LED Xmas lights

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On Sun, 20 Nov 2005 20:33:19 GMT, snipped-for-privacy@sympatico.ca (Jason D.) wrote:

Naah, it's still 60 HZ. It's just on half of each cycle and off half of each cycle. 8.33 ms on, 8.33 ms off still adds up to 60 HZ.
Now, if it were full wave rectified you'd get 120 HZ.
I think it's more noticible because the LED actually turns off during the half cycles....an incadescent bulb stays hot enought to still give off some light.
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On Sun, 20 Nov 2005 20:33:19 GMT, snipped-for-privacy@sympatico.ca (Jason D.) wrote:

The output of a LED string (half-wave rectifier) on 60Hz AC will be 60Hz flicker with a duty cycle slightly less than 50%.
Diodes will not change the frequency.
A diode will convert AC to pulsating DC. Another diode will NOT chop THAT in half. Even if it did, you would just be halving the duty cycle not the frequency.
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snipped-for-privacy@sympatico.ca (Jason D.) writes:

Still 60 Hz. :)
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Jason D. wrote:

If the 60 Hz (as others have corrected you) flicker bothers you how do you watch TV (59.97 Hz vertical) or go to the movies (48 Hz ficker rate)?
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On Mon, 21 Nov 2005 05:16:48 GMT, Steve Kraus

Thanks for the correction. :)
60hz is what I can still see on monitors at 60hz and LEDs on AC especially LED xmas strings. Bugs me.
TVs CRT has phorphors that is too slow to make it barely objectable.
When time permits I'm taking down two old fluorescents down was magnetic ballast & old housings and put up new one with electronic ballasts and proper housings that reflects lights towards floor. These items are already bought and waiting to put up.
Cheers, Wizard
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Have you actually seen LED xmas lights with this in person? They have a 50% duty cycle and zero phosphor persistance, the flicker is not subtle, particularly if you move your eyes and catch the lights in your peripheral vision. A TV does flicker, but the phosphor persistance makes it far less apparent. A computer monitor with low persistance phosphors running at 60Hz is unbearable though, instant headache.
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James Sweet wrote:

It prolly wouldn't take a very big electrolytic capacitor to level those current pulses out, huh?
Easy to do fer your own strings I guess, but that doesn't help for all the others you have to look at.
Fortunately that 60 Hz flicker rate is above what triggers photosensitive epilepsy in most persons sensitive to that.
http://www.epilepsy.org.uk/info/photo.html
Jeff
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Don't even need a capacitor, a simple bridge rectifier changes it to 120Hz flicker, pretty much not noticeable unless you scan your eyes past it quickly.
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James Sweet wrote:

My thinking wuz that by using just a cap, you wouldn't change the total energy delivered to the string of LEDs significantly, but with full wave rectification you'd double it and possibly blow an LED if the original design had them powered pretty close to their safe limit. Capice?
Jeff
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Jeff Wisnia wrote:

Good thought! But you have to put a diode somewhere to protect the electrolytic capacitor from seeing AC. I guess you could "sacrifice" the 1st LED and put the capacitor after it. But, there are parallel and/or series resistors in the sockets, so you have to be carefull. A separate diode would probably be best.
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Art Todesco wrote:

That shows another this Jew doesn't know about Xmas, I've never examined a string of LED lights to determine just how they work. <G>
I'd assumed there was a half wave rectifier and maybe a series resistor there already there, to avoid exceeding the LED's PIV ratings.
But maybe they avoid that with a parallel diode across each LED and a single series resistor for the whole string, that's similar to what I found inside one of our white LED nightlights when it started acting funny because the LED was having a nervous breakdown. (It stayed dark even though plugged in, but flickered on when I switched an inductive load on the same circuit - a bathroom vent fan.) Soldering in a new LED fixed that sucker.
Anyway, you got my point, full wave rectification would most likely double the power dissipated in the LEDs if everything else in the circuit stayed the same.
Jeff
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Jeff Wisnia wrote:

Adding a single diode and capacitor will also increase the current because the capacitor will bring the DC voltage up to the peak value of the AC line, usually considered to be 177 volts. As I had a string of 70 (2 series circuits in the same direction) on the bench, I tried a single diode and capactor. With a capacitor as small as 2.3uf, the string current still went up significantly and the LEDs got much brighter. BTW, in the stores they have a display string (5 or 6 LEDs) with a momentary button to show how they look. No blinking here as this display is run on a battery burried in the cardboard display.
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Art Todesco wrote:

All sounds reasonable, maybe some more LEDs in the string and a cap large enough to remain charged close to peak voltage would be an economical way to avoid *any* flicker. <G>
I think we've about "saucered and blowed"* this one by now, but I am curious about how those series strings handle the reverse voltage across the LEDs. Do they have diodes or resistors across each one or do they just depend upon the summed inverse breakdown voltages keeping the reverse current from ever getting large enough to blow an LED? I'm guessing maybe that's it, do ya happen to know the answer?
Jeff
* An old "down Maine" expression referring to method of cooling a too hot cup of coffee by pouring some into the saucer and blowing across it, returning it to the cup when it's ready to drink.
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They depend on the latter, don't think for an instant that these things are over-engineered, or even well engineered.
Really though, in practice I've found LED's to be far more tolerant of reverse voltage than the specs would indicate.
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For an example of overdone xmas decorations, see:
http://media.putfile.com/WizardsofWinter-SM
(broadband connection suggested)
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Kraus wrote:

I sometimes see flicker in TVs and monitors set to 60 Hz. I usually see flicker in monitors set to 56 Hz.
I also only sometimes see LED lights flickering when they have 60 Hz flicker. I suspect that the flicker gets more visible when the light source is more intense - possibly explaining why LEDs are more likely to visibly flicker at 60 Hz than TVs and monitors are.
Movies have a high duty cycle and a much shorter offtime than halfwave rectified LEDs.
- Don Klipstein ( snipped-for-privacy@misty.com)
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Don Klipstein wrote:

I'm going to say one word, and you decide if you still like your theory.
Persistence.
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wrote:

Check out the Philips brand ones at Target.
They resemble the Forever Bright ones of the past 2-3 years. The manufacturer mentioned on the package sounds familiart to me. However, LEDs get improved gradually over the years, and I suspect they also had to get brighter to get Philips to put their name on them.
At Target, they show some in operation - see if they're bright enough for you. I saw some with bare LEDs - those had somewhat narrow beams and were weak outside their beams, while ones with "bulbs" over the LEDs shone more equally in all directions.
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I really wanted to buy some LED last year, they were real cheap at an after Xmas sale at a Nursery. The white ones, (the only ones I was interested in) were too "cool" for my taste. I love the yellowy glow of regular incandescent "white" lights, seems homey, warm & inviting. The LED's looked harsh & "office-light" like for my tastes so I took a pass.
FWIW... they must be popular, as from what I could see Target or mass marketers, from what I could see, sold out before Christmas
I hope they soon come out with a warmer version, for I can't wait to be done with cheap incandescent xmas lights buring out and driving me absolutely crazy. .
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Yesterday I tried experimentally running one of the Forever Bright strings off a bridge rectifier, no capactitor. It seemed only slightly brighter. That's puzzling because although instantaneous brightness would stay the same having twice as many flashes per unit time ought to seem something like twice as bright.
Also, I noted an alleged 10V drop across the bridge (121V in, 111V out) but I think this may be a figment of my DMM's imagination, perhaps unable to give an accurate RMS reading of the pulsating full wave rectified DC. I guess I should dig out my analog VOM. And maybe visually compare a couple of C9's incandescent bulbs (considerably more current draw than an LED string) on either side of the rectifier.
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