The RGB Lamp mechanicals are based on a little rectangular lamp purchased at IKEA. The CREE LED module is placed on a 1/2 inch think heat sink designed for the module, and that module is mounted inside the lamp in place of the standard screw bulb mount.

There are several challenges with this design. First, the LED module must not exceed its thermal specs during operation. Second, the lamp must not expose human eyes to the direct output of the LED module. Finally, we need to find a way to mount both the module and the electronics.

Thermal issues

The datasheet for the 0.5 inch thick heat sink specifies a 60 degree C rise in temperature when dissipating 9W with natural convection. That is 60/9 or 6.67 C/W. Since the CREE LED module will be limited to 350 mA at about 3.2 V by the control circuitry, maximum power dissipation is 1.12 W per LED, or 4.48 W for the entire module. In ambient air, the 0.5 inch heatsink will see a rise over ambient of about 30 degrees Celcius. So if room temperature is 30 C (86 F), the temperature and the juntion beween the module and the heatsink should be about 60 C. I failed to locate a maximum junction temperature for this module, but 60 C is not too high for most other LEDs out there. To ensure good thermal contact, the module was secured to the heat sink with machine screws and a bit of thermal paste.

In operation, the lamp when tested in operation suggests the heat sink is actually performing a bit better than the data sheet numbers suggest.

Eye protection

The problem with the IKEA lamp used is that it is open through the top. This is an advantage thermally because the lamp acts like a chimney, allowing cool air to enter through the opening in the bottom -- which was left open for ventilation purposes -- and warmer air to rise out of the top. The downside is that a person could easily look into the lamp from above. These high power modules can cause retinal damage, so this is a problem. The solution was to mount a few inches above the LED module a diffuser. The diffuesr spreads the light nicely while also blocking direct visual exposure of the module LEDs.

Mechanical mounting notes

This was a quick project, so the mechanicals are of the basic prototype variety. The electronics are mounted on perfboard in a small project box, and a short section of 28 AWG cat 5e cable delivers current to the LED module. A DC wall wart is used for power, and the heat sink is mounted in the lamp with hot glue.