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PlanetChristmas Magazine September 2014

These types of repairs can be hard to track down. You can also make a homemade tester and check each bulb one by one. The Blob If you still have no lights when plugged in even after you’ve used the LED tester then it’s probably THE BLOB. We’ll discuss what’s happening on that big piece of plastic a little later in this article. Shortening the LED String You’re going to have to deal with the BLOB. Inside is a diode circuit that changes AC to DC. Testing of the circuit or diode when in line can be completed but the numbers read by the meter will more than likely not be right. You need to Calculate the power needed or use a meter and potentiometer (pot) to figure it out. LED Calculations Here are some basic terms: 1. Peak Voltage - Peak voltage out of the full wave rectifier from 120VAC RMS equals 170V minus the voltage dropped by the diodes. This would result in about 167V. • Rounded voltages, 120VAC *1.414 = 170V. • 170V - 2.8V = 167V 2. LED Voltage and Current 22  PlanetChristmas  |  September 2014 • Voltage varies by manufacturer and color for LEDs. The most common current is near 20mA. You must look up the specifications for your LED string. • In my examples below, I’ll be using: Red = 2.0V, 17mA or 0.017A 3. Resistor Calculation (Example using a shortened string of 26 Red LEDs.) • Calculate voltage drop of LEDs. 2.0V x 26 = 52V • Calculate peek voltage minus voltage dropped by the LEDs. 167V – 52V = 115V • Calculate Resistor(s) needed to drop remaining voltage. Ohms - 115V / 0.017A = 6.764KOhm (Use next higher standard value, for example, 8.2KOhm) • Watts - 115V * 0.017A = 1.955Watt (Use a minimum of 2 watt resistor) 4. Rectification – You will need 4 diodes per string to make the full wave rectifiers. I use 600V/1A diodes. Mouser part number 821-1N4005 5. Here is a link to a handy online calculator which I use: http://www.horrorseek.com/ home/halloween/wolfstone/Lighting/ litlec_LEDCalc.html Limitations: This example does not utilize any voltage doubling circuits. You are limited to using the number of LEDs that add up to or just below 167V. For example, I could build a string using a maximum of 83 red LEDS which drop 2.0V each. This would utilize 166V of the available 167V. Other LED colors drop higher voltages. For long strings, you must divide the string in half and build full wave rectifiers for each section. Tip: When purchasing resistors, you can use one or more. Rather than just using the one 8.2K ohm resistor, you could use two resistors, one on each end that adds up to at least 6.8K ohms or just over that value. I’d also recommend purchasing resistors in Inside the blob


PlanetChristmas Magazine September 2014
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