Powering 720 leds?

Discussion in 'General Electronics Chat' started by usmc96gt, Apr 14, 2010.

  1. usmc96gt

    Thread Starter New Member

    Apr 14, 2010
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    Ive been working on a project to light a wall and am a little confused on powering it. It consists of 36 strips of 20 leds. The leds have a voltage drop of 3.3v, and require 20ma. I know how to power each strip individually in parallel with resistors. But, ive been looking around and see there is much cheaper ways. Ive been looking at Buckpucks but dont know what the total ma draw would be. Any suggestions on the easiest and best way to power this. Also Id like to power them all from 1 power source and dimming would be required. Thanks in advance for all your help.
     
  2. SgtWookie

    Expert

    Jul 17, 2007
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    That's a lot of LEDS!

    Are you wiring them up yourself, or do they come in strips?

    One of my favorite suppliers has a 24v 6.5A power supply on sale right now for $15:
    http://www.mpja.com/email/04-06-10a.asp?r=%%ref%%&s=26
    Plenty of power for what you want to do.

    You could wire the LEDs in strings of 5, with a 390 Ohm 1/4W resistor in series. The 5 LEDs in series would drop a total of 16.5v, and the resistor 7.5v.
    That would give you about 19.2mA per LED.
    You would then need 4 of these series strings per strip. Each strip would draw 76.8mA, for a total current requirement of about 2.765 Amperes.

    To vary the brightness, you could use a linear regulator like a 7812 to power a 555 timer circuit wired for PWM (pulse width modulation) control of a power MOSFET. If the 555 timer were running about 200Hz or faster, you wouldn't notice any flickering of the LEDs.
     
  3. usmc96gt

    Thread Starter New Member

    Apr 14, 2010
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    no im going to be making the strips myself, there smd leds btw. Sounds pretty easy, how about adding dimming to them. I would like to be able to dim all the leds together. Is there any way to get away from using resistors completely? **edit** guess i caught you mid post. Im still working on learning circuit design, can you lead me to a schematic for the dimmer your talking about
     
    Last edited: Apr 14, 2010
  4. SgtWookie

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    Jul 17, 2007
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    That's going to be a lot of soldering. You'll need to have boards made, or make them yourself.

    If you want to avoid resistors, then you will have to use current regulators, which will be more expensive and more complex than just simple resistors.

    The problem is that as LED's warm up, their Vf (forward voltage) decreases. As the Vf decreases, the current increases, which heats them up even more. You wind up with a situation called "thermal runaway", and eventually blackened blobs of plastic instead of LEDs.

    Resistors are the cheap way to limit current when the LEDs are supplied with a constant voltage.
     
  5. usmc96gt

    Thread Starter New Member

    Apr 14, 2010
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    yea the soldering isn't a problem and I already have the boards worked out it. The problem was efficently powering them once all the strips were connected.
     
  6. SgtWookie

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    Jul 17, 2007
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    I see. Can you post the board layout for a strip?

    .PNG format images are preferred; it's compact, no special software is needed to view them, and they are not "lossy" like .jpg format images.

    You can use the "Go Advanced" and "Manage Attachments" buttons to upload them from your computer.
     
  7. SgtWookie

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    Jul 17, 2007
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    Have a look at the attached schematic.

    It consists of the 24v power supply, an LM317 regulator to drop the 24v down to 12v, an LM555 timer, a power MOSFET and associated parts for PWM control of the LED's intensity.
     
  8. usmc96gt

    Thread Starter New Member

    Apr 14, 2010
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    the leds aren't going to be going on a printed circuit board, just simple gluing and soldering. the strips are used for backlight and wont be in view. Looking at that circuit it seems the brightness is set by a resistor, is it possible to add a potentiometer and vary it like that?
     
  9. SgtWookie

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    Jul 17, 2007
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    You're going to glue and solder SMT LEDs? Yikes - that's going to be a real bear.

    R7 - R10 limit the maximum current through their strings of LEDs.

    R4 is the potentiometer that controls the on/off ratio of the 555 timer. R4 controls the brightness for all of the LED strings.
     
