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confusion over LED forward voltage rating

Discussion in 'LEDs and Optoelectronics' started by Raul, Oct 26, 2012.

  1. Raul

    Raul

    33
    0
    Sep 25, 2012
    In this product description (image infra) the Forward Voltage is referenced two times and each time it is different
    The first time it is 2.2 to 2.6 and the second in the table titles Electrical/Optical characteristics it is rates at 3.0 to 3.6
    Can you shed any light on what seems to me to be a discrepancy?
    Blue 3-0Vf 150mA.JPG


    If you wish to see the original content it is here:
    http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=120991873657

    I've looked at his other 8mm straw hat LEDS and no such discrepancy exists. So maybe it's an error in the product description? I emailed the vendor to see what he says.

    thx
     
    Last edited: Oct 26, 2012
  2. screwball

    screwball

    89
    1
    Jan 9, 2012
    As far as I know, there are only 2 forward voltage ratings for most LEDs (and components for that matter), the typical and absolute max

    See what the vendor says or if someone knows different on here
     
  3. gorgon

    gorgon

    603
    23
    Jun 6, 2011
    For a blue LED I would think that 3.0V to 3.6V is correct. The typical value is between those margins. I would assume that the difference is just a printing error in the summary.
     
  4. gorgon

    gorgon

    603
    23
    Jun 6, 2011
    Normally you should have 3 values, min, max and typical.
     
  5. Laplace

    Laplace

    1,252
    184
    Apr 4, 2010
    One expects a red LED forward voltage to be in the 2 volt range and a blue LED forward voltage to be in the 3 volt range because it takes more energy to produce blue photons than red photons. Other blue LEDs I have seen all have their forward voltage at 3.2 V or greater. It would make sense to change the "2" into a "3".
     
  6. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,174
    2,690
    Jan 21, 2010
    I'd go for the 3.0 to 3.6 value.

    I suggest the "specs" are a mashup of mismatching information.
     
  7. Raul

    Raul

    33
    0
    Sep 25, 2012

    This vendor states it as a range with no optimal. I guess there is an assumption that you can pick your poison and run with it within that range.



    Yah, that's what I would have assumed judging from all the Blue LEDS I've looked at.
    This is a 150 mA forward current 8mm LED.

    The suggestion that
    and
    are probably correct. He cut and pasted the charts to the Ebay site but the Application data may be something he or some one else generated apart from the charts.

    I bought some warm white and yellow 8mm straw hats from him and really like what I built with them. An 11 foot long, slender 31 count LED light bar for my entertainment room.
     
    Last edited: Oct 26, 2012
  8. screwball

    screwball

    89
    1
    Jan 9, 2012
    Indeed, woops :eek:
     
  9. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,174
    2,690
    Jan 21, 2010
    No, it means when you run them at the specified current, you should expect the voltage across them will be within the range specified.

    If you're using a resistor as a current limiting device, you should perhaps assume the middle value, but check by measuring the current (possibly by measuring the voltage across the resistor) to ensure you've got it close to right.

    You get a range of voltages because the actual voltage is rarely a consideration.
     
  10. Raul

    Raul

    33
    0
    Sep 25, 2012
    Didn't know that.
    thx
     
  11. gorgon

    gorgon

    603
    23
    Jun 6, 2011
    If you design a mass produced product using this LED, you should design it with a current limiting resistor value that will give you a current not exceeding the max continuous current allowed at the minimun guarantied forward voltage. The max forward voltage will then return the minimum current for that resistor value.

    If you only make a single device, you can measure the forward voltage for the batch of LEDs you have, and select the resistor based on the actual voltage range you find.

    With a 'typical' value given, this will be a statistical most probable value. You will still need to take care for the actual range, if you don't do a preselection of the LEDs with a narrower range criteria before production.
     
  12. CocaCola

    CocaCola

    3,635
    4
    Apr 7, 2012
    In addition to what gordon just stated the life expectancy of the product can be considered... The LED at the 'recommended' current should last a 100,000 hours (or whatever in that range) if this product is only going to have a consumer life of a few hundred hours (key chain lights) or a few 1000 hours (most novelty toys) then getting this optimal limiting is not a huge overall concern... Pushing a little over the recommended will almost certainly still get you enough life in the expected application if it's only a fraction of what the LED could theoretically last... On the flip side if it's just a visual indicator where intensity is not crucial driving in on the lower side it almost certainly OK...

    And last but not least if true long term life is desired, IMO a constant current driver should seriously be considered, if you are driving multiple LEDs constant current driver ICs are not horribly costly in the application...
     
  13. Raul

    Raul

    33
    0
    Sep 25, 2012
    That makes things a little more complicated.
    So what is the process:
    Take the rated voltages bread-board a string of the LEDs in question up like they will in real life with the resister that fits the rated voltage and take a measurement and then adjust accordingly adding or attenuating resistance?

    What if one is slothful and just uses the lower voltage in the range, builds with that resistor that mathematically gets him the lower rated voltage and runs with it?

    Or stated another way:
    Is it reasonable to assume and therefore can one rely on greater precision in LED manufacture as compared to that of a decade or so ago and just trust the specifications?
     
    Last edited: Oct 28, 2012
  14. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,174
    2,690
    Jan 21, 2010
    Remember, the voltage across the led DOES NOT MATTER. It's the current.

    Measure the voltage across the resistor you use to limit current and as long as it establishes that the current is OK, then you need do no more.

    You use a typical voltage in determining the value of the resistor. That voltage may or may not be right and it may or may not be consistent across the LEDs (but it usually is)
     
  15. laver

    laver

    3
    0
    Oct 28, 2012
    thank you
     
  16. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,174
    2,690
    Jan 21, 2010
    Just run your calculations past us before you wire things up and we'll check your math.

    It's never a problem using too large a value resistor (low current), but a resistor too low in value can destroy (or damage) your LEDs.
     
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