Resistor-free LED cluster

Discussion in 'General Electronics Chat' started by takao21203, Dec 3, 2012.

  1. takao21203

    Thread Starter Distinguished Member

    Apr 28, 2012
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    I have built a LED cluster from Piranha LEDs.

    No resistors are used at all. A laptop PSU is used to supply 20V.
    8 yellow LEDs are used in series, and 10 red LED in series.

    The PCB heats up so much that forced cooling is required.
    Especially the red LED strings heat up too much.
    But using 11 LEDs result in much less brightness. So I think I will reduce it a bit with a regular diode. Otherwise, the strings go off after a while when they have heated up.

    The yellow strings don't heat up so much (larger LED case).
    It's maybe not worth to mount so many 3mm red LEDs, as one single 10W LED could do the job much better.

    I'm waiting on 10W LEDs to arrive here...

    So it seems to be possible to use LED strings without resistor. There also seems to be considerable internal resistance.
     
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  2. Sparky49

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    Jul 16, 2011
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  3. takao21203

    Thread Starter Distinguished Member

    Apr 28, 2012
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    I have inserted a small glass diode into each of the red strings. Much less heat developement. Electronic supplies of course will always maintain a constant voltage.
     
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  4. takao21203

    Thread Starter Distinguished Member

    Apr 28, 2012
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    Yes I saw that one. I have to complete my large 768 LEDs matrix first.
     
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  5. Audioguru

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    Dec 20, 2007
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    Your LEDs get hot and will burn out soon because their current is higher than the ABSOLUTE MAXIMUM ALLOWED CURRENT.
    You must use a circuit or a resistor to limit the current to 20mA or less.

    If the 20V supply is accurate and all your red LEDs have the minimum forward voltage of 1.8V then with 10 in series the voltage is too high. Then the current is also too high. Then the heat is also too high. Your current could be 45mA (more than double the absolute maximum current spec of 20mA).

    If your 10 red LEDs have the maximum voltage of 2.5V each then they will not light with your 20V supply. Another reason to use a series resistor (with fewer LEDs).

    In the datasheet, look at the graph of forward voltage vs current. It shows a voltage of 1.3V at 20mA WHICH IS WRONG! 1.8V is supposed to be the minimum, not 1.3V.
    The idiots made the graph WRONG!

    You bought cheap Chinese Piranha LEDs with an absolute maximum current spec of 20mA. I was given Name-Brand Philips Luxeon SuperFlux LEDs with an absolute maximum current spec of 70mA. The cases are the same.
     
  6. Sparky49

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    Jul 16, 2011
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    Na, it's cooler without a resistor.
     
  7. Audioguru

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    Dec 20, 2007
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    Why don't you understand? LEDs are not a simple 2.0V lightbulb.
    Their voltage requirements are all different even it they have the same part number. Your red LEDs might be 1.8V, 2.5V or anywhere in between.

    You do not know the forward voltage requirement for your LEDs (each one is different) so the unlimited current might be way too high or nothing.

    You made A SINGLE circuit. If MANY circuits are made like that then some will not light and others will get very bright and very hot and burn out soon.
     
  8. gerty

    AAC Fanatic!

    Aug 30, 2007
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    Do yourself a favor and measure the current through one of the strings, that way you'll know at what current the leds will burnout at.
     
  9. Audioguru

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    The resistance of the current meter will reduce the current.
    The current is nornally measured with the voltage measured across the current-limiting resistor but here there is no resistor.
     
  10. gerty

    AAC Fanatic!

    Aug 30, 2007
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    The resistance of my meter in the current mode (10a range) is only .2Ω
    edit: in the 200ma range the resistance is 1.2Ω
     
  11. John P

    AAC Fanatic!

    Oct 14, 2008
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    If you look on eBay you can find lots of constant-current drivers for LED strings, which generate the correct current with very little heating. However, if you're using large LEDs and you have a lot of them close together, you'll still get heating in the LEDs. Whether it's a problem depends on construction details.
     
