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LED currents?

Discussion in 'Electronic Design' started by Jan Panteltje, Dec 26, 2007.

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  1. I was just soldering a LED in a PIC circuit so I can test
    some software I am writing.
    Now I want low brightness, so I derated, 680 Ohm in series, 5V,
    3 V LED drop, green LED.
    I bought some of the brightest greens ones some time ago...
    Switched it on, had to close my eyes: 2.9 mA WAY to bright!!!

    So what do you guys use for LED current these days for a normal
    non blinding clear transparent green LED (say on indicator)? < 1mA?
     
  2. Jamie

    Jamie Guest

    how about PWM it?


    --
    "I'd rather have a bottle in front of me than a frontal lobotomy"

    "Daily Thought:

    SOME PEOPLE ARE LIKE SLINKIES. NOT REALLY GOOD FOR ANYTHING BUT
    THEY BRING A SMILE TO YOUR FACE WHEN PUSHED DOWN THE STAIRS.
    http://webpages.charter.net/jamie_5"
     
  3. Winfield

    Winfield Guest

    Use a wide-viewing-angle diffused-lens LED,
    for an overall pleasant useful appearance.
     
  4. Guest

    Yeah LEDs are very efficient these days, almost blinded myself with a
    stupid SMT red LED on one of my projects.
    It was just supposed to be a power LED, not an optical warfare
    component...
     
  5. DJ Delorie

    DJ Delorie Guest

    For "blinky light" indicators, I use 1k per volt Vcc. So, 3.3k for
    3.3v, 4.7k for 5v, 12k for 12v, etc.

    That's less than 1mA current (due to the Vf drop), but it's enough to
    easily tell that the LED is on without casting that warm glow
    throughout the room.

    Example: for the average green LED (2.0V Vf) at 3.3v: (3.3 - 2.0) =
    1.3v across the resistor, 1.3v / 3300R = 0.4mA

    5v -> 0.6mA

    There is quite a range of "lumens" for LEDs, I like to buy the
    brighter ones because I can drive them directly from GPIO ports
    without the extra transistor.
     
  6. If the green LED is one of those InGaAlP ones, half a milliamp is
    probably about right for "normal LED indicator lamp" brightness. At that
    current, voltage drop is about 2.9 volts, so the dropping resistor would
    be about 3.9K or 4.7K or so.

    I have had some better ones work just fine at .3 milliamp.

    InGaAlP green LEDs tend to have nominal wavelengths around 520-530 nm.
    Their color varies a little with current, and at half a milliamp expect a
    pretty lime green or greenish lime green color.

    - Don Klipstein ()
     
  7. Joerg

    Joerg Guest


    Does anyone know what those super-bright LEDs in the Harborfreight head
    lamp can stomach over the long haul? There is no part number on them.
     
  8. Tim Williams

    Tim Williams Guest

    Gosh, I don't think I've *ever* seen an LED (singles, not a special type)
    with a part number on it.

    And those that do have numbers are hard (impossible?) to find references
    to, last I tried.

    Tim
     
  9. Henry Kiefer

    Henry Kiefer Guest

    Just test it. But I would wonder if they last longer than 100 hours.

    - Henry
     
  10. Most "regular size" white LEDs are characterized at 20 mA, and I hear a
    few life figures of 6,000 and 10,000 hours, though sometimes 50,000 hours
    for some.

    I have seen some with a halflife appearing to me about 4,000 hours at 30
    mA.

    If you don't mind faster aging, my experience is that most don't mind 40
    mA too much and you have a fair chance of getting away with 50, especially
    with some heatsinking on the negative leads (such as large copper areas in
    a circuit board).

    Keep in mind that the usual white LEDs are nonlinear, with maximum
    efficiency when moderately underpowered (maybe 5 mA or so for "regular
    size" / "low power" ones).

    - Don Klipstein ()
     
  11. Yes that would work, but I need timers for other things too...
    And that would add a user control.
    Not a bad idea... maybe add a photo cell and measure room light....
    But it defeats simplicity :)
     
  12. Thank you Winfield, I will have a go at some catalogs.
     
  13. LOL
     
  14. Yes, looks like I will have to go to lower currents too.
     
  15. Thank you Don, that confirms also what DJ Delory suggests I use.
    Yes I think these LEDs are those InGaAlP, they are a different green then some older
    ones I have, I will play with the current a bit to see if the color changes.
     
  16. Eeyore

    Eeyore Guest

    Since WHEN did green LEDs have a Vf of 3V @ a couple of mA ? It'll be
    more like 1.6V.

    So I = 3.4/680 = 5mA

    If you bought an ultra-bright one no surprise. Also watch out for the
    view angle.

    Often about 3mA with a run-of-the-mill LED and 10mA if I want it to be
    bright.

    Graham
     
  17. Joerg

    Joerg Guest

    Well, that's why I was asking since they might drive them a bit past
    their usual limits :)
     
  18. Joerg

    Joerg Guest


    Thanks, Don, that's good information. So I'll keep them to 20mA then.
     
  19. In the long run they will ALL go dead.


    Best regards,
    Spehro Pefhany
     
  20. Joerg

    Joerg Guest

    Why is that? Sloppy processes or something like that?
     
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