Paralleling PSU

Discussion in 'General Electronics Chat' started by MWG, Nov 30, 2009.

  1. MWG

    Thread Starter Member

    Apr 15, 2009
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    I had a quick search around on here and i don't think anyone has asked this question before

    I have a couple of ATX PC power supply's which are SMPS

    No1 can supply 95A @12V and 30A @5V
    No2 can supply 20A @12V and 30A @5V

    Basically i need a supply capable of atleast 110A @12V and 40A @5V

    Can i simply parallel the supplies together to achieve this?
    Also with all that capacity for current what would happen if the supply were to short to ground? would it turn off or do something spectacular?

    Thanks
     
  2. SgtWookie

    Expert

    Jul 17, 2007
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    If their output voltages weren't perfectly matched, they would "fight" each other over what the voltage was supposed to be. No.1 would win on the 12v issue, but on 5v it would literally be a dead heat. :eek:

    You might use some resistance in the output to limit the current flow, but that would sort of defeat the purpose of the supplies to begin with.

    Supplies have foldback current limiting; if it exceeds their ratings, it will shut down.

    Of course if you're trying to do something silly like PWM an electrolysis cell, they'll trip immediately.
     
  3. MWG

    Thread Starter Member

    Apr 15, 2009
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    I was worried there would be a problem with them trying to fight each other, i was thinking of diodes to control the direction but i'd need a silly amount so its not worth it

    The supply will be split into 9 feeds of 5V and 12V, the thing i was concerned with was one of the cables shorting to ground and trying to carry all of the current available. Each feed would be via 2mm^2 cable so would only be good for 15A maybe before it started to get overloaded, i wanted to know if the supply would shut off before anything bad happened?

    I wanted to keep all of the feeds common but i'm now thinking of splitting it so 6 are on the larger of the two power supplies and the remaining 3 are fed from the smaller one
     
  4. studiot

    AAC Fanatic!

    Nov 9, 2007
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    I think you will find the official advice was that you can't parallel them.

    If you are actually supplying different pieces of kit / modules you can use one supply for each module. If this is possible it might be a way forward.
     
  5. Wendy

    Moderator

    Mar 24, 2008
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    Diodes and resistors will isolate one from the other, but one of them will be taking the bulk of the load, and if they are very far apart in voltage (computer PSUs are not that exact) only one will take the load, die, then the other will fry. You might be able to compensate with the resistors I mentioned, they will drop some of the voltage, but they will need to be high wattage low ohmage (which suggests a cluster of parallel resistors).

    The idea with the resistors is they drop enough voltage at 20A that the output of the resistor is the same as the other power supply. This will require tweaking. You could get a 0.01Ω resistor (at 20A it will drop 0.2V and need to be 8W), put it on the lower of the two voltages (20A load required), the put another similar resistor on the higher voltage power supply (again loaded with 20A). This doesn't have to be at the same time. Then start putting 1Ω resistors on the low voltage power supply until the output of the two resistors match. If there is more than 0.2V difference between the power supply a larger resistor with more wattage can be used.

    You can make high wattage resistors by paralleling smaller units (but don't group them as a bundle, they need to breath air). For example, qty 100 1Ω ¼W resistors will make a 40W 0.01Ω resistor, or you can use nichrome wire.

    The wattages I'm talking about will melt solder, so take that into account. If you don't have much experience you may need to rethink this. You are better off with supply rated for the job.

    Gee, I think everyone else said that in much shorted sentences. :D
     
    Last edited: Nov 30, 2009
  6. studiot

    AAC Fanatic!

    Nov 9, 2007
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    I wouldn't recommend trying to connect by way of dropping resistors at these current levels.

    The problem is simply one of the differing internal resistances of the different power supplies. A dropping resistance will only be correct at one supply current level and wrong at all others, so unless you have a constant load don't try it - even a small variation could have disastrous effects at these current levels.

    One way to combine to power supplies is to feed both outputs into some form of energy storage device take output from there.
    Another way is to combine the outputs in series and use the higher voltage supply to drive a voltage convertor to supply the required voltage at higher current.

