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Thread: Could use some help with my newbie setup

  1. #1

    Default Could use some help with my newbie setup

    Thanks for taking the time to read this;

    I've got a simple setup, 18650 battery with a driver (cree XRE-Q5/XPE XP-E/XBD XB-D driver 17mm 20mm DC3-4.2V 700MA)
    and LED (CREE XPE XP-E w-3 w 3535 SMD High Power LED Emitter Diode)

    Works fine, untill I fully charge the battery and then the LED gets so hot it melts the solder.

    I don't get it, it's the right driver for the LED?

  2. #2
    Flashaholic* RetroTechie's Avatar
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    Default Re: Could use some help with my newbie setup

    Both driver & LED need cooling. How is the LED mounted? Hanging in the air on a few wires? On a small pcb with wires attached? Some other way? Same question for the driver.

    At 0.7A (and ~3V for white power LED like XP-E) the LED consumes ~2W. It converts maybe 1/4..1/3th of that into light. The remainder (so around ~1.5W in this case) is shed as heat. LED must be mounted in such a way that that heat can be shed. Otherwise LED will get hotter & hotter as you observed. That a LED is rated for 3W only means it can 'process' 3W of power. It does NOT mean it can make thermal losses (input power that's not converted into light) magically disappear! This is the reason power LEDs like this usually come mounted on a small pcb using aluminium or copper as base material. Pcb is attached to a larger heatsink like the body of a flashlight (usually with thermal paste in between to get rid of air gaps), and that larger heatsink transfers the heat to your hand and/or ambient air.

    Same story goes for the driver as it also has losses. Although usually the LED produces most heat.

  3. #3
    Flashaholic* DIWdiver's Avatar
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    Connecticut, USA
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    Default Re: Could use some help with my newbie setup

    Quote Originally Posted by RetroTechie View Post
    Both driver & LED need cooling. How is the LED mounted? Hanging in the air on a few wires? On a small pcb with wires attached? Some other way? Same question for the driver.

    At 0.7A (and ~3V for white power LED like XP-E) the LED consumes ~2W. It converts maybe 1/4..1/3th of that into light. The remainder (so around ~1.5W in this case) is shed as heat. LED must be mounted in such a way that that heat can be shed. Otherwise LED will get hotter & hotter as you observed. That a LED is rated for 3W only means it can 'process' 3W of power. It does NOT mean it can make thermal losses (input power that's not converted into light) magically disappear! This is the reason power LEDs like this usually come mounted on a small pcb using aluminium or copper as base material. Pcb is attached to a larger heatsink like the body of a flashlight (usually with thermal paste in between to get rid of air gaps), and that larger heatsink transfers the heat to your hand and/or ambient air.

    Same story goes for the driver as it also has losses. Although usually the LED produces most heat.
    100% agree with all of this, with one exception. Current XPE2 datasheet shows luminous flux at 700mA to range from 106-227 lm, depending on binning. Forward voltage at this point is typically 3.05V, for about 2.14W (darned close to what you suggested). Thus we get between 50 and 106 lm/W. At 50 lm/W you are getting less than 1/7th of the electrical energy converted to light, thus more than 6/7 converted to heat. At 106 lm/W you are converting around 39% to light. EasyWhite might be a little different, but won't be better.

    Thus the range of possible heat outputs is quite a bit larger than you suggested, and we would like to know more specifically which LED is selected in order to make the best calculations we can.

    cocobob - Today, 106 lm/W SUCKS and you can do a lot better.

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