LED Heat dissipation problems

Discussion in 'The Projects Forum' started by DJ Lockjaw, Feb 19, 2014.

  1. DJ Lockjaw

    Thread Starter New Member

    Feb 19, 2014
    3
    0
    Hello,

    I've been having some problems making a star projecting light box. I'm using a pair of these LEDs in series: http://www.superbrightleds.com/moreinfo/high-powered/vollong-3w-white-high-power-leds/896/

    Their relevant specs are
    Power dissipation 3W
    Continuous forward current: 750 mA
    Forward voltage: 3.6
    The power supply that I have chose is a 9v 800mA supply, which should be more than adequate. I've tried a few different online calculators for resistance required in the circuit, and I came up with 2.4 ohm. I figured that I would be on the safe side and choose 3 10W resistors in series; this should give me 3.33 ohm (actual measured resistance is like 3.8).

    The problem that I have been having is extremely excessive heat. The resistors are extrememly hot. I made the move to the excess wattage handling due to blowing an array of 1/4W resistors that totaled 3W of total dissipation. I'm a little new to circuit building and I have no idea what is going on. Please help!
     
  2. crutschow

    Expert

    Mar 14, 2008
    13,026
    3,236
    You results seem out of whack. I calculate only 474mA current and 853mW dissipation in the three resistors, which should make the 10W resistors only slightly warm. Post your circuit diagram.
     
  3. burger2227

    Member

    Feb 3, 2014
    190
    24
    Did you mean three 10 ohm resistors in PARALLEL to get 3.33 ohms? Four would give you 2.5 that you need to drop 700 ma current by 1.8 volts. Series LED's each get half of the voltage. Parallel, they get half of the current.

    Use a metal case or heat sinks too.
     
  4. jjw

    Member

    Dec 24, 2013
    173
    31
    Is the 9V supply regulated?
    Have you measured the voltage?
     
  5. DJ Lockjaw

    Thread Starter New Member

    Feb 19, 2014
    3
    0
    @burger2227 Yes, I meant parallel. Ditzy me.
    @jjw The power supply is not regulated. It's rated at 9v, but I just measured across the tip and got about 10.5v.
    @crutschow I attached a napkin sketch of the schematic.
     
  6. burger2227

    Member

    Feb 3, 2014
    190
    24
    What voltage does it read with the load on it? Are the LED's bright enough?

    If you are using a wall transformer power pack, it may need some capacitor filters. Try adding another 10 ohm resistor too.
     
  7. DJ Lockjaw

    Thread Starter New Member

    Feb 19, 2014
    3
    0
    Thank you all for your help. I just tore the whole thing out and re-wired it. I don't know if something was shorting or what, but it's working great now.



    Thanks!
     
  8. burger2227

    Member

    Feb 3, 2014
    190
    24
    GREAT!

    You may be able to use less powerful resistors too. Just check the heat they generate. Warm is OK if you need the space.
     
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