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ATX PSU 24v mod

Discussion in 'Power Electronics' started by bigone5500, Aug 23, 2014.

  1. bigone5500

    bigone5500

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    Apr 9, 2014
    Has anyone ever had any luck making a 24v power supply from a computer PSU?
     
  2. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    I haven't done it but I can tell you what you need to do.

    You need to re-wind the 12V secondary of the transformer with twice the number of turns. You can remove the 5V and 3.3V secondaries, so you'll have plenty of room on the bobbin.

    You may need to change the rectifier diode(s) on that rail because their voltage ratings may not be high enough. You'll have to change the smoothing capacitors to ones with higher voltage ratings.

    You'll need to change the voltage feedback circuit so that it monitors the new rail instead of the 5V or 3.3V rail like it did before, and change the voltage threshold from 3.3V or 5V to 24V.

    It would be REALLY helpful if you could find a schematic for the power supply so we can check that there's nothing else funny or special about it that might cause a problem. Having a schematic would also make it easier to specify what changes are required.

    How many amps are you hoping to get from the 24V rail?
     
  3. Gryd3

    Gryd3

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    Jun 25, 2014
    Should be able to use the -12V line and +12V line.
    http://pinouts.ru/Power/atxpower_pinout.shtml

    Keep your current use to less than the -12V line as it is usually far less than the +12V line.

    Would this be a problem Kris?
     
  4. davenn

    davenn Moderator

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    on a 500W ATX PSU the -12V rail is only 1A compared to 22A for the +12V rail

    its a serious mismatch. being restricted to 1A is almost a waste of time
    Tho a lot of work to do Kris's suggested mod, at least you would end up with a 24V supply of ~ 10A capability :)

    Dave
     
  5. bigone5500

    bigone5500

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    Apr 9, 2014
    I can tell that this would be a waste of time. Easier to buy a 24v supply.
     
  6. bigone5500

    bigone5500

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    Apr 9, 2014
    I was hoping that it could be done somewhat easy. I am only looking to get a couple amps @ 24vdc.
     
  7. davenn

    davenn Moderator

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  8. davenn

    davenn Moderator

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  9. bigone5500

    bigone5500

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    Apr 9, 2014
  10. davenn

    davenn Moderator

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    there ya go :)

    tell us how ya get on
     
  11. Rleo6965

    Rleo6965

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    I'm just curious. Was it possible to rewind a ferrite transformer with 1:2 winding ratio. primary will be connected to original ferrite transformer of for 12V winding. This will result 24V on secondary winding . Just add new components filter circuit and rectifier to produce 24V.
     
  12. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    Yes it is possible. I described this in post #2 on this thread.
     
  13. bigone5500

    bigone5500

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    Well, the supply I thought I had was actually from an old HP printer and is rated at 30vdc @2.5A. I don't know if I could use this or not.
     
  14. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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  15. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    If you have a more modern computer PSU with multiple 12V outputs the possibility exists (in theory) to change one or more of these additional 12V outputs to be references to something other than the common ground.

    I say "in theory" because these power supplies are not built in a way that makes doing modifications like this easy (or indeed safe).
     
  16. Pablo Díaz Enrico

    Pablo Díaz Enrico

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    Feb 8, 2015
    Before of all, I hope this can help anyone. Sorry for my English...
    To obtain 24 V for ATX psu you can (as mines):
    1) Add two of them in series.
    2) Mod one (more or less) as follow:
    Remove the extra components for 3.3V, -5v and -12V (optionaly 12v ones).
    Connect the +5v output diodes at the output of the +12v rail from the transformer (the +5v rail will be the new +24v one).
    In my first intent, I taken an ATX psu with an AT2005 pwm supervisor, and, with a pair of circuits taken thanks google, I'd replace the resistors from Vref to get 2.5V from the new output.
    This psu is actually not fully tested yet, but can manage a load of 60W without drop any mV. I connected a 24V lamp and its output remains constant. My first idea was to mantain the modified +12V rail (from the old +5V) to the fan, but this output goes high with the load on the other rail, which is reasonable. In fact, I plan to add a switching regulator (like the LM2586-12) to move the fan and discard all the rails but the +24V modified.
    If anybody wish more details, tell me if I can help.
    Thanks for this great place!
    Regards from Argentina.
     
  17. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    If you connect two power supplies in series you need to ensure that the output ground is floating.
     
  18. Pablo Díaz Enrico

    Pablo Díaz Enrico

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    Feb 8, 2015
    Oh, yes, when you put two or more power supplies in series, they share the input voltage but not the ground-chassis, then, the ground-chassis is only connected to the bottom power supply. The ground of the upper power supply must be completely isolated from chassis (floating).
     
  19. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    If this involves removing the protective earth then you are risking a deadly fault
     
    Gryd3 likes this.
  20. Pablo Díaz Enrico

    Pablo Díaz Enrico

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    Feb 8, 2015
    The idea of two psu in series is not to remove the protective earth. This is connected to the entire case (psu cases screwed one by the another), then, the ground of the bottom psu is to earth and its +12v are connected to the ground of the upper psu, isolated from earth. Psu's voltages are added without loose security. I have a couple of burns in my figers that allways remeber me that electricity can be very, very dangerous.
     
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