Yet another LED driving question

Discussion in 'The Projects Forum' started by Nrets, Aug 27, 2013.

  1. Nrets

    Thread Starter New Member

    Jan 31, 2008
    9
    1
    Hi guys,

    I know these sorts of questions get asked a lot here, and I've done a lot of searching, but am still not convinced I have a good answer to my question.

    I would like to drive a quantity of 30-50 3W LEDs (700ma). Brightness will be controlled using PWM from an Arduino that will be slaved to a RaspberryPi.

    Here's what I need:
    • I have a 400W PC power supply that I would like to use as my power source. I'm pretty set on this as I already have it and it has more than enough power for the application.
    • I would like to be able to control groups of LEDs or individual LEDs through BJTs or FETs (whichever makes the most sense).
    • I would like to be able to use PWM from the Arduino to control the brightness of the LEDs.
    • The LEDs will be heat sinked and coupled with force convection through the heat sink

    Now, here are my questions for the community:

    • What driver circuit is best suited to limit the current from a constant voltage PC power supply?
    • Would I need a driver for each group of LEDs or each LED, or does this just depend on the driver?
    • Would it be smarter to use the 12V rail and chain the LEDs in series in groups of 3?
    • Can the driver best suited for this application be switched using PWM to control brightness?

    I hope this isn't overwhelming, but I also hope to hear some great ideas. Thanks in advance
     
  2. LDC3

    Active Member

    Apr 27, 2013
    920
    160
    You will notice that this question has appeared before.
    1. You have a solid power supply.
    2. MOSFETs have lower resistance when turned on fully than BJTs.
    3. Easily done when using transistors.
    4. Just ensure that the heat sink doesn't get too hot.

    5. A fixed current supply would be best to control the LEDs.
    6. You would need a driver for each string of LEDs.
    7. You can have 3 LEDs in series if the voltage drop for the 3 is less than 9 V (or 3 V each).
    8. Easily done when using transistors.

    To properly design the drivers, we need to know the voltage drop for the LEDs and the number of LEDs in each string. Each string could be controlled separately, or grouped in any combination.
     
    Nrets likes this.
  3. Nrets

    Thread Starter New Member

    Jan 31, 2008
    9
    1
    Thanks for replying in spite of the fact that some of these questions have been answered elsewhere.

    Most of the LEDs are:
    Forward Voltage: 3.2-3.8V (not sure why there is such a large range)
    Max current: 700 mA
    Power at max: 3W

    They are these: http://www.ebay.com/itm/141010516015?ssPageName=STRK:MEWNX:IT&_trksid=p3984.m1439.l2649

    They haven't arrived yet, but I am somewhat skeptical since they are not the high quality CREEs and I don't know what to expect. Also, the viewing angle is quite large and I hope to get some optics for them.

    If I don't like them, I'll switch to something better.

    So I'll combine all the 12V lines in the PC power supply and get one master 12V line. This 12V lime will then connect to the individual LED driver boards (which I will make), and as either part of these drivers, or on the grounding end of the LEDs, I will palce MOSFETs for PWM control and general swithcing?
     
  4. Metalmann

    Active Member

    Dec 8, 2012
    700
    223


    I've bought 200+ of those cheapies over the last few months, and haven't had one fail yet. The only time they did not function, is when I overloaded them.:D

    They are great for experimenting or using in your projects.
     
    #12 likes this.
  5. LDC3

    Active Member

    Apr 27, 2013
    920
    160
    The forward voltage varies from the manufacturing of the device. It would be best (but not necessary) to determine the forward voltage of each unit. Likely, you will want to operate them at less than 700 mA to increase lifetime; it will decrease luminance, but you will probably not be able to see it.

    Yes, there is a common 12 V line connecting to each string (consisting of a resistor, LEDs and a MOSFET). All the MOSFETs will then be connected to the common ground (or return). The switching is controlled by a another connection to the MOSFETs.
     
  6. Nrets

    Thread Starter New Member

    Jan 31, 2008
    9
    1
    Okay, but wait. I don't want to be using resistors for my current limiters in this situation, do I? Shouldn't I be using something more advanced and efficient that can handle the power draw without getting extremely hot? I was thinking like a simple buck driver connected to each line.
     
  7. LDC3

    Active Member

    Apr 27, 2013
    920
    160
    Not necessarily. For 3 LEDs with 3.2 V minimum for each one, the voltage drop is at least 9.6 V. So the resistor needs to drop (12 - 9.6) V or 2.4 V. The maximum current is 700 mA, so the resistance is 2.4 V / 0.7 A or 3.4 Ω and the power dissipation is 1.68 W.
    These are 3.5 Ω resistors with 5 W rating for $2.20 each.
    A constant current circuit would need a similarly priced resistor and a voltage regulator, total price is between $3 and $5 (I think).
     
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