LED confusion need clarification, not asking about resistors

Discussion in 'The Projects Forum' started by burton, Mar 12, 2011.

  1. burton

    Thread Starter New Member

    Mar 12, 2011
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    I have been researching LED circuits for the last couple of days and I keep getting conflicting information. So I figured I would post to get clarification.

    Simple answers will do for me as I just need clarification, not a lesson as I already have read the viewpoints below but have found conflicting views.

    For LED's in series

    • Can you mix LED's of different voltages in the same string without affecting the brightness of each led in the string so long as the IF is the same for each LED in the string. (I believe the answer is NO based off information I read here but other websites say go for it)




    • Should I use a LM317 to regulate the current if I am using a AC/DC power converter in the house, LEDS will not be pulsing just continuously on? (or should it be voltage reg? Projected 24v @ 40w 1.66A to power project)

    Thats all I need clarification on, the project will not be built until I get my tax return but I will post pictures when its done.

    Thanks!
    -Burton
     
  2. Kermit2

    AAC Fanatic!

    Feb 5, 2010
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    CURRENT regulation is the way to go.

    A series string of LED's of different types/color is possible IF the current specs for the LED's are THE SAME. All 20mA, or All 10mA, but not a mix of them.
     
  3. burton

    Thread Starter New Member

    Mar 12, 2011
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    Thanks for your response, some follow up questions for you when you have time.

    So I can mix LED's with fV from 1.6 - 3.6 so long as I run them at their tested 20mA values despite some of the their maxes being 30mA?

    With the current regulator each string should pull the same fV drop if running on one LM317 for two+ strings right? (this to prevent one string pulling more current than the others causing it to die out earlier)
    Example:
    24v > LM317 > string with 20fV AND a string with 19fV (string with 20fV will die first correct? or does this only apply to LM317 as a voltage reg and not current reg?

    Thanks for your input!
    -Burton
     
  4. Bernard

    AAC Fanatic!

    Aug 7, 2008
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    Yes & yes. 3rd question - 19V string has lower resistance, so will draw more current than 2o V string. With several strings, might wish to go with V reg & current limiting resistor for each string.
     
  5. Wendy

    Moderator

    Mar 24, 2008
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    You can mix colors as long as they are rated for the current. Current is the thing that will blow an LED, not voltage.

    LED brightness is not linear. If you drive a 30ma LED with 20ma it will not be 2/3 as bright. There would be some difference, but you would have trouble seeing it. This is one of the reasons PWM is used with LEDs.

    Driving a 30ma LED with 20ma means it's already long life will just be that much longer. You are putting less strain on them long term. Conversly, a driving a 30ma LED with 32 ma will shorten the life a little, but you would have trouble measuring the reduction.

    The voltage drop of the LED only sets a minimum voltage. Generally you need at least a volt or two more than the LED needs to power them. I don't agree you need current regulation, a resistor will work fine, as long as the DC voltage is stable. People have commented about thermal runaway on LEDs, it is somewhat exaggerated. There are times a current limiter is need, like when the DC voltage isn't very stable.

    By a power converter I assume you are talking a wall wart. It is just an unregulated power supply (usually, but not always). You will need either voltage or current regulation.

    I wrote an article for beginners about LEDs. Chapter 1 and the 1st half of chapter 2 has most of the information you would need...

    LEDs, 555s, Flashers, and Light Chasers
     
  6. burton

    Thread Starter New Member

    Mar 12, 2011
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    I have read your guide bill, twice :), it was very informative.

    I dont mind running the IF (max) 30mA with the 20mA as the results for the optoelectric testing is at 20mA and represents the Luminosity which attracted me to the LED in the first place. The spec sheet I listed above as an example has max IF continuous as 50mA but they optoelectric testing was done at 20mA and the Luminosity rating from this test is what they company states the brightness of the LED to be.

    Yes the AC/DC power supply is a wall wart, it is a old toshiba laptop power supply :) I would have to see if I can track down a spec sheet to see how stable it is, then decide if I will need to go with a regulator(s). Might just go with regs to be safe, would hate to blow a ton of LED's.

    quote: Bill_Marsden

    The voltage drop of the LED only sets a minimum voltage. Generally you need at least a volt or two more than the LED needs to power them

    I am designing the circuit based on the "typical" vF from the spec sheets given, the only other vF on these pages is the vF MAX. This is the proper way to do this correct?


    Thanks for all the replies!
    -Burton
     
  7. Wendy

    Moderator

    Mar 24, 2008
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    Laptops are generally very regulated, with switching power supplies.
     
  8. burton

    Thread Starter New Member

    Mar 12, 2011
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  9. Wendy

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    Mar 24, 2008
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    The PDF you show has several models, about 8 of them. Which one are you talking about.

    All of them have enough juice to power a large number of LEDs. All are 40 watts. Figure a white LED using 20ma at 3.6Vf is 0.072 watts.
     
  10. burton

    Thread Starter New Member

    Mar 12, 2011
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    ah sorry, thought my original post had 24v in it some where ...

    24v 40w is my desired spec
     
  11. Wendy

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    Mar 24, 2008
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    A good choice. The higher voltages allow more LEDs per chain, which increases efficiency a lot. One of these power supplies will handle almost 500 LEDs, more if they aren't white or blue. How many LEDs are you wanting to light up?

