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Can you please tell me what I'm doing wrong and how I should fix it

Discussion in 'Computers, Cabling & Other Miscellaneous Hardware' started by vincerules, Jul 4, 2013.

  1. vincerules

    vincerules Full Time Elf

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    OK, I am trying to get a handle on the power/wiring. I will use my biggest load(most lights) as the example.

    I am outling my rooflines using 2811 10 sections/meter, which is .64 amps/meter according to this thread.
    The power supply I am using is this one from Ray Wu which is 12v/350W and from the sticker it says 29A.

    80% max load = 23.2A
    85% max load = 24.65

    For my rooflines, I have 420 total pixels.
    So I divide that by 10 to get meters = 42.
    42 * .64 = 26.88

    So if my math is correct, I am overloaded and should split these into two power supplies, correct?

    Now onto the cabling. I am using this 18 gauge speaker wire from Monoprice.
    I inject power every 50 pixels(or standard 5M length), so the most 1 cable run would need to power is 100 pixels (since I would inject between 2 sets of 50). Conductor resistance is .023

    The longest run is probably ~20 feet away, but lets use 10 meters for ease of math and extra length room.
    10 * .023 = .23ohms * 2 = .46ohms total return circuit resistance
    100 pixels = 10 meters of lights * .64/ampspmeter = 6.4 amps
    6.4 * .46 = 2.944v
    12v - 2.944v = 9.056
    24.6% voltage drop, I'm assuming that is not acceptable.

    Or should I be doing it for only 50 pixels since the circuit does not actually go through all 100 pixels and back but instead gets split into 2 sets of 50.
    50 pixels = 5 meters * .64 = 3.2
    3.2 * .46 = 1.472v
    12 - 1.472 = 10.528
    12.3% voltage drop. Is that accetable?

    Do I need to use thicker gauge wire for the longer runs?

    Is my math correct? I am doing this while reading through the ACL manual (for the 1000th time) and trying to make sense of it all.

    Thank you for taking the time to read this and any help you can give.
     
  2. AAH

    AAH I love blinky lights :) Community Project Designer

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    Based on the 2811 spec which current limits the leds in the pixel to 18.5mA your current for 400 pixels should equal 420x3x0.0185=23.31A when every pixel is turned on to white. I would be happy enough to use the 1 power supply to run those 420 pixels.
    As for your power injection / voltage drop, as long as you haven't dropped much more than 2V anywhere along the pixel run you should get very uniform colours with the 12V 2811 pixels. The way in which you inject will affect the voltage drop. If you just run the power injection wire in parallel with your pixels and tap off at every injection point you will have a greater voltage drop than if you bring each injection point back to the power supply.
    The power injection with 12V pixels is nowhere near as critical as it is with 5V ones and with the 3 leds per chip in the 12V strip means that anything above 9V at the chip will give you sufficient volts to reach the 18.5mA where it current limits at.
     
  3. OP
    OP
    vincerules

    vincerules Full Time Elf

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    Awesome, thanks for the reply AAH!

    For injecting, I run a new cable from the PS to the injection point every time.

    Thanks again!
     
  4. damo1271

    damo1271 Full Time Elf Generous Elf

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    This raises an interesting conundrum!
    Which is more cost effective and which is simpler?
    Having a more runs of thinner wire back to the PSU vs fewer runs of heavier wire?

    The maximum wire length Vince can run is around 15m if he needs to keep the voltage drop less than 2V. If any run is more than 15m he needs 'fatter' wire! (lower resistance/m)

    Given the linear relationship of Ohms law, then if the price of cable having 1/2 the resistance, (ie 12.5mOhm/meter) is less than double the price of the 23mOhm/m wire, then would it be better to use less runs of the heavier guage cable and power two strings off each run? (Same equivalent voltage drop)
    It also makes for less wires in the display and simpler wiring back at the power supply!

    For a few pixel runs it probably makes no real differnece, but when I read about the people who have kilometers of wiring in their display it may make a big difference in cost.
    After all, less money on wires = more money for lights. :)
     
  5. jediknight2

    jediknight2 Full Time Elf

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    I used 2801 pixels last year and got away with injecting every 200 pixels. They were "homerun" injections, meaning every injection came directly from the PS. I used 18 gauge SPT2 wire and some left over speaker wire for my injections and everything ran like it should.
     

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