The following simple formula represents the total current (IT), and the illustration offers a demonstration.

To determine the individual branch currents, it is necessary to know whether or not all the resistors have the same value.

Current with Equal Resistor Values: The current divides equally. Divide the total current by the total number of branch resistors to determine the current flowing through each branch. The following illustration and calculation demonstrate this procedure.

Current with Unequal Resistor Values: The current is greater through the branch with the least resistance.

Parallel circuit voltage is easy to determine because it is the same across each resistor and/or load. The illustration shows a parallel circuit with voltmeters indicating the voltage across each resistor to be the same as the source battery.

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Ohm's Law

10:20 / Posted by tech data / comments (0)

There is a definite relationship between the three primary electrical characteristics: current, voltage and resistance. A German mathematician, George Simon Ohm, formulated this relationship in the 19th century. His law (Ohm's Law) stated that current is directly proportional to voltage and inversely proportional to resistance. The following formula was derived from that law:
Current = Voltage/Resistance or I = E/R
Current (I) in amps: Voltage (E) in volts: Resistance (R) in ohms
Figure 7. Ohm's Law
Ohm's Law is the basic formula used in all AC and DC electrical circuits. So if you know two of the three characteristics, you can calculate the third one.
Electrical designers use it to determine how much voltage is required for a certain load, like a motor, a computer, or even a house full of appliances.
DC Circuits
We can use a simple DC circuit here to demonstrate Ohm's Law. Before we do any calculations, however, let's briefly discuss the symbols that will be used in our circuit diagrams.
Voltage Symbol: The terminals of a battery are symbolically indicated on an electrical drawing by one or more pairs of lines. The longer line represents the positive terminal, and the shorter line the negative terminal.

Resistance Symbol: Resistance is represented in one of two ways: either an open rectangle or a zigzag line. Resistance in a circuit can take the form of many different components from light bulbs to motors. Most of these components have their own unique
Figure 9. Resistance Symbol (Resistor)

symbols. For now, we will use the zigzag line symbol to represent the loads.

Series CircuitUsing the simple circuit shown, assume that the voltage supplied is 12 volts, and the resistor provides six ohms of resistance. To determine the current, use the following Series Circuit
Using the simple circuit shown, assume that the voltage supplied is 12 volts, and the resistor provides six ohms of resistance. To determine the current, use the following formula.
E Voltage (volts) =I R

Another example of a simple DC circuit is a flashlight. Batteries in the flashlight provide the DC voltage source, the inside of the battery case usually acts as the conductor, and the lamp bulb is the load.

The flashlight has an ON and OFF switch which controls the flow of electricity. Because there must always be a complete path for current to flow, the switch stops the flow when it is in the OFF position. Why? Because the circuit is open when the switch is OFF. When the switch is ON, the circuit is complete and current flows, lighting the bulb.

The simple circuits above are called Series Circuits, which means all loads are connected one after another in a series. If a conductor or a load is broken, it opens the circuit. This condition does not allow the current to complete the circuit and makes the entire circuit dead. A good example of this is the old design for holiday lights. If one bulb was burned out, the entire string would not light. the entire string would not light. Figure 12. Series Circuit
Take a look at the next series circuit. The voltage is unknown, but can be calculated using Ohm's Law, E = IR. The current (I) is four amps as shown, but the resistance has to be calculated. In a series circuit, when more than one resistance is in the circuit, the resistances are added together to get the total resistance (RT). The RT is 12 ohms. Given these two values and Ohm's Law, the voltage is 48 volts.


Now is a good time to talk about how current and voltage behaves in a series circuit. The current value is the same in every part of the circuit. An Ammeter can verify this.
Voltage, on the other hand, does not remain constant throughout the circuit. Voltage values can be measured across each resistor or load. This is called the Voltage Drop. The total voltage (VT) is equal to the sum of all the voltage drops in that circuit. A Voltmeter can verify this. The formula is:
(VT) = V1 + V2 + V3 ...

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Basic fundamental of Electronics

10:00 / Posted by tech data / comments (3)

The cliché, "opposites attract," is certainly true when dealing with electrical charges. Charged bodies have an invisible electrical field around them. When two like-charged bodies are brought close together, they repel each other. When two unlike charged bodies are brought closer together, their electrical fields work to attract.
Characteristics
When we look at the flow of electricity, we need to look at its characteristics. There are three main characteristics of electricity:
Current

The flow of free electrons in the same general direction from atom to atom is referred to as current and it is measured in Amperes ("amps" or "A"). The number of electrons that flow through a Conductor's cross-section in one second determines amps. Current can be expressed in a number of different ways, such as:


When discussing current, the direction of current flow needs to be considered. There are two different theories about this:

Conventional Flow:

This theory states that electrons flow from positive to negative. Benjamin Franklin theorized this when very little was known about electricity. It states that an invisible fluid known as electricity tended to flow through a wire from the positive to the negative. Ben's theory became the convention (hence the term "conventional current") in electrical theory, mathematics, textbooks and electrical equipment for the next hundred years.

