A cheap, current-regulated Luxeon Star driver design

The circuit itself is pretty simple, and is just a minor variation on an application found in the LM317 datasheet.

Here’s the analysis: It’s convenient to think of the LM317 as a 1.25V voltage source where pin 2 (Vout) is at 1.25V with respect to pin 1 (ADJ). The 3.9 ohm current programming resistor across that 1.25V source will always carry a current of I = 1.25V / 3.9ohms = 320 mA. If you connect a load between ADJ and ground, Kirchoff’s current law requires that the current delivered to that load be 320 mA.

I picked 320 mA as the target design current because it’s close to, but comfortably beneath, the rated 350 mA current for the Luxeon Star. If you want to adapt this circuit as a constant current supply for some other load, just do the calculation for R = 1.25V / the target current.

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