When the voltage on the anode is more positive than the voltage on the cathode, current will flow through the diode.  If the voltage is reversed, making the cathode more positive, then current will not flow through the diode.  (unless the peak reverse voltage rating is exceeded).

In the drawing below a single diode is used to rectify the alternating current.  A half-wave DC output is produced.  (click on drawing to view with animation)

On the first half of the cycle the diode allows the electrons to flow through it powering the DC load.  On the second half cycle the diode blocks the flow of electrons and the load receives no flow at all.  This is what causes the flicker effect of cheap LED lights.

When we use a four diode bridge rectifier , we solve the problem of having no flow to the load for half of the time.  (click on drawing to view animation)

On the first half of our cycle the flow electrons passes through the bridge allowing electrons to flow to the load in one direction.  Then on the second half of the cycle the flows passes through the bridge recifier following a different path, but still sends the electron flow in the same direction to the load giving it twice the output of a single diode rectifier.  In the animation above, the cycles are moving at 1/8 cycle per second, while in actuality, this is happening 480 times faster at 60 cycles per second.  An RC filter (a resistor and a capacitor wired in parallel with the DC load) further smooths the output reducing the flicker even more.  So to answer our original question, LED Christmas lights that make use of full-wave bridge rectifier technology are worth spending  a little more for.

     Maybe a tax on a bad idea might be more appropriate.   With the growing concern over contamination caused by the improper disposal of CFL’s (compact fluorescent lights), the suggestion of charging a 25 or 50 cent deposit per bulb has come up.  Which, on the surface sounds like a fairly good idea.  But lets make some comparisons:  A CFL contains a small amount of mercury (4mg give or take).  When improperly disposed of, that 4mgHg can contaminate 5600 gallons of drinking water.  That’s huge!  On top of that, during a five year life, the emissions from a coal fired plant to produce the energy to run the bulb is an additional 2.4mgHg.  Now, contrast that with 10mg of emissions needed to operate a conventional incandescent bulb for that same five years.  By these figures the CFL isn’t quite as bad as it’s been made out to be in comparison to the incandescent bulb that most of us typically use.  So why should we penalize someone who is trying to use less energy and cause less emissions?  Why not add a small tax on those who buy incandescent bulbs to help offset the cost of recycling CFL’s? 

     The first thing we must do regardless of how it’s paid for, is to educate the population to the dangers of improper disposal of mercury.  CFL’s are not the worst culprits.  The 4mgHg in a fluorescent bulb is small compared to the 500mg found in your typical thermometer or the 3000mg in an old household wall thermostat! 

     An even better solution to this problem is the LED light.  It uses substantially less energy than a CFL to produce the same amount of light and contains no mercury at all.  As LED technology increases so does the efficiency of the light output (lm/w).  On the same path as the computer chip, the increasing in LED technology is exponential.  With the brighter and whiter LED light, its uses are ever expanding.  They are not just good for watches and signaling, but everything from LED replacement light bulbs to LED Christmas lights.  With their lack of mercury, their low power consumption, and extremely long life, not to mention, a score of other benefits, it seems that the LED may be a solution to a small part of a very big problem.