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3W LED. Why doesn't P=VI?

Discussion in 'LEDs and Optoelectronics' started by h2k, Aug 25, 2013.

  1. h2k

    h2k

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    Aug 25, 2013
    I have some 3W LED's that I purchased on eBay. They say they are 2.2V-2.6V and 750mA. So simple math says this is 1.95W max. This is far below the claimed 3W.

    So what gives? Is it maybe 3W peak?

    In a related question, how should one choose a heatsink. For example, why use a 3W heatsink if it is actually running at 2W. This allow me to save weight on my project.
     
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    What colour are they?

    There are three possibilities:

    1) Vf is not 2.2-2.6V, it is actually 3.2 - 3.6V or higher.

    2) They're not really 3W LEDs

    3) The If is actually higher.

    For longer life you might want to de-rate them anyway. You might run them at 2W.

    You choose a heatsink that will maintain the temperature of the LEDs at a reasonable value. There is no such thing as a 2W heatsink or a 3W heatsink. (or even a heatsink suited to a 2W or a 3W LED).

    To correctly determine the heatsink, you need to determine the maximum ambient temperature, the maximum desired junction temperature, and do some calculations to determine the minimum heatsink based on it's measure of effectiveness (degrees C per watt).

    If you're mounting your LEDs on a cool metal surface, you may not need any further heatsink. If they're operating in a high temperature area, you might need to go to far more extravagant efforts to keep them cool.

    You're probably operating between these cases.

    Remember that free airflow over a heatsink is required for it to operate efficiently (i.e. up to its specs)
     
  3. h2k

    h2k

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    Aug 25, 2013
    Thanks :)

    The color is red, 660nm. Here is the part.
    http://www.ebay.com/itm/10-pcs-3W-H...773?pt=LH_DefaultDomain_0&hash=item56581e3b7d

    I guess I can just run them at a derated value, but I will only be using them about 1 hr/day so not sure if I need to do that. I need about a 3 year life out of it...so about 1,000 hours. The problem is which of the 3 options should I consider?

    1) If it is at the wrong voltage, will it even turn on? I can get them to turn on in the specified voltage range.
    2) Not sure how to determine this, if even possible.
    3) I suspect they could run at a higher current...but what if one uses a LED constant current driver? I suppose you are stuck. Not much wiggle room for V*I it would seem unless I am wrong about option 1.

    As far as heatsinks go, you are right. They will not have air flowing over them, but they will be in the open (no housing). So far, they get pretty warm to the touch. After about 5 seconds, it becomes uncomfortable to touch. I really don't know what the junction temperature should be though :/ I can detach the heatsink and measure it, but what is the proper value? I suppose if it making good contact, it shouldn't be too much higher than the heatsink itself.
     
  4. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Your response suggests you're NOT using a constant current source to power the LED. You should be.

    As a first try, select an appropriate resistor to run them at about 20mA. From a 12V supply, a 390 ohm resistor should be fine. Then read the voltage across the LED.

    If your LED heatsink gets too hot to comfortably leave your finger on, then I would say it's too hot.

    You are correct, if things are connected properly, the LED should only be slightly hotter than the heatsink.

    If you're unsure of why constant current drivers are required, read this:

    https://www.electronicspoint.com/got-question-driving-leds-t256849.html
     
  5. h2k

    h2k

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    Aug 25, 2013
    Thanks for the link, that does help clarify a few things.

    For now I am just connecting them in series and have a 100W - 100Ohm resistor to adjust the current. It's just a test setup really.

    But the numbers won't add up. I adjusted the resistor until I got 0.75A in the circuit. From that, I have 2.4V across the LED. The total power to the LED is 1.8W. This is just 60% of the claimed 3W.

    I was able to increase the current to 1.0A and it ran fine for 30min. So what does this mean? In reality, you are right and I will want a constant current driver for the final use. But how do I select it? If I use a 700mA driver, I won't get the power I desire.

    I guess I am still confused about the item on eBay (all of the LED's there have the same "issue" where V*I do not equal P. If you run it with a constant current driver at the stated value, you won't get the rated power.
     
  6. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    It's a case of people who don't care selling (often) to people who know no better.

    I can do no more than point you back to the original three points I made.

    Running it at a higher current may be OK if it's really a 3W device, and ensuring you have sufficient heatsinking (so that it doesn't overheat) will also help.

    Thanks for taking the time to read parts of the LED tutorial.
     
  7. h2k

    h2k

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    Aug 25, 2013
    Thanks for all the help. I really appreciate it.

    Sounds like I just need to get a constant current driver rated for 700mA and just accept what I get out of it. Then get a heatsink to accommodate that. I will get a thermocouple thermometer to check it out. Looks like I don't want to exceed roughly 130-140F for red light. Sounds like I will need some fans.

    Side note: I read somewhere that if LED's are not run at their rated current, the wavelength can shift (and hence resistance dimmers are not recommended). Has anyone else experienced this or is the shift so minimal (less than 5nm or so) such that it is not that important?

    EDIT: I got a response from the eBay seller about this. The response does not quite makes sense, but I think I get it now. Their white LED is also a "3W" at 750mA. But the voltage is 3.5V (as opposed to 2.4V for red). They said something that the effect red has a lower voltage. I know V is not IR for a LED, but I thought P was still VI. Either way, I think I get it. The "3W" designation does not literally mean power (unless I am really missing something). Rather, it is just a relatively rating compared to the 1W, 5W, 10W, etc. LED's. I guess.
     
    Last edited: Aug 26, 2013
  8. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    The change in wavelength is likely to be so small you won't notice it.

    Forced air cooling can be a problem because there's more to go wrong and your heatsink is more likely to get clogged with dust over time.
     
  9. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    As I said

    Unfortunately you have shown a capacity to both do math and to think.
     
  10. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

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    I see that the seller's location is Hong Kong. Pigeon English responses are not unusual. ;)

    Make sure you use Heat Sink Compound between the base of the LED and the heat sink. Failing to properly do so will render the heat sink less than fully effective.

    Chris
     
  11. h2k

    h2k

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    Aug 25, 2013
    For test purposes, I just screw the LED to the heatsink.

    I was planning on using a thermal epoxy (arctic alumina) for the final setup since it won't be messy (although it does have a higher thermal resistance than the standard silver pastes).

    Do you know of any good pastes that don't get "messy". In other words, the silver stuff I am used to gets everywhere. Any since the final product may be lightly handled, I am afraid it will get on your fingers, then clothes, etc.
     
  12. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Almost anything (except chocolate) :)
     
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