Cheap multi-color LED Christmas light strings.

My wife bought some multi-color LED Christmas-light strings, 50 lights for $10 per string. We used them to decorate our traditional wreath over the door, but I was disappointed with their brightness. Checking them on the bench, I found that having no bridge rectifier, they operated on only one half of the AC line cycle, and with 50 LEDs in series, they needed 125 to 135 volts to draw a even modest 10mA current.** Their conduction angle was also less than one might like.

BTW, half-cycle operation makes a 50 or 60Hz flicker, not a 25 or 30Hz flicker as many web-sites state.

Clearly, adding a bridge rectifier would double both the LED's flicker rate, and light output. But I elected to add a storage capacitor after the bridge (200uF, for 2 to 3 strings), and feed the LED string with DC. This would make the conduction angle 100%, and further increase the light output.

My in-situ measurements: 162 volts and 42mA per string, about 5x the original estimated current. Yep, they looked about five times brighter. At least that's what I told my wife, after she said they didn't look _that_ much brighter.

** The string has 20 yellow LEDs, and 10 each red, green and blue. I guess the band-gap voltages add up to ~ 110V.
Reply to
Winfield
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42mA is a lot of average current for a small LED, especially the blue and green ones.

Best regards, Spehro Pefhany

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"it\'s the network..."                          "The Journey is the reward"
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Reply to
Spehro Pefhany

I hooked up 10 high brightness LED's to my power supply. They're all run through 1K resistors and it's drawing 30ma at 5VDC, so if if you divide it out by the 10 LED's it would mean approximatley 3ma per LED.

Reply to
T

Winfield wrote in news:7d3dab1c-8efd-4abc-877a- snipped-for-privacy@i12g2000prf.googlegroups.com:

42 ma seems a bit high for the LEDs,doesn't it?
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Jim Yanik
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Reply to
Jim Yanik

Where did you get the lights? Price sounds good.

You might be buying a few more strings if the lights burn out from the higher current. Do a burn-in test yet?

M
Reply to
mrdarrett

Maybe for indoors, but chilled LED's (outside in the cold) can probably last longer with higher current.

D from BC

Reply to
D from BC

Fine, now suppose you were to put them in series at 42mA as in the above situation, what would you calculate the current per LED to be?

Best regards, Spehro Pefhany

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"it\'s the network..."                          "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
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Reply to
Spehro Pefhany

D from BC wrote in news: snipped-for-privacy@4ax.com:

most common,small LEDs max at around 25ma,would cold allow almost double that current? It doesn't seem likely. But a pulsed LED would allow a greater max current,as the duty cycle would be less than for 'pure' DC. less than 50% for half-sine.

I guess it all depends on what LED chip they are using.

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Jim Yanik
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Reply to
Jim Yanik

Given a fixed maximum power dissipation, the average current has to be

*less* for pulsed current because there is more voltage drop at higher currents.

And on the packaging.

Best regards, Spehro Pefhany

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"it\'s the network..."                          "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
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Reply to
Spehro Pefhany

"The light that burns twice as bright burns half as long ..." -- Eldon Tyrell

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Paul Hovnanian	paul@hovnanian.com
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Reply to
Paul Hovnanian P.E.

could you use something like this?

formatting link

martin

Reply to
Martin Griffith

My wife sweet talked me into making her an LED XMass string, some years back. She had tagged along with me on a visit to the old Eli Heffron / Solid States Sales surplus shop in Cambridge, MA, US, and, while I was looking at used test equipment, bought herself a couple bags of red and green LEDs. She raided my stash of blues when we got home (Eli's didn't stock them) and demanded that I make them into XMass lights for her.

So, got a 5 volt wall-wart and a bag of 150 ohm resistors, one ballast resistor per LED, and soldered it all together. She wrapped the string around a small evergreen tree that she's had growing indoors, "bonsai" style, for a good many years, and lit it up for the holidays. Looked pretty, but, took up way too much time soldering....

W Letendre

Reply to
WJLServo

This trick dates from at least the 1960's, when you could drmatically improve an electric shaver's performance with a diode and a capacitor.

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Many thanks,

Don Lancaster                          voice phone: (928)428-4073
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Reply to
Don Lancaster

The little incandescents (~1mm diameter) are more attractive than most current LEDs on a mini tree.

Best regards, Spehro Pefhany

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"it\'s the network..."                          "The Journey is the reward"
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Reply to
Spehro Pefhany

42 mA would usually appear less than 4.2 times as bright as 10 mA because:

a) Nonlinearity in human vision. Keep in mind that a 60 watt incandescent lightbulb looks 60% as bright as a 100 watt one, while being only 50% as bright.

b) The LEDs would run hotter and their efficiency would suffer

c) Efficiency of the LEDs vary with instantaneous current. I expect the reds and yellows to mostly operate fairly linearly, possibly be extremely slightly improved except for their higher temperature because their efficiency is less at less than a few mA, and without the smoothing capacitor a small percentage of the energy going into the LEDs goes in during low current operation that impairs the reds and greens. However, I expect this to be outweighed slightly by higher temperature.

Same story for the green ones if they are the dimmer very yellowish green. If they are brighter non-yellowish green, then they will behave like most blue ones - see below:

Most blue LEDs, especially turquoise blue ones and very deep blue ones, have a different nonlinearity where efficiency is maximized at a few mA (for the lower power size ones, as opposed to ones rated 150 mA or more). With a smoothing capacitor, they will be receiving current near the maximum instantaneous current received without a smoothing capacitor, so their efficiency will probably be reduced measurably (though maybe as little as a few percent) by use of that capacitor.

Of these factors A, B and C, I would give A the most weight.

- Don Klipstein ( snipped-for-privacy@misty.com)

Reply to
Don Klipstein

In general, modern high brightness low power LEDs, when given 25 mA average current, give maximum efficiency with steady DC as opposed to pulsing. This is especially true with the InGaN ones, and really especially with blue and green InGaN ones.

- Don Klipstein ( snipped-for-privacy@misty.com)

Reply to
Don Klipstein

That's true, but these LEDs really do look much brighter in-situ, I suppose because the LED light output is being compared with the surrounding reflected light from street lights, porch lights, the moon, city sky light, auto lights, etc., where the absolute light level certainly does count, as opposed to being compared with no light.

Hasn't been to be too bad at 40mA. I'll take measurements and report back.

BTW, many years ago I made a very intense light source from hundreds of "ordinary" red LEDs (with the exact wavelength we needed to process bags of blood), and ran them at 150mA. The light output vs current began to drop above 80 to 100mA, but it was still effective to run them at 150mA for maximum light. I soldered the LEDs to a copper bar and used a huge fan to remove heat as we could manage. It worked quite well, lots of light, far more than you could stand to look at (and we got very rapid blood processing, minutes instead of hours). We called it the "LED Blaster" and my technician affixed a sign, like the laser warning signs, saying DANGER LED LIGHT.

It worked well, except every now and then one of the LEDs would explode. No warning or dropping light level, etc., it would just explode into many small bits of plastic. (The guys at the Red Cross, who were performing the blood tests, were not amused.)

Reply to
Winfield Hill

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