  10. usmc96gt

    Thread Starter New Member

    Apr 14, 2010
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    haha yea its really not too bad. Ive made quite a few mock ups to get the light evenly dispersed and I just do it like an assemble line one one step at a time. Now how dim compared to full brightness would the 200hz threshold be? I dont need 0-100% dimming, probably just 70-100% max. btw sorry for so many questions and thanks again. What is/was your mos?
     
  11. SgtWookie

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    Jul 17, 2007
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    I see.

    Dimming will be from around 99% to around 3%. The timer is running at about 200Hz, +/- the tolerance of the cap C1 and resistors R3, R4.

    The 1N4148 across the wiper to the low end of the pot is what makes the PWM really wide. Might as well leave the really wide PWM feature in, you might like it.

    I was a 6657; that was a Vietnam-era MOS. Radar/missile fire control tech on F-4J/S Phantoms.
     
  12. awright

    Well-Known Member

    Jul 5, 2006
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    Look into the National Semiconductor LM3445. It is designed to drive high power LED arrays directly off the line in conjunction with a regular triac-based incandescent lamp dimmer.

    awright
     
  13. awright

    Well-Known Member

    Jul 5, 2006
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    Look into the Natioanal Semiconductor LM3445. It is specifically designed to drive LED strings off the line in conjunction with an ordinary incandescent lamp dimmer. I believe there is a demo board available that would simplify construction of the complete circuit.

    Since the circuit does not provide isolation from the line you would have to be very careful about insulation and preventing personnel contact with the large array of LEDs (or provide an isolation transformer on the input).

    awright
     
  14. SgtWookie

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    That regulator is not safe, as it is not isolated from mains power by a transformer.

    Furthermore, our original poster is using low power LEDs, not high power LEDs.
     
  15. Wendy

    Moderator

    Mar 24, 2008
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    Have you read this?

    LEDs, 555s, Flashers, and Light Chasers

    Chapters 1 and 2 cover most of what you need to know.

    Resistors are also the simpliest approach, one component per string instead of 2 or more. You can get SMT resistors too.

    If you insist on not using a PCB I'd go with copper tape. It would be a lot simplier than wire. Either way, you're going to have to have mad soldering skills.
     
  16. retched

    AAC Fanatic!

    Dec 5, 2009
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    Your soldering skills arn't going to be the only things that are mad.

    I think Bill is right by suggesting copper tape. You can also use extra copper tape as "radiator fins" to help pull some of the heat away from this monstrosity. ;)

    You may even consider strip board. It is basically copper tape already laid down. Im not sure if they have undrilled strip board for SMD components or not. But either way, It would save your wire cutters.
     
  17. SgtWookie

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    If he were going to use SMD resistors, they would need to be thick film 1206 or larger, 1210 preferable, for the power dissipation requirement. They would also need to be mounted on a PCB with wide traces.

    I like the idea of cutting up stripboard. But, you wouldn't even need that - just single-sided plain copper PCB ripped into short, narrow strips.

    My favorite local store is a place called Skycraft Parts and Surplus. They recently had several hundred sheets of very thin FR4 for prototype PCB's, roughly 1'x1.5', some had copper on one side, some both sides. It's so thin that you can cut it with a paper cutting board or heavy-duty scissors. I don't know where to buy board like that online. It's only about 20-24 mils thick, copper included.
     
    Last edited: Apr 15, 2010
  18. retched

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  19. SgtWookie

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    Good find, retched.

    http://www.goldmine-elec-products.com/prodinfo.asp?number=G17456
    They have a minimum order of $10. Without calculating, get 3 or 4 sheets.

    What's the recommended pad size of your LEDs? Better yet, what's the mfgr and part number, or a link to the datasheet?

    They also have size 1206 390 Ohm resistors, 20 for $1.
    http://www.goldmine-elec-products.com/prodinfo.asp?number=G246R
    You'll need eight of those. 720 LEDs/5 = 144 strings, /20 resistors per pack=7.2 packs, you'll need the extras.
     
  20. usmc96gt

    Thread Starter New Member

    Apr 14, 2010
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    wow you guys are awesome here, heres a link to the leds there data sheet is on the page. So how would I easly etch all those strips, seems like that would be more time consuming than just glueing and soldering. Also the supplies have started comming in, im just waiting on the dimmer components now. Ill post some pictures of progress and the completed wall
     
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