Fall is in the air and a fresh, eager group of Advanced Auto students have gathered in my classroom. As I explain the four units we'll teach this year - electrical/ electronics, engine performance, brake systems and suspension/ steering & alignment - I see no reaction in the students' eyes.

In years past, I've heard comments such as, "Why do we have to learn about electronics? I took this class so I could learn mechanics!" I have always used this moment as a teaching opportunity to explain the complexity of the modern automobile to our young techs-in-training. When I heard no reaction after introducing our curriculum, I still felt I had to explain the need for our electrical/electronics section. I was pleased to tell them we had acquired five General Motors Corp. (GM) Specialized Electronics Trainers (SET) recently. "You will," I explained, "receive the exact same training that all GM technicians get when they go to the training centers." Now I saw some reaction.

This month in Tech to Tech, I'd like to share with you some of the information being presented in my advanced classes this year.

Students in my curriculum are required to build circuits shown in schematics and test them using a Fluke digital volt/ohm meter (DVOM). After we build a strong basic electrical foundation in our students, we move on to more advanced electronic concepts. For this article, I will simply share some of the core data presented in today's electronics courses. This article will not only give you some insight as to what future techs are learning, but also serve as a refresher course to any automotive electrical/electronics training you may have received in the past.

Properties of Electricity

Today's technician must be able to define the properties of electricity, including EMF potential (voltage), current (amperes), resistance (ohms), and power (watts). One should also be able to explain the difference(s) between AC and DC current.

In my class, students conduct a compass current detector experiment in which they use a compass to indicate current flow in a circuit. These hands-on projects are great learning tools, but again, for our purposes here, I can only share some information with you.

Detecting Current Flow

A number of electrons gathered in one place constitute an electrical charge called an electrical potential or "voltage." Voltage is measured in volts (V). Since it is used to "move electrons," an externally applied electrical potential is sometimes called an "electromotive force" or EMF. Potential, voltage, and EMF all mean the same thing.

Voltage is often described as an electrical "pressure" that drives electron flow or current. This pressure is known as electromotive force, or EMF. A battery and generator are automotive devices used to provide the pressure, or voltage, required to operate electrical components.

This pressure, or voltage, exists only when there is a higher potential of electrons at one point than at another point. This difference in potential is voltage. Therefore, voltage is pressure available to push electrons from the power supply through the circuit and back to the power supply.

There are two types of voltage: direct current, more commonly called DC; and alternating current, more commonly called AC.

Direct current is best described as a direct or continuous flow of electrons in one direction. Most automotive systems use DC because it can be stored electro-chemically in a battery.

Alternating current is best described as an alternating - or back and forth - flow of electrons. Automotive generators produce AC potential, which is easier to produce in a generator due to the laws of magnetism, but extremely difficult to store. Generators incorporate special circuits that convert the AC to DC before it is used in a vehicle's electrical system.

Current, Flow and Electron Theory

The movement of electrons in a circuit is the flow of electricity, also called "current." Current is measured in units known as amperes or amps (A). An amp expresses how many electrons are moving through a circuit at a given time. The time interval we use in electronics is the second. The more electrons moving through a circuit, the higher the amperage.

Current flow is usually shown as flowing from the positive terminal to the negative terminal. This current flow is called the "conventional theory." Another way of describing current flow is called the "electron theory," which states that current flows from the negative terminal to the positive terminal. The conventional theory and the electron theory are two different ways of describing the same current flow.

Essentially, both theories are correct. The electron theory follows the logic that electrons move from an area of many electrons (negative charge) to one of few electrons (positive charge). However, in describing the behavior of semiconductors, we often describe current as moving from positive to negative.

The important thing to know is which theory is being used by the service literature you happen to be using. Service manual schematics use conventional current flow theory.

Conductors

Electrons move along a path called a conductor. They move by traveling from atom to atom. Materials that make it easy for electrons to move through them are called "good conductors." Examples of good conductors include aluminum, copper, silver and gold. A material is a good conductor if it has many free electrons, or electrons that can be easily removed.

Other materials make it difficult for electrons to move through them. These are called "poor conductors" or "insulators." A material that is a good insulator keeps its electrons tightly bound in orbit. Examples of insulators include rubber, wood, most plastics and ceramics. No material is a perfect insulator, and some insulators (such as wood) do conduct some current flow when wet.

