HOME  |  NEWS  |  BLOGS  |  MESSAGES  |  FEATURES  |  VIDEOS  |  WEBINARS  |  INDUSTRIES  |  FOCUS ON FUNDAMENTALS
  |  REGISTER  |  LOGIN  |  HELP
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
Platinum
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!



Partner Zone
Latest Analysis
Former DARPA official and Google executive Dr. Kaigham Gabriel believes sensor companies think too much like suppliers and need to bring their products closer to the consumer.
One way to keep a Formula One racing team moving at breakneck speed in the pit and at the test facility is to bring CAD drawings of the racing vehicleís parts down to the test facility and even out to the track.
Most of us would just as soon step on a cockroach rather than study it, but thatís just what researchers at UC Berkeley did in the pursuit of building small, nimble robots suitable for disaster-recovery and search-and-rescue missions.
Engineers at Festo were inspired by how a caterpillar builds its cocoon when designing its new 3D Cocooner printer.
Design engineers need to prepare for a future in which their electronic products will use not just one or two, but possibly many user interfaces that involve touch, vision, gestures, and even eye movements.
More:Blogs|News
Quick Poll
The Continuing Education Center offers engineers an entirely new way to get the education they need to formulate next-generation solutions.
SEMESTERS: 1  |  2  |  3  |  4  |  5  |  6 |  7 | 8 | 9


Focus on Fundamentals consists of 45-minute on-line classes that cover a host of technologies. You learn without leaving the comfort of your desk. All classes are taught by subject-matter experts and all are archived. So if you can't attend live, attend at your convenience.
Next Course June 28-30:
Sponsored by Proto Labs
Learn More   |   Login   |   Archived Classes
Twitter Feed
Design News Twitter Feed
Like Us on Facebook

Technology Marketplace

Copyright © 2016 UBM Canon, A UBM company, All rights reserved. Privacy Policy | Terms of Service