LED First Light

Wow, LED’s are amazing!  I just fired up my Cree XM-L2 LED with a 3-cell NiMH battery pack and a 0.91 ohm resistor to keep the current under control.  The circuit runs at 500 mA, which corresponds to 1.4 W for the LED and 0.4 W for the resistor… or 75% circuit efficiency and probably just shy of 200 lumens.  With 2000mAh batteries I should get about 4 hours of light.


In fact I’m a little surprised I didn’t destroy that poor LED.  Surface mounting the LED to the thermal substrate was definitely the hard part.  I couldn’t get enough heat for the solder paste to reflow with my soldering iron so I had to bring out the butane lighter.  The lighter did the job after about 10 seconds, but it left a lot of charred flux on the substrate.  It may be time to invest in a heat gun!

Parts from Digikey:

  1. Cree XM-L2 cool white LED: XMLBWT-00-0000-000LT60E3CT-ND 
  2. NiMH AA batteries (3): P220-ND
  3. Battery holder: SBH331AS-ND
  4. Resistor, 0.91 ohms: P0.91W-2BK-ND

Parts from Mouser Electronics:

  1. Bergquist Company Thermal Substrates – MCPCB: 804936
  2. Reflector optic: C12512_RITA-A
  3. Wahl Solder Paste ISO-Tip: 7459
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