  12. KJ6EAD

    Senior Member

    Apr 30, 2011
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    If you want to make large arrays of LEDs that last, you need to control the current and employ some thermal management techniques such as spacing the LEDs from each other, making sure the leads connect to a large copper pour area on the circuit board and creating large vent holes in the board under each LED.

    It's a good thing that you got a picture of the LEDs all lit up while you could. :p
     
  13. takao21203

    Thread Starter Distinguished Member

    Apr 28, 2012
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    I already said the red LEDs are fine now, having a small glass diode for each LED string. I have tested carefully in advance, at first with 13 red LEDs.

    The yellow LEDs strings can also light up with 9 or 10 LEDs.

    They are NOT from a chinese vendor.

    This is only an experimental circuit. Particulary I want to figure out if I can mount power LEDs on phenol PCBs, and install a cooling fan on the backside. This time I simply tied it with wire!

    I get some 10W LEDs soon.

    The LEDs without resistor are not a problem as long as the circuit is matched to suit actual LEDs, and the voltage remains constant. Normally you also would not want a laptop PSU to output a higher voltage very suddenly.

    Some months ago I also destroyed 2 of my 5 3W red LEDs. The crystal lights up a bit, and then they will be gone very quickly.

    The fan I use now is a big fan from an ATX supply, 20V are a bit too much for it, I reduced it with one 3W red LED. Still spinning too fast but for today I keep it that way.

    My pineapple plant will be happy, as one of the larger CFLs reached end of life, so most of the remaining CFLs are now defunct. I removed the CFLs from the plastic boxes. Which also were bulky, dangerous (240V), and developed a lot of heat.

    Soon I will also have the 10W LEDs and then I can compare light output. It is also a lot of work for instance to solder 1000 LEDs. I installed only 70 3mm LEDs which I had around here for a while. I have some 100pcs more but not sure if I want to install more 3mm LEDs.

    There are also 3x red 3W LEDs and that can't be matched to 20V. Each LED needs about 2.5V!

    Actually the PSU has an output voltage of 19.5V
    The red LED chains run at 18.9V. Heat dev. is normal now.
     
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  14. takao21203

    Thread Starter Distinguished Member

    Apr 28, 2012
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    Forced cooling is neccessary at least for power LEDs. The yellow LEDs heat up a lot but still not too much, could be used without a cooling fan.

    Voltage, current, and heat developement all must be controlled, yes.
     
  15. Sparky49

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    Jul 16, 2011
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    Can I have an Irish pineapple please?
     
  16. gerty

    AAC Fanatic!

    Aug 30, 2007
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    Humor us and measure the current:eek:
     
  17. Markd77

    Senior Member

    Sep 7, 2009
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    I'm not sure why you would want to use a diode instead of a resistor to reduce the voltage to the LEDs. Diodes have negative temperature coefficients so they are worse for the purpose than resistors.
     
  18. Audioguru

    New Member

    Dec 20, 2007
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    The minimum forward voltage of your 10 red LEDs is 1.8V BUT YOU POWERED THEM WITH 20V! Then of course their current was way too high without a current-limiting resistor.
    What would happen if all 10 of your LEDs were the max of 2.5V and you fed them 20V? NO LIGHT!

    Your meter is 1.2 ohms so it barely limits the current.

    Chinese flashlight manufacturers buy millions of LEDs, measure the voltage of each one, label them and stack them so the current-limiting resistor can be small. You did not doo dat. Instead, your circuit is for ten 2.0V light bulbs.
     
  19. Wendy

    Moderator

    Mar 24, 2008
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    It is one of those myths that persist, treating LEDs as light bulbs. It is probably maxing out the power supply too, which is saving the LEDs for a little while longer. A component that is normally rated for many hundreds of thousands of hours is now down to a thousand hours or so, if you're lucky. It's not doing the power supply much good either, its lifespan is being affected. The heat is telling you something.

    Most folks design for fail safe. When it finally goes out all the LEDs will have been damaged, not just the one that burned out. At that point fixing the design will come too late for those components, including the power supply. The damage is cumulative.
     
  20. Audioguru

    New Member

    Dec 20, 2007
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    Then please attach the detailed datasheet.
     
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