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Christmas light Blues

NickD on Thu December 09, 2010 9:58 AM User is offline

Country of Origin: China

Blue LED's to be precise, 50 in a series string, which one is bad. Local utility gives us money for buying LED's, never thought I would see the day when we are paid to use less.

Normally connect a wire to the neutral of a U-ground plug to clip my voltmeter on, pull a lamp in the center of the string, if I get a voltage from the plug end, know the first half is good, problem is in the second half, should read the full 120VAC with no load. Split the second half in two for the same thing, cuts that out to about 12 suspect bad bulb.

Wife likes multicolored bulbs, have red, green, yellow, clear, and blue, so have ten of each color in a 50 lamp string. My split method didn't work as practically no voltage anywhere, so based on history, just pulled all ten blue lamps and individually tested them on a current limiting power supply set at 10 volts at 10 milliamperes, the voltmeter on the power supply drops to the forward saturation voltage of the lamps. For some reason, good blue LED's drop 4 volts if where all the other colors only drop 2 volts on the average, so this means they are operating at twice the power. No wonder why they burn out

Out of the ten blue bulbs, 8 were bad, a couple were shorted out, four were completely opened, and two still worked, but measured at over 6 volts, they were on their way out. Box said warranted for five years, needed the UPS label, store receipt, but shipping cost was far greater than buying a new all blue set. All the lights worked before putting them on the tree, but after a couple of day, dead. Really can't troubleshoot these on the tree, constantly get lost, removing the string is the biggest PITA especially when buried under all those ornaments. Maybe we should go back to candles or just project a photo unto a white wall, really takes the joy out of Christmas.

Ha, feel awkward using the word Christmas, but until our congress changes the name of that national holiday, should be safe to use.

mk378 on Sat December 11, 2010 1:10 PM User is offline

Most LED strings have diodes built into the plugs in order to apply (pulsing) DC to the string. It's usually a full wave bridge with two of the diodes in the plug end and the other two in the socket provided to attach another string to the far end. Ballast resistors are also typically used, though I would think there may be a few with resistors in each LED instead.

I also have a really cheap string that just has a resistor and applies AC to the LED's, using the LEDs themselves as a half-wave rectifier. Probing such a string could disrupt the distribution of reverse voltage and blow LEDs. A single LED can usually only withstand about 5 volts reverse voltage.

Many strings of 70 LEDs have pairs of LEDs in parallel then 35 such pairs in series. Failure (open) of one LED will not kill the whole string, but the other one in the pair is under stress with twice the current.

Blue LEDs operate at higher forward voltage because blue photons have more energy than red ones. LEDs work by forcing electrons across an energy gap and converting that energy to a photon. The operating voltage of the diode needs to be at least slightly more than the eV energy of the wavelength produced. There is a difference between red yellow and green LEDs too but it is not as noticeable.

Edited: Sat December 11, 2010 at 1:11 PM by mk378

NickD on Mon December 13, 2010 6:42 AM User is offline

I'll have to take the blame for this for mixing different colored LED's in the same series string. Sylvania is using a capacitor as a ballast where its Xc is determining the series current, with a diode. The P-GaN semiconductor material used in blue LED's does exhibit roughly twice the forward drop resulting in lower life, so they use a lower current in all blue sets.

These are C-6 bulbs, with a colored base, using a clear LED with the blue colored results in a lighter blue color, but works. These things have plenty of warning labels on the light sets, like not sticking the plug in your ear or causes cancer in California rats. But nothing about using different colors in the same string.

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