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Electronics Separating the grounds on seperate 12V rails in a PSU?

Discussion in 'Modding' started by karbonKid, 1 Jul 2007.

  1. karbonKid

    karbonKid Minimodder

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    Hey everyone,

    I'm hopefully going to be running a 24V peltier element in my new system, but I'm not prepared to spend my entire budget on a Meanwell PSU. However, I still want an elegant solution. I was reading a tutorial on another site where they disconnected the grounds from 2 PSUs (ie. made them floating grounds) and then connected the 5V rails in series.

    I'm wondering, would it be possible to do the same thing across seperate rails on a single PSU? For example, if I got hold of one of these:http://www.thermaltake.com/product/Power/PurePower/w0099/w0099.asp, which has 2 12V rails, floated one of the rails and connected it in series with the other rail - I would have 24V, right? Is this possible? This would be perfect, as 250W should be enough to power my 170W pelt.

    And yes, I am aware that this PSU uses a pass-through ATX connector from your mainboard to sync itself with the PC's main PSU, but this can easily be bypassed as I will be using the PSU to power the pelt, and nothing else.

    Thanks for your help.
     
    Last edited: 1 Jul 2007
  2. karbonKid

    karbonKid Minimodder

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    I've been thinking about this a bit more, and I don't think it should be that hard to do. Just to outline the concept more clearly, here is what I plan to do:

    The forums don't allow whitespace, so I'll embed an image of my makeshift diagram
    STOCK PSU
    [​IMG]

    I think the main obstacle will be the PSU's over-voltage protection. How hard would this be to reconfigure this?
    Also, I will need to eliminate the PSU's syncing fuctionality - as I understand, it takes readings from your PC's PSU and adjust itself accordingly in order to syncronise voltages and not damage any components.

    Does this seem like a viable project, or just a waste of time?

    Thanks again
     
  3. ConKbot of Doom

    ConKbot of Doom What's a Dremel?

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    separate the rails? most likely not the easiest thing to do. Modify the rails so that each one can output 24vdc, you probably could.

    pretty much all ATX power supplies run off a TL949 pwm control IC, so search around online, see what you can find. 24V from a computer PSU isnt that unreasonable, Ive heard of people pushing to 30-40v with only replacing a few components.

    Also, how many watts is the peltier you want to use?
     
  4. karbonKid

    karbonKid Minimodder

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    Thanks for the help, I'll look into that. The pelt is 172W dQmax @ 24.6V so i'm guessing it'd consume somewhere in the region of 240W at max volts.

    EDIT: The peltier consumes 11.3A, so input power is 271.2W @ 24V - looks like I'll have to look at something more powerful in terms of a donor PSU. The FSP Booster (300W) is approaching the price of a Meanwell, but I'd rather get this done on the cheap, even if it takes me a while to figure out.
     
  5. cpemma

    cpemma Ecky thump

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    This will be moved to Disaster Mods if you try linking two +12Vand 0V rails in series. All the +12V and all the 0V wires go to the same two points inside the PSU, so connecting between 0V and +12V will blow the supply, and "isolating" a 0V wire will just leave a bit of wire going nowhere.

    For simple diagrams, draw in a fixed-width font (Courier) in Notepad, then C&P to inside [ code ] tags.

    Code:
      5V------+
              |
              +-------- 0V
    
     
  6. Skulldragger

    Skulldragger What's a Dremel?

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    Can't you just use 2 PSUs connected in series? PSU1 +12V out to Pelt connector / GND to PSU2 +12v / PSU2 GND to Pelt connector. In 24v systems used in a UPS they link 2 12v batteries exactly like this, some use 24v circuits to charge them, some use 12v to charge each battery separately.
     
  7. karbonKid

    karbonKid Minimodder

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    I do not intend to simply piggy-back one rail onto another. I want to actually seperate the second rail from the common ground (ie. at a much earlier stage than connector level) and then connect its ground to +12v from the first rail. I realise it's not going to be easy, but it has to be possible. Apparently this is quite a similar technique to that used when biasing an amplifier:confused: Oh, and thanks for the code tag suggestion :)

    Yeah, I could do that, but it would be less than elegant. I want something that fits in a drive bay. The technique you mention, however, is where I got the idea for this mod, except instead of using seperate PSUs, I want to use seperate rails on a single PSU.
     
