Constant Current Power Source For LEDs

Discussion in 'The Projects Forum' started by jj_alukkas, Jul 29, 2009.

  1. jj_alukkas

    Thread Starter Well-Known Member

    Jan 8, 2009
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    5
    I would like to drive about 15 normal orange led's from an LM317 in constant current mode. So would it be safe to parallel all the LED's and set the current limit resistor equivalent to the current for 1 LED x 15 (say 12mA) ?? Would I end up blowing any of the LEDs?? I intent to use a 5v power source. Would that suffice taking into account the voltage drop for LM317?
     
  2. bertus

    Administrator

    Apr 5, 2008
    15,648
    2,347
    Hello,

    Putting leds parallel is a bad idea.
    The forward voltages of the leds will not be the same (even from the same batch).
    The led with the lowest forward voltage will get the most current and burn afther some time.
    When this led is burned the remaining leds will get more current.
    Then again the led with the lowest forward voltage will get the most current and burn afther some time.
    When this led is also burned the remaining leds will get even more current.

    And so on.

    Greetings,
    Bertus
     
  3. Jack_K

    Active Member

    May 13, 2009
    115
    0
    If you put a resistor in series with each LED you can parallel them. That will limit the current to each LED to the amount it requires. Then you don't need a constant current source, just a constant voltage.
     
  4. jj_alukkas

    Thread Starter Well-Known Member

    Jan 8, 2009
    751
    5
    I was about to solder a resistor for each LED which is conventional so thought if this would be better. I was sure that using a single resistor for a bunch of LED's would result as explained by bertus.. But wasn't sure about this one.. Anyway thanks guys.. Back to old school..
     
  5. Wendy

    Moderator

    Mar 24, 2008
    20,766
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    A simple resistor per LED and a regulated power supply is usually the best approach. Simplicity and elegance works.

    The only reason to make it more complicated is if your voltage source varies a lot, such is in a car, or if you have to drive a LOT of LEDs and have a relatively high voltage (say 24V) to do it with, but even then if the voltage source is stable resistors are best.
     
  6. jj_alukkas

    Thread Starter Well-Known Member

    Jan 8, 2009
    751
    5
    I went with constant voltage mode when I had to drive 25 blue LEDs for my car and had an LM317 do it and it still works fine since a year.. The interesting fact is that the blue LED's there run at 1.92V and unlimited current.. It appears to me as working in full intensity.. There I used that mode cos of the battery voltage issue and the tiresome soldering I'll have to do cos I have to fix all the LEDs in a very limited space and a resistor per leg would make problems...

    But here I have an mobile charger with an output of 5v 700mA which gives a steady out.. So thought a constant current mode would do go if it worked of course..!! But anyway I have a low voltage to have minimum dissipation even if I used a regulator chip.. But I find resistors are best here.. Anyway Thanks for all your ideas..
     
  7. THE_RB

    AAC Fanatic!

    Feb 11, 2008
    5,435
    1,305
    I don't know why people keep saying you can't parallel LEDs from the same batch. :( Almost every cheapy commercial LED light you buy in the stores these days does it.

    LEDs have quite a soft Vf curve, and in batches they will generally have Vf within 1% or 2% of each other (measure some if you don't believe me) and they have a positive coefficient of Vf to heat anyway.

    You can't parallel vastly different LEDs (like different colours), but you CAN safely parallel LEDs if they are the same type and batch or if they measure similar Vf on a multimeter (within a few percent of each other).
     
  8. jj_alukkas

    Thread Starter Well-Known Member

    Jan 8, 2009
    751
    5
    I think we can parallel them, provided we provide enough room for variations when designing the circuits or in other words, not pushing the LED's to their extreme limits when used parallel.. Cos the voltage drop for LED's from the same batch are about a maximum of upto 30mA sometimes.. And that ain't good.. But it is really dumb to parallel different colours together..
     
  9. Wendy

    Moderator

    Mar 24, 2008
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    The real problem is long term drift. You'd like it to work 3 months (or more) later. The commercial units would just as soon have them burn out as soon as the warrantee is expired. It's a matter of good design or cheap construction.
     
  10. Ron H

    AAC Fanatic!

    Apr 14, 2005
    7,050
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    This is wrong. Just measure one if you don't believe me.
     
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