|MadSci Network: Engineering|
Hi, Nathan. You've asked more than one question. Let's take them one at time. A rectifier is an electric check valve. A check valve is a pneumatic or hydraulic device allowing only one-way flow. One-way flow is the key. An early vacuum tube rectifier was called a Fleming Valve. Electrons would flow from a cathode to an anode but not the other way because the cathode was heated and the anode was not. Vacuum tubes are still called valves in the United Kingdom. Today's rectifiers are almost all solid-state based on the PN junction. A PN junction is made of a single crystal (usually silicon) with impurities called "dopants" that change the nature of charge flow in the dopant area. If you put a voltage across a cube of pure silicon, half the current will flow in the form of electrons free of atoms, and half will flow in the form of holes (places where the crystal "thinks" an electron should be). The holes are effectively positive charges. So, if you dose one half the cube with "donor" dopant, 99.9% percent of charge flow becomes electrons, and the material is now called "N." The other half can be dosed with "acceptor" dopant, and 99.9% of charge flow becomes holes and the material is called "P." The border is called a PN junction. A PN junction rectifies because only voltage polarity that pushes holes and electrons toward the junction causes current flow. The holes and electrons "recombine" at the junction. The opposite polarity pulling holes and electrons away from the junction, prevents current flow because when the holes come to the attached wire they cannot go further. Metal does not allow "hole flow." Try this site: http://www.knowledgelabs.com/KlTraining/PptPres/Taps/sld020.htm As to battery chargers, you have it right. The simplest battery charger is just that — a transformer to reduce AC voltage, and a rectifier to create a pulsating DC to charge the battery. But some battery chargers are fancier. They recognize the differences among a totally depleted battery, a partially charged one, and a fully charged one. The following site shows a battery charger that displays all three modes: http://www.aaroncake.net/circuits/charger1.htm Charger designers recognize whether the target battery is tolerant of overcharging and design accordingly. Under your car hood, your voltage regulator is diligently taking care of your battery by varying field current to your alternator to match the battery charging needs. Larry Skarin
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