    Either way requires building a third power supply, which rather defeats the object - you might as well just drive it directly from the mains.
     
  7. MWG

    Thread Starter Member

    Apr 15, 2009
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    OK i'll scrap the paralleling of them then. As i said the supply will have 9 feeds from it so they can be split between the 2 quite easily

    Is it unreasonable to say that modifying the circuit in the larger supply to allow for more current is not feasible either?
     
  8. blueroomelectronics

    AAC Fanatic!

    Jul 22, 2007
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    What needs 110A @ 12V?
     
  9. MWG

    Thread Starter Member

    Apr 15, 2009
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    Hard Drives, i will need to supply 45 of them and their spin up current is ~2A @ 12V each, after spin up this drops away to a value i am unsure of

    Also cooling fans which use 2A @ 12V of which there will be 6 maybe 8, i'd like to have some overhead to make sure nothing burns out as i don't know what the fan spin up current is.

    During use the drives require around 0.8A @ 5V each, there is also 9 circuit boards which require 0.5-1A @ 5V (i don't have a figure for this so am guessing)

    Neither of the power supplies i have can cover this by themself hence wanting to join them together. I do have the option of trying to split the load between the 2 supplies without having to join them though as all the wiring will be custom so i can prety much do what i want. Its looking like i need a larger second supply
     
  10. blueroomelectronics

    AAC Fanatic!

    Jul 22, 2007
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    What on Earth needs that many drives? What type of drives are they? How do you even control them?
     
  11. SgtWookie

    Expert

    Jul 17, 2007
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    Seems like just managing the load better would get you there.
    1) Power up the fans.
    2) Turn power on to a block of 15 drives.
    3) Wait for them to spin up; perhaps 15-20 seconds.
    4) Turn on power to the next block of 15 drives.
    5) Wait for them to spin up.
    etc.
    Then turn power on to the boards last.

    You could start with a fairly large block of drives, but you would have to reduce the number of drives in the last block or two when total current load is fairly high.
    [eta]
    Did a spreadsheet; if you power things up as I suggested in sequence, your 95A supply will easily handle the load based on your supplied data:
    Fans x 8, - 2A running each, startup current unknown, assumed to be 5x specified.
    Drives x 45 - 2A starting, 0.8A running each.

    If you wanted faster startup, you could
    1) turn on the fans
    2) wait 10 seconds
    3) turn on 30 drives
    4) wait 20 seconds
    5) turn on the remaining 15 drives
    6) wait 20 seconds
    7) turn on the boards.
     
    Last edited: Dec 1, 2009
  12. studiot

    AAC Fanatic!

    Nov 9, 2007
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    What you want to do is not a problem to split between two or more power supplies. There is no reason that the drives, fans and boards should have common power rails, just a common zero.

    As the the Boss says you will need to work out a management scheme, but it need not be too difficult.

    Just make sure that individual boards can't run if their respective fans fail.
     
  13. MWG

    Thread Starter Member

    Apr 15, 2009
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    I was kind of hoping for it to be able to all come on at once, Once on its meant to stay on until it needs maintenance so it shouldn't be too much hassle to power it on in stages as you'd have physical access to it.

    Oh well, thanks for your input guys
     
  14. SgtWookie

    Expert

    Jul 17, 2007
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    If you want to power up everything at once, you'll need about 150A.

    If you wanted to manage the start-up sequence, your single 95A supply will be more than adequate.

    The start-up sequence can be automated, so in the event of a power failure and subsequent restart, it will work properly.

    Otherwise, you are going to need to buy another larger power supply, as your current No1 and No2 supplies won't be enough.
     
  15. campeck

    Active Member

    Sep 5, 2009
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    I like the idea of flicking on all the switches in sequence. That just sounds cool!
    you can tell all your friends about how your powering up the hyper drive or something.
     
  16. MWG

    Thread Starter Member

    Apr 15, 2009
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    The second power supply was not definitely going to be used, it was just one i have lying around. I think i'll change it to a larger one just to make balancing out load easier

    I still need to split the 5V load as i'll need more than 30A, i'll have to sort it out as i build as i usually do
     
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