MadSci Network: Engineering
Query:

Re: How does a rectifier work?

Date: Mon Dec 20 12:32:45 1999
Posted By: Lawrence Skarin, Faculty, Electrical Engineering, Monroe Community College
Area of science: Engineering
ID: 943233619.Eg
Message:

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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