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KenL
User Rank
Gold
Re: One warning
KenL   4/30/2013 11:47:49 AM
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Speaking of TV & movies - film projectors typically have a frame rate of 24 fps, which is chopped to give a 48Hz repetition.  This is right at the eye's threshold when the brightness is low, but unacceptable for higher brightness.  Europeans have had to live with 50Hz refresh rates on CRT TVs for a long time - very annoying - even with the phosphor persistence.

Does anyone know if half-wave LED strings are used in Europe, with their 50Hz power?  That would be even worse than 60Hz.

In the early days of AC power, the frequency was 25Hz, which would make annoying flicker even with incandescent lights.

 

 

KenL
User Rank
Gold
Cheap vs commercial LEDs
KenL   4/30/2013 11:35:03 AM
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The key word here is "cheap".  You can spend more and get commercial quality LED strings that do include a bridge rectifier. I have seen these used on trees at the local mall.  With the bridge rectifier, the flicker is much less noticable because it is at 120Hz and because the duty cycle is much closer to 100%.  But I can still see the flicker.  Your eyes are more sensitive to flicker in your peripheral vision, so look to the side and you will become more aware of it.  Flicker seems to be much less of a problem with fluorescent lights because of the phosphor persistence.

I have been able to demonstrate the flicker by taking pictures of the bulbs with my digital camera while panning the camera.  I can then easily see the on/off pattern of the streaks of light.  Half-wave lights have less than a 50% duty cycle.  The full-wave lights look like about 80% or 90%.

The high power LED fixtures I've seen at the local grocery store and gas station don't have any visible flicker, because they apparently use filtered DC and high frequency switching.  

You can also see flicker on the LED tail-lights on many cars if you scan your eyes across them.  

Next question - how to get rid of the flicker on your neighbors' outdoor lights.

 

 

 

 

 

D. Sherman
User Rank
Silver
One warning
D. Sherman   4/30/2013 11:13:05 AM
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I like the idea, which is mostly a discovery of the fact that the light designers cut costs to the bone by eliminating a rectifier and accepting a 60 Hz rather than 120 Hz flicker rate. In the early days of AC power and movies it was determined that a 60 Hz flicker was just on the threshold of annoyance. Old-style TVs and incandescent bulbs smooth it out somewhat due to the persistence of the phosphor and thermal inertia of the filament, which LEDs don't have.

My concern, which should have at least been given a mention in the article, is that someone might inadvertently plug some device other than an LED light string into the rectified outlet. I'm sure that, as presented, this would never receive any safety agency approval. The problem is that if any device that utilizes an input transformer or induction motor is plugged into the rectified source, the transformer or motor will look like a near-short to DC and will at best blow the device's internal fuse, if one is provided, and at worst, will overheat the transformer or wiring (including this adapter with its small-gauge wire) and start a fire.

If nothing else, the homeowner should affix a prominent red tag to the socket, reading "LED lights only!" as a reminder to anyone who might inadvertently use it for something else.

willie92708
User Rank
Silver
Rewire the sets?
willie92708   4/30/2013 10:45:57 AM
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The main problem with running the cheap Xmas LED lights on a full wave is that 1/2 of the set is wired for the positive half cycle and the other half of the set is wired for the negative half cycle.  So if you run a set on a full wave you light up only half of all the LED's.  Reverse it and you light up the other half of the LED's.  You could rewire all the sets for one polarity, but that's a time consuming PITA.  Also, as others have mentioned you are doubling the average forward current of the LED's, which may or may not cause the life to become too short for your needs.

 

My solution was far more complex.  I reduce the AC line slightly with a 60 Hz power transformer, rectify, filter, then chop with a 1KHz MOSFET full bridge.  The 60 Hz flicker becomes 1KHz and no one complains.

marty48
User Rank
Iron
Safety
marty48   4/30/2013 10:25:36 AM
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This is a clever idea.  I agree with others that the typical holiday LED strings are very poor quality and may fail when the duty cycle is doubled.  I would suggest safer construction since lethal volages are involved.  Mositure is often a consideration during the holidays too.  Electrical tape is subject to deterioration over time and damage due to sharp edges.  Even though LED strings are fuse protected, the described rectifier and wiring are not.  I would suggest buiding this in a grounded water proof enclosure and provide fuse protection.

Jim_E
User Rank
Platinum
Excellent idea
Jim_E   4/30/2013 10:19:24 AM
NO RATINGS
Excellent idea! 

The LED flicker drives me crazy, so I'm going to have to make some of these up for the strings that I recently purchased after Christmas.  Maybe the light quality will be improved enough to use them inside.

Larry M
User Rank
Platinum
Re: This can be even better
Larry M   4/30/2013 10:03:09 AM
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Ramjet wrote: "The negative DC lead out of the bridge is going to drive to that negative (relative) voltage giving you double the voltage DC as compared to simple half wave voltage"

No.  A full-wave rectifier flips those negative peaks over so all the peaks are on the positive side. There are just twice as many of them.

Ramjet
User Rank
Silver
Re: This can be even better
Ramjet   4/30/2013 9:56:36 AM
NO RATINGS
I stated it poorly.

Your diode rectified voltage will be half the Peak to Peak AC voltage.

You are blocking the opposite polarity peak.

The negative DC lead out of the bridge is going to drive to that negative (relative) voltage giving you double the voltage DC as compared to simple half wave voltage.

Larry M
User Rank
Platinum
Re: This can be even better
Larry M   4/30/2013 9:20:34 AM
NO RATINGS
Ramjet wrote: "The half wave was going to be 1/2 the VAC voltage, you are more than doubling it."

You've got that wrong. A scope trace of half-wave recitified AC across the LED string will look like a bunch of humps from 0 to peak voltage with gaps between them of the same width as the humps.

A trace of full-wave rectified AC across the LED strings will look like a bunch of humps of the same amplitude as the half-wave humps but instead of gaps there will be a continuous string of humps--the gaps will be filled in by humps.

Hence:
  • The peak voltage will be the same between half-wave and full-wave cases.
  • The peak current will be the same between half-wave and full-wave cases.
  • The power dissipated in the LEDs will be twice as great in the full-wave case because the duty cycle is doubled.

As noted, as long as the LEDs don't get too warm this is probably not a significant life limiter.

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