Understanding a computer's PSU volts/amps

Discussion in 'The Projects Forum' started by viper_1986_1986, Feb 20, 2010.

  1. viper_1986_1986

    Thread Starter New Member

    Feb 20, 2010
    2
    0
    I am a computer guy, I don't go down to the waves and all that stuff but I want to learn.

    So i got myself a DMM and been ALWAYS making sure it is safe to TRY something. :)

    Well my current "lets do this..." project is to make a old ATX PSU give 24v and more amps than that its rated for.

    I THOUGHT this could be done by taking 2 differnt 12v rails and run them in parallel that the total volts would jump to 24v as well as the max amps go from 4A to 8A.
    But after attemping this the volts is staying the same.

    After research i learned, and checked myself, that all the 12v rails come from the same location in the PSU. But the 4pin is ona 2nd rail. (being a computer person i know this was done in 2002 as a "ONLY FOR MOBO" for extra CPU power)
    But even if using that and another 12v line i still stay at 11.2ish volts and never go up.

    What gives?
     
  2. BMorse

    Senior Member

    Sep 26, 2009
    2,675
    234
    All the power leads coming from the PSU have a common ground, so you will not get double the voltage no matter how many of those 12 volt rails you tie together, but if you tied 1 12 volt rail to another , the voltage will stay the same but the current capabilities would be both rails added together....


    My .02
     
  3. russ_hensel

    Well-Known Member

    Jan 11, 2009
    818
    47
    I have done this sucessfully by putting the supplies in series. Note: Do this with only one of the voltages ( i have used the + 12 ). The supplies do have a common ground, which you must eliminate. In the supplies I have used ( generally older ones ) The common ground comes from a connection between the supply and the metal case which is connected to the third plug. Opening the case ( beware of charged capicators inside ) remove the main board from the case and remount using nylon nuts and bolts and remove any other conducting connection to the case ) Check with an meter that this is successful. Put the sucker back together. Now you can connect any given 2 voltages, one from each supply in series. In principle you should have to do this with only 1 of the two supplies.

    A further question is: is this a good idea. I am not completely sure, but it certainally has to be done with care, inside the psu it is pretty easy to get deadly voltages.
     
  4. viper_1986_1986

    Thread Starter New Member

    Feb 20, 2010
    2
    0
    Ok so the volts would stay the same, and the max amps would go up.
    The unsafe thing to doing this is that the wire is rated for the 9amps, NOT 18, or whatever the TRUE max amp would be.

    By doing this do i need double GROUND wire going back? Or would just the single ground work?
     
  5. BMorse

    Senior Member

    Sep 26, 2009
    2,675
    234
    Here is a way to modify 2 PSU to get 20 to 24 vdc out of them.....

    seriesPS.gif


    Hope this helps.... just be careful when working with mains power....

    Brent Morse
     
  6. russ_hensel

    Well-Known Member

    Jan 11, 2009
    818
    47
    This is the parallel connection. Current may not be shared correctly between the supplies, a small series resistor for each can help.

    Also should note that may supplies will not work without some other connections, perhaps a power on connection and/or some load on the supply, often must be on the +5 connection. There is a lot to google out there.
     
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