8amp cc driver help

Discussion in 'The Projects Forum' started by nickarpik1440, Nov 16, 2010.

  1. nickarpik1440

    Thread Starter New Member

    Nov 16, 2010
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    looking to build a driver for an led project im working on.
    needs to be constant current, adjustable and capable of up to 8 amps of output. can anyone lend a hand on this one?
    -Nicholas
     
  2. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    Can you post aschematic of the planned setup for the leds?
    That way we can see how to configure the constant current source.

    Bertus
     
  3. nickarpik1440

    Thread Starter New Member

    Nov 16, 2010
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    Its a single led rated 8amps at 5v.

    is anymore info needed?
    -Nicholas
     
  4. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    If you want to do this efficient, you will need a special driver chip.
    The LM3433 or LM3434 could do the job.
    See the attached datasheets for more info.

    Bertus
     
  5. nickarpik1440

    Thread Starter New Member

    Nov 16, 2010
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    efficiency isnt the concern, safe current is, i have adequate cooling for the led so thermal shutdown safety features arent a concern.
    just to be clear is that pdf showing a circuit that would suit my needs? if so where can i buy it, trying to keep the cost in the $20 ballpark.
    -Nicholas
     
  6. SgtWookie

    Expert

    Jul 17, 2007
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    I'm not quite sure why you'd say that, as high efficiency is all the rage nowadays. Efficiency is one of the big reasons to go from incandescent to LEDs.

    Modern synchronous buck regulators like Bertus linked to are very efficient as well as providing excellent current regulation. They are a bit challenging for novices to understand.

    I don't know if you've gone through the math, but an LED rated for a typical Vf of 5v @ 8A will be dissipating 40 Watts of power. That's a LOT of heat to get rid of. I'm thinking that you're going to need a large copper heat sink with fan-forced cooling. You'll know if your cooling wasn't good enough if your LED turns into a black blob of molten plastic.

    That's another reason why you need an efficient regulator; less power dissipation in the regulation circuit itself. You might build a really cheap, inefficient linear regulator circuit for a few dollars, then spend a fortune on a heat sink large enough to keep it from melting down.

    Well, just the IC alone costs around $7, depending on whom you buy it from, plus shipping. It's in a DFN package, which isn't exactly the most hobbyist-friendly; the pads are on the underside of the IC. You'll have to make a circuit board in order to use it. The circuit shown is a "typical implementation"; a generalized schematic where you select parts for your particular application.

    I was going to suggest purchasing an evaluation board, but those run ~$200/ea, or 10x your budget. If this is going to be some kind of production run where you're building lots of them, that would be a way to go to get you up and running quickly.
     
  7. SgtWookie

    Expert

    Jul 17, 2007
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    Why don't we start off with what you're planning on using for a power supply?

    8A is a fairly hefty load, even at 5v. A buck-type converter would need more voltage than that as input.
     
  8. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
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    What is the input? 110VAC/220VACHousehold Line voltage, or 12V/13.8V automotive?

    What range of adjustment are you looking for, full 0-8A? How do you want to adjust it? Standard pot (very inaccurate for desired range), multiturn pot, digital input, etc?
     
  9. wayneh

    Expert

    Sep 9, 2010
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    This is the salient point. Designing and building a linear regulator to do what the OP wants is trivial, on paper, "except" for the heat issue. But a linear supply starting at 12v would be dissipating more heat in the controller than in the LED itself, which is already a big number.
     
  10. nickarpik1440

    Thread Starter New Member

    Nov 16, 2010
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    the plan is to power a phatlight pt54 off either a 18650 battery array or a regulated switched power supply. the pt54 is mounted to a copper base and that copper base is mounted to a tec which is then mounted to an aluminum heatsink which is cooled with a 26cfm fan. that about all i have worked out so far for this project.
    -Nicholas
     
  11. thatoneguy

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    The Datasheet states a max current of 16 Amps, 8 Amps being "typical display".

    Is this an attempt to make a home theater projector? The power supplys for the LEDs would be modulated with the video signal
     
  12. SgtWookie

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    You need to post more data, like a link to a datasheet.
    Well, the 18650 batteries I've seen will hold up for roughly 3 seconds in your application. So, the regulated switching power supply it is.

    Hope you won't be offended when I suggest that your cooling efforts probably won't be enough, and they will probably consume more power than the LED itself.
     
  13. wayneh

    Expert

    Sep 9, 2010
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    The TEC is worse than useless if you're not trying to go below ambient. You'd do MUCH better to cool directly with the heatsink and fan. A TEC makes far more heat than it moves, eg. 100 watts more heat while moving 10.
     
  14. nickarpik1440

    Thread Starter New Member

    Nov 16, 2010
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    interesting, this is all new to me, as for the 18650 i was speaking of making an array, not just 1 single 2.5amp dinky battery. the tec i suppose would be overkill, i guess its back to the metal shop to make a newer larger heatsink.
    as for the amperage of the led, if it can handle 16 amps i would prefer a driver capable of 13 amps unless someone thinks its safe to go beyond that and stress the die to its limits.
    heres the datasheet i found.
    http://www.luminus.com/stuff/conten...c/pds_001051_pt54_summary_datasheet_rev07.pdf

    -Nicholas
     
  15. thatoneguy

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    Are you trying to build a projector (what the IC was designed for), or just a super bright LED?
     
  16. retched

    AAC Fanatic!

    Dec 5, 2009
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    An LED at those currents will burn your retinas PERMANENTLY in WAY LESS than 1 second if you look into it during operation.

    Use a cover when building and handling at close ranges.
     
    thatoneguy and SgtWookie like this.
  17. SgtWookie

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    Jul 17, 2007
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    Thanks, retched. I forgot about the safety aspect.

    An LED that bright will toast your eyeballs in a heartbeat. It's different from looking at the sun; your eyes don't respond in the same way to LED light as they do to incandescent or thermonuclear light (like from the sun) - LED light is quite different. Your retinas can be damaged quite quickly from exposure to high-power LED emissions.
     
  18. thinus

    New Member

    Nov 26, 2010
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    Hi, I have also discovered the LM3434 to use for our design, to drive an LED at 6A (Vled ~14V). This driver seems from what I read to be pretty impressive.

    However, I struggle to get the design to actually regulate the current. The current just goes up until the Vled reaches the supply voltage. I have stated some measurements in this new thread. http://forum.allaboutcircuits.com/showthread.php?p=300851

    I would appreciate if someone who actually got a LM3434 constant current design working could comment or suggest something there.

    Regards
     
  19. thatoneguy

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    A perfect current source has infinite voltage.

    The current source hitting the supply line is called "clipping" in audio amplifiers.

    You need a higher voltage power supply, though I'm still unclear on the purpose of driving an 8 Amp (84W @6A) LED, I would like a photo of it in operation though.
     
  20. thinus

    New Member

    Nov 26, 2010
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    The goal is to drive a Phlatlight csm360 LED to replace a 400 Watt factory light. Not to highjack this thread, I have out the photo with more explanation here: http://forum.allaboutcircuits.com/showthread.php?t=46113
     
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