LED ARRAY-Series or Parallel?

Discussion in 'The Projects Forum' started by JDR04, Aug 15, 2012.

  1. JDR04

    Thread Starter Senior Member

    May 5, 2011
    I've got a small project I would like to tackle. I'd like to assemble 50 UV led's onto a vero/strip board.

    Theres seems to be a lot of conflicting information on the web about series and parallel connections so I'm not sure what to do. Ideally I'd like to power them with 12V from my bench power supply.

    Below are the specs on the leds.

    Forward Voltage (V) : 3.2 ~ 3.6V
    Reverse Current (uA) : <=30
    Max Power Dissipation : 80mw
    Max Continuous Forward Current : 30mA
    Max Peak Forward Current : 75mA
    Reverse Voltage : 5~6V

    I'm going to use this for exposing photoresist boards. Can anybody make a few suggestions please before I waste my hard earned cash. Thanks guys - JDR04
  2. #12


    Nov 30, 2010
    I thought you could do this kind of work.

    Speaking in an average sort of way, you can put (3) LEDs in series and stay under the 12 volt limit with 1.8 volts leftover. You have to use a resistor on that 1.8 volts to limit the current. If you want to run the LEDs as hard as they will go, that requires 30 ma. 1.8 volts/.03 amps is 60 ohms.

    Because the LED spec is +/- .2 volts each, You should figure for 3.2 volts per LED.
    12 - (3.2 x 3) = 2.4 volts leftover.
    2.4/.03 = 80 ohms. Standard 5% resistors can be bought with 82 ohms in them.
    Of course, you can measure the LED voltages after you buy them and get exact on the resistor value.

    You will have to make 50/3 strings of 3 LEDs. That requires 17 series strings.

    Is this what you needed?
  3. JDR04

    Thread Starter Senior Member

    May 5, 2011
    Thanks for that. I kind of knew originally what I wanted to do but when doing my research I found a lot of people suggesting parallel configurations instead of series which is what I originally wanted to do.

    Having you guys around kind of gives me a bit more confidence. Thanks for the help -JDR04
  4. THE_RB

    AAC Fanatic!

    Feb 11, 2008
    From 12v I would use a LM317 and a resistor to give a constant current source. The LM317 CC source will take up about 3v, giving you about 9v to run the LEDs.

    If they are 3v per LED they need to be in pairs (series) at about 6v each pair, then a small resistor in series with that pair.

    So that's 25 pairs of LEDs (=50) and 25 resistors, and a LM317 and current setting resistor.

    The benefit of using constant current supply is that the LED brightness will be constant and exact, which will be of benefit if this UV LED array is used for photo developing etc. :)
  5. Markd77

    Senior Member

    Sep 7, 2009
    Here's mine:

    I matched the LED forward voltages of each string of 3, it didn't take too long and if I'd randomly selected them and got unlucky there would have been quite a big mismatch:
    I don't think there is much point in using a single current regulator, the currents in the pairs of LEDs can still be different to other pairs.