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

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AnandY
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Gold
Re: Question
AnandY   4/30/2013 2:27:47 AM
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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.

tdesmit
User Rank
Iron
Half-wave vs Full-wave
tdesmit   4/30/2013 8:41:43 AM
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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.

Tom Drechsler
User Rank
Silver
Re: Necessary component
Tom Drechsler   4/30/2013 8:49:55 AM
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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.

3drob
User Rank
Platinum
Re: Product improvement
3drob   4/30/2013 8:52:00 AM
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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).

Ramjet
User Rank
Silver
This can be even better
Ramjet   4/30/2013 9:00:09 AM
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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.

Watashi
User Rank
Platinum
Re: Half-wave vs Full-wave
Watashi   4/30/2013 9:03:38 AM
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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).

William K.
User Rank
Platinum
Re: This can be even better
William K.   4/30/2013 9:15:26 AM
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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.

Larry M
User Rank
Platinum
Re: This can be even better
Larry M   4/30/2013 9:20:34 AM
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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.

Ramjet
User Rank
Silver
Re: This can be even better
Ramjet   4/30/2013 9:56:36 AM
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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.

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