Luxeon BuckPuck usage and DC Voltage Regulators

Discussion in 'General Electronics Chat' started by K22, Jan 24, 2008.

  1. K22

    Thread Starter New Member

    Jan 18, 2008
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    In looking to complete a high powered LED project, I came across the Luxeon BuckPuck as a current regulator used for driving the Luxeon LEDs. My hope is to be able to change the brightness of the LED, and it sounds like the BuckPuck allows you to do just that, but I can't quite seem to make out how their sample circuit allows you to adjust it using an IC. Their sample circuit on page 7 of their data sheet (http://www.luxeonstar.com/3021-buckpuck.pdf) apparently has a piezo transducer??

    Can anyone explain how efficient dimming can be controlled via an IC? There is no need for infinite adjustability, just 2 or 3 settings.

    Also, would using a LM327 as a positive voltage regulator allow me to efficiently step up from 5 volt to 12 volt for use with the BuckPuck? The spec sheet says that with an external transistor it can handle up to 1A, but I don't know if there are other implications or not :confused:

    Thanks
     
  2. K22

    Thread Starter New Member

    Jan 18, 2008
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  3. rwmoekoe

    Active Member

    Mar 1, 2007
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    the data sheet gives just the way you want to control the output current.
    the sample circuit in pic. 15 of page 7 gives just that.
    you can put on several I/O pins of the PIC with defferent values of resistors to the CTRL pin to get the different brightnesses, by choosing either one or none of the I/O pins to high state at a time, while the others to high impedance.

    (nb: what looks like a piezzo transducer, is actually an x-tall resonator for the PIC microcontroller)
     
  4. K22

    Thread Starter New Member

    Jan 18, 2008
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    Ahh, so if my functions don't require the use of a PIC, I can leave the 3 capacitors and the resonator out? Is the '0.1' capacitor necessary? Are the other 2 just used in conjunction with the resonator to generate a clock signal for the PIC? and finally, does it matter if I ground the logic circuit to the current regulated output, or the original voltage source?
     
  5. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    Why can't you just control it with a pot, as shown in Fig. 9 of the datasheet?
     
  6. Søren

    Senior Member

    Sep 2, 2006
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    Hi,

    Very efficient according to the datasheet ;)

    Fig. 2 gives the relation between the voltage fed to the [Control] pin as a graph and page 3 tells you that:
    The LED's start to dim when the control voltage is 1.65V (+/-5%) and they shut off at a voltage of 4.2V (+/-5%) So it's an inverse relationship - a higher voltage gives a lower LED output.

    (And since it seems a linear relation, the LED's are probably around 50% at a voltage of 2.9V).

    The input impedance of the [Control] pin is 1k according to p.3 and the reference voltage is 5V (+/-5%) as long as Vin is > 5.25V and the ground reference for an external control circuit is LED- (LED minus).

    This should be the info needed for the design of an external circuit.


    LM723 is a linear regulator and as such can only step down (that said, a died in the wool hardware hacker could make a switcher out of it, but it would not be as efficient as a real switcher.

    The BuckPuck is a step down switcher and it would be a bit of overkill to put a step up switcher in front of it IMO.
     
  7. K22

    Thread Starter New Member

    Jan 18, 2008
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    Yeah, I realized that the LM723 wouldn't work too well after having a look at one of the other threads. I also did a bit of reading and it looks like the voltage can be stepped up a little bit, but not significantly while maintaining current draw. Plus, with batteries, it seems as though they may not be able to maintain a high enough current to go with a stepped up voltage.

    I was hoping to step up the voltage because I have a battery pack at 6V, but the spec sheet on the buck puck mentioned requiring forward voltage + 2v to work properly, and the LED I'm hoping to use is 9.6. I didn't want to spend the extra money on batteries, but it really does seem to be the best idea.

    Thanks for clarifying the dimming control and efficiency. I'll admit I glazed over several of the pages since I wasn't quite sure what I was looking at :)
     
  8. Søren

    Senior Member

    Sep 2, 2006
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    Hi,

    9.6V? What LED is that?

    Instead of throwing your money at the BuckPuck, why not use a driver that can go directly from the supply that you have? Maxim (among others) has several chips for that purpose.

    If you've got a link to a datasheet for the LED (or at least the power specs), I'll see if I can find a better alternative than the BuckPuck for you.
     
  9. thingmaker3

    Retired Moderator

    May 16, 2005
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    I'd guess we're dealing with an array. Maybe three in series at 3.2V each.
     
  10. K22

    Thread Starter New Member

    Jan 18, 2008
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    Well, I'm looking at a 3 LED array made up of Luxeon Rebel LEDs. They come independently as surface mount modules, but they also make a 'Star' which can have one or 3, and is hand solderable (up to 540 lumens!). I imagine they're hooked up in series, so 3.2V for each, I haven't actually looked at the individual ones.

    I decided to go with the buckpuck just because it was there to be honest with you. It's not the smallest thing out there, but ah well. There might be one that works better, but it also allows for simple current control through a logic circuit, so I just stopped looking.
     
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