Luxeon Assembly
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Circuit Design for Luxeon LED

 

Luxeons want to be driven by a constant current. The design current is 350 ma. At that current, the forward voltage drop is about 3.4 Volts. The old design used ballast resistors individually selected for the voltage drop of each chain of 6 LEDs. The design with the high power LEDs has a single chain of five LEDs with a combined voltage drop of 16 to 20V. It made sense to use an active current regulator. While the current master supply voltage is regulated to 24V DC, a current regulator opens the path to using an unregulated DC supply. The figure below shows the schematic of the driver circuit.

 

The LM317T is a three-terminal regulator used in a current-control mode. It will adjust its output voltage such as to produce a voltage drop across the sense resistor of 1.25 Volts. Using a 3.9 Ohm resistor, the resulting current is 321 ma, safely below the maximum rating of 350 ma. The .1 uF capacitor quenches oscillations when long supply leads are used.

The five LEDs and the regulator IC are mounted to the flat side of the heat sink with clamps and machine screws. Heat sink compound under the LEDs and a mica washer under the regulator ensure good heat transfer.

The additional components are mounted onto a small printed circuit board, set at a right angle to the heat sink. A few jumper wires complete the circuit. The supply voltage wire penetrates the heat sink and is terminated with a 2.5 mm DC jack.

The photo to the right shows the completed fixture without the rear cover ready to be installed.

The light fixtures of the pillar lights which are larger (twice the number of LEDs) will be rebuilt using the same principles.

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