Basic DC TheoryDC SOURCESAs the junction is heated, the electrons in one of the metals gain enough energy to become freeelectrons. The free electrons will then migrate across the junction and into the other metal. Thisdisplacement of electrons produces a voltage across the terminals of the thermocouple. Thecombinations used in the makeup of a thermocouple include: iron and constantan; copper andconstantan; antimony and bismuth; and chromel and alumel.Thermocouples are normally used to measure temperature. The voltage produced causes acurrent to flow through a meter, which is calibrated to indicate temperature.RectifiersMost electrical power generating stations produce alternating current. The major reason forgenerating 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 foroperation. If we are to operate these devices from ordinary AC outlet receptacles, they must beequipped with rectifier units to convert AC to DC. In order to accomplish this conversion, weuse 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. Thepolarity of the applied voltage determines if the diode will conduct. The two polarities areknown as forward bias and reverse bias.ForwardBiasA diode is forward biased when the positive terminal of a voltage source is connected to itsanode, and the negative terminal is connected to the cathode (Figure 4A). The power source’spositive side will tend to repel the holes in the p-type material toward the p-n junction by thenegative side. A hole is a vacancy in the electron structure of a material. Holes behave aspositive charges. As the holes and the electrons reach the p-n junction, some of them breakthrough it (Figure 4B). Holes combine with electrons in the n-type material, and electronscombine with holes in the p-type material.Rev. 0 Page 5 ES-02
Integrated Publishing, Inc. - A (SDVOSB) Service Disabled Veteran Owned Small Business