  8. SteveyG

    SteveyG Electromodder

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    :hehe: :hehe:

    It definitely isn't without a complete redesign. All of the switching circuitry supplied from the same supply and so is referenced to the same ground also (mains earth) and usually the rectified mains is stepped down to a suitably lower voltage before being further regulated to 3.3, 5, 12V etc.

    Modifying the PSU to output 24V is much more doable for sure. You may as well make your own PSU if you're going to try to separate the grounds.
     
  9. cpemma

    cpemma Ecky thump

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    There isn't a "second rail" for any one voltage - just a single HF transformer secondary winding with various taps for +12V, +5V, 0V, -5V, -12V. Thinner wire at the negative voltages end limit their current. Rectified, smoothed, and out with all similar wires from one PCB pad. The feedback regulation system is to get one rail's output (+5V) right and the others all fall into line by turns ratio, AFAIK.

    There's a limit to how much you can tweak the voltage, to get 24V you'll need to rewind the secondary AFAICS.
     
  10. ConKbot of Doom

    ConKbot of Doom What's a Dremel?

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    I was under the impression that in multiple 12v rail PSUs there was a separate winding/rectifier/sensing circutry for each 12v 'rail'.

    EDIT:

    http://4hv.org/e107_plugins/forum/forum_viewtopic.php?19632.0
    may be a good read for you, looks like 24v may be in range of 'hacking' without extensive modification, but depending on the PSU, just barely, and with a careful touch. Dont know how your amperage will look though.
     
    Last edited: 3 Jul 2007
  11. karbonKid

    karbonKid Minimodder

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    Thanks, ConKbot, thats a really useful link. My eyes are tired atm, but I'll have a proper read of it tomorrow.

    And thanks to everyone who'se helping me out here, this is a great community :thumb:
     
  12. cpemma

    cpemma Ecky thump

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    Can't see anything in the ATX12V spec allowing that
    I wish the links worked at your link; the schematics I have show only the +5V line being from each side of a centre-tap ground - that's the line with the highest current capacity so makes sense.

    Whoops, I checked the Thermaltake link, it's not a standard supply. :duh:
     
  13. karbonKid

    karbonKid Minimodder

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    Yeah, it's designed to power graphics cards only, hence only 12V rails. It syncs voltages with your main PSU by passing your mainboard's ATX connector through itself and, I assume, syncing to the 12V line instead of using sense wires like a standard PSU. FSP makes a similar supply which may be more suitable for my needs, as it supplies 300W (and its less messy as it senses from a single molex instad of the 24-pin ATX connector), but again, that 300W is split across 2 rails :duh:
     
  14. Butcher9_9

    Butcher9_9 What's a Dremel?

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    y dont you just run the peltier at 12 volts

    they work more efficiently at lower voltages and there will be less chance on condensation
     
  15. karbonKid

    karbonKid Minimodder

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    I will most probably be undervolting the peltier, but i'd like to have the flexibility to run it at full power when i'm benching, etc. I'm going to be using a 40mm peltier, and with the best peltier of this size I could find, i need 24V to be able to achieve a 172W delta-Q. In normal use, i'm looking to run it somehwere around 18-20V. Condensation isn't a problem, I'm going to insulate everything properly.
     
  16. karbonKid

    karbonKid Minimodder

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    I'm beginnig to think i might aswell just buy a meanwell, as I'm not going to get an ATX PSU with the required amperage for less than that price, and plus the meanwell requires no modding (except to fit it into the case).
     
  17. DrunkenSlob

    DrunkenSlob What's a Dremel?

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    Am I missing something here?! :confused:

    Doesnt an ATX supply have -12v?

    if so the potential difference between +12v - -12v == 24v


    Hope that helps :clap:
     
  18. SteveyG

    SteveyG Electromodder

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    Yes, you're missing something. Look at the output capabilities of the negative supplies and you will see...
     
  19. karbonKid

    karbonKid Minimodder

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    Exactly. The pelt I am looking at requires 11.3A, and most -12V rails provide less than 0.5A, AFAIK.
     
  20. Pegasus

    Pegasus What's a Dremel?

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    Have you considered using one of those car battery chargers? Some of the larger models have a 12V / 24V switch.
     

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