Wires

A wire in an automotive harness is made up of a conductor and an insulator. The metal core of the wire, typically made of copper, is the conductor. The plastic (or other material) jacket that coats the core is the insulator.

Under normal circumstances, electrons move a few inches per second. Yet, when electrical potential is applied to one end of a wire, the effect is felt almost immediately at the other end of that wire. This is so because the electrons in the conductor affect one another, much like billiard balls in a line.

Resistance

Resistance is the opposition to the movement of electrons, or current flow. Resistance is measured in units called ohms. Most resistance sources are designed into the circuit and are known as loads, such as light bulbs or motors. As a matter of fact, all electrical devices, including wires, have some resistance.

As resistance works to oppose current flow, it changes electrical energy into some other form of energy such as heat, light or motion.

Transistors, Transistor Types

A diode is only one type of semiconductor. By combining several kinds of semiconductor material, we can create transistors. Like diodes, transistors control current flow. Transistors can perform practically all the functions that were once performed by vacuum tubes, but in much less space and without creating as much heat. Transistors are used in many automotive applications, including radios, integrated passive components (IPCs), body controllers and other solid state switches.

There are many kinds of transistors. They can be divided into two major groups: bipolar and unipolar (also called field effect transistors or FETs). While there are several differences between the two types, the most important difference for our purposes is this:

field effect transistors vary voltage to control current flow Bipolar transistors are more common in automotive circuits, so we'll concentrate on them.

Transistor Construction

Like diodes, transistors contain a combination of N-type and P-type material. However, transistors contain three groupings of material instead of two. The three groups are arranged so that N-type and P-type materials alternate (either as an NPN or a PNP group). In practical terms, this means diodes have two leads while transistors have three. Figure 1 is a symbolic representation of transistor construction.

Emitter, Base and Collector

In the illustration, the material on the left is called the emitter. The material sandwiched in the middle is the base. The material on the right is the collector.

The symbols on the top of Figure 1 are the schematic symbols for a transistor. The arrow indicates current flow direction (using conventional theory), and is always on the emitter. The arrow points in a different direction depending on whether the transistor is PNP or NPN.

FETs also have three sections; they are referred to as the gate (which approximates the function of the base), the source (similar to the emitter) and the drain (similar to the collector).

Basic Function

A transistor works by using the base to control the current flow between the emitter and the collector. When the transistor is turned "on," current can flow in the direction of the arrow only. When the transistor is "off," current can't flow in either direction.

Understanding transistor operation is easy if you think of a transistor as a relay.

Base Paths

It's important to realize the base leg of a bipolar transistor controls the flow of current. Although it accounts for only a small amount of the total current, it is current flow through the base that allows current to flow from emitter to collector.

PNP or NPN Transistors?

There's an easy way to identify the kind of transistor without thinking about the movement of electrons or electron holes. Just remember that the arrow always points toward the N material and away from the P material. So, for a PNP transistor, the arrow points inward toward the base. For an NPN transistor, the arrow points away from the base.

In automotive circuits, NPN transistors are much more common than PNP.

Transistor Operation

When you're trying to understand how a transistor functions in a specific circuit, there are two facts you must remember:

First, an NPN transistor is turned on by applying voltage to the base leg, and turned off by removing voltage from the base leg. This is very similar to the operation of a relay, which is turned on and off by applying and removing voltage to the coil.

Second, the current through the base circuit is always much smaller than the current across the collector circuit. Changing the base current a little results in a big change in the collector current. The current through the emitter circuit is always the largest of all. In fact, the emitter current must be equal to the base current added to the collector current. Put another way, the current in the emitter circuit is split between the base circuit and the collector circuit.

If you don't have a solid grasp of these electrical/electronic concepts, there are many excellent training opportunities out there.

Class dismissed.

Brian Manley is a vocational automotive instructor for the Cherry Creek school district in Aurora, Colo. He is an ASE master certified automobile technician and a former member of the National Automotive Technicians Education Foundation (NATEF) board of trustees. He can be reached at manley_brian@hotmail.com.

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