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Thread: Custom built Pixel String project

  1. #1
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    Default Custom built Pixel String project

    Hi, I'm building a custom string of WS2811 nodes in order to control the specific length between nodes. Each string will be 25' with 30 WS2811. Currently I am testing with 5v LEDs but may move to 12v for my final product. I purchased 3pin pigtails with 22 gauge wire, and am now concerned I should have bought the larger connectors with 18 gauge wire. I would really appreciate if someone could point me in the right direction regarding what gauge of wire I should be using, as well as how many of these strings I could potentially mate before needing to inject power. Ultimately I will have 20 of these strings all together for some mini trees in a display and am trying to figure out how to best manage the power.

    To be clear, these won't be the standard 8mm WS2811 Nodes chopped up and spliced back together at the desired lengths, but a custom built WS2811 F5 circuit for each node. I've got nodes done, just need a little help with the cabling.

    I hope I've explained this well. I've just started in this hobby and am hoping to get this project ready for this next holiday season.

    Thanks in advance!

  2. #2
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    Default Re: Custom built Pixel String project

    Whether custom built or off the shelf, The tri colored LEDs will draw current. You use the correct value in your power calculations Keep in mind that current flow through the wire changes at each pixel. Wire voltage drop is based on wire length between pixels and the current through the wire at each segment. The longer the distance between pixels, the thicker the wire you will need to use.


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  3. #3
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    Default Re: Custom built Pixel String project

    Thanks! So the strings will be 30 LEDs with 10" spacing, Total of 25'. I am hoping I can get 90 LEDs (3 Strings) per power drop, more would be great!. I was planing to run 2 conductor 16 gauge for my power injection. So would using the 22 gauge on the LED stings be ok, or should I plan for 20 or 18 gauge for the strings?

  4. #4
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    Default Re: Custom built Pixel String project

    You would still need to inject every 70" or so


    2021 New Tune-to sign. New 40x27 matrix at the house. Retiring the Strip based matrix. Updating some of the old window frames to the new house. Adding two new songs.
    2020 Full sized show reworked for the new location. Only adding (famous last words) 13 RBLs that I finally got converted to using pixels
    2019 - Just moved into a new home (yet another change of plans). Will be dim but not dark. Too much to do at the new place to leave time for a show. Dim show (3000 pixels) had regular visits most nights.
    https://www.youtube.com/channel/UCyX...ttrsZNARkUce0Q

  5. #5
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    Default Re: Custom built Pixel String project

    Do the calculations...

    Assuming that you would want to accommodate 100% on for every RGB LED using 22 AWG wiring for the string...

    The precise way of calculating the voltage drop across the string would be to add up the voltage drops in each 10" inter-pixel segment. However, using average current (i.e. current averaged over all of the wiring segments) which is half of the near-end current and total resistance is close enough (as would using total near-end current and average wire resistance, amounting to the same thing).

    Take length L in inches and assume 10" separation between pixels...

    The average current draw over that length is L/10 * .060A / 2 ( L/10 is the number of pixels, .060A is draw per pixel, and the '2' division factor is used to get average current from the maximum current at the power source).

    The resistance over that distance is L/12 * 16mΩ/foot * 2 (the 12 is used to convert inches to feet, the 16mΩ is the resistance per foot of 22 AWG wire, and the factor of 2 is take into account the voltage drop over both the power and ground wires).

    The voltage drop at the far end of the string is then L/10 * .060A /2 * L /12 * 16mΩ * 2 = L * L * 8e-6 (L is in units of inches, the voltage drop is in units of volts).

    For 25' string (i.e 300") this would result in a voltage drop of 0.72V from the near end to the far end, which might be a bit much for 5V strings but likely OK for 12V strings.

    Anyone see any major flaws in either the logic, the algebra or the calculations?
    Phil

  6. #6
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    Default Re: Custom built Pixel String project

    That is what my calculations come up with as well: 4.27V at the end of the string. But this assumes the wire is actually 22AWG and there is no leader from the PSU to the string (need to calculate that drop as well). At 4.27v, I would inject somewhere in the middle of the string and at the end of the string. For three strings that is 2 power injection points per string and one power injection point at the end of the last string, for a total of seven injection points.

    On my house I feed my long runs like this using 12v or 24v on the feed line with 3A buck converters to inject 5v.

    In your case I would have a 16AWG feed line running with the pixel strings at 24V. Seven injection points with 2A Buck converters. You only need 1A units but I prefer the extra capacity to keep the heat down on the converters. If you use 24V on the feed line, the max current on the feed line would be 1.4A and that would have a negligible voltage drop over the length of the feed line.


    2021 New Tune-to sign. New 40x27 matrix at the house. Retiring the Strip based matrix. Updating some of the old window frames to the new house. Adding two new songs.
    2020 Full sized show reworked for the new location. Only adding (famous last words) 13 RBLs that I finally got converted to using pixels
    2019 - Just moved into a new home (yet another change of plans). Will be dim but not dark. Too much to do at the new place to leave time for a show. Dim show (3000 pixels) had regular visits most nights.
    https://www.youtube.com/channel/UCyX...ttrsZNARkUce0Q

  7. #7
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    Default Re: Custom built Pixel String project

    ok. So each string is for 1 2' tall mini tree which will line a walkway, so 10 per side (2 runs). I'd rather not have 2 power injections on each tree. I was hoping to inject every other tree on each run (6 per 10 trees). So it sounds like I need to up the wire gauge at very least, if not switch to 12v LEDs. Thanks for the help on this. I'm trying to work out the kinks before building rather than building ten of thees strings and realizing I cant even get through half a tree without injecting power!

  8. #8
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    Default Re: Custom built Pixel String project

    Ok. So if I up my wire gauge to 20awg with a resistance of 10mΩ, that should drop me to a loss of .45v at the end of the string. Am I doing that math correctly? Is that an acceptable for 5v leds? I also doubt I’ll be running them at 100% so I assume that will help. I also like the idea of running 12v or 24v as a feed line along my string run and dropping to 5v with a buck converter. Could I run a 16 or 14 awg from my psu at 12/24v all the way down my run of 10 mini trees (1 string per tree) and inject power off that main feed at each string with a 5v buck?

  9. #9
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    Default Re: Custom built Pixel String project

    Let me make sure that I have this straight ... you have 10 trees, each with one 25' string of 30 RGB LEDs?

    If so, and the string has 5V at the first LED (or possibly at the 10" lead to that first pixel), the pixel at the other end of the string would have 4.55V across it when all LEDs are on 100%. The outstanding question is whether the voltage at the injection point is actually 5V, or whether it would be somewhat lower than that due to the drop along the power lead-in wires.

    As for using a feed line buck converters, similar calculations would apply there to determine the gauge of the feed line wires.
    Phil

  10. #10
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    Default Re: Custom built Pixel String project

    That is correct. In fact I will have a total of 20 trees, 10 along each side of a path. To be clear, none of this is built yet, and I am trying to figure out the best plan to accomplish this goal. If I used power bucks, I would probably build them into the tail at the base of each tree so I could potentially change the layout year to year and never have to worry about getting a 5v line to the tree. I could also run a 12v/24v power feed line to each group of 5 trees if that made more sense for outputs off a controller.

    I guess the first step is really to determine the gauge of wire I should use for building the 30 led string And get one built as a sample to test how it performs.

    I’m open to any suggestions if there is a better way to accomplish this. Probably not the best choice as my first project, but I am excited to see this come to fruition!

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