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LED Forward Current vs Forward Voltage

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Discussion in 'LEDs and Optoelectronics' started by tjk84, Feb 14, 2021.

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  1. Feb 14, 2021 #1
    tjk84

    tjk84

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    Feb 14, 2021
    In an LED datasheet, what do you use the forward voltage vs forward current curve for?
    Shouldn't there be multiple curves on this graph because there are different voltage bins (different forward voltages)?

    If I'm using a constant current source, will there be differences in relative luminous intensity if I'm using LEDs in series that have different forward voltages.....or does the relative luminous intensity curve vs forward current curve apply to every voltage bin?
     
  2. Feb 14, 2021 #2
    Nanren888

    Nanren888

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    Nov 8, 2015
    Can you explain "voltage bins"?
    .
    Generally, given a colour, the led will have one curve. Nominally, to a reasonable approximation at low current, a fixed voltage, but actually one curve.
     
  3. Feb 14, 2021 #3
    dave9

    dave9

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    Mar 5, 2017
    Different LED bins don't matter near as much as the token graph provided, because the human eye isn't that sensitive to small % changes.

    Yes, there will be differences in output. If that is important to you, change the design. For most people, it isn't important enough to change the design, and if the design is changed from that point, it is using more LED die of same size at lower current.

    Ultimately you can ignore your question. ;) lol, just pick the led based on the target outcome and forget about the rest besides design ing for that. Small % differences, are far less important than the outcome.
     
  4. Feb 15, 2021 #4
    Harald Kapp

    Harald Kapp Moderator Moderator

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    Nov 17, 2011
    LEDs are current driven. Of course different LEDs will give different intensities and/or color temperature even when driven by the same current (e.g. series connected). This is where binning plays a role, but binning is is done for brightness/color temperature vs. current. Not for voltage.

    The graph voltage vs. current usually shows the typical curve, possibly best and worst case, too. This graph can be used to calculate e.g. a series resistor or the voltage drop when using a constant current source. You'll have to live with the uncertainty in voltage drop resulting from this procedure.
    Note that voltage drop vs. current depends also on temperature and age of the LED.

    For applications where the exact LED current is not that much relevant, use the typical voltage drop from the graph at the desired current to calculate the series resistor. Simply accept that this current has a comparatively high tolerance due to the effects described above.
    For applications where LED current needs to be exact, use a current source. The voltage drop at the set current is only an indicator required to determine the necessary compliance voltage of the current source in this application. It doesn't have to be exact. You'll have to leave some headroom (reserve) in voltage anyway to account for variations in voltage drop due to the effects mentioned above.

    As a note on the side: reading a value from a datasheet's graphed curves to better than 10 % is imho a challenge anyway.
     
    tjk84 likes this.
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