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Electronics 24V of a power suply, by the idea of a friend it goes BOOM!

Discussion in 'Modding' started by DXR_13KE, 10 Jun 2006.

  1. DXR_13KE

    DXR_13KE BananaModder

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    i have a friend, i told him i could get 24 volt of a standard power suply, i did not end my frase and he exclamated the folowing: "that easy, you just plug your power suply in series" i sayd WTF how? and he says like this: "plug it like this, conect the 12volt conection of one molex and conect it to another molex ground then use the first molex ground as the ground and the second 12V line as the 24V line". inside my brain thunder sparkled as i designed it and simulated it.

    as i see it i would be short circuting the power suply sending the 12V line to hell or even making the power suply blow up.

    i did not laugh at his face at the moment as i pondered....what if.....he did it and it worked. what do you think?

    as for the 24V of your power suply remember that -12 volt line in that fat mobo connector? just plug the 12V to the -12V and you get 24V just dont do this without something to limit the current like a resistor or a fan or something like that.
    find it here
     
  2. hitman012

    hitman012 Active Member

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    His logic is flawed. This does work with batteries, chaining the ground of one into the 12V output of another and creating a voltage of 24V across the pair. However, with a power supply, the grounds are common - you cannot float the 0V rail of a particular connector without shorting the entire unit.

    Furthermore, the power supply functions in a completely different way; even if he could somehow float ground at 0V, one of two things would happen, depending on how it worked. Either the power supply would continue generating voltages as normal, meaning that the potential between the 12V rail and ground would be actually 0 volts, or it would attempt to generate everything relative to his elevated ground - i.e. adding 12 volts to every input voltage. Does he like the smell of burning electronics? :eyebrow:

    The -12V method, while feasible, seems a bit dodgy to me; generally, the power supply doesn't like to sink much current into rails other than ground. It's fine for a few light loads (like a 7Ved fan), but things running at 24 volts might push the envelope a bit.

    In other words - let him blow it, just make sure you get it on video :thumb:
     
    Last edited: 10 Jun 2006
  3. cpemma

    cpemma Ecky thump

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    You can get 24V (or -12V, 0, +12V) with loads of amps but you need
    1. Two power supplies
    2. Circuit board, heatsinks and ground wires of each isolated from the case and mains safety earth (but leaving the cases connected to mains safety earth - very important!)
    Then you can connect 12V on one to 0V on the other, and if you've done everything right it won't blow up. If you've made a mistake, :duh:

    People are doing it to run power amplifiers with +/-12V rails.
     
  4. DXR_13KE

    DXR_13KE BananaModder

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    hitman012 i want to film him blowing up a power suply :D if i do i will post it here.

    yes cpemma, that would work, but my friend says its possible in one power suply.

    heatsinks? what for? were would you put them?

    we at my uni use a power suply that has 2 variable outputs and with we put the + of one into the - of the other and set both to 12V we efectively create +/-12V and 0V or a 24V power supply.

    interestingly i was to put this in the electronics forum but i thought it might not belong here. thanks for moving it.
     
  5. Smilodon

    Smilodon The Antagonist

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    on two powersupplies WITHOUT the ground wire connected (on the 230V side) and the chassis not touching each other it would work. Taking two lines of the same PSU wond work (feel free to laugh of him). They are simply connected to the same output inside the PSU in most cases.

    besides, on an ATX PSU all the ground wires are connected together ang grounded in the chassis.

    The only way of getting 24V from a ATX PSU is to use the -12v line as "ground". this wont work unless the PSU is under load on the 12v (and 5V?) line.
     
  6. whypick1

    whypick1 The über-Pick

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    DXR, you friend kind of sounds like he has no clue what the hell he's talking about. Yes, you can pull out 24v from a standard ATX PSU doing what Smil said, but the -12v line usually can't supply all that much current. What cpemma's talking about would allow for a high-current 24v supply, but it's not terribly practical.

    What he was talking about regarding step 2 is that all of those parts will have a ground connection on them (voltage regulators and other transistor looking ICs commonly have the tab that the heatsink attaches to tied to one of the pins, usually the ground one). You'd have to remove the ground connection of all of those so that when you connect the positive voltage from one supply to the "ground" of the other, you don't violate Kirchoff's Voltage Law (I've found professors don't like it when you try to violate the laws of physics to solve homework problems).
     
  7. cpemma

    cpemma Ecky thump

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    Yes, but that's dangerous... a fault and the case could end up live. :eeek:

    Actually you only need to mod one PSU, the one that will have the floating output ground.
     
    Last edited: 11 Jun 2006
  8. DXR_13KE

    DXR_13KE BananaModder

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    please define "floating output ground" on a standard PSU.
     
  9. Smilodon

    Smilodon The Antagonist

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    True, but safe living is boring, right?

    (you have no idea on how many un-grounded computers there is out there.)
     
  10. gideon

    gideon New Member

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    messing with powersupplies and than unground them.... seems to be a bit suicidal to me :worried:
     
  11. g0th

    g0th New Member

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    It's exactly as cpemma said - we want to leave the mains earth connected to the case of the PSU so that we're not compromising the electrical safety, but disconnect this from the 0V rail.

    Basically, all you need to do is Dremel away the copper pads on the PCB where the PCB mounting holes are, this is where the 0V rail is connected to the mains earth.
     
  12. Starman97

    Starman97 New Member

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    Rather than spending the money for a second PC power supply and then doing some questionable wiring to get 24V, why not just get a 12v->24V DCDC converter module and be done with it?

    If you use the +12v -12V trick to get 24V, remember, you are limited by the current rating on the -12V supply, (about 500mA typical) you also have to be careful if whatever you are powering at 24V has a metal case since this will be connected to -12V, if it touches the PC case, you will short the -12V supply to GND.

    Connecting a 2nd supply's GND lead to the 1st supply's +12V lead means that 2nd supply has to be electrically isolated when you mount it. The GND of a PC power supply is electrically connected to the power supply case and to the 3rd GND lead of the mains power cord. You would have to cut the mains ground wire, a bad idea, and mount the supply in the system case with nylon standoffs. The entire second supply case would be +12V hot from the first supply.
    Anything conductive that touches the 2nd supply would see +12V at 10-30 Amps from your first supply. Your best bet would be to encase the second supply in a non-conductive box and mount the box to your PC case.

    The other issue is that most PC supplies expect some sort of load on the primary outputs, since your second supply is now offset by +12V from case Ground, you cant use any of the other outputs and would have to put some power resistors on it's outputs to properly load the supply. Those need to be isolated as well.

    So, again, just get a DCDC power module or a seperate 24V supply, it's much easier.
     
  13. DXR_13KE

    DXR_13KE BananaModder

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    i know its much easier to do that but the point is that a power suply goes kaboom if you do what my friend says. :D
     
  14. Smilodon

    Smilodon The Antagonist

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    the easiest and most effective will be to get a 24V poweradapter.

    converting 12v to 24v at high currents isn't THAT easy or cheap
     
  15. Starman97

    Starman97 New Member

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  16. ConKbot of Doom

    ConKbot of Doom New Member

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    Gotta watch disconnecting the ground on a PSU, the leakage current in the SMPS can be a real shocker :naughty:
     
  17. PowerPackedPC

    PowerPackedPC New Member

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    seems interestingly dangerous to me, but id rather keep my PSU in one piece
     

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