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Step 5Add Resistors and Power Rails
Next, grab your bag of 2.7 ohm 1/4 watt resistors. Trim one side of the resistor down to 1/4" and trim the (-) side of the LED triplet down to 1/4". Solder the two shortened leads together. Repeat until all of the LEDs triplets have a resistor. The reason this resistor is 2.7 ohms is due to the current regulation circuit. The LM334 (the main component in the current regulator) has a reference voltage of 64 mV. A simple ohm's law calculation can determine this resistor value. 2.7 ohms ensures that each LED gets exactly 23.5 mA.
Take some 18 AWG copper wire and being to solder it in place along the sides of the LEDs. Leave approximately 4 inches of extra wire on each end to work with at the end. Assuming you are starting with the positive rail, hold the copper wire against the LED's lead just slightly above the flattened area. This flattened area is a small "warning" that soldering below it can damage the LED, so try and stay above it. Now that the copper is held against this spot, fold the lead over the copper wire and solder. If you are bulding the 87 LED model, repeat this process 29 times until all of the LED triplets have a tie to the power rail. Then repeat this process for the resistors. The resistors do not have a "safety mark", so try and remain at least 1/4 inch away from the resistor when soldering. You will notice a 10 degree angle on the LEDs in this cross-section. This would be an ideal design that could make the lamp usable without a diffuser. However, this angle is not a requirement. It is difficult to drill accurate angles in the plexi-glass. In the bulbs that were built, the angle was omitted and the holes were simply drilled straight through.
Take some 18 AWG copper wire and being to solder it in place along the sides of the LEDs. Leave approximately 4 inches of extra wire on each end to work with at the end. Assuming you are starting with the positive rail, hold the copper wire against the LED's lead just slightly above the flattened area. This flattened area is a small "warning" that soldering below it can damage the LED, so try and stay above it. Now that the copper is held against this spot, fold the lead over the copper wire and solder. If you are bulding the 87 LED model, repeat this process 29 times until all of the LED triplets have a tie to the power rail. Then repeat this process for the resistors. The resistors do not have a "safety mark", so try and remain at least 1/4 inch away from the resistor when soldering. You will notice a 10 degree angle on the LEDs in this cross-section. This would be an ideal design that could make the lamp usable without a diffuser. However, this angle is not a requirement. It is difficult to drill accurate angles in the plexi-glass. In the bulbs that were built, the angle was omitted and the holes were simply drilled straight through.
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Aug 27, 2009. 6:42 AMskrubol
says:
2.7 ohm is probably not enough for current limiting on the individual strings of 3. Your one string that is connected to the reference of your regulator will have the proper current, but the rest of the strings may have significantly different currents, especially if the temperature is different at one end of the tube as it is at the other. On its own, the end with the regulator will be warmest of course, so it shouldn't be a big problem, as the reference LED's will be the warmest (highest current.) If you have LED's with different Vf's, you may get significant variations in current between strings though.
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