LED array driver.

Discussion in 'The Projects Forum' started by DarkMavis, Aug 24, 2014.

  1. DarkMavis

    Thread Starter New Member

    Aug 24, 2014
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    so, I made an array of IR LED's. It's a 7x4 array. The LEDs have a forward voltage of 1.4 to 1.5 volts, and forward current of 140ma. I need to be able to run the array off a 6v lead acid battery (at 6.8v fully charged) But I'd like to be able to run it off an unregulated wall-wart adapter, too, AND do away with the wasteful resistor in each string.
    I bought an adjustable voltage and current regulator, but, without buying one for each string (don't have enough space for that anyway) I still need the current limiting resistors.
    So, what's the best way to power this array? Do I use a voltage regulator first, then a separate current regulator for each string? Or is there an IC that can do all this?
    I want to be able to accurately set the maximum current for each string, accept input voltage from say, 6 to 9 (although the wider the range that still works the better I guess) be as efficient as possible and not be super complicated or expensive. Oh, and I'm working with a space about the size of a deck of cards.
     
  2. ronv

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    Last edited: Aug 24, 2014
  3. DarkMavis

    Thread Starter New Member

    Aug 24, 2014
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    But surely, if I use a single regulator, I still need to use the resistors, or if one string is drawing slightly more current and getting hotter, therfore drawing more current until it dies, won't the remaining 6 strings now be getting the current intended for 7? Or do these regulators just set the MAXIMUM current, rather than try to hold at a specified current? I bought something similar http://www.ebay.co.uk/itm/201059502388?_trksid=p2059210.m2749.l2649&ssPageName=STRK%3AMEBIDX%3AIT But I thought it would be better to regulate each series string independently. Also, I'd like to use something I can build myself as I'll be making a Youtube video, and, if the regulator is home-made, it #1 looks cooler, #2 means I can get more videos out of the build (Therefore more ad revenue) and #3, Youtube commentors are know-it-alls, and I don't want hundreds of 14 year olds correcting my bad design!

    I saw a design that used a voltage regulator to regulate current, and it used a transistor for each string, but it also needed a separate resistor for each string to set the current, again, not very efficient.
     
    Last edited: Aug 25, 2014
  4. mcgyvr

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  5. ronv

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    Not always is it a problem. His LEDs are pretty well matched (+/- 7% or so) and he will have 7 strings in parallel. So several things happen:
    1- you need to be pretty unlucky to get all the low voltage ones in one string and all high voltage ones in all the others. But even if you did the initial imbalance is only about 10ma. 15 mw. Assuming they are standard LEDs the temperature rise will be only about 3C in the hot strings and a voltage drop of .025 volts but only a drop of .5C in the cold ones. A difference of 3.5C will result in about another 3 ma of heating in the hot string and so on, but the seven strings saves it. If it were only 2 strings it wouldn't work. This is how they make high power LEDs.
    But having said all that, individual control is still better, but he doesn't like all the "stuff" to do it.
    Dark, the buck regulator can only drive 2 in series because it needs a couple of volts to regulate.
     
  6. DarkMavis

    Thread Starter New Member

    Aug 24, 2014
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    I didn't post the specs of the LED's I'm using, but I can go find them. I don't remember if it listed the tolerance or not though
     
  7. DarkMavis

    Thread Starter New Member

    Aug 24, 2014
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    Oh, nevermind. Yeah, the forward voltage is listed as 1.4 - 1.5v so...7%. That's better than the 20% ones I used to use. Why is it better? I mean, these are from the US and not China, but still...They are 3 chip LEDs, so...are those 3 chips run in series in Each LED? Cus I guess the averaging of 3 different chips would result in a tighter tolerance. I'm totally reading that current mirroring article right now, but I'm not very far into it so I'm not sure if it's something that can be done with more than 2 strings. The name suggests it isn't.
     
  8. ronv

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    If there are 3 chips that make up the one LED they are in parallel. Hard to say why they are better, but it is easy for a manufacture that tests their LEDs to bin them by Vf at the same time.
    You can have several reflecting mirrors, but each string needs it's own transistor and resistor.
    Do you need the 4 X 7 or could it be a 3 X 9. The 4 X 7 forces you into a boost regulator to get enough from your 6 volt battery to run a string.
    http://recotana.sakura.ne.jp/recotanablog/wp-content/uploads/2007/11/irdriver12ver1.pdf
     
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