lm3915 vu meter to drive 1 watt luxeon leds

Discussion in 'The Projects Forum' started by burton2663, Jul 19, 2011.

  1. burton2663

    Thread Starter New Member

    Jul 18, 2011
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    hi

    i am testing some luxeon 1watt leds to run on the 3915, i got some 3904s and tip13c. using 8v... does anyone use these components to drive the leds? a diagram would be appreciated tnx
     
  2. nickelflipper

    Active Member

    Jun 2, 2010
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    The most efficient and proper way to drive high power leds is to use constant current. An adequate buck regulator, amongst the dozens, would be a zxld1350.
     
  3. iONic

    AAC Fanatic!

    Nov 16, 2007
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    Aside from the optimal method of driving an High-Power LED, what was your thought or intentions on using the 3914 Dot-Bar LED driver?
     
  4. KJ6EAD

    Senior Member

    Apr 30, 2011
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    By the way, the LM3915 has logarithmic voltage steps, the LM3916 has Vu meter steps.
     
  5. iONic

    AAC Fanatic!

    Nov 16, 2007
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    But the LM3914 will be more easily found than the LM3916.
     
  6. burton2663

    Thread Starter New Member

    Jul 18, 2011
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    i have some 2sc1061 & lm317, tried them both but the brightness is still not that good...how do i increase brightness using these parts?
     
  7. SgtWookie

    Expert

    Jul 17, 2007
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    You say "Luxeon 1W LEDs", but you don't give any other specifications.
    We need to know what the typical Vf is, and the current rating.

    The LM317 when used as a current regulator has a minimum dropout of around 3v; 1.7v from IN to OUT, and another ~1.25v from OUT to ADJ.

    You say you have 8v available. Is this from a regulated power supply? What is the wattage or VA rating or current output rating?
    How many of these 1W Luxeons are you planning on driving at once?

    What do your existing circuits look like? Can you post a schematic, preferably in .png image format?
     
  8. burton2663

    Thread Starter New Member

    Jul 18, 2011
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    hi

    i just base it on the basic vu meter diagram using 3915, im now using 12volts 1 ampere plugin supply

    i am getting another 12 volts 5 watts supply

    the 317 specs is here http://www.ebanxts.com/index.php/tag/current-regulator-with-ic-lm317

    The red and yellow voltage is :2.2-2.6 v
    Green voltage is :3.0-3.5V
    All the leds current is:350mA

    tnx
     
    Last edited: Jul 22, 2011
  9. SgtWookie

    Expert

    Jul 17, 2007
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    If you want to operate 10 Luxeon 350mA LEDs at full brightness, then you will need at least a 3.5 Ampere supply just for powering those LEDs.
    350mA x 10 = 3.5A. This does not include power for the IC or your aux LEDs.

    With a 1A supply, you do not have enough current available to drive more than 2 at full brightness. 2*350mA = 700mA; there would only be 300mA left for the 3rd LED, and none for any others. The 12V 5W supply you are talking about will supply up to 417mA, which would be enough to power just one LED.

    However, it is a Very Good Thing that you have not yet been able to supply full current to your LEDs, as you are operating them with no heat sinks. This will destroy them VERY quickly if you operate them anywhere near their power rating. You may have already damaged some or all of them by overheating. See your link to the LM317 specs for examples of heat sinking those LEDs.

    Now let's look at the regulation problem. You want to use LM317's for voltage regulation.
    You say the red and yellow LEDs have a Vf of 2.2v to 2.6v @ 350mA. Let's assume 2.4v for the moment.
    You're powering them from 12v.
    So, 12v - 2.4v = 9.6v; this will have to be dropped across the regulator. 9.6V*0.35A = 3.36 Watts total, of which about 0.44 Watts will be dissipated in the resistor from OUT to ADJ, the remaining 2.92 W in the regulator itself. That's going to require a heat sink, and your resistors will need to be 3.6 Ohms and rated for 1 Watt or more.

    An LED with a Vf of 2.4v and an If of 350mA is dissipating 840mW. The LM317 current regulator will be dissipating 3360mW, for a total power dissipation of 4200mW, and will only be 20% efficient; 80% of the power will be wasted as heat.
     
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  10. iONic

    AAC Fanatic!

    Nov 16, 2007
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    I could be wrong, but I do not recall the Luxeon Red LED's with a Vf of 2.2V - 2.6V.
     
  11. SgtWookie

    Expert

    Jul 17, 2007
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  12. iONic

    AAC Fanatic!

    Nov 16, 2007
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    2.85V. That sounds more reasonable for a Luxeon High-Power LED.
     
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