led help

Discussion in 'The Projects Forum' started by ol'trusty, Nov 2, 2012.

  1. ol'trusty

    Thread Starter Member

    Sep 9, 2009
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    I bought some led bulbs and drivers from ebay and I am abit confused:mad:

    This is what I got:
    10 pcs 3w 750ma 2.2-2.4 forward voltage
    1 pcs Power: 30W 350mA-1000mA
    30V-36Vdc forward voltage

    I also purchased 2 30-36v drivers 1.5a

    I think I can connect the 10w straight to the driver and be good to go..

    For the rest I am not sure if the driver would work, can I put a resistor to drop the excess voltage? for example 36v-(10X2.2) / 750ma.

    How should one go about connecting multiple leds to a driver? (series parralel?)

    Shouldnt the driver state the constant current that it is putting out ?
    It just says 0-1.5A which is max current.

    I must be missing a few pieces of knowledge

    pics attached

    Thanks for some insight
     
  2. wayneh

    Expert

    Sep 9, 2010
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    3,027
    Here are slightly cleaned up pictures. Not sure it helps.

    A couple things: Those LEDs will need good heat sinking to avoid burning them up, especially if you want to run them anywhere near the rated max current.

    You need to understand the details of that driver before you hook anything up. LEDs need to be driven at a controlled current. The available voltage needs to exceed the forward voltage (as in your series example) but after that it's all about current control. What are the controls on your driver?

    Avoid using a series resistor if you can because it will waste a lot of power and get very hot, but absolutely use one if you need it to protect against over-current, since this will destroy your LEDs and/or the power supply.

    [​IMG]

    [​IMG]
     
  3. ol'trusty

    Thread Starter Member

    Sep 9, 2009
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  4. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
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    I suggest you purchase the right driver or add more LED's to the series string.
    Are you SURE its 2.2V Vf.. Seems low. Most LED's like that are in the 3.1-3.7V range.

    The 0-1.5A is because its a dimmable driver. Supply the dimming port with 10V and its outputting 100% (or 1.5A) give it 5V and it will output 750mA
     
  5. wayneh

    Expert

    Sep 9, 2010
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    That driver is made for a specific type of LED - a 35V high power (50W) LED. I can't tell how it behaves but I'll guess it provides PWM control for that bulb. I continue to speculate it will be supplying a 30-36V square wave at variable duty cycle. The control knob likely adjusts the duty cycle.

    If it was me, I'd want to protect the LEDs so I'd assume the driver might supply continuous 36V and design around that. So if you want to drive your LEDs at 0.5A (safer and longer lasting than at the rated 0.75A), you need to drop 14V across a resistor at 0.5A. By Ohm's law, that's a 28Ω resistor. But power dissipated by that resistor will be 0.5A x 14V = 7W. You'd need to spec it for at least 15W, again to avoid running it too hot and too near it's rated max. Personally, I'd try some larger values as well (maybe 47Ω and even 100Ω) until I was more comfortable that nothing was overly stressed. Adding a few more LEDs would reduce the power dropped by the resistor.

    Those LEDs will be dissipating about 1W each so will get very hot without a proper heat sink.
     
  6. crutschow

    Expert

    Mar 14, 2008
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    You should be able to drive the 30W LED directly from the driver. The others need to be in series with a resistor, as wayneh noted.
     
  7. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
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    yuck.. thermal runaway here you come..
     
  8. crutschow

    Expert

    Mar 14, 2008
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    You may get excessive heating of he LEDs if they aren't properly heat sunk but I don't see any path for thermal runaway. :confused:
     
  9. ol'trusty

    Thread Starter Member

    Sep 9, 2009
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    OK , So I will connect the single led straight to one of the drivers and the rest to the other driver all in series with a resistor.
    Will update when I get everything setup and working :cool:
     
  10. wayneh

    Expert

    Sep 9, 2010
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    I think mcgyvr is concerned that the OP hasn't once uttered the word "sink". Just guessing.
     
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