LED Driver Compatability

Discussion in 'The Projects Forum' started by indimensionn, Dec 21, 2012.

  1. indimensionn

    Thread Starter New Member

    Dec 21, 2012
    Hi All, I'm new here and could use some help determining whether DMX LED drivers I just purchased will work with some leds I'd like to drive.
    My project employs a battery powered system of many different leds which are DMX controlled. The drivers that I'm using will be this:


    What I'd like to drive are a bunch of these:


    What I'm not sure about is whether or not I need to calculate and include resistors for This and if so is it best to share resistors? Clearly I know just enough to be dangerous with this stuff so any help would be greatly appreciated.

  2. Dodgydave

    Distinguished Member

    Jun 22, 2012
  3. KrisBlueNZ


    Oct 17, 2012
    Yes you can do this.


    D1~3 are optional and are for ESD (electrostatic discharge) protection for the LEDs. It's very unlikely you'll have a problem with this, because of the capacitance in the wiring. But you can use them if you want. They're a lot cheaper than the LEDs.

    The DMX controller provides a positive voltage (shown as "+V" on the bottom pin of the plug at the left), which connects to the three anodes of the RGB LED. (These are indicated by holes on the lead wires.)

    The cathodes of the RGB LED connect to the individual outputs through limiting resistors R1~3. When the controller wants to illuminate a LED, it brings the corresponding output down to 0V. This causes current to flow out the +V pin, through the LED (in the anode, out the cathode), through the current limiting resistor and into the controller output, which conducts current to ground. (This is conventional current flow, positive to negative.)

    Calculating the current limiting resistors is done using Ohm's Law. First you need to calculate the voltage across the resistor.

    I'll assume you're using a 12V supply to the controller outputs. This is preferable to using 24V, since it means less power will be dissipated in the current limiting resistors. I'll assume this supply is actually 12.0V, not "12V" which sometimes means 13.8V in battery circuits.

    The voltage across a current limiting resistor will be 12.0V minus the LED's forward voltage. (There will be a small amount of voltage dropped by the output circuit but it will be negligible. I'm assuming a MOSFET output, which is a reasonable assumption.) From the data sheet, the forward voltage for the red, green and blue LED components are:

    Red: 2.2V (typical). Resistor voltage = 12.0 - 2.2 = 9.8V.
    Green: 3.5V (typical). Resistor voltage = 12.0 - 3.5 = 8.5V.
    Blue: 3.5V (typical). Resistor voltage = 12.0 - 3.5 = 8.5V.

    Now you can calculate the resistance using R = V / I where V is the voltage across the resistor, and I is the current through it, in amps. Assuming you want to run them at maximum current, this is 0.15 amps. So the resistances are:

    R1 (red) = 9.8 / 0.15 = 65.3 ohms; nearest greater preferred value is 68 ohms
    R2 (green) = 8.5 / 0.15 = 56.7 ohms; nearest preferred value is 56 ohms.
    R3 (blue) same as R2.

    Next you need to calculate the power dissipated in each resistor, using the formula P = V I.

    R1: 9.8 x 0.15 = 1.5 watts
    R2,3: 8.5 x 0.15 = 1.3 watts.

    So you will need resistors rated for at least 2 watts. I would use 5W wirewound resistors; they have a large surface area (a rough surface) and won't overheat easily. You could get away with using 2W resistors if you KNOW that you're not going to be running the LEDs at full power steadily for any length of time.

    You can also use two or more resistors in series or parallel to make the desired resistance value. See http://en.wikipedia.org/wiki/Resistor#Series_and_parallel_resistors. You will need to calculate the power dissipation in each resistor separately to work out the power rating you need.

    Also, the LED data sheet warns that you will need to heatsink the LED if you run it at maximum current for longer than one second.

    This will not guarantee that the LED will emit WHITE light when all three colours are at maximum. The data sheet does not state that the LED's light will look white when all three elements are driven at the same current. You should check this visually. If one colour is predominant, you have to reduce the current in that element, by increasing the resistance of the series resistor for that element.