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LED array getting hot

Discussion in 'LEDs and Optoelectronics' started by MattP, Oct 14, 2011.

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  1. MattP

    MattP

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    Oct 4, 2011
    I've bought a 48 LED array for peanuts from China, and it seems pretty well designed.

    From what I can see, there are 16 strings of 3 SMD LEDs, each string with a 181OHM resistor before to limit the current. Now, these panels are supposed to be run off 12v (from a car), but doing so results in the LEDs getting very hot. :confused:

    Reducing the voltage does help, but the brightness decreases drastically. This is a real shame as they're quite a bargain. Is there anything I'm missing, or perhaps something I can do to fix the heat?

    I can post some pictures of the panel if that helps.
     
  2. BobK

    BobK

    7,682
    1,686
    Jan 5, 2010
    With a 181 (unlikely, did you mean 180) ohm resistor it is not likely they would get hot. Are you sure it is not a n 18 Ohm?

    Also what color are the LEDs? Red LEDs have a voltage drop of about 2V, while blue or white have a voltage drop of about 3.6. I would guess they are the latter if 3 are wired in series to run of 12V.

    bob
     
  3. MattP

    MattP

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    Oct 4, 2011
    Hi Bob, they're warm white LEDs, and you're right, they're 18 Ohm not 181 according to my multimetre (they're SMD resistors, and actually say 181 on them so I just assumed).

    I have a similar panel that's the same configuration, but it uses white LEDs (rather than warm white) which are the normal kind (not surface mounted). It also gets warm (too warm for an LED, I think) but it doesn't get anywhere near as hot as these SMD LED panels.

    Currently I'm running them at 9v and they're very slightly warm to the touch. They're quite a bit dimmer than at 12v though, but by no means dark.

    I'm very puzzled! I can't see anything wrong here, and I've tried two different power adapters with the same results.
     
  4. nepow

    nepow

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    Jul 18, 2011
    Hi Mattp, you have a multimeter so can you measure the DC current? you can then work out the current draw! ... also some of the higher brightness led's no available draw more current and can get quite warm, usually though they have some kind of heat sink!! can you identify them by searching the net.
     
  5. MattP

    MattP

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    Oct 4, 2011
    With the multimeter set to 10A, it reads 0.07 with the panel connected up to 9v, and 0.25 with it connected up to 12v.


    I'm surprised at the stark increase in current. Is that normal?
     
  6. nepow

    nepow

    99
    1
    Jul 18, 2011
    That's 250mA @ 12 volts / 16 = 15.6 mA per string... so they're not over driven it would seem!!
     
  7. MattP

    MattP

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    Oct 4, 2011
    Very odd. They do get very warm indeed, and are too hot to touch comfortably at 12v. They're only mildly warm at 9v though.

    Do you think it would be better for me to just run them at 9v despite the light loss, or do you think that it's fully within the normal operation to get this hot?

    I've attached a picture of what the panel looks like, and its SMD LEDs. :)
     

    Attached Files:

  8. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,412
    2,780
    Jan 21, 2010
    Keeping them cool will make them last longer.

    Lifetime vs. brightness is therefore your issue :)

    If you can manage to mount them so that the heat is transferred to something else (without shorting the power) then you can have the best of both worlds.
     
  9. Resqueline

    Resqueline

    2,848
    2
    Jul 31, 2009
    What Steve said. Glue a heatsink on back of it - if it can be done w/o shorting it out.
    What has often been forgotten by the Chinese LED bulb designers is that closely packed LED's will warm each other, neccessitating a current derating.
    At 12V the panel draws 3W which is a lot of power for the small size, even if the current is only 15.6mA per string and some power is given off as light.
    It simply "has" to get hot as it is, but since the LED chips can stand quite a bit of temperature their lifetime may still be acceptable, depending on the application.
    Oh, and the resistors are (and have to be) 180 ohms btw.. If they could be mounted externally the temp's would be reduced somewhat.
     
  10. MattP

    MattP

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    Oct 4, 2011
    Okay, thanks for the insight. I think I'll run them at 9v, as I don't really have the space in my project to deepen the panels' profiles with heatsinks (plus the heatsinks will add to the cost).

    They still kick out a lot of light at 9v though, for the price/power consumption!
     
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