LED array with varying voltages/currents

Discussion in 'The Projects Forum' started by wemcod, Oct 26, 2011.

  1. wemcod

    Thread Starter New Member

    Oct 26, 2011
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    Hey guys, I'm currently working on an LED array to build a grow light for some plants in the winter and I need some help with the getting everything right. I'm fairly comfortable with electronics, fixed a lot of things, built a lot of things, but this one is stumping me and all my google searches.

    I have 150 Red, 100 (actually 99 now, I think I might of just burnt one :( ), 25 white, and 8 high voltage red led's.

    if you clicked those links, you'll find out they dont actually give a lot of info on them. for the most part, I'm going to go with the tried and true voltages of
    red=2v, 20mA
    blue=3.3v, 20mA
    white=3.3v, 20mA
    red cree=2.2v, ??mA

    I'm not sure about the cree's amperage, one of the comments on the site suggest 130mA (1.3A), so I feel comfortable running with that if anyone wants to weigh in.

    now my main problem:
    I know how to wire up a large multitude of LED's on an array, but I'd like to get these all on to the same array, and what would be even better is maybe 2 or 3 identical arrays.

    I have an old PC PSU, with 12V, -12V, 5.0V, 3.3V, and 28VS(?) outputs (all dc of course).

    I've used http://led.linear1.org/led.wiz before, to great success, but it doesn't help with multiple led's of differing amperage/voltages. So like I said, I'd love to have 2, maybe 3 identical arrays, but I'm alright with one big one if that's easier on anyone. thanks in advanced for the help!
     
  2. SgtWookie

    Expert

    Jul 17, 2007
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    You really should measure the Vf of a number of the LEDs with a known current going through them, so you can get a good idea what the Vf really is.

    You can easily make a constant current regulator using an LM317 and a 62 Ohm resistor connected between the OUT and ADJ terminals. The regulator will then limit current to roughly 20mA (1.25v/62 Ohms).

    This current regulator configuration is shown in National Semiconductors' LM117/LM317 datasheet in the Application section, somewhere around page 17 I think.

    If you use that and a 12v supply, it will be very easy to measure your LED Vf's. I would measure at least a dozen of each to get a reasonable statistical sample.

    Post the measurements of what you read.

    I am NOT fond of that LED "wizard". It gives some very poor recommendations. For example, tell it you have 50 LEDs that have a Vf of 2v and a 10v supply. It will tell you to use 5 LEDs in series with a 1 Ohm resistor for 10 strings, which is wrong.
     
  3. ke5nnt

    Active Member

    Mar 1, 2009
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    I want to give you the benefit of the doubt and assume it's a typo, but just to be on the safe side, let me correct you. 130mA is not 1.3A, it is in fact 0.13A. Don't try to pump 1.3A through it.

    Additionally, like SgtWookie said, knowing Vf makes it quite a bit easier to design. Generally speaking, if you had 30 LEDs all with a typical Vf of 2V and a If of 20mA, with a 12V power supply you can only run 6 in series before you're out of voltage. Each LED will produce a voltage drop. However, current is the same everywhere in a branch of series components, so whether you have 1 LED in that branch or all 6, you're still only going to draw 20mA for that branch.

    From the power source, 12V across all branches, 20mA in each branch. Each LED drops 2V, you would need (30/6) = 5 branches of LED strings with 6 LEDs in each string and a very small (near 0 Ohm) resistor to limit the current to 20mA in each branch. 5 branches at 20mA each, total circuit current for an array of 30 LEDs arranged that way would be 100mA (0.1A).

    With various numbers of LEDs that operate at different Vfs and Ifs, probably the most practical way to do it would be to arrange the LEDs so that each branch is constructed of similar LEDs (those with the same forward current). If 2 LEDs in series both operate at 50mA but one has a Vf of 3.2V and the other 2.0V, I'm pretty sure you can run them in series no problem (Vs-Vfled1-Vfled2 = voltage left over to calculate resistor value for 50mA of current). 12V - 2.0V - 3.2V = 6.8V left over divided by 50mA = 136Ohm resistor to limit current.

    So, multiple arrays on the same board or however you're doing it with each array being made of LEDs that have an equal forward current and the sum of each forward voltage in that string can't be more than the voltage source. And remember that the more branches you add, the higher your total current will be.

    Theoretically, without all the quirks about LEDs, you can have a 200V power supply that drives 100 2V LEDs in series at 20mA total current, or a 2V power source that drives 100 2V LEDs in parallel at 20mA each for a total current of 2 Amps. Hope this helps.
     
  4. ke5nnt

    Active Member

    Mar 1, 2009
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  5. SgtWookie

    Expert

    Jul 17, 2007
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    No, no NO! That's what the led "wizard" does, and it's simply wrong.

    You must have a voltage drop across the resistor, or the current regulation will be very poor. As the LEDs warm up, their Vf will decrease. The voltage drop across the resistor will help to compensate for that, so the current only increases slightly.

    If the power supply is well-regulated, you can take ~0.7v or 10% off the output voltage, and THEN see how many LEDs you can operate in series from that.

    So, for a 12v supply, you'd have ~11.3v for the LEDs, needing at least 0.7v drop across the resistor. Let's say you had LEDs with a Vf of 2.2 @ 20mA.

    11.3v / 2.2v = 5.13636... Take the integer of the result, that's 5 LEDs in series.
    Rlimit = (Vsupply - (Vf * number_of_LEDs)) / Desired_Current
    Rlimit = (12v - (5*2.2))/20mA
    Rlimit = (12-11)/0.02 A
    Rlimit = 50 Ohms. 51 Ohms is the closest standard value.

