Resistor values for LEDs

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

  1. Rosbife

    Thread Starter New Member

    Jan 17, 2013
    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


    Feb 11, 2010
    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"?