    By being a regulated power supply you can also use resistors. My personal cost for resistors is around 2¢ each, plus 8.25% tax. If you look in the swap shop forum I have a standing offer to get a bunch, you may be able to do better. If you are outside the USA shipping will eat up the savings.
     
  12. burton

    Thread Starter New Member

    Mar 12, 2011
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    I actually reverse engineered the led quantities, pwr supply etc from an existing product I do not own and cost too much lol

    So each module has

    • 23 x 660nm typ 1.9v, IF max 30mA
    • 10 x 605nm typ 2.0v, IF max 20mA
    • 2 x 470nm typ 2.2v, IF max 30mA
    • 1 x 525nm typ 3.6v, IF max 20mA
    • 1 x 400nm typ 3.5v, IF max 30mA
    • 1 x 730nm typ 1.6v, IF max 50mA
    Each above have spec sheets and have been tested at 20mA to get the advertised brightness.

    Now each Unit has 5 of these modules, so 190 per unit.
    And each power supply will run 4 of the units in series, so 760 in total per power supply.

    I will be making a total of 8 units, 1520 LED's or 40 modules of 38 LED each. Good thing I use to manufacture laboratory equipment for a couple years in the past.

    Each unit is suppose to consume <= 10Watts at 24v

    Given my pre-configuration this should be easy to obtain if I don't use the max mA per LED. And given each LED spec chart was tested at 20mA it would be more desirable to run them all at 20mA to get the desired result.
     
  13. Wendy

    Moderator

    Mar 24, 2008
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    OK, start thinking how you will lay them out, and post the results. It doesn't have to be pretty, many people draw pictures by hand then use scanners or digital cameras to post schematics. Whatever works for you.
     
  14. burton

    Thread Starter New Member

    Mar 12, 2011
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    Ok less try this again.

    One unit laid out. Four units connect to one 24v 40w regulated power supply in series via patch cables with DC power plugs I will create.

    The array layout was chosen based off of the information I have gathered on an existing product I wish to duplicate for personal use; I do not own this product.

    http://img862.imageshack.us/i/completeunit.gif/
     
  15. wayneh

    Expert

    Sep 9, 2010
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    For each string of LEDs in series, I'd recommend testing with a larger resistance resistor, and only reducing the resistance as guided by a current reading on a multimeter. The Vf is not so predictable that you can design around it with precision, and properly lighting LEDs is all about the current, not the voltage. You don't want to hit, say, 25mA on one string and 15 on another. They might look the same, bu the lower current string would last a lot longer.

    It doesn't really matter, but are you aware that many of the LEDs will not produce visible light? I guess I'm curious why you need both UV and IR along with the visible light.
     
  16. Wendy

    Moderator

    Mar 24, 2008
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    Two points:

    If you are going to lay them out like that it would be better to included the Vf voltage on the schematic, it is too much trouble to do a basic check adding up basic voltages.

    This site has a really good local hosting service, you don't need to use on of the others. Local attachments don't have the same size limitations, the pictures can be large. .gif or .png type files are preferred.

    It looks like you have the basics down. I don't know what you are trying to do, but your reasons are your own.
     
  17. burton

    Thread Starter New Member

    Mar 12, 2011
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    The spectrums included are based of an existing product geared toward photosynthisis.

    The main frequencies needed are 400, 470, 525, 612 (sub 605 as its graph is closer to 612), 660, and 730.

    http://www.uic.edu/classes/bios/bios100/lecturesf04am/lect10.htm

    I have double checked the voltages several times but I can put the vF for each next to the LED's and upload it here. Will take a minute.
     
  18. Wendy

    Moderator

    Mar 24, 2008
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    One last point, you can use the resistor to actually measure current. Measure the voltage across the resistor, then use Ohm's Law to calculate the current. It makes an excellent quick check when you power up a chain.

    I = V ÷ R
     
  19. Wendy

    Moderator

    Mar 24, 2008
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    Actually, and I am no expert nor will I pretend to be, the subject has come up. There are only two frequencies you need. It was discussed in these threads.

    Help with LED grow lights

    300W LED Grow Light

    Need help with simple LED grow light circuit: Cash reward

    Simple LED Project

    The colors will be off, but according to the other folks it is all the plants really want.

    Might be a good opportunity for an experiment. Perfect for some kids science fair.

    Good luck with your project, I would like to know how it works out.
     
  20. burton

    Thread Starter New Member

    Mar 12, 2011
    17
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    Re-attached ... I will keep that in mind about measuring the current. Should I try to make each string identical by changing out the resistors as needed?


    You guys have been a great help thanks so much :)




    Background on the BLUE / RED spectrum use.

    The array I am duplicating is from a company who has been in the business over 17 years, in fact it was the first company to come up with LED grow lights.

    Most people will use two colors, blue and red, citing a nasa study which they claim uses the same two spectrum to grow plants. However the nasa study did not have any blue LED's and instead used a supplemental white light source to cover the blue spectrum. Blue LED's had not been invented at the time of the study, and red LEDs were cheap.

    Further more the plant did "grow" in the nasa study. However studies have shown if you only use red / blue the plants will not mature properly, might not fruit, have a off taste, might not flower, color could be off, etc.

    Most people I have read using the blue / red combo are pot growers ... almost exclusively. I intend to grow vegetables, fruits, and herbs in my garden, and by far the number one grow light for this purpose is made by the companies design I am duplicating :)
     
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