    If the supply is poorly regulated, you need to decide how much variance you can tolerate in the intensity of the LEDs. If the regulation is too poor, you really ought to look at a different solution, such as a switching boost regulator.
     
  6. Wendy

    Moderator

    Mar 24, 2008
    20,765
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    An LED tutorial...

    LEDs, 555s, Flashers, and Light Chasers

    What you have marked as high voltage are not, they are high current. Most LEDs (for a color) are pretty close to each other, it is the current that varies. One of your high wattage LEDs is equal to around 300 of the regular LEDs.
     
  7. wemcod

    Thread Starter New Member

    Oct 26, 2011
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    0
    I'll be honest, I'm learning an incredible amount from you guys! It might take me a minute and a definition lookup or two, but damned if I'm not learning a lot.

    I don't know how much closer I actually am at knowing what I'm supposed to be doing, but I'm at least learning! haha

    oh, and something I should've stated before in my first post, I was hoping to have the 3 different led's (red/whit/blue) mixed around in one array (11x25, having 25 columns of R-B-R-B-R-W-R-B-R-B-R) though, the keyword here was "hoping" lol. the cree's I'd probably have on a separate circuit, the PSU has multiple 12v outputs.

    for the record, I saw some mention of of the power supply being poorly regulated. should I be worried about that with a PC PSU? I've built a multitude of PC's and I've never had any problem of the voltages being variable, at least, nothing more variable than +/-.01V at any moment.
     
  8. ke5nnt

    Active Member

    Mar 1, 2009
    384
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    Thanks for clearing that up wookie. I seem to remember now someone having mentioned that resistor values near 0 ohms is basically useless for this function for the reasons you stated, just had forgotten about that. I don't mean to pass on misconceptions when trying to aide others.

    Regards.

    [edit for the sake of replying to OP without a new post]

    Yeah this stuff is crazy, and it's easy to get addicted to LED projects. Tons to learn and always fun.
    One thing to remember, you mention PC PSUs being poorly regulated. In the case of computers, the types of components on motherboards and GPUs etc can be sensitive to changes in voltage and current. You'll find that a lot of hardware like motherboards and GPUs have component-level regulation to ensure that they operate where they should. As far as they're concerned, variations in a poorly regulated power supply shouldn't matter, because they're designed to handle the fluctuations.

    With regards to wanting a mix of colors in one array, it shouldn't be a problem. I mentioned earlier that different forward voltages in a string is okay as long as the forward currents of the diodes are the same. You'll find when you "shop" a product line of LEDs, that they are generally classified in forward currents or power ratings. What that means for you is that, in a specific type of LED, you can have Red, Blue, Orange, Yellow, White, and Green LEDs that are all "100mA LEDs" or 0.5W LEDs etc. Regardless of color within' this line, the forward currents should be the same, the only difference is that Red Orange and Yellow will probably have a forward voltage (typical) of around 2.0-2.2V while the Blue, White, and Green will have a forward voltage of around 3.2-3.4V. It would be okay to mix these in a single string.

    For the sake of explanation and argument, if you wanted to mix different currents into one array, you could always get creative with PCB layout or the way you wire things up so that different currents "appear" to be in the same array, but are actually separate circuits within' the same array. Really just depends on how much effort you want to put into "cheating".
     
    Last edited: Oct 27, 2011
  9. wemcod

    Thread Starter New Member

    Oct 26, 2011
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    ledschem.png


    would this even be at all possible? just a quick and dirty schematic of what I envisioned. I've always been wrong about my resistor placement.


    oh, and ke5nnt: I'll be using that guide to power the cree's, thanks!

    edit: oops, pay no mind to that random floating line with a resistor on it! artifact from me working on the image.
     
    Last edited: Oct 27, 2011
  10. SgtWookie

    Expert

    Jul 17, 2007
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    No, you have far too many LEDs in series.

    I guess I just wasted my time typing the formulas in my last post?

    Take 11.3v.
    How many of each LEDs can you have in 1 string that adds up to 11.3v or less?
    Tell us ALL possible combinations of the red, blue and white 20mA LEDs that you can have with that supply voltage.


    Plan on using the 5v supply for the Cree-like LEDs; you will need a fair amount of load on the 5v supply for the 12v supply to regulate properly.
     
    Last edited: Oct 27, 2011
  11. Dadanddave

    New Member

    Jul 18, 2013
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    0
    Gday from down under All,

    This is making some very interesting reading. Just wondering if there is a way I can use these on-line calculators to work on a parallel / series circuit.

    I am working with some boards that come from an LED light unit that has 6 panels of 9 LEDs wired with 3 LEDs in series linked in parallel. applying 12 V through a 1K ohm resistor, I get 96 mv across the resistor.

    With a 3.3 K ohm in the circuit, I get a reading of 290 mv.

    I am getting good light out of the panel but am trying to work out how many of the panels I can place on a parallel circuit?

    If this does not make sense I will post a diagram of what I am working on.

    Thanks in advance for any replies.
     
  12. wayneh

    Expert

    Sep 9, 2010
    12,090
    3,027
    First, you have revived a very old thread. You should have started your very own. Perhaps a mod will come along and break this out into a new thread.

    The limit on panels in parallel is the current that your power supply can deliver. You haven't told us what that is. You can keep adding load until you're at ~80% of rated current.
     
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