Electron Flow: This theory states that electrons flow from negative to positive. When more was known about the behavior of electrons, scientists discovered that electrons actually flow from negative to positive. Because electrons are negatively charged, it follows that they are attracted by positively charged bodies and repelled by negatively charged bodies.

Despite the fact that it has been positively determined that electron flow is the correct theory, the conventional flow theory still dominates the industry. Either theory can be used as long as the orientations are correct. Conventional flow will be used from this point on in these training modules unless otherwise stated.

Voltage
Voltage is the force that is applied to a conductor to free electrons, which causes electrical current to flow. It is measured in Volts or "V". Current will flow in a conductor as long as voltage, the electrical pressure, is applied to the conductor. Voltage is expressed in a number of ways:
There are two methods that voltage forces current to flow:
Direct current:
With this method, the voltage forces the electrons to flow continuously in one direction through a closed circuit. This type of voltage is called Direct Current (DC) voltage. Batteries and DC generators produce DC voltage.

Alternating current:
With this method, voltage forces electrons to flow first in one direction, then in the opposite direction, alternating very quickly. This type of voltage is called Alternating Current (AC) voltage. A generator is used to produce AC voltage. The voltage generated by utility companies for our home, factories and offices is AC voltage.

Resistance :
This is the third characteristic of electricity. The restriction to the flow of electrons through a conductor is called resistance and it is measured in Ohms and abbreviated "W", the Greek symbol Omega. Resistance is expressed in a number of ways:In general, there are four factors that affect the amount of resistance in a conductor:

  • Material
  • Length
  • Cross-Sectional Area
  • Temperature

Material:

We know that the amount of electron flow depends upon how readily particular atoms give up their electrons and accept new electrons. Materials that permit this are called conductors. Copper, silver and aluminum are considered good conductors. Materials that don't readily give up electrons, which restricts the flow, are called Insulators. Rubber, glass and porcelain are considered good insulators.


Conductors and insulators perform a very important team function. An electrical cord to a lamp, for example, has a copper wire conductor on the inside with a rubber-coating insulator around the outside. Free electrons flow along the copper wire to light the lamp while the rubber coating keeps the free electrons inside to prevent shock and other problems.


Length:
The longer the conductor, the more resistance in the conductor. Resistance is increased or decreased in proportion to the conductor's length. For example, a 2-foot long conductor would have twice the resistance of a one-foot long conductor. Cross-Sectional Area: As the cross-sectional area of a conductor increases, the resistance decreases, and vice versa. For example, if the area of a conductor is doubled, the resistance is cut in half.
Temperature: Usually when the temperature of a conductor increases, the resistance increases. The temperature factor is not as predictable as the other factors, but it must be considered when dealing with electricity.

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Fundamentals of Electricity

09:52 / Posted by tech data / comments (0)



Welcome to Module 1


This module will cover the fundamentals of electricity in a practical way, and will not be complicated by complex theory and mathematical calculations. The module will present a number of different topics. You will be introduced to information that will be used in later modules.



Like the other modules in this series, this one presents small, manageable sections of new material followed by a series of questions about that material. Study the material carefully, then answer the questions without referring back to what you've just read. You are the best judge of how well you grasp the material. Review the material as often as you think necessary. The most important thing is establishing a solid foundation on which to build as you move from topic to topic and module to module.



Introduction to Electricity


The technical term electricity is the property of certain particles to possess a force field which is neither gravitational nor nuclear. To understand what this means, we need to start simply. Everything, from water and air to rocks, plants and animals, is made up of minute particles called atoms. Atoms consist of even smaller particles called protons, neutrons and electrons. The nucleus of the atom contains protons, which have a positive charge, and neutrons, which have no charge. Electrons have a negative charge and orbit around the nucleus. An atom can be compared to a solar system, with the nucleus being the sun and the electrons being planets in orbit.


Electrons can be freed from their orbit by applying an external force, such as movement through a magnetic field, heat, friction, or a chemical reaction.A free electron leaves a void, which can be filled by an electron forced out of its orbit from another atom. As free electrons move from one atom to another, an electron flow is produced. This electron flow is the basis of electricity.

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