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Thread: String length, amps and power injection

  1. #1
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    Default String length, amps and power injection

    Happy New Year all! Newbie here who recently got bit by the pixel bug after looking at some great Christmas lights in my area, and am trying to plan for next year. Goal #1 is a house outline, using 12v pixels and a F16V3 controller which is sort of centrally located.

    My question is about string length, wire gauge, and power injection. As I am still VERY green in the pixel / electrical arena, any comments, advice or wisdom imparted would be most appreciated!

    Say I have a string of 12v pixels coming right off my controller that is 280 pixels long, fed by a standard 18ga pigtail getting V+ from the controller. Conventional wisdom says to power inject every 100 pixels, but I watched a video by Bill Porter (madsci1016) "xEssentials E10 power injection made easy", and his comment was no 12v pixel should be > 100 pixels away from any point of power. Based on this, for my 280 pixel string, I should be able to inject at pixel 190 only - which would be ~30' away from the controller/PSU that would feed it. I would use 14ga landscape wire for this power injection point due to distance.

    However, this is where I am having trouble. 280 pixels @ .06a / pixel == 16.8 amps. Questions:

    1) By adding one power injection line, I am halving the current/amp load / wire, so the lead wire and injection wire are effectively carrying 8.4amps each, right?

    2) From what I gather, most pixel wiring is NOT 18ga, but rather more like 22ga or worse. If 22ga wire is only capable of safely carrying 3a of power, it seems like this is a bad idea since the wire in the 1st 50 pixels would be carrying over twice their rated capacity?

    3) What do I use for a fuse on the controller port and V+ on the injection wire in this scenario? 10 amps? I read somewhere that it was not recommended to increase the sizes of the fuses on the controller - as there could be good reason they are what they are - for example wiring traces or component tolerances. Thoughts?

    4) Do I need to be concerned about the pigtail wire size here? I'm guessing no, but what if pixel#1 were 20' from the controller? Does the increased length dictate that a beefier pigtail be used?

    Thanks for any and all insights!

    - pc

  2. #2
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    Default Re: String length, amps and power injection

    The .06 amp/pixel is at full brightness white. Most people adjust the brightness to less than 50% (mine are 35%) so the power is generally reduced by the same amount.

    I have purchased all of my pixels from DIYLedExpress.com and believe their pixel wire is closer to 18awg than to 20awg. So depends on where you get your lights as to how close the gauge is to 18awg.

    I use 5 amp fuses on all of my power distribution boards with the exception of one run where I have a 7 amp fuse. For 12v pixels I try to inject every 200 pixels which theoretically fits Bill's recommendation. For 5v pixels I inject every 100 pixels. Each of these cases will result theoretically in 3.5amps max per power distribution output.

    No, you should not worry about pigtail wire size. My understanding is that the cable capacitance causes more problem than the resistance of the wire.
    Kevin

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    Default Re: String length, amps and power injection

    Quote Originally Posted by kev View Post
    The .06 amp/pixel is at full brightness white. Most people adjust the brightness to less than 50% (mine are 35%) so the power is generally reduced by the same amount.
    FYI: The AVERAGE power consumed by the individual pixels vs Controller brightness settings is NOT linear. (aka 50% brightness is actually more than 50% current on average). When you set a pixel to 50% power, the pixel draws 0.06A for 50% of the time. If every pixel happens to be on at the same instant, they will draw 0.06A x NumberOfPixels in that time slice. This causes the same amount of voltage drop in your feed wires (at that moment) as if you were at 100% brightness. while the on vs off times are pretty fast, and the pixels are not in sync with each other, they can still coincide. Keep in mind that pixels are a constant current device and they are either on (and drawing full current) or they are off (drawing minimal but not zero current). A white pixel at 50% controller setting does not draw 0.03A. It draws 0.06A 50% of the time with 100% more startup spikes which exceed 0.06A.


    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/user/MartinMueller2003

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    Default Re: String length, amps and power injection

    Never exceed the current rating of the controller port (aka increase the fuse size). Best to learn how to deal with power injection and REMOVE the fuses from your controller (you dont need V+ from the controller ).

    Using your 280 pixel example, you can power inject at Pixel 100 and 200 and only bring V- and data from the controller to the first pixel. This would allow you to run at full brightness without issues. Yes your pigtails need to support 9A each. And the runs from the injection points should be dedicated to supplying power to only the single injection point (or you need to make the wires handle 16A which means using 12AWG wire).


    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/user/MartinMueller2003

  5. #5
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    Default Re: String length, amps and power injection

    Thanks for confirming and the great information. As I am no EE, I am trying my best to understand how everything fundamentally works, and to ensure my setup is done properly and safely.

