Kilo-Lumen bike headlight

Step 4 Machining the Heat Spreader

The heat spreader is a crucial component of this project. The heat spreader must conduct 10-15 watts of heat to the outer housing which acts as a heat sink. This means that the dimensions of the spreader are critical. Gaps between the spreader and the housing will result in poor heat conduction resulting in a potential overtemperature condition within the LEDs.

I used a Harbor Freight table-top milling machine to shape the heat spreader. It didn't come out perfectly but it came out close enough and with the use of screws on the side of the housing, it works fine.

The front face of the spreader is just as flat as possible and has holes for power supply wires. Additionally there are holes tapped for 2-56 scrwes used to hold the LEDs in place while the epoxy cures.

I overdid it with the silver epoxy, evidenced by the look of a flooded surface. The compression of the screws ought to maintain a reasonably thin interface between the LED PCB and the spreader. I reccomend using an ink roller and rubber ink pad to spread a 1-2 mil layer of silver epoxy.

The back of the heat spreader is machined out to make room for the driver circuit. The first driver I used required a small pillar for heat sinking. I blew this driver up, and changed to another type (the BlueShark) which comes with a copper heat spreader. I reccomend this driver over the Taskled maxFlex (which I burned out) simply because the Blueshark uses a potentiometer for brightness control. The maxFlex is a really great board but I like the variable control.

Alignment of the LEDs is critical. The square lenses I used are almost 1 inch square. This means there is little room for error in the alignment of the LEDs. Careful layout with a set of calipers should provide adequate alignment. This is another useful feature of the scrwes, as they keep the LEDs in alignment while the epoxy cures.

This heat spreader goes into the blue housing and is secured on the sides by screws. The sides provide the majority of the heat conduction and as such, the wider the cross section the better (hence the 1 inch thickness of the spreader).