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LED arrays

Discussion in 'LEDs and Optoelectronics' started by Pennywise, Apr 7, 2013.

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  1. Pennywise

    Pennywise

    1
    0
    Apr 7, 2013
    Hi, I tend to use an online calculator any time I want to design an LED array, so I have no idea, really...
    I mean, I recently wanted a 28 LED array, running off a 6 volt battery, so I used the online calculator. I then tried the same led specs and number of LEDs with 12v just in case I changed my mind.
    Instead of 7 parallel rows of 4 LEDS, this time the calculator suggested 4 parallel rows of 7. (I think the resistor values were pretty much the same) I noticed that the 12v array was drawing much less current. What the hell? Surely it should be the same or thereabouts? Surely the array can't draw half the current and still be as bright?
    These online calculators spit out all sorts of numbers, like current drawn by the array, power dissipated by the LEDs, and by the resistors...etc. So, which numbers do I need to be looking at to design an array that is as efficient as possible? I was under the impression that getting the array to work out so that it had the lowest value resistors possible would be more efficient, but someone pointed out that that isn't really the case. (Though I think that was more of a safety issue, or maybe to do with how the battery voltage drops as the battery dies)
    Is there even a "smarter" online calculator that can give reasons why one solution is better than another? I mean, the math is simple enough that a computer should be able to do it. (and I truly HATE math)
    Oh, and the forward current they ask for...is that the MAX forward current as per the data sheet, or is that the current you WANT each LED to see in the finished array?
     
  2. duke37

    duke37

    5,364
    769
    Jan 9, 2011
    You do not need a calculator if you can do division. Have you got a slide rule?

    What leds are you using? To get a string of 4 leds to run on 6V, then each led must run at 6/4V = 1.5V (hard maths). I do not know such leds.

    The total current will be the current/string times the number of strings.
    If you have 4 strings instead of 7, then the total current will come down to 4/7.
     
  3. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,418
    2,788
    Jan 21, 2010
    That's fine. Just remember that thes4e calculators are dumb and can lead you astray...

    Each string draws the same current. But you have less strings, so less current in total.

    Sure can! :) The current that matters is the current flowing through a LED (think of current like the flow of water). It doesn't matter if the same current flows through one or a million LEDs, they will be the same brightness.

    However if you have two strings of LEDs, it again doesn't matter if the same current flows through one or a million LEDs in each string, there is now twice the total current flowing.

    What limits you from placing all LEDs in the one string (which would seem to be the best thing to do) is that the voltage you can provide must be great enough to do so.

    The calculator's prime task is to do the various calculations which (in essence) calculates how long a string can be, and arranges things to have the fewest it can.

    There's nothing in particular. Much of that stuff is just for your information. If The LED can operate safely from 20mA, then knowing the power dissipation might be interesting or (as you've found) might also be distracting.

    The power dissipated in the resistor(s) helps you choose an appropriate resistor, and the value of the resistor(s) is VERY important.

    Both opinions are correct. However for low current LEDs, the safety factor is more important. It is important that the current is not too high when the batteries are fresh, and too low when they are flatter.

    My simplified rule of thumb is that I like to see at the very least, 1V across the series resistor, preferably 2V or higher. At 20mA, that means I prefer to see around 100+ ohms, but never less than 50 ohms.

    For batteries I like to see higher values. For regulated power supplies, lower values are OK in most cases (but I still would be very cautious about less than 50 ohms).

    Unfortunately the best calculator is your brain.

    If the on-line calculator can show you the LED current if the input voltage varies by +/- 10% to 20% (and the variation will be much greater than 10% or 20%), then you can make sure the LEDs will still be visible at the lower end, and be within the maximum continuous current at the high end, then you're fine.

    (Or you can calculate it, but that's math...)

    You use the current that you WANT to flow through the LED.

    But make sure that it will not exceed the max if the battery voltage is high (as in a fresh battery) or if the LED forward voltage is at a minimum -- i.e. when it gets hot
     
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