Gadget Freak

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

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William K.
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Re: This can be even better
William K.   4/30/2013 9:15:26 AM
ramjet, the peak output of a bridge rectifier is equal to the peak-to-peak input, less the diode drops. The apparent increase does not appear without the output capacitor. Likewise for a transformer driven full wave rectifier. So the LED strings are already set to handle the voltage. Where the difference occurs, and why this device eliminates the 60Hz ripple, is that the strings in question only use one half of the waveform, so that the string is only lighting on the positive peaks, which gives the flicker. The bridge rectifier supplies positive peaks for both half cycles, thus raising the flicker to 120 Hz, which the eye does not follow.

There does appear a problem at this point because the string system is now dissipating twice as much power, since the energy per second has doubled. That is the reason to consider lowering the voltage input to the string.

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Re: Half-wave vs Full-wave
Watashi   4/30/2013 9:03:38 AM
You could always add a capacitor to eliminate the ripple. 

Also, a current limiting resistor in series with the LEDs would help the life cycle issue (if it becomes an issue).

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This can be even better
Ramjet   4/30/2013 9:00:09 AM
This can be further inmproved with a Cap and a Voltage regulator.

Output of a Full wave bridge is 1.417 X the input VAC. the string divides it over the number of bulbs but the resultant current is likely to exceed the rating.

The half wave was going to be 1/2 the VAC voltage, you are more than doubling it.

Adding a Cap on the Bridge output will smooth out ALL the ripple / flicker but raise the voltage even more. Thus the need for a voltage regulator. I'd set it at 48 VDC to avoid any hassle from the regulatory folks.

Another possibility, if this is a 3 wire string, the AC may be split between 2 sets of opposing polarity. If this is the case, rewiring them to series will reduce the need for regulation.

User Rank
Re: Product improvement
3drob   4/30/2013 8:52:00 AM
The lifespan of LED's on holiday strings (in my experience) is freakishly short.  BOM cost is most definately paramount, so I think they tend to use LED "seconds" or output from questionable chinese firms.  Even with good LED's, 100k hour mtbf with a well lit house will result in a measurable # of LED failures per season.

As to the full wave rectifier, I'm surprised it works at all.  Certainly doubling the fequency will help with percievable flicker.  But, the strings I've used seem to be set up as two anti-parallel strings (each half lights up on alternating cycles).  I would suspect such a string would only light half the LED's with a full bridge.  I'll have to give this a try (perhaps my assumption of anti-parallel isn't correct).

Tom Drechsler
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Re: Necessary component
Tom Drechsler   4/30/2013 8:49:55 AM
I had no idea that seeing the flicker was so rare. The same effect is true with fluorescents, but more so. Before LCD screens, I would always change the refresh rate at any monitor I used because they would beat with the office lights. I don't see the flicker in LEDs so I must be less susceptible.

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Half-wave vs Full-wave
tdesmit   4/30/2013 8:41:43 AM
Others have alluded to it, but no one has mentioned (that I saw anyway) the reason for the flicker; the cheap stings of LED's use a single diode to produce half-wave rectification. adding the bridge make it full-wave. This doubles the ripple frequency (from 60 to 120 Hz). Other's have mentioned, too, that the added brightness will probably mean a shorter life. I'd let the string run for a while with the bridge, and then check to see how warm/hot they are. If the temp rise isn't too great, the life-span effect would probably be small. Heat is definately the biggest enemy of LED's.

Tom D.

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Re: Question
AnandY   4/30/2013 2:27:47 AM
According to studies about 1 in 4,000 people are highly susceptible to flashing lights cycling in the 3 to 70 Hz range. Such obvious flickering can trigger ailments as serious as epileptic seizures. Less well known is the fact that long-term exposure to higher frequency flickering (in the 70 to 160 Hz range) can also cause malaise, headaches, and visual impairment.

User Rank
Re: Product improvement
AnandY   4/30/2013 2:14:46 AM
Today LED lighting is not limited to festival lightings, they are also used in day-today life. old-fashioned incandescent or harsh fluorescent are replaced by bright and shiny new LEDs. The local supermarket, the office where you work, the train station, restaurant or department store—all of these businesses are seeing the benefits of this rapidly developing technology. The light produced by LED bulbs is brighter and warmer and more focused than ever before, and business owners love the savings to their energy bills.

TJ McDermott
User Rank
Re: Product improvement
TJ McDermott   4/29/2013 11:02:32 PM
The answer to just about any why question in this context is going to be money.  Granted, the rectifier is quite inexpensive.  The margin on such products is probably quite low - adding the rectifier while staying cost-competitive won't work.

There's also the cost of getting this now-new product tested by UL so a company can affix the all-important UL label.

OR, the company can just counterfit the label...

User Rank
Re: Product improvement
greg   4/29/2013 10:55:16 PM
the lifespan of LEDs is freakishly long as it is. shortening it to, say, a quarter, would likely still outlive any of us.

no, i havent done the math.,,

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