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New LED light fixture is too bright

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Discussion in 'LEDs and Optoelectronics' started by hwurl, Oct 31, 2012.

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  1. Oct 31, 2012 #1
    hwurl

    hwurl

    2
    0
    Oct 31, 2012
    Hi All,

    First time poster and really hoping you all can help with a question on modifying an LED light fixture.

    I just bought this new light fixture from Ikea that my wife and I love, but unfortunately its far too bright for our application. There are other lights on the circuit that are different output, so dimming everything won't work. Anyway, I had to take it apart to see what I could do to make it work for us.

    I have attached pictures of the fixture, a single LED and the power supply. I would like to remove six of the nine LED's from the circuit, but I am worried that there will then be too much current for the remaining three LED's. Am I right to be concerned? The power supply says it is "constant current" and I know that LED brightness is dictated by current instead of voltage like a light bulb.

    I am not sure of the resistance of these LED's but wonder if I measured the resistance across the terminals and then installed a corresponding resistor in their place if that would solve the current issues?

    Also, the fixture has a rating of 20W on the UL sticker. Does that mean that these are 2.2V LED's (2.22x9=20)? The power supply has a listed load rating of 22-30W. Does this mean that is it's maximum, or is that the only range that it will work within?

    I'm not terribly electronically inlined, so please keep your answers simple.

    Your help is greatly appreciated!!!:)
     

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    Last edited: Oct 31, 2012
  2. Oct 31, 2012 #2
    (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,505
    2,852
    Jan 21, 2010
    OK, that's a constant current power supply, so (in theory) all of those LEDs are in series and you can short any one (or more) out and it will not affect the light output of the others.

    I would try shorting out one momentarily and see what happens. Do any others also go off? Does the brightness of those which stay on change?

    I would imagine that this light is pretty bright.

    edit: there may well be a minimum voltage that this power supply has to drive. If so, you won't be able to reduce the number of LEDs past some point. I can't tell you how you'll know you've reached or exceeded this point.
     
    Last edited: Oct 31, 2012
  3. Oct 31, 2012 #3
    CocaCola

    CocaCola

    3,635
    5
    Apr 7, 2012
    After just 'dealing' with and 'fixing' a constant current LED light this week, a few words of advice...

    Note the "Load: 22 - 30W" on the driver, if it's anything like the driver I just dealt with it will need that 22W load or else it will go into error mode and flash or shut down...

    What that means is that you can't just remove the LEDs as that would reduce the load, resistors (IMO) are really impracticable in this instance...

    There are 9 LEDs in the light fixture, based on the power supply happy middle of say 24-26 Wattts we can take a guess that each LED is dropping about 3.75 - 4 @ 700mA Volts... Example (3.75V * 0.7A ) * 9pcs = 23.6 Watts

    So with every LED you remove you need to replace it with something that consumes about 3.9V @ 700mA to keep the system balanced... The most cost effective way and the way that I just did this was with some 1N400x diodes in series (use 1n4002 - 1N4007 to be safe) these diodes drop about .7V each, so you will need to put 5 or 6 diodes in series for each removed LED to maintain the load... I know it sounds and might even look impracticable to series wire 5 or 6 diodes for each LED but at 2 cents each they are really the most cost effective way to do it and if you line them up in an organized fashion it's not that bulky... They also allow for easy tweaking of the load, as you can step up or down .7V with ease...

    You could use properly rated Zener diode(s) to do the same abut again they are not as cost effective, nor as easy to obtain locally in most areas...

    FYI if you want the diodes in an instant Radio Shack sells a random 25 pack of 1N400x diodes for $2, remove the 1N4001 and you can use the rest... You technically can use the 1N4001s as well, but they are rated at 50V and your supply will push 42V, I like a little more headroom...

    Just mind the polarity of the Diodes and the removed LEDs, and it's simply a matter of swapping out each removed LED with a series of 5 or 6 1N400x diodes and the supply is happy...

    Just my two cents...

    **Edit

    I think my math is a pretty solid guess, and based on that I suspect that even 1 or 2 removed could cause it not to function... Also based on my very recent experience and some similar experiences on Google searches most of these commercial drivers will flash or blink (basically constant resetting) when they can't regulate properly...
     
    Last edited: Oct 31, 2012
  4. Oct 31, 2012 #4
    (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,505
    2,852
    Jan 21, 2010
    If you use diodes to drop the voltage here, they will be forward biased, so the inverse rating doesn't matter.
     
  5. Oct 31, 2012 #5
    KJ6EAD

    KJ6EAD

    1,114
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    Aug 13, 2011
    There is a possibility that the supply has a lower limit to it's output voltage such that as you short (bypass) additional LEDs the supply voltage may not drop correspondingly and excess current would be driven through the remaining conected LEDs. If you have a multimeter, you could measure the output voltage with all 9 LEDs on then divide by 9 to determine an expected drop for each LED as you short them. If you continue to monitor the voltage as you short additional LEDs, you'll be able to tell if this problem occurs.

    Edit: I see we were all typing at the same time.
     
    Last edited: Oct 31, 2012
  6. Oct 31, 2012 #6
    gorgon

    gorgon

    603
    24
    Jun 6, 2011
    The 22-30W @.7A indicates that the minimum output voltage is 31.5V. The OP should measure the output voltage with a full LED ring, and the voltage dropped over each LED assembly. Replace or shunt each LED assy with a number of 1N400x diodes in series, adding up to a voltage slightly less than the LED assy, if you shunt. Please leave air around each diode to cool it down since you are bleeding 0.5W in each diode.
     
  7. Oct 31, 2012 #7
    hwurl

    hwurl

    2
    0
    Oct 31, 2012
    Thanks so much!

    I really appreciate your time.
    I will pick up some diodes and see if I can make the packaging work. How hot are they going to get?

    Thanks again.
     
  8. Oct 31, 2012 #8
    gorgon

    gorgon

    603
    24
    Jun 6, 2011
    Based on some datasheets 0.5W will give 25-50K + ambient, from 50C to 75C at 25C ambient. It depends on how they are mounted.
     
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