LED Array - Need tips

Discussion in 'The Projects Forum' started by Serpente, Apr 11, 2014.

  1. Serpente

    Thread Starter New Member

    Apr 11, 2014
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    So I'm kind newish to electronics and am looking to build an LED array as my third project so far.

    I have about 1000 of these LEDs: http://au.element14.com/cree/c503b-rcs-cw0z0aa1/led-red-t-1-3-4-5mm-5-1cd-630nm/dp/1855524http://

    My current plan is to use a 19.5V, 3.95A AC adapter to power a series/parallel array. The layout that I have come up with is 9 LEDs in series with a 100ohm 1/4 resistor, and 111 rows in parallel for a total of 999 LEDs.

    I also want an on/off switch and a "dimmer". Originally I had planned to use a potentiometer, but it seems that may not be ideal for an LED array.

    I'm hoping that someone with a bit of experience can comment on the validity of my current design and propose a good method for dimming.

    Thanks in advance.
     
  2. John P

    AAC Fanatic!

    Oct 14, 2008
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    I'm not sure that 9 LEDs in series will run properly on 19.5V. You might have to cut back to 8.

    You'd be best off to use a digital control for dimming. One easy way is an LM555 oscillator driving a power transistor, most likely a MOSFET. A basic 555 oscillator won't go all the way to zero or 100%, but you can find variant circuits that will do it.

    The question I most want to ask is, are you really up for all that wiring?
     
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  3. Serpente

    Thread Starter New Member

    Apr 11, 2014
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    Thanks for the reply. Could I ask for the reasoning behind 9 LEDs potentially not working properly on 19.5V? Is it that they will be underpowered, or is there another potential issue?

    Thanks for the dimming suggestion; I'll look into it. They don't need to dim all the way down to 0, but I would like the dial to go up to 100% or close to it.

    As for the wiring, at this stage I'm willing; however, that may not continue to be the case all the way through lol. On the topic of wiring, could I potentially join up the LEDs just with solder if they are side by side, or do I have to literally wire them together?
     
  4. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    Is it just a panel of red light? 2.1 volts x 9 will be 18.9 volts. You will be fine if your power supply does not fall below that number at full load. If it does, lights out.

    You will also need a current limiting resistor for each series of 9 LEDs. Your choice of 100 ohms means 19.5-18.9)/100= .006 amps (6mA). Kind of dim. What's the point of using an array of 1000 dim LEDs when each could be run at 5x more current? I'll bet 200 running at 30 mA will be brighter than 1000 running at 6mA.

    Any control needed or are you just making 'grow lights'?
     
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  5. burger2227

    Member

    Feb 3, 2014
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    At 10 ma per LED you'll be running 10 amps of current. That will require something special to dim them. Bright white or blue LED's normally use 3 volts up to 20 ma each.
     
  6. Metalmann

    Active Member

    Dec 8, 2012
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    "On the topic of wiring, could I potentially join up the LEDs just with solder if they are side by side, or do I have to literally wire them together?"




    Don't know what your whole setup is, but I have hooked up several 1W and 3W Hi Power LEDs, connected only with solder.:confused:
     
  7. SgtWookie

    Expert

    Jul 17, 2007
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    2.1v * 9 = 18.9; leaving 0.6v as GopherT stated.
    0.6v/20mA = 30 Ohms. For long life of the LEDs, you could operate them on reduced current, say 18mA, using 33 Ohm resistors. 0.6v/18mA = 33.333... Ohms. You'll need one resistor per string of LEDs.

    18mA * 111 strings = 1.998 Amperes current required. Burger2227 forgot that you will be operating in strings of 9 LEDs when they made their current requirement calculation.

    Here's a logic level power MOSFET that would be much more than adequate for switching:
    http://www.mouser.com/ProductDetail...24NPBF/?qs=sGAEpiMZZMvsw8vHdI9FumJKlxro%2bjWf

    I suggest that you at least use some kind of perfboard to wire your LEDs up on, as trying to do them freeform without any support might leave you with a big mess, and it could be difficult to keep them from shorting against one another.
     
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  8. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
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    Max Vf could be 2.6V then 2.6V x 9 = 23.4 which is greater than the power supply voltage..
     
  9. GRNDPNDR

    Member

    Mar 1, 2012
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  10. Serpente

    Thread Starter New Member

    Apr 11, 2014
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    So many replies. Thanks guys.

    This is a working link to the LEDa: http://au.element14.com/cree/c503b-rcs-cw0z0aa1/led-red-t-1-3-4-5mm-5-1cd-630nm/dp/1855524
    Not sure what happened to the previous one.

    The project is a "light therapy" machine for my GF, which is essentially a whole bunch of red LEDs at 630nm. The whole thing looks and sounds like pseudo-science, but she spends money on it and I needed a new project so building this seemed like a win/win.

    @GopherT
    I didn't realise that the 100ohm resistor would limit the current so much. Still getting the hand of all the calculations. I wanted to slightly under power them to increase their lifespan, but not by that much.
    I am simply making glow lights with a simple on/off switch and a dimmer knob.


    @SgtWookie
    Really helpful info. 33.33...ohm resistors is probably what I was trying to originally get at. And yea, I'm building on a perfboard.

    The power supply looks to be an old laptop adaptor. I just checked it again; it's actually 19V.

    I'm guessing I should drop down to 8 in series with around a 120ohm resistor, or can I push 9 with a 5.6ohm resistor?

