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Need help modifying a PC PSU

Discussion in 'Power Electronics' started by marshach, Apr 5, 2016.

  1. marshach

    marshach

    2
    0
    Apr 5, 2016
    Hello. I would like to try modifying a PC PSU to work in my PC (HP xw4600), but I have basically no experience with electronics. The only reason I am thinking of trying this is that my PC requires a proprietary PSU. To my knowledge, there is only one compatible model, and it is only 475W. Also, the only ones available are used/"refurbished". I want a PSU with more than 475W, and I would rather not buy a used one. Even if a used one is in good working order, who knows how much more life it has remaining? So, I've compared the pinout of a standard ATX/BTX PSU with the proprietary PSU my PC uses. Diagrams of the standard pinout are readily available online. I obtained the pinout for my PC's PSU from the "HP xw4600 Workstation Service and Technical Reference Guide". The image below shows the pins side-by-side, with the standard on the left. The obvious differences are on pins #12, #20 and #23. On pins #10 and #12, the standard is "+12 VDC". On the proprietary PSU, it's "+12 V-B". I searched for that on the Internet, but did not find an explanation of the "V-B". Below the image showing the pin configurations is some additional information from the service and technical reference guide for my PC that seems relevant.



    [​IMG]
    [​IMG]
    [​IMG]
    My questions are:
    1. To modify the pinout on a standard ATX/BTX PSU, would I just need to remove pins #12 and #23, and get a ground into pin #20? If so, to get a ground into pin #20, can I create a new ground pin by soldering a piece of wire to one of the other ground wires? Is it okay to use one ground wire for two pins?
    2. What does the "V-B" in +12 V-B mean?
    3. In the "Maximum Current Per Rail" chart, it shows different maximum currents for different +12 V rails. Does that mean that the number of amps on each rail is proprietary as well as the pinout? Might a standard ATX/BTX PSU deliver more continuous current than is specified in the table?
    4. In the "Maximum Current Per Rail" chart, it shows +12 VCPU and +12 V-D. I assume the VCPU is for the CPU, but what does "V-D" stand for?
    5. Regarding the information in the "warning", is that to avoid damaging the motherboard or the PSU, or both? If I were to use a standard ATX/BTX PSU, it is possible it would deliver enough watts to damage the motherboard, or will the motherboard and other components only pull what they need?
    Like I said, I have no experience with this stuff, so excuse me if my questions seem ignorant. Any help would be appreciated. Thanks!
     
    Last edited: Apr 5, 2016
  2. Gryd3

    Gryd3

    4,098
    875
    Jun 25, 2014
    Playing with fire. V-B could simply be a secondary 12V bus, or it could be managed differently if the PSU is on/off/standby .
    If I were you, I would only try this if you can stomach the thought of burning your main-board...

    I've done something similar with an older HP slim desktop that used a proprietary plug, but it was a spare computer that was currently collecting dust. To me, it was worth the risk.
     
  3. marshach

    marshach

    2
    0
    Apr 5, 2016
    Thanks for the advice
    I really want to try it, but I think you're right. It's my only PC, and in addition to destroying it, I could literally start a fire. I would like to understand this, though: What determine/controls the number of amps on a rail?
     
  4. Gryd3

    Gryd3

    4,098
    875
    Jun 25, 2014
    Typical construction of a computer power supply is a switch-mode power supply that regulates multiple voltages as you have seen. The capacity of the 'rails' is determined by the regulators in the PSU. A more capable design can sustain and provide higher currents for a particular voltage rail. Sometimes the power needs to be distributed either due to design or budget constraints. In this case, the 5V or 12V rail is commonly split into two or more... so there are now multiple regulators (rails) responsible for providing 12V or 5V. This is simply to help distribute the load. When done, the rails are normally split so that one rail goes to the Main-board, and the other rail goes to other components.

    You can replace a multi-rail supply with a single rail supply. You should also be able to replace a single-rail with a multi-rail. However... the pinouts here are important. In a proprietary setup, many of the standards can go out the window... You may find that the additional rails are not simply for more power or distributing a load, but are a design feature to allow the computer to be powered 'off' but still offer full power to USB and/or other peripherals. Replacing a multi-rail PSU with a single-rail PSU in this case could prevent certain features from working or cause instability. On the flip side... replacing a multi-rail PSU with another multi-rail PSU with a different pinout / rail could cause the seperate rails for a specific voltage to be inadvertently tied together, which of course would be a bad thing xD"
    If we knew exactly what the other rail was... we could recommend a solution, but for the time being I would suggest against it. As far as a fire is concerned, it's plausible. So if this was done and worked... I'd certainly keep an eye on it..
    Luckily for my modification, the pinout was standard... it only had a proprietary connector on it.
     
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