    I understand what you are saying regarding the power injection, and not running V+ from the controller and instead using wiring dedicated to power injection. If I may indulge, to better my understanding of the electrical properties here, I have some follow-up questions:

    1) When using the pixel power calculator (http://spikerlights.com/calcpower.aspx), and I add a 5' 14AWG lead wire, and inject at pixel-192 using a 30' 14AWG injection wire, it shows 16.8 amps load on the lead wire. Is this wrong?

    2) Regarding not carrying V+ from the controller at all. Since neither the aforementioned power calculator handles this scenario, nor does blinkyflashy.info, how is amperage calculated in a circuit such as this? For example, in the circuit below, what is the value of amps1 and amps2? How would this change if the "amps2" wire were moved to the end of the string (after the "80 pixels")?

    Thanks,
    -pc

    circuit.png
    Attached Images Attached Images

  6. #6
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    Default Re: String length, amps and power injection

    set your brightness at 35% and you wont need to inject power at all. completely individual choice though. i have 3 outputs that are 300 pixels each, no power injection at all.

  7. #7
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    Default Re: String length, amps and power injection

    Good safety practice is to design for worst case. So, assume your controller has a bug and that bug might inadvertently turn on all your pixels to full brightness and leave them there at 3:00am when you are asleep.

    IMHO, 12V pixels are not a good idea. I thought they were when i first started looking at this stuff but the more I looked the more concerns I had about their safety. First, a 12V pixel has to dissipate a considerable amount of heat. More energy is dissipated by the regulator than is used to generate the light. The main reason 12V pixels are popular is because people assume it reduces or eliminates power injection. It does neither. This is because you require the same current (60mA per pixel) when running 5V or 12V.

    So, my recommendation would be to use 5V pixels and design your wiring for full brightness.

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    Default Re: String length, amps and power injection

    Quote Originally Posted by puppycrack View Post

    1) When using the pixel power calculator (http://spikerlights.com/calcpower.aspx), and I add a 5' 14AWG lead wire, and inject at pixel-192 using a 30' 14AWG injection wire, it shows 16.8 amps load on the lead wire. Is this wrong?


    circuit.png
    Yeah, there is a bug with the web page. The first time you run it (close and open the page) it works correctly. After that, it does not appear to calculate the load current properly.
    Last edited by steve123; 01-02-2021 at 07:32 PM.

  9. #9
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    Default Re: String length, amps and power injection

    I agree on designing for worst case. I want my display to be safe at 100% brightness, regardless of whether or not I actually run that bright. Regarding your comments about 12v safety - do you have any links or references that discusses the safety ramifications of them? Or do you have knowledge of how things can go wrong with 12v vs 5v - assuming a properly designed system (proper AWG, fusing, etc...)?

    Also, please correct me if I'm wrong, but don't 5v pixels require power injection every 50 pixels, as opposed to 12v which require injecting every 100 pixels? This would reduce power injection, no? Also, the lights I am starting with, and was planning on 12v pixels, are for my roofline, which have quite a few long runs. If I used 5v pixels in a 280 pixel string, that would require 5 injection points (50, 100, 150, 200, 250). I realize I could run one 12ga bus wire for everything, but it would require more power tie-ins to the string. I'll reiterate though, that I've only been really digging into all this for about 10 days now (didn't know what a pixel was before that), so *please* let me know if I'm mistaken or otherwise ignorant of the something.

    I *am* planning on using 5v anywhere things are dense, or in props though.

    Thanks,
    -pc

  10. #10
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    Default Re: String length, amps and power injection

    Quote Originally Posted by puppycrack View Post
    I agree on designing for worst case. I want my display to be safe at 100% brightness, regardless of whether or not I actually run that bright. Regarding your comments about 12v safety - do you have any links or references that discusses the safety ramifications of them? Or do you have knowledge of how things can go wrong with 12v vs 5v - assuming a properly designed system (proper AWG, fusing, etc...)?

    Also, please correct me if I'm wrong, but don't 5v pixels require power injection every 50 pixels, as opposed to 12v which require injecting every 100 pixels? This would reduce power injection, no? Also, the lights I am starting with, and was planning on 12v pixels, are for my roofline, which have quite a few long runs. If I used 5v pixels in a 280 pixel string, that would require 5 injection points (50, 100, 150, 200, 250). I realize I could run one 12ga bus wire for everything, but it would require more power tie-ins to the string. I'll reiterate though, that I've only been really digging into all this for about 10 days now (didn't know what a pixel was before that), so *please* let me know if I'm mistaken or otherwise ignorant of the something.

    I *am* planning on using 5v anywhere things are dense, or in props though.

    Thanks,
    -pc
    There are no safety issues between 12 and 5 volt. And yes, 5v must be injected more. Very few people even use 5v anymore because of that. I have several very dense snowflakes, all use 12 v pixels, no issues at all.

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