    Thanks for the link GRNDPNDR. I'll bookmark it, but I'd rather try building one first.
     
  11. SgtWookie

    Expert

    Jul 17, 2007
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    This would be unlikely in the extreme; the other extreme being all of the minimum Vf being in series. Having 9 LEDs in series, your standard Monte Carlo random distribution would have the Vf's evening out, until all of the strings had a nearly identical total Vf, which will be very close to the "typical Vf" times the number of LEDs in series.
     
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  12. SgtWookie

    Expert

    Jul 17, 2007
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    For a 19v adapter, use 8 in series, and a 120 Ohm resistor should be good.
    2.2v * 19mA = 41.8 milliWatts, so even a 1/10 Watt resistor would be rated high enough.

    The Vf of LEDs will decrease with an increase in temperature, so don't pack them in too tightly.

    Don't stare at super-bright LEDs; they can damage your vision.
     
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  13. FuzzballJack

    New Member

    Apr 12, 2014
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    I am very much intrigued by your project. I don't exactly know what the goal of your GF is. This project will only benefit skin. NASA first discovered these beneficial effects on skin. But they used visible red (650nm) and infrared (800 to 1000 nm).
    Infrared penetrates the skin much deeper to organ depth. This is the kind they use in trauma recovery to aid the recovery of muscles and organs in the months after surgery. It gives off warmth inside your muscles (I have experienced this, it's a quite spectacular feeling).

    See this Specifications Chart of such a product.
    http://www.imageupload.co.uk/P1n
    See the infrared and the far red light? Also pulsed light has very beneficial effects. e.g. 4.6 Hz, 9.2 Hz, 73.2 Hz, 146.5 Hz, 293 Hz, 586 Hz, 1171.9 Hz, 4687.6 Hz.

    So your setup is only red 630nm (not 650nm nor infrared). Be aware of that.
    That being said I admire your determination for this project if you succeed.
    Be sure to use thick enough wire between the parallel strips (0,5mm² for example). Don't use standard fine electronic wire.
    As said you will need a kind of holder to hold all of those leds. A half PE 1meter diameter pipe with 5mm holes drilled into them (1000 holes! :) ). This is so the leds don't get tangled up.
    Use correct calculations as given here. Try out 3 parallel strips for starters and check if it even works as desired so you won't be disappointed when it isn't working after you completed the build.
    And a last comment. That will be one bright SOB! 5.1 candela leds x 1000. A whopping 5100 candela! At 30° that is about equivalent to a 100W incandescent lightbulb. Be sure to use sunglasses!

    Let us know your progress in this project. Enjoy!
     
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  14. Serpente

    Thread Starter New Member

    Apr 11, 2014
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    @SgtWookie
    Thanks for the great info once again.

    @FuzzballJack
    Well my GF goes and pays money to use professional omnilux machines - something like 90AUD for one session. She has some little home machine that wasn't working and wanted to buy a new one. I looked at it and said "I bet I could build one 10 times as strong at a fraction of the price", and thus the project was born. In the beauty industry, the red light supposedly helps with collagen production and preventing/treating acne. I don't really care so much about that; I just like to build thing. The research I did set suggests that the effective wavelength is 320-360nm for this application. Various products use various wavelengths in this range.

    Thanks for mentioning the wire; I didn't really think about the thickness. I plan to mount them all on several PCBs and create a sort of LCD monitor style mount for them. If they cover a fairly large area I'll use 3 panels with the two outside ones hinged to the centre one so they can be angled inward.

    I will definitely try out a few rows first. Not really up for wiring 1000 or so LEDs only to find out that it doesn't work lol.

    I joke with my GF that "this thing is going to be so strong that it will burn your face off". She's practically scientifically illiterate so she gets worried lol. Anyway, the dimmer is there just in case it actually is too bright.

    I'll post progress, pics and questions along the way. I have the LEDs and power supply. I just need to get some appropriate PCBs and wiring to start testing it out. I really should get a benchtop power supply. Speaking of which, I've had my eye on this one for some time now. What do you guys think? I just need something for hobby use.
    http://www.wavecom.com.au/product_view.php?id_product=683
     
  15. Wendy

    Moderator

    Mar 24, 2008
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  16. burger2227

    Member

    Feb 3, 2014
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    So far I see a need for about 10 amps of power and a supply of 3.9 amps. Even your dream power supply is only 5 amps. The price is a little high, you can get pretty good ones for a lot less!
     
  17. Serpente

    Thread Starter New Member

    Apr 11, 2014
    15
    0
    Where are you getting 10 Amps from Berger?
    I have 1000 LEDs with a fV of 2.1V and 20mA.
    Putting 8 in series which = 20mA per row of 8, and then 125 rows of 8s which is 125 x 20mA = 2500mA. The 3.9 amp power supply should be more than enough...unless I have missed something?
     
  18. Evil Lurker

    Member

    Aug 25, 2011
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    If you are going for 1000 LED's you might want to consider driving them at mains voltage.

    Also, 320-360nm if I recall correctly is in the UV spectrum and would probably burn the skin/eyes over a finite exposure time.
     
  19. Wendy

    Moderator

    Mar 24, 2008
    20,766
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    Read our ToS, your last advice would get this thread closed. That is not allowed here.

    This can be found in our Terms of Service (ToS)
     
  20. burger2227

    Member

    Feb 3, 2014
    190
    24
    Why not remove his post instead of posting the TOS? If that were the case one nut case could close all of the threads by just mentioning the mains...oops :(
     
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