Basic DC Theory
DC SOURCES
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.
Rectifiers
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.
Forward 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.
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