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PlanetChristmas Magazine September 2014

An LED light string is typically made up of multiple sets of LEDs wired in series Look closely at a light string. The blue box in the schematic shows one set of LEDs in series. The yellow oval shows a cross-section where you see three wires. The red oval shows the cross-section where you see two wires. Understand this simple principle and you understand how LED strings are configured. mains power How to find that bad LED LED light strings are typically wired the same as incandescents. Each light can handle a low voltage so if you connect enough of them together in series they will work at the high voltage com-ing out of the wall outlet. Incandescents don’t care about voltage polarity and work fine on the AC coming out of a wall outlet. LEDs much prefer DC voltage (like in your car) but luckily for testing purposes, AC voltage will work fine. If you look at a cross section of the wires of a typical light string, there are two wires going to the plug at each end but three wires almost everywhere else along the string. Look at the schematic and you’ll understand why. Most light strings handle multiple groups of lights wired in series. When an LED goes bad, that explains why a portion of the string goes out. The series of lights where the bad LED exists has a broken circuit so they all go out while the other circuits stay on. For testing we focus on the part of the string that’s dark and specifically just the wires entering and exiting the light sockets. From here on out it’s all about developing your own testing technique. Using the test clips to check one, then two LEDs 30  PlanetChristmas  |  September 2014 mains power


PlanetChristmas Magazine September 2014
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