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Connecting the Negative (Ground) on two PSUs - wire size & fuses ?

Discussion in 'Power Supplies' started by davrus, Jul 12, 2014.

  1. davrus

    davrus Silent Elf

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    What size wire should be used to cross connect the -ve between two (or more) power supplies ?


    In theory, there shouldn't be anything flowing between them as the reason it exists is to provide a return path for the Data signal, but .....


    And the associated question - is there a reason to put a fuse on the connection ?


    I know, you fuse all the positive side, and in general you use a fuse to protect the cable, so in the case something goes wrong, is it a good idea to protect the cross connection ?
     
  2. AAH

    AAH I love blinky lights :) Community Project Designer

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    Theoretically you don't need any other ground connection other than that provided by the pixels and the power injection wiring. If you provide a power injection point at the last pixel on 1 power supply and the 1st pixel on the next 1 you really shouldn't require a 100% direct wire between the 2. As far as fusing the -ve there isn't really much point especially if all the +ve's are fused.
     
  3. fasteddy

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

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    Agree, I have never connected the 2 power supplies directly and instead done it all at the injection point. The reason being is you are trying to achieve the same ground reference point and if you do it back at the power supply only, then different cable resistance due to cable lengths may alter the reference point between the 2 grounds at the injection point. So I see no real need for it in most situations
     
  4. OP
    OP
    davrus

    davrus Silent Elf

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    Now I am getting confused.


    I thought that I had read that if you are injecting power from a different power supply, that you break the positive connection from the first one, so that there is no feedback from one to the other.
     
  5. fasteddy

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

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    You are correct you don't connect the +V together between power supplies, but you must connect the ground wires together at the injection point. There is no real need to connect the grounds together at the power supply.
     
  6. plasmadrive

    plasmadrive Full Time Elf

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    Allow me to confuse you further. :p

    It was brought to my attention that if you are injecting power for an entire 5 meter strip from two separate supplies, that there is most likely enough resistance in the PCB to allow for equalizing of the two power supplies without causing them to fight each other. I have not tried it but it sure make perfect sense if the supplies are putting out the same voltage within say .5v. Closer they are to the same voltage the better of course..

    There is a down fall to this approach, especially when the LEDs are dark. But there is up to a 2.5v drop on full white from one end to the other on the strips I have used.. so I really don't see a technical issue.. but I would fuse both supplies if you try this..

    I still think the best way is a single supply with DC-DC converters.. Yeah.. I know.. I am kinda stuck on that tune, but it works really well and has a lot of upside to it..
     
  7. OP
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    davrus

    davrus Silent Elf

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    I was sold on DC-DC converters, and I already have a few. So I thought why not use them for my 5v strips ... and then I realised, my converters are max 2A (could do 3A if I got some heatsinks) ... and my 5V strips need 7.2A. So I would have to have power at each end, and also break them in the middle for a 3rd power feed, so decided to get some 5V power supplies, and have them up on the roof, being a simpler solution, and not increasing my exposure to water !
     
  8. i13

    i13 Senior Elf

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    Don't do this until someone confirms that I am right.


    I interpret what AAH and Fasteddy are saying to mean, for example, the correct way to power a 5V strip with a power supply at each end would be to cut the positive line half way along the strip (so each supply powers half of the strip) and then connect the supplies as normal with fuses on the positives. The negatives will be connected through the strip and you won't need a wire running straight from one power supply to the other.
     
  9. plasmadrive

    plasmadrive Full Time Elf

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    I used many of the converters last year on the Plasma Icicles but I bought the ones that were already big enough and waterproof. They weren't that expensive and they worked really great once I sorted out their manufacturing issues. They were low cost, potted, short circuit protected and highly regulated.. I think I paid about $5-6 each if I remember right.. I ran 20 or so of them. At that price it wasn't worth me worrying about making power supplies weather proof.

    I used ones from this company.. they now have these 8 amp 5v units.. http://www.prodctodc.com/dc-buck-converter-12v24v-to-5v-8a-step-down-voltage-converter-waterproof-module-p-134.html#.U9l5gWPzKos
     
  10. plasmadrive

    plasmadrive Full Time Elf

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    I believe that is what they are saying.. not sure it has to be don't though.. needs to be tried the other way by one of us that has the equipment and know how to do it and record it.
     
  11. multicast

    multicast Senior Elf Generous Elf

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    For *most* switching power supplies, even a very small voltage difference between two different Power Supplies is problematic. Because The way the regulation works, if the voltage is "over" the PSU's set point it will stop putting energy into its output circuits. This is how they regulate.. the first thing will happen is that the psu with the highest voltage output will end up taking all of the load. ( the last thing that will happen will be the magic smoke will come out of the box, and then everything will stop. )


    If you really do want to run PSU's in parallel you can, but you need PSU's that are designed to do this. and generally this is only the bigger ones ( 1000+ W )..


    If you are using one PSU to power multiple segments of the same strip, then there is no need to break the + between the sections, but there are some reasons why you might want to as well. ( think what happens if a feeder wire breaks, current will flow on another path, and you might just end up melting copper ).


    If you are using more than one PSU that is not intended for parallel operation, you *must* keep the sections separate.


    If you are unsure, draw a picture, post it here, and someone will help you out!!
     
  12. OP
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    davrus

    davrus Silent Elf

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    Thanks everyone.


    To summarise:
    1. you do not need to have a fuse on the common (-ve) side.
    2. Don't connect the common ground (-ve) across the power supplies, as it will happen at the injection point.
    3. Never cross connect the positive side.
     
  13. fasteddy

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

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    And here is a picture that confirms this

    [attachimg=1]
     

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  14. plasmadrive

    plasmadrive Full Time Elf

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    I don't know if I agree about that "must" part... but I have about 4 different types of supplies that are not intended for parallel operation. I will try to test this out today. Theoretically I see your point and with the LEDs dark it could be an issue since there would be no real Vdrop... With a load I am not so sure.. however, most of the LEDs spend a great deal of time dark.. As I said before, that would be the most probable issue..

    I have always used paralleling supplies for high current, but when that point was brought up to me it seems like it might have some legs.. Of course with the cheap supplies from China that most are using you are most likely right on the money..

    The old balancing resistor act was mostly for the older linear supplies.... Still I am going to see what I can test out. I have some Meanwell, Cosel and Lambda supplies to play with.. I also still owe a test on the DC converter efficiency.. where does all the time go?
     
  15. David_AVD

    David_AVD Bite my shiny metal ass!

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    Running power supplies in parallel (that aren't designed for parallel operation) is just asking for trouble. Just keep the positive wires separate and have one less thing to cause a problem in your layout.
     

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