Results 1 to 10 of 10
  1. #1
    mtbr member
    Reputation:
    Join Date
    Dec 2006
    Posts
    10

    lead heat dissapation question

    i'm making a cree light would like to have a thermally actuated switch up with the led chips in order to protect them from overheating.

    anyone know what such a thing might be called in electrical engineering speak?

  2. #2
    mtbr member
    Reputation: California L33's Avatar
    Join Date
    Jul 2005
    Posts
    699
    They do make thermal switches, or thermal resistors. I've never heard of anyone using one in a home brew LED system, though it shouldn't be that difficult to design the circuit. Size might be an issue as these are generally used in things like coffeemakers and other heating devices to prevent fires in case of malfunction and most light makers are trying for the smallest package possible. It would be much easier to just build a robust heat sink that doesn't need thermal protection, and switch the thing off when in doubt.

    Edit: I believe in engineer-speak a thermal resistor is called a 'thermisistor' though I'm not sure about the spelling. The circuit would have to be designed to that as the resistance changed with heat, the power to the LEDs is reduced, something that should be possible with a relatively simple transistor circuit- but a power transistor with its own heat sink, again adding to the size. If you just used a switch then you could be pounding down the mountain and have everything go dark.
    To the troll mobile, away...

  3. #3
    mtbr member
    Reputation: heatstroke's Avatar
    Join Date
    Jul 2003
    Posts
    1,008
    The bflex has what you are after.

  4. #4
    some kind of hero...
    Reputation: airman's Avatar
    Join Date
    Jan 2004
    Posts
    543
    Quote Originally Posted by nmr8
    i'm making a cree light would like to have a thermally actuated switch up with the led chips in order to protect them from overheating.

    anyone know what such a thing might be called in electrical engineering speak?
    I've seen two different kinds of these switches. The one I use is a Klixon by Texas Instrument. It comes in various sizes (I use 4mm) in 5 degree increments. It may be overkill since it works to protect both temperature and short circuit.

    The other one is a polyswitch - which I've never used - and it is temperature activated only. I think Digikey has the polyswitches. The polyswitch is used as part of the protection for Nimh packs from some of the larger night lighting companies.

    Another device is a thermal fuse... It is a one-time-only self destructing element ...

    Hope that helps.

    Good Luck
    GEVELTERSCHMIDT RACING

  5. #5
    mtbr member
    Reputation:
    Join Date
    Oct 2007
    Posts
    170
    The taskled drivers using the UIB2 firmware have settings that allow power step down levels to be set to protect things if it all gets a bit warm

    Temperature monitoring of driver board. If closely coupled with the light's body, it can be user configured to force the current output to reduce. Optional trip points of 50, 60, 70, 80, 90C or off.
    from http://taskled.com/bflex.html

    The maxflex boost driver is soon to have the same firmware and functionality

  6. #6
    mtbr member
    Reputation:
    Join Date
    Dec 2006
    Posts
    10
    Quote Originally Posted by California L33
    If you just used a switch then you could be pounding down the mountain and have everything go dark.
    yeah, that's a good point. ideally it should dim with overheating. that's going to be hard i think, because the bflex is a current regulator.

    is it possible to get the bflex integrated circuit with the thermistor close enough to the leds? i have an mr11 aluminum plate and i figure i need a lot of that plate's surface area to conduct heat away, and that there might not be enough room to get the bflex controller close enough that the thermistor would be accurate.

    has anyone put their bflex board right next to the leds?

    i wish bflex had a connection for an external thermistor. i wonder if the stat led pins on the atmel chip are reconfigurable general i/o pins and a thermistor could be hooked up there.

  7. #7
    mtbr member
    Reputation:
    Join Date
    Dec 2006
    Posts
    10
    Quote Originally Posted by airman
    I've seen two different kinds of these switches. The one I use is a Klixon by Texas Instrument. It comes in various sizes (I use 4mm) in 5 degree increments. It may be overkill since it works to protect both temperature and short circuit.

    The other one is a polyswitch - which I've never used - and it is temperature activated only. I think Digikey has the polyswitches. The polyswitch is used as part of the protection for Nimh packs from some of the larger night lighting companies.
    the polyswitch sounds like what i'm thinking of. i knew i'd seen something in the makita pack i rebuilt just didn't know what it was called. (there was a thermistor and a small boxy looking thing inside the pack, fwiw.)

  8. #8
    some kind of hero...
    Reputation: airman's Avatar
    Join Date
    Jan 2004
    Posts
    543
    Quote Originally Posted by nmr8
    yeah, that's a good point. ideally it should dim with overheating. that's going to be hard i think, because the bflex is a current regulator.

    is it possible to get the bflex integrated circuit with the thermistor close enough to the leds? i have an mr11 aluminum plate and i figure i need a lot of that plate's surface area to conduct heat away, and that there might not be enough room to get the bflex controller close enough that the thermistor would be accurate.

    has anyone put their bflex board right next to the leds?

    i wish bflex had a connection for an external thermistor. i wonder if the stat led pins on the atmel chip are reconfigurable general i/o pins and a thermistor could be hooked up there.
    The bflex just keeps getting better!

    I use an older version of the bflex and didn't mount it in the led housing since I did not know what temperature problems I might have.

    My home built light is a triple K2 and what I've found is that while I'm riding I can run it at 1 amp without any heating issues. When I stop, it only takes a couple of minutes for the temperature to start to run away. I just tap the switch on the bflex to lower the intensity and it works fine... I went to great lengths to provide heat sinking and it has worked out for me. With the bflex temperature monitoring it would be fine to mount in the led assembly since it probably won't need to restrict current unless you're stopped - and you won't need the full output under those conditions. The temperature setting for current reduction will take some testing, but the cooler you keep this stuff, the longer it will last. The Cree led will also degrade significantly over time if you run too hot.

    Good luck!
    GEVELTERSCHMIDT RACING

  9. #9
    mtbr member
    Reputation:
    Join Date
    Dec 2006
    Posts
    10
    Quote Originally Posted by nmr8
    i'm making a cree light would like to have a thermally actuated switch up with the led chips in order to protect them from overheating.

    anyone know what such a thing might be called in electrical engineering speak?
    done.
    http://picasaweb.google.com/nnmmrr88/DiyCreeXrEXlamp

    skipped the thermal switch, smushed the bflex temp sensing ic up against the heatsink, set the temp threshold to 50c, and decided to hope for the best and not to further worrry about it over heating.
    Last edited by nmr8; 12-21-2007 at 04:25 PM.

  10. #10
    mtbr member
    Reputation: Snowdawg's Avatar
    Join Date
    Nov 2007
    Posts
    62
    I think you want is called a thermistor. As the temp changes, the resistance changes also. I think it is theoretically possible that you can use it with a buckpuck to lower the current as the temp goes up.

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •