Supplying lots of power for lots of LEDs

Discussion in 'General Electronics Chat' started by stoopkid, Oct 17, 2011.

  1. stoopkid

    Thread Starter Member

    Mar 3, 2011
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    I'm going to be controlling a whole lot of LEDs with an arduino. The way I've got it set up I need to run them each in parallel, no series'. I plan on running them at 10mA and there may be anywhere from 300 to 500 of them, I don't know quite yet. I wanted to use a laptop power supply or something because of the high amp output but I was told that it is inefficient regulating a high voltage laptop power supply down to 5v for LEDs. I would be fine with a 5v 5A power supply but they are super expensive(>$50), and I can get a laptop power supply for about $10.

    Is a laptop power supply really a bad idea with a high power voltage regulator? How should I do this, and can I do it without too much expense?

    Thanks
     
  2. colinb

    Active Member

    Jun 15, 2011
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    Maybe you can explain in a little more detail what you're trying to do. Are you driving groups of LEDs independently or are do you have just a single circuit of LEDs that is on or off together? Driving LEDs in parallel is inefficient since you will really need a resistor on each LED to make it robust and that means wasted power. Why not put at least a few LEDs in series when you aren't controlling each LED individually and you have some higher voltages (more than 9 V for instance) to use.
     
  3. stoopkid

    Thread Starter Member

    Mar 3, 2011
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    There will be 32 channels that each have maybe 10 or more LEDs each. The problem is that some channels will have different numbers of LEDs and all the channels will have inconsistently different colors, so the overall voltage drops will all be different. I can't imagine it being very easy to keep each channel the same brightness this way. There will be two mirrored sides, but geographically, it would require tons more wire to make both sides run in series. Many of the LED's will be up to 2 feet away from each other.
     
  4. KJ6EAD

    Senior Member

    Apr 30, 2011
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  5. stoopkid

    Thread Starter Member

    Mar 3, 2011
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  6. KJ6EAD

    Senior Member

    Apr 30, 2011
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    A load resistor establishes a current that some supplies require for regulation. I wouldn't waste power with a load resistor, especially not a 15W one. Have you considered using several of these in parallel? They each need a power cord but you have the advantage of redundancy if one should fail. There are other electronic surplus vendors such as HSC, Alltronics, etc.

    http://www.allelectronics.com/make-a-store/item/PS-5111/5.1-VDC-1.1-AMP-SWITCHING-POWER-SUPPLY/1.html

    Power cord for above:
    http://www.allelectronics.com/make-a-store/item/LCAC-390/6-2-COND.-UNIVERSAL-POWER-CORD-NON-POLARIZED/1.html
     
  7. stoopkid

    Thread Starter Member

    Mar 3, 2011
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    Hmm yeah, but I would need like 5 of those :(

    That seems a little much
     
  8. kubeek

    AAC Fanatic!

    Sep 20, 2005
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    How about a computer power supply?
     
  9. stoopkid

    Thread Starter Member

    Mar 3, 2011
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    definitely too big... unless they make really little ones I don't know about.
     
  10. kubeek

    AAC Fanatic!

    Sep 20, 2005
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  11. stoopkid

    Thread Starter Member

    Mar 3, 2011
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    Hmm yeah that could work... Will I have a problem if it's not a regulated power supply?
     
  12. kubeek

    AAC Fanatic!

    Sep 20, 2005
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    It is a switching one so it most probably is regulated.
     
  13. Potato Pudding

    Well-Known Member

    Jun 11, 2010
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    You are describing 50 Amps at 5 volts for a continuously on set of 500 LEDs at 10mA.


    I think you need to look at the pulse current capacity of your LEDs.

    Don't light more than 1 in 5 of your LEDs at any moment and rotate through those channels, rapidly as in >>100kHz.

    They will all look lit up as far as the human eye can tell. Your power supply requirement drops to 10A.

    Anything more than 10 amps is probably not something to take on unless you have lots of experience. Even lower would be better and possible.

    I assume you want to multiplex these. To run 500 LEDs. 20 by 25 channels&strings would still be within single Microcontroller range. If you want to add some logic ICs to the circuit, you would not need a Micro with 45 outputs. Turn it into 2 x 25 x10 banks/channels/strings and you could do it with a 12 output microcontroller.

    The system I just described would only have 1 out of every 50 LEDs *lit* at any moment so that your supply would only need to be 1 Amp. The LEDs should still appear lit as normal if done properly.

    Good Luck.
     
  14. kubeek

    AAC Fanatic!

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    Potato, check your numbers 0.01*500 is 5, not 50.
     
  15. SgtWookie

    Expert

    Jul 17, 2007
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    As Kubeek wrote, your math is off a tad.
    500 * 10mA = 5A, not 50A.

    100kHz? Sure you didn't mean 200Hz? At 200Hz, a person wouldn't be able to see any flickering at all.

    It was already 5A, but if you are thinking that running at 20% duty cycle will look as bright as 100% duty cycle at the same current, I'm afraid that you are mistaken.

    Actually, using 74HC595 shift registers and some circuitry, you could do it with just a couple of outputs, and a lot of bit shifting. I really wouldn't want to do it that way though...

    They may appear lit, but at 1/50th the brightness they would appear at 100% duty cycle.
     
  16. colinb

    Active Member

    Jun 15, 2011
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    I'm using the Lambda Regulated +5VDC and +3.3VDC Power Supply (only $6 and even less sometimes when it's on sale) from The Electronic Goldmine and it's an amazing cheap solution. Only downside is that it doesn't have an enclosure, so I had to make my own to ensure isolate line voltages for safety.
     
  17. Wendy

    Moderator

    Mar 24, 2008
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    When thinking LED power supplies think in terms of wattage.

    If you have 500 LEDs, each drawing up to 20ma (3W LEDs draw 0.7A) you will need 10W, an absolute minimum, 20W is better. You will also need 500 matching resistors, and low voltage (5V is excellent). There is a reason LEDs are run in series, it increases their efficiency and reduces their parts count enormously.
     
  18. Adjuster

    Well-Known Member

    Dec 26, 2010
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    If you could even restrict yourself to driving the LEDs in twos or threes of the same colour, each with its own resistor / current limiter, you could use a higher voltage lower current supply. If the cost must be kept to a minimum (how much are the LEDs?), you may have to make some compromises.

    Note however that quite cheap power packs rated around 5V 2A are made for such things as netbooks and wireless routers. Two of those might suit: you would have to check details like regulation and idle current though.
     
  19. Potato Pudding

    Well-Known Member

    Jun 11, 2010
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    Never post before finishing your first cup of coffee. :D
    Thanks for the reality check.

    As for how to make them brighter with 1/20 the duty cycle, that was why I asked to check the LED's pulse current rating. Which of course means increasing the momentary current which means you don't reduce your current requirements very much.

    That and using inductors with snubbing diodes across the inductor and LED instead of resistor to drop the current is what I had in mind but that also falls under the heading of don't post before the first cup of coffee.

    It might be workable but ease and cost are thrown away.

    Start with one mistake like thinking that you need to avoid 50 amps and a whole lot of ridiculous stuff follows.
     
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