2 Electric Current

It has been proved that electrons (negative charges) move through a conductor in response to an electric field. “Electric current” is defined as the directed flow of electrons, and the direction of electron movement is from a region of negative potential to a region of positive potential. Therefore, electric current can be said to flow from negative to positive.

Conductors

Any material that will allow an electrical current to flow through it is an electrical conductor. Conductors are used in automotive equipment to carry electric current to all of the electrical equipment. The electrical properties of a substance depend mainly on the number of electrons in the outermost shell of each atom. The maximum number of electrons in an outer shell is eight. When there are less than four electrons in the outer shell of an atom, these electrons will tend to be free. This condition allows the free motion of electrons, making the substance a conductor (Figure 6-3).


Figure 6-3 — Conductors.

Electrical energy is transferred through conductors by means of the movement of free electrons that migrate from atom to atom within the conductor. Each electron moves a short distance to the neighboring atom where it replaces one or more electrons by forcing them out of their orbits. The replaced electrons repeat this process in nearby atoms until the movement is transmitted throughout the entire length of the conductor, thus creating a current flow. Copper is an example of a good conductor because it has only one free electron. This electron is not held very strongly in its orbit and can break away from the nucleus very easily. Silver is a better conductor of electricity, but it is too expensive to be used in any great quantity. Because of this, copper is the conductor used most widely in automotive applications.

Insulators

Any material that blocks electrical current flow is an electrical insulator. Insulators also are necessary to keep the electric current from taking a shorter route instead of going to the intended component.

Whenever there are more than four electrons in the outer orbits of the atoms of a substance, these electrons will tend to be bound, causing restriction of free electron movement, making it an insulator (Figure 6- 4). Common insulating substances in automotive applications are rubber, plastic, and fiberboard.


Figure 6-4 — Insulators.

Semiconductors

A semiconductor is an electrical device that acts as a conductor under certain conditions and as a nonconductor under other conditions. The most popular of all semiconductors is silicon. In its pure state, silicon is neither a good conductor nor insulator. But by processing silicon in certain ways, its conductive or insulative properties can be adjusted to suit just about any need. When a number of silicon atoms are jammed together in crystalline (glasslike) form, they form a covalent (sharing) bond. Therefore, the electrons in the outer ring of one silicon atom join with the outer ring of other silicon atoms, resulting in a sharing of outer ring electrons between all of the atoms.

 
Figure 6-5 — Covalent bonding of silicon.

Figure 6-5 shows that covalent sharing gives each atom eight electrons in its outer orbit, making the orbit complete. This makes the material an insulator because it contains more than four electrons in its outer orbit. When certain materials, such as phosphorus, are added to the silicon crystal in highly controlled amounts, the resultant mixture becomes a conductor (Figure 6-6). This is because phosphorus, which has five electrons in forming a covalent bond with silicon (which has four electrons in its outer shell), will yield one free electron per molecule, thus making the material an electrical conductor. The process of adding impurities to a semiconductor is called doping. Any semiconductor material that is doped to yield free electrons is called N-type material.


Figure 6-6 — Phosphorus-doped silicon.

When boron, which has three electrons in its outer ring, is used to dope the silicon crystal, the resultant covalent bonding yields seven electrons in the outer shell. This leaves an opening for another electron (Figure 6-7). This space is called a hole and can be considered a positive charge, just as the extra electrons that exist in N-type semiconductor material are considered a negative charge. Materials that have holes in their outermost electron shells are called positive or P-type materials. To understand the behavior of P-type semiconductors, it is necessary to look upon the hole as a positive current carrier; just as the free electron in N-type semiconductors are considered negative current carriers. Just as electrons move through N-type semiconductors, holes move from atom to atom in P-type semiconductors. Movement of holes through P-type semiconductors, however, is from the positive terminal to the negative terminal. For this reason, any circuit analysis of solid-state circuitry is done on the basis of positive to negative (conventional) current flow.


Figure 6-7 — Boron-doped silicon.

When a source voltage, such as a battery, is connected to N-type material, an electric current will flow through it (Figure 7-8). The current flow in the N-type semiconductor consists of the movement of free electrons, the same as the current flow through a natural conductor, such as copper. When a current source of sufficient voltage is connected across a P-type material, an electric current will also flow through it, but any current flow in a P-type semiconductor is looked upon as the movement of positively charged holes. The holes appear to move toward the negative terminal, as the electrons enter the material at the negative terminal, fill the holes, and then move from hole to hole toward the positive terminal. As is the case with the N-type semiconductors, the movement of electrons through P-type semiconductors toward the positive terminal is motivated by the natural attraction of unlike charges.


Figure 6-8 — Hole movement theory.

Diodes

A diode is a device that will allow current to pass through itself in only one direction (Figure 6-9). A diode can be thought of as an electrical check valve. Diodes are constructed by joining N-type material and P-type material together. The negative electrical terminal is located on the N-type material and the positive terminal is located on the P-type material.


Figure 6-9 — Diode operation.

When a diode is placed in a circuit, the N-material is connected to the negative side of the circuit and the positive side of the circuit is connected to the P-material. In this configuration, which is known as forward bias, the diode is a good conductor. This is because the positively charged holes in the P-type material move toward the junction and fill these holes using them to move across the P-material. If the connections to the diodes are reversed, current flow will be blocked. This design is known as reverse bias. When the diode is connected backwards, the positively charged holes are attracted away from the junction to the negative terminal and the free electrons in the N-material are attracted away from the junction to the positive terminal. Without the presence of holes at the junction, the electrons are not able to cross it.

Zener Diodes

A zener diode is a special type of diode that conducts current in the reverse direction as long as the voltage is above a predetermined value that is built into the device during manufacturing (Figure 6-10). For instance, a certain zener diode may not conduct current if the reverse bias voltage is below 6 volts. As the voltage increases to 6 volts or more, the diode suddenly will begin to conduct reverse bias current. This device is used in control circuits, such as voltage regulators.


Figure 6-10 — Zener diode operation.

Transistors

A transistor is an electrical device that is used in circuits to control the flow of current (Figure 67-11). It operates by either allowing current to flow or not allowing it to flow. Transistors operate electronically and have no moving parts to perform their function. This design allows for a longer operating life of the component. The major automotive applications of transistors are for electronic ignition systems and voltage regulators.


Figure 6-11 — Transistor configurations.

The PNP transistor is the most common design in automotive applications (Figure 6- 12). It is manufactured by sandwiching an N-type semiconductor element between two P-type semiconductor elements. A positive charge is applied to one of the P-type elements. This element is called the emitter. The other P-type element connects to the electrical component. This element is called the collector. The third element, which is in the middle, is made of N-type material and is called the base. The application of low current negative charge to the base will allow a heavy current to flow between the emitter and the collector. Whenever the current to the base is switched off, the current flow from the emitter to the collector is interrupted also.


Figure 6-12 — Transistor operation.

The NPN transistor is similar to the PNP transistor (Figure 6-12). The difference is that it is used in the negative side of the circuit. As the term NPN implies, the makeup of this transistor is two elements of N-type material (collector and emitter) with an element of

P- type material (base) sandwiched in between. The NPN transistor will allow a high current negative charge to flow from the collector to the emitter whenever a relatively low current positive charge is applied to the base.

Test your Knowledge

1. In a semiconductor, what type of material is doped to yield free electrons?

A. O-type
B. P-type
C. N-type
D. Y-type

2. What type of electrical device is used in electrical circuits to control the flow of current and operates by either allowing or not allowing current to flow?

A. Transistor
B. Diode
C. Resistor
D. Thermistor