LED array with LEDs in series, varying forward voltages

Discussion in 'The Projects Forum' started by LEDwire, Mar 1, 2015.

  1. LEDwire

    Thread Starter New Member

    Mar 1, 2015

    I'm wiring an LED array with 3w LEDs that I ordered online. Their specs give a forward voltage range of 2.2-2.4 volts, but I'm finding it's more like 2.1-2.6 when I test them. I'm wiring them in several series, with 5 per series with a 3.9 Ohm resistor, ~13v power supply, figuring about .384 amps per series.

    I wanted to know if I need to test each LED individually for its voltage drop so I can group them accordingly and more or less maintain a constant total voltage drop for each series (e.g. not getting several 2.1 fV in a series or several 2.6 fV in a series). Will the brightness of the various LED series be changed if I happen to get several with a higher or lower forward voltage lumped in one series? Or will this not be a problem if the power supply stays constant and the voltage is high enough to light all of the LEDs up?

    The main thing is I want to maximize/keep consistent the brightness.

    Thanks for any help.
  2. LEDwire

    Thread Starter New Member

    Mar 1, 2015
    I started another project that used the same set up, same LEDs, and asked a question in another forum several months ago about that same question, and I was told I needed a resistor because of thermal runaway.
  3. MikeML

    AAC Fanatic!

    Oct 2, 2009
    Bad advice!
  4. crutschow


    Mar 14, 2008
    Agree. The LED forward voltage varies with temperature and thus the current will also undesirably vary with LED temperature (both ambient and that due to self-heating) if no series resistor or other current-regulating circuit is used.
  5. Dodgydave

    AAC Fanatic!

    Jun 22, 2012
    you can put 5 in series and use a J113 fet as a constant current limiter .
  6. ronv

    AAC Fanatic!

    Nov 12, 2008

    If this is really the most important to you I would put fewer in series and try to match the voltage drop of the strings.
    The light output will vary with the current so you would like to keep it constant. Using a larger resistor and fewer LEDs will help this.
    You can try the math with say 3 or 4 per string of the highest and lowest voltage drop, and calculating the resistor for say 0.5 amps with the low voltage ones then seeing the difference with the high voltage ones. The down side to this is the resistor needs to be pretty high power.
    The best would be a constant current LED driver. They can be pretty cheap on Ebay.
  7. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
    Varying Vf and only a fixed resistor for current limiting will equal inconsistent brightness.. Will you notice the difference.. probably not.
    A constant current driver is the solution as it will maintain a constant current for the LEDs as the Vf changes. A fixed resistor cannot..

    If I was doing it I would ensure that even in the worst case VF my LEDs current rating is not being exceeded.
  8. LEDwire

    Thread Starter New Member

    Mar 1, 2015
    Thanks all. Your help is much appreciated.