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Gadget Freak Case #239: Bridge Rectifier Eliminates LED Light Flicker

Bernard R. Smith Jr. created a bridge rectifier that eliminates the flicker in LED light strings.
Click here for build instructions, BOMs, and more.
NO RATINGS
4/29/2013 | 3 comments
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Greydragon
User Rank
Iron
Re: I double dare ya
Greydragon   5/6/2013 10:37:45 PM
Actually, your assumpions are erroneous.  Christmas tree light strings are warranted for three years if you follow the fine print that states the intended use is for 8 hours a day and six weeks of annual usage.  Considering the poor design practice of simply series wiring LED's for a maximum forward voltage drop of 130 V, even that caveat is generous.  What kills these LED strings most often is not heat or excessive current but inverse voltage spiking.

Let me explain:

LED's typically exhibit zener-like conduction that can cause spiking.  When operating from 110-140 VAC, common in most households, cumulative spiking, as the self-rectifying diodes switch on and off through the 60 Hz sine wave exceed the 5 v inverse limits of the diodes and they accumulate damage at a rate far in excess of their predicted 100,000 hour lifetime.  Full wave rectification will benefit these strings because although they are theoretically capable of blocking the reverse bias of AC, their individual inverse voltage rating is only 2 V greater than their forward bias.  But simple full wave rectification is not enough.

Practically speaking, the addition of a moderate filter capacitor, a current limiting resistor and a shunt resistor will extend the lifetime of a "normal" Christmas tree string to its rated lifetime.  You see, the "raw" output of a full wave bridge also contains spikes and twice the frequency of the input AC.  More importantly however, is the ouput voltage rises to a peak voltge which averages 1.41 times the input voltage.  Another $0.25 worth of parts is needed for good design.

How do I know this?  Well, I work with LED's and I like to practice what I preach.  I have been running a typical, Home Cheapo, made-in-China, string of white LED's CONTINUOUSLY for over 4 years and they are just a bright and just as flicker-free as when they were first put into service.  Please remember that although an LED is actually a diode, it is also a DC light emitting device, NOT a rectifier.  LED's are intended to be run on DC and the purer, the better.  They are "happiest" being fed DC at their rated current.  Think of an LED as a very fragile current sink and you will stay out of trouble.  Properly treated, they really do last a lifetime.

mrdon
User Rank
Gold
Re: I double dare ya
mrdon   5/6/2013 3:03:50 PM
NO RATINGS
Howman,

I agree. Doubling the frequency does minimize the flickering because of reduced output ripple voltage from the bridge rectifier. The LED flickering will be diminished because of minimized ripple voltage. As noted, modifying AC devices is at the risk of the consumer. Extreme caution should be taken in consideration with this Gadget Freak product. 

Howman
User Rank
Iron
I double dare ya
Howman   5/3/2013 6:24:57 PM
NO RATINGS
So instead of the LED string lighting up on once per cycle for just one half cycle (60Hz), they light up twice per cycle (120Hz).  I can definitely see why this would reduce perceptable flicker.

Just beware that this gadget now doubles the the power drawn by the LEDs causing all the components to run hotter.  This probably voids the manufacturer's warranty on the lights (Who cares?  Their cheap Christmas lights, right?)  But heaven forbid a fire would result.  The insurance company may deny your claim if they found out you did not power the lights as the manufacturere intended.

Please use this with extreme caution!

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