Split 5v External PSU 4 ways

Discussion in 'General Electronics Chat' started by stigzler, Jan 27, 2016.

  1. stigzler

    Thread Starter New Member

    Jan 27, 2016
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    Hi folks. New here + reasonably new to electronics - can solder + hack, but anything finer I need help with!

    Building a games rig out of an old console case. I'm looking to have the PSU outside of the unit to keep temps low.

    The key component is an Odroid XU4 (http://www.hardkernel.com/main/products/prdt_info.php?g_code=G143452239825)
    It's rated at 5V/4A. I've been advised that the voltage/A needs to be super steady:

    "5Volt +/-5% tolerance is required.
    So the PSU must have well regulated output at 4.75V ~ 5.25V range."

    What I'm seeking to do is also power other components in the unit. Thankfully all at the same voltage:

    Powered USB2 Hub - 5V 3A
    Powered USB3 Hub - 5V 1A
    HDMI Audio splitter - 5V 1A

    Thus, I'll need a new PSU (I'm guessing rated at 5V 9A??). My questions are:

    1) How do I split the 5V supply 'cleanly' in the unit - that is to make sure the Odroid is protected. I've been advised against something like this, due to 'voltage drops' or something:
    [​IMG]
    2) Considerations around buying the 5V 9A PSU? Need anything in particular?

    Been re/searching for a couple of days now and still none the wiser. Thanks + hope someone can help!
     
  2. wayneh

    Expert

    Sep 9, 2010
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    I wouldn't size the power supply to just barely meet the ratings. It should have some overhead to avoid overheating and such. So if I KNEW I needed 9A, I'd shop for something rated to 10-12A or more.

    But the problem is that those hubs are not likely running at their rated limits, and so it's hard to know what is actually going on. If you want to be safe and don't know the answer, go for at least 10A.

    I don't see any problem with the octopus strategy if the supply is rated to keep up. If the cables are poorly made with narrow gauge wires, you will see some voltage drop especially at currents over 1A. But if it is done right, it would be fine.
     
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  3. stigzler

    Thread Starter New Member

    Jan 27, 2016
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    Thanks for the reply, Wayne. Good steer on the 10-12A PSU - will look for one higher rated.

    Yes - advised against the "octopus" leads (I know the name of these now!):

    "This kind of splitter cable dont work well with the XU4.
    I have tested one some time ago.
    The wire are very thin.
    There is a big voltage drop under load (over 1V) and it get very hot with one XU4 connected."
    So when you say "if it's done right" - what would that entail?

    Thanks again.
     
  4. wayneh

    Expert

    Sep 9, 2010
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    Supplying 4A over a wire 2 feet long, with a voltage drop of <0.2V requires at least 22 gauge. See here for a handy calculator. Don't forget that your ground return line is part of the total length.

    I'd prefer 16-18 gauge, like standard lamp zip cord or speaker wire.
     
  5. stigzler

    Thread Starter New Member

    Jan 27, 2016
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    Thanks again. So, I'm looking at a PSU like this:

    https://www.adafruit.com/products/658

    IN terms of accommodating a voltage drop - wouldn't most of that occur in lead from the psu to my unit?

    Past that, the length of the wires from the unit barrel(?) plug to the components therein will be minimal (20cm max).

    Or are you saying look for a splitter lead with thick wires? That being 16-18 guage?

    Forgive me, quite a newb to electronics.
     
  6. wayneh

    Expert

    Sep 9, 2010
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    Their will be voltage drops everywhere, in proportion to the product of length and gauge (with higher gauge being thinner wire with higher resistance). If it's long, it had better be fat.
     
  7. stigzler

    Thread Starter New Member

    Jan 27, 2016
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    OK - so got another idea....picoPSUs

    I can amend my design to use:
    1x12Vx2.5A USB3 Hub (i.e. lose the usb2 hub - just use usb2 devices in usb 3 ports)
    1x5Vx1A HDMI splitter
    1x5Vx4A Odroid

    I've see a 120W picoPSU with ratings: 5V @ 6A, 5VSB @ 1.5A, 3.3V @ 6A, 12V @ 7A, -12V @ 0.05A. Example here:

    http://www.fullspeedit.co.uk/picopsu-12 ... -368-p.asp

    So this should work, right? I'd assume being a 'pc psu' the voltages will be regulated and in this instance, the Odroid have its own rail.

    One question though, I'd have to put the Odroid on the 6A 5V line, but would I need to activate the 5VSB line to power on the other devices? If so - how could I do this?
     
  8. stigzler

    Thread Starter New Member

    Jan 27, 2016
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    and the trail goes cold... :(
     
  9. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    Get a pc atx psu. 5V and 12V at 22amps easily, dirt cheap..
    ...
     
  10. stigzler

    Thread Starter New Member

    Jan 27, 2016
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    Thanks Dave, but doesn't really fit my unit, that. Need to psu outside the unit due to limited space - thus my considering the 12V switched power supply/picoPSU route. Anyone know if the PicoPSU route will work?
     
  11. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    If an atx is no good, what voltages do you need at what current?
     
  12. stigzler

    Thread Starter New Member

    Jan 27, 2016
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    thanks - as per post #7
     
  13. RogueRose

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    Oct 10, 2014
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  14. stigzler

    Thread Starter New Member

    Jan 27, 2016
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    Thanks Rogue, but not really what I'm after - not sure it's going to do the job as outlined above. I'm still curious about the picoPSU and whether that would work.
     
  15. wayneh

    Expert

    Sep 9, 2010
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    It sounds decent. They mention derating the 5V supply for fanless operation. Since your loads are 5A out of the 6A spec before derating, that makes me a little nervous. If those load ratings are worst cases and not everyday loads, it's probably fine. Only you can answer whether you will really be running at 5A long enough to potentially overheat the power supply.
     
  16. stigzler

    Thread Starter New Member

    Jan 27, 2016
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  17. Dodgydave

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    Jun 22, 2012
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  18. wayneh

    Expert

    Sep 9, 2010
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    Using a regulator can only increase the overall load on the supply, so I'm not sure it's the best solution. It will distribute some of the 5V load onto the under-used 12V section. Maybe someone that knows power supplies can speculate if this is an improvement compared to just using the 5V section up to near capacity.
     
  19. stigzler

    Thread Starter New Member

    Jan 27, 2016
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    Apologies Dave, but my knowledge of electronics is poor. What is the difference between this and the thing I posted?

    Thanks Wayne. Where do these legendary PSU experts hang out? And do they eat newbies?
     
  20. wayneh

    Expert

    Sep 9, 2010
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    The device Dave linked is cheaper and more versatile, but otherwise the same idea, and it doesn't add (much) to the total load because it is an efficient DC-DC converter.

    There are plenty of helpful folks here that know power supplies in and out, but it may not be possible for them or anyone to know how your particular supply is designed and would behave.

    But anyway your supply is rate to 120W total, which is plenty. Since the efficient converter doesn't add any more total load, it removes any doubt about powering some of your 5V amp demand off of the 12V section via the converter.
     
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