How to determine Imax for an unknown LED?

Discussion in 'General Electronics Chat' started by iONic, Nov 2, 2010.

  1. iONic

    Thread Starter AAC Fanatic!

    Nov 16, 2007
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    I have some Luxeon LED's, Red, Green, Cyan. There are 4 and 8 LED's on a single aluminum board meant to run off 12V DC. I believe the forward current is probably 350mA, but am not sure. I searched for all the strings of "number-text" on the boards and came up blank.

    Is there a way to determine the Imax of these LED's? I am also guessing that the Forward Voltage is between 3.2V and 3.7V.

    Thanks
     
  2. bertus

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  3. iONic

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    I don't belieyve they are listed as part numbers. They may have been assembled by a third party.
     
  4. DonQ

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    It's called "Destructive Testing", and it can be loads of fun!
     
  5. Potato Pudding

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    Jun 11, 2010
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    Ramp up the current until they get bright, and then don't bother going more than 2x or 3x that current.

    So if they reach near full brightness at 50mA meaning they are bright and don't seem to get much brighter with 20% more current at 60mA then I would say try to keep them around 50mA. LEDs are nonlinear. You might be able to drive the example LED with 150 mA of current but it won't be that much brighter. It will certainly not be three times as bright. You are likely to just burn it out quickly.

    If you ramp up the current slowly (which is easy to do with a variable resistor or fixed resistor and variable power supply) you should find their optimum current area just by observing carefully.

    Once you have that optimum value - use it. Maximum rating usages are for people who are very inexperienced with electronics and don't understand; maximum ratings are not a recommended normal operating value. Maximum ratings are where the part is likely to start stressing heavily and failing.
     
  6. iONic

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    Potato Pudding,
    It's funny, cause your suggestion about ramping up the current was exactly what I was considering as I logged onto the site. The unly difference is that I was thinking of ramping it up till the voltage drop was about 3.2V, then measuring the current. These LED's are a Minimum 150mA+ at normal operating conditions, but more than likely 320mA. If they are 320mA LED's I would run them at about 300mA constant current regulated or some PWM method.

    Thanks for the insight. At least it backs up my thinking a little. I can always use that!
     
  7. SgtWookie

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    When current is in excess of the maximum, the color of the light emitted will change slightly - that's with LEDs that are cast in clear plastic. If you exceed the maximum current rating, you will permanently reduce the maximum brightness of the LED.

    PWM by itself doesn't limit peak current. You can control average intensity with the use of PWM, but the maximum current still needs to be limited by some means.

    A switching buck, boost, or buck-boost regulator that uses an inductor is an effective and efficient way to limit the current.
     
  8. iONic

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    Yes...I knew that. The PWM would come after I determine a reasonable current level for these LED's. I didn't intend to imply that PWM would be a stand-alone "current limiter", although looking at my word choice I may have done just that.

    i
     
  9. SgtWookie

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    I figured you probably knew that already. However, we have plenty of beginners on here that may not; so I simply wanted to make it clear that the current has to be controlled/limited by some means.
     
  10. iONic

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    It turns out that the current regulating circutry for the 3 LED's in series was supplying 400mA. I'd probably drop that to 350mA to be safe.
     
  11. Audioguru

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    Instead of guessing, why don't you use LEDs that have a datasheet?
     
  12. hgmjr

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    That sounds like a reasonable compromise. Still it is always a bit risky when you deal with components for which you do not have a manufacturer's datasheet.

    hgmjr
     
  13. iONic

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    Yes, I know. They were part of an assembly and the datasheet for the assembly did not specify who made the LED's or what the regulating circuit output was.
    The whole light assembly cost less than the three LED's alone, thus I got free current regulator circuits and Heat-sinks to boot. My main acquisition was the heat sinks for a different LED setup.
     
  14. hgmjr

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    Sounds like you don't have a lot of money invested in the LEDs. That should allow you to relax and enjoy playing with them. If they don't last as long as you would have liked at least you would have had the experience of experimenting with them.

    Sometimes here in the forum we forget that this is mostly all for fun. Breaking eggs is part of making the omelet.

    hgmjr
     
  15. Audioguru

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    Why?
    I design circuits to last as long as is possible. I designed many circuits and one had tens of thousands manufactured. Not one failed except one that had its quad opamp mounted backwards and another with a shorted electrolytic capacitor.
    I never over-stress a component. I am proud that my designs are reliable.
     
  16. hgmjr

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    Actually that is my point. Most of the members here are in this for the fun and learning of playing with electronics. There is less emphasis on making things that last forever and more emphasis on relaxing and having fun. If something fails then that is part of the learning experience.

    hgmjr
     
  17. spinnaker

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    And I have a fried Pickit 3, a couple of LCD displays and a couple of PICs to prove it. :)
     
  18. hgmjr

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    Most of our members will be able to relate their experience with letting the smoke out of parts. I permanently damaged a $900 EL display when I powered it up with the voltage reverse. It was my first experience with EL displays and the pins of the connector were not clearly marked. I learned an important lesson from that mistake. It never happened again.

    hgmjr
     
  19. thatoneguy

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    What is the intended use?

    I am of the mind that if 150mA drives them "bright enough" for the purpose, then run them at that.

    Ignore above when it comes to CR123 sized Sunshine, or if you are a member of candlepower forum, but I repeat myself.
     
  20. iONic

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    The part I purchased is a fairly reputable supplier, thus their specifications are lifetime operating expectancy promises are probably fair. That's why I cut back 12.5% from the 400mA to 350mA. I may decide to cut it back 20%, but to cut back more than 60% would be overkill. That's like using my CPU heat sink and cooling fan on one end and my vacuum cleaner on the other!

    Since I have several of these assemblies, I will check the current on each of them. If they are consistent, then I see no reason to play in the gray of mediocrity. I'm not mass marketing anything.

    @AudioGuru,

    I understand fully and respect your commitment to the quality that you represent and wish that there were many more like you. If I were to sell a design or product to someone I would strive to be as ethically clean as you.

    iONic
     
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