Find data info for RGB LEDs

Discussion in 'General Electronics Chat' started by stoopkid, Dec 8, 2011.

  1. stoopkid

    Thread Starter Member

    Mar 3, 2011
    136
    1
    I just got some common anode RGB LEDs off a Chinese manufacturer. I asked for a data sheet or model number and they just gave me this little picture, which seems to have info that I would not have expected, namely the 3.5v for each color.

    Is there a way to test what the actual properties of the LED are? Would a multimeter give me the LED's voltage, or something else arbitrary? Also is there a way to find the proper amperage? Am I better off just assuming the details are similar to most other of this type?

    Thanks
     
  2. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    You could use the 25mA number.

    Get yourself an LM317, and a 47 Ohm resistor, or better yet, two 100 Ohm resistors in parallel to make 50 Ohms.

    Connect the 47 or 50 Ohms from the OUT to the ADJ terminal.

    Then connect the IN terminal to a voltage source; 7v to 12v.

    You now have a constant current regulator that will output 25mA from the ADJ terminal.

    Connect the ADJ terminal to your RGB LED's anode.

    One at a time, connect a cathode to the supply return; and measure the voltage across the LED with your meter.

    Repeat with all three colors.

    Try out a dozen of them to see what the Vf is. You may find that they vary a fair bit. Write the numbers down as you go.
     
  3. stoopkid

    Thread Starter Member

    Mar 3, 2011
    136
    1
    Excellent, that sounds simple enough. Is it safe to assume it's 25mA? Aren't LEDs typically 20?
     
  4. Wendy

    Moderator

    Mar 24, 2008
    20,766
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    Modern LEDs can be 20ma or 30ma. I usually aim for 20ma myself, just in case. Thing is, while you may shorten the lifespan a little it is still going to be good for 1000's of hours.
     
  5. stoopkid

    Thread Starter Member

    Mar 3, 2011
    136
    1
    Cool, thanks guys.

    Could someone post the formula for the resistor for a particular current? When I search for it, they mention desired voltage which I don't understand in this situation.
     
  6. Wendy

    Moderator

    Mar 24, 2008
    20,766
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    LEDs, 555s, Flashers, and Light Chasers

    Bill's Index

    [​IMG]

    A favorite trick for higher current LEDs. 12Ω for R1 is 0.104A. Every resistor in parallels adds to the current, so a second 12Ω is another 0.104A. Each resistor is dissipating just over an eighth of a watt, which means it is ideal for ¼W resistors. Qty 9 of 12Ω resistor will be .936A. High power resistors can run from 300ma to 1.2A nowdays.
     
  7. Markd77

    Senior Member

    Sep 7, 2009
    2,803
    594
    For a situation with just the LED and a resistor it is:
    R=(Vin-Vf)/I
    I is in Amps, but unless you use a circuit like SgtWookie's, you don't know Vf (the LED voltage) at a known current.
    I'll bet that the red LED is nowhere near 3.5V, probably a typo.
    You can just calculate the resistors for the minimum forward voltages in the table, then measure the Vf of the LEDs (at that current) and calculate the current from the voltage across the resistors.
    Vf does vary with current and temperature.
     
  8. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
    6,357
    718
    Do you have a constant voltage/constant current supply?

    Even a 3A one is a must for the bench. Set the output to 10mA and measure Voltage for R, G, B, set to 15mA, repeat measurements, set to 20mA repeat voltage measurement, set to 25mA, repeat measurement. Enter the values into OpenOffice Calc (free) or M$ Office (RipOff)

    Do that for about 10-15 randomly grabbed LEDs out of the box and they should group into a nice curve plot in excel if you enter the data as you go. columns for current, rows are for rgb1rgb2, etc
     
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