Resistor values for LEDs

Discussion in 'General Electronics Chat' started by Rosbife, Mar 25, 2013.

  1. Rosbife

    Thread Starter New Member

    Jan 17, 2013
    9
    0
    Hello everyone, I have a pretty simple question.
    I am designing the brake lights for a car, and it is pretty simple - red square LEDs (datasheet attached) to form a circular shining area of at least 15 sq. cm., and power will be coming from a 12V battery. Must be seen in bright, direct sunlight.

    So right now what I've got in mind is a design with 8 parallel subsets of 3 LEDs in series with a resistor (subset example attached), as each LED has a forward voltage of 3.2 typically (@20mA).

    I know that to calculate the resistor value all you've to do is divide the remaining voltage (supply voltage - 3*(FV)) by the forward current at that voltage drop (20mA), which gives us (12-3*3.2) / (0,020 ) = 120 ohms.

    Now, is this a value I should trust, or is there anything big I'm missing? Because I have this other gadget with these same LEDs, from the same source, and they're pretty shiny with a 1k resistor, which gets me kind of confused seeing as how that is over 8 times less current driving the LEDs.
     
  2. Leigh

    Member

    Feb 11, 2010
    20
    3
    Yes, you can trust the math, but there are other considerations, eg temperature, and the actual properties of the leds you have (maybe non-typical). However, what current is flowing through the 1k resistor when you see the leds "pretty shiny"?
     
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