More power injection clarification

Discussion in 'Power Supplies' started by bdt, Sep 11, 2017.

  1. bdt

    bdt Apprentice Elf

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    Looking for some electronics theory clarification; Almost every time a power injection question is asked someone always reminds us that the -V between the power supplies should be connected and the +V should not. I understand the reasoning, however, pixels are constructed with positive and negative busses; so actually the +Vs are connected??? Each wire on 100 ws2811 LEDs with 18awg wire is ~1.6 ohms. So, following recommendations; 5v pixels should be power injected every 100 pixels (~1.6 ohms) and 12v pixels should be power injected every ~150 pixels (~2.4 ohms). If reduced to a simple circuit there could be less then 1.6 ohms between +V on separate power supplies.

    Is 1.6 ohms really enough resistance to adequately separate power supplies? Does it really matter? What is the proper way to isolate the +V between power supplies?

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  2. fasteddy

    fasteddy I have C.L.A.P Global Moderator Generous Elf

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    Keeping it simple without too much theory then 1.6 ohms is not much resistance at all but could be considered a 'load' that separates the power supplies and hence a reason to leave them connected. But due to the low resistance then the power supplies will be sharing that load. Now the issues is that not everything is made equal and this is even more the case when using the cheap Ray Wu power supplies, so you will have power supplies with different characteristics (like voltage output) than the other power supplies its connected with and thus the power supply load will not be evenly distributed with some taking on more of the load than others. This reduces power supply life. If connecting power supplies together for load sharing was acceptable then there would be no need for load sharing supplies that are designed to keep the balance right.

    The proper way to inject is to not connect the +V of the previous light string when injecting power to the next string. The Ground wire is kept connected between the strings so they all have the same reference as if the ground was not equal to each other then you would see ground loops occur and this is when your lights start to act real funky with crazy flickering
     
  3. David_AVD

    David_AVD Bite my shiny metal ass!

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    Your power supplies will effectively be in parallel. Not many power supplies are rated to do this.

    You may get away with it, but if it were me I would not do it.
     
  4. OP
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    bdt

    bdt Apprentice Elf

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    Are you saying the voltage drop across the resistance (~1.6 ohms) of a string of lights is sufficient if the power supplies are set close to the same voltages? That makes sense, the voltage across the light string will fluctuate depending on the current draws of the individual power supplies. I can see how that will over work the power supplies, however, 1.6 ohms would be significantly better than 0 ohms. Is my logic sound?
     
  5. David_AVD

    David_AVD Bite my shiny metal ass!

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    I'm saying that feeding the output of one power supply into another is not good unless they are designed for parallel operation.

    You seem determined to connect them this way anyway so not sure what else I can say.
     
  6. OP
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    bdt

    bdt Apprentice Elf

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    I don,t mean to sound determined, the T splitters used for power injection are wired this way.
     
  7. fasteddy

    fasteddy I have C.L.A.P Global Moderator Generous Elf

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    Are these T splitters designed for power injection or designed to split the signal so you have separate runs. I always cut the +V pin off any T-Way splitter if being used for power injection

    The T way's may be straight through but this doesn't mean its correct. As I mentioned some may consider 1.6 ohms to be enough load resistance to separate the power supplies, but regardless most would be using these with the cheap Ray Wu power supplies and running them in parallel may cause issues in the long run including heating up the cabling and pixels because the wire is being treated as a load,.

    So with anything electrical do you want to take a chance because it seems to work OK for you, because it may very well be OK for you but for someone else it may not be, as all conditions would not be the same.
     
  8. David_AVD

    David_AVD Bite my shiny metal ass!

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    Modern (switch-mode) power supplies have a feedback loop that maintains the output voltage. Putting them in parallel can lead to instability unless they were designed for that in mind.

    The other issue is how do you fuse that parallel mess? One fuse blows and the total load transfers to the remaining power supply, then the fuse for it blows.

    Then you're left with no working lights where as you would have had a chance with independent supplies.

    If you don't fuse the power supplies and an overload occurs, they could start cycling through shut down / start up cycles with each one kicking the other in the teeth multiple times per second. Not really a good thing.
     
  9. ezellner

    ezellner New Elf

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    The T splitters are intended to put power into the end of one string and the beginning of the next rather than have to have two separate power feeds to 2 strings. For power injection, you can use a T between two strings IF the power is coming from the same power supply.
     
  10. OP
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    bdt

    bdt Apprentice Elf

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    Thanks for your responses. I should have clarified my initial question by giving a little more background. I’m going to rewire much of my display and I recently got the impression from some of the online discussions that injecting power using T splitters with multiple power supplies is ok. I tried to word my question such that I was surprised because I didn’t think there was much difference between hooking multiple power supplies directly together vs. having them connected between pixel strings via a T splitter. I was trying to understand the theory behind it when there really isn’t a difference. – thx again.
     
  11. sweepa

    sweepa New Elf

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    Can I chime in on this conversation if the + is only connected at the beginning of the string does that mean a single + wire needs to be run from the PSU to the injection spot providing th enegative is connected at the beginning of the strings or does each injection spot require a positive and a shared negative connection?
     
  12. David_AVD

    David_AVD Bite my shiny metal ass!

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    Power injection requires positive and negative connections.

    However, I you only had one spare wire, then the negative one would be the one to double up on.
     

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