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cree xm-l t6 bicycle light?

Discussion in 'LEDs and Optoelectronics' started by Codyf1113, Jun 18, 2013.

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  1. Codyf1113

    Codyf1113

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    Jun 18, 2013
    Looking around on ebay and I see these bicycle lights every where. It looks like a nice light. Said to be 1000 lumens. But I've asked few of the ebay sellers if there a heat sink for the light, and they all told me there wasn't. Only thing the light has is vents for some air flow but would that really be enough to protect the leds from over heating? I've used flash lights before off ebay and they all got hot and became very dim and were no longer bright because of it.

    I know people here say they had no problems with the light but I don't have much faith in it. Here more about the light http://www.bikeforums.net/showthread.php/882778-cree-xm-l-t6-bike-light/page2

    Whats everyone opinion about this?
     
    Last edited: Jun 18, 2013
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    Sounds like they rely on airflow from the moving bike to keep them cool.

    If they have a metal case and you can use a metal mount to connect them to a metal part of your bike, you may be able to conduct a significant amount of heat into the larger (thermal) mass of the frame of the bike.
     
  3. BobK

    BobK

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    Jan 5, 2010
  4. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    In this case it is almost certainly the thermal resistance from the junction to the "case" of the device.

    This means that if the case is held to a constant temperature, the junction's temperature will rise by 2.5 degrees C for each watt of energy it dissipates.

    Now, with a LED, the power is not quite If x Vf because some of that power gets converted to light. However it's close enough to If x Vf that it makes very little difference.

    If the LED junction needs to be kept below 100C, and the ambient temperature is 30C and the power being dissipated is 10W, then...

    We have an allowable temperature rise of 100 - 30 = 70C
    We have a power dissipation of 10W
    Therefore our budget is 70/10 = 7 degrees C per watt
    We know that the thermal resistance to the junction is 2.5 degC/W an that our mounting might add an extra 1/2 to 1 degC/W.
    So that leaves us with 3.5degC/W to play around with.

    In still air, we would require a heatsink with no more than 3.5 degC/W rating (I'd be looking for 2 degC/W to allow for poor placement, dust and dirt, etc.

    However, in moving air, the performance of the heatsink will improve rapidly. You still require no more than 3.5 degC/W, but this can be achieved with a far smaller heatsink (and from what is said on the forum we were pointed to, the case of the torch is sufficient).

    An advantage of "forced air" without a fan (ie. relying on the movement of the vehicle) is that there is no fan to impede airflow when it's off or broken, and there is (in general) less build-up of dust and rubbish.

    However, mounting the device to a large metal part of the bike (it may just be the handlebars) using something which will conduct heat readily will improve the thermal performance. This will allow the lights to remain on with the bike stationary for long periods. As a practical issue it probably means that if you fail to turn them off they won't burn up and die.
     
  5. BobK

    BobK

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    Jan 5, 2010
    Thanks Steve, good explanation.

    Bob
     
  6. BobK

    BobK

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    Jan 5, 2010
    From the looks of those lights, the case is the heat sink.

    Bob
     
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