BASIC DC THEORY 1 next>
When most people think of DC, they usually think of batteries. In addition to
batteries, however, there are other devices that produce DC which are frequently
used in modern technology.
A battery consists of two or more chemical cells connected in series. The combination of
materials within a battery is used for the purpose of converting chemical energy into electrical
energy. To understand how a battery works, we must first discuss the chemical cell.
The chemical cell is composed of two electrodes made of different types of metal or metallic
compounds which are immersed in an electrolyte solution. The chemical actions which result
are complicated, and they vary with the type of material used in cell construction. Some
knowledge of the basic action of a simple cell will be helpful in understanding the operation of
a chemical cell in general.
In the cell, electrolyte ionizes to produce positive and negative ions (Figure 1, Part A).
Simultaneously, chemical action causes the atoms within one of the electrodes to ionize.
Due to this action, electrons are deposited on the electrode, and positive ions from the electrode
pass into the electrolyte solution (Part B). This causes a negative charge on the electrode and
leaves a positive charge in the area near the electrode (Part C).
The positive ions, which were produced by ionization of the electrolyte, are repelled to the other
electrode. At this electrode, these ions will combine with the electrons. Because this action
causes removal of electrons from the electrode, it becomes positively charged.
A simple DC generator consists of an armature coil with a single turn of wire. The armature coil
cuts across the magnetic field to produce a voltage output. As long as a complete path is present,
current will flow through the circuit in the direction shown by the arrows in Figure 2. In this
coil position, commutator segment 1 contacts with brush 1, while commutator segment 2 is in
contact with brush 2.
Rotating the armature one-half turn in the clockwise direction causes the contacts between the
commutator segments to be reversed. Now segment 1 is contacted by brush 2, and segment 2 is
in contact with brush 1.
Due to this commutator action, that side of the armature coil which is in contact with either of
the brushes is always cutting the magnetic field in the same direction. Brushes 1 and 2 have a
constant polarity, and pulsating DC is delivered to the load circuit.
A thermocouple is a device used to convert heat energy into a voltage output. The thermocouple
consists of two different types of metal joined at a junction (Figure 3).
As the junction is heated, the electrons in one of the metals gain enough energy to become free
electrons. The free electrons will then migrate across the junction and into the other metal. This
displacement of electrons produces a voltage across the terminals of the thermocouple. The
combinations used in the makeup of a thermocouple include: iron and constantan; copper and
constantan; antimony and bismuth; and chromel and alumel.
Thermocouples are normally used to measure temperature. The voltage produced causes a
current to flow through a meter, which is calibrated to indicate temperature.
Most electrical power generating stations produce alternating current. The major reason for
generating AC is that it can be transferred over long distances with fewer losses than DC;
however, many of the devices which are used today operate only, or more efficiently, with DC.
For example, transistors, electron tubes, and certain electronic control devices require DC for
operation. If we are to operate these devices from ordinary AC outlet receptacles, they must be
equipped with rectifier units to convert AC to DC. In order to accomplish this conversion, we
use diodes in rectifier circuits. The purpose of a rectifier circuit is to convert AC power to DC.
The most common type of solid state diode rectifier is made of silicon. The diode acts as a gate,
which allows current to pass in one direction and blocks current in the other direction. The
polarity of the applied voltage determines if the diode will conduct. The two polarities are
known as forward bias and reverse bias.
A diode is forward biased when the positive terminal of a voltage source is connected to its
anode, and the negative terminal is connected to the cathode (Figure 4A). The power source’s
positive side will tend to repel the holes in the p-type material toward the p-n junction by the
negative side. A hole is a vacancy in the electron structure of a material. Holes behave as
positive charges. As the holes and the electrons reach the p-n junction, some of them break
through it (Figure 4B). Holes combine with electrons in the n-type material, and electrons
combine with holes in the p-type material.
When a hole combines with an electron, or an electron combines with a hole near the p-n
junction, an electron from an electron-pair bond in the p-type material breaks its bond and enters
the positive side of the source. Simultaneously, an electron from the negative side of the source
enters the n-type material (Figure 4C). This produces a flow of electrons in the circuit.
Reverse biasing occurs when the diode’s anode is connected to the negative side of the source,
and the cathode is connected to the positive side of the source (Figure 5A). Holes within the
p-type material are attracted toward the negative terminal, and the electrons in the n-type material
are attracted to the positive terminal (Figure 5B). This prevents the combination of electrons and
holes near the p-n junction, and therefore causes a high resistance to current flow. This
resistance prevents current flow through the circuit.
Half-Wave Rectifier Circuit
When a diode is connected to a source of alternating voltage, it will be alternately
forward-biased, and then reverse-biased, during each cycle of the AC sine-wave. When a single
diode is used in a rectifier circuit, current will flow through the circuit only during one-half of
the input voltage cycle (Figure 6). For this reason, this rectifier circuit is called a half-wave
rectifier. The output of a half-wave rectifier circuit is pulsating DC.
Full-Wave Rectifier Circuit
A full-wave rectifier circuit is a circuit that rectifies the entire cycle of the AC sine-wave. A
basic full-wave rectifier uses two diodes. The action of these diodes during each half cycle is
shown in Figure 7.
Another type of full-wave rectifier circuit is the full-wave bridge rectifier. This circuit utilizes
four diodes. These diodes’ actions during each half cycle of the applied AC input voltage are
shown in Figure 8. The output of this circuit then becomes a pulsating DC, with all of the waves
of the input AC being transferred. The output looks identical to that obtained from a full-wave
rectifier (Figure 7).
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