Power supply voltage bump

Discussion in 'General Electronics Chat' started by SomeDude15, Apr 5, 2015.

  1. SomeDude15

    Thread Starter New Member

    Apr 5, 2015
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    I have a HP server power supply that has a DC output of 12 volts, 75 amps at 900 watts off of a 110 AC input. How can this be modfied to increase the output voltage to 13.8? Attached are 2 pictures. 1 picture is of the contacts on the power supply that plug into a server. The second picture is off the same model power supply that has be modified to give the 13.8 volts output Im looking for. Can anyone help me identify what was done in picture 2 that may be what's increasing the voltage?

    1 7.JPG 2 57.JPG

    The resistor in picture 2 on the left side contacts is used to complete a connection to turn on the power supply with it being not plugged into a server. The 2 wide contacts are the negative and possitive contacts. In picture 2 there is something between the negative and possitive contacts and I need help identifying what it is and what it does so I can duplicate it on more power supplies.

    Thanks in advance for help.
     
  2. LDC3

    Active Member

    Apr 27, 2013
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    It would help if you could gives us the numbers on the device and maybe a clearer picture to show what it is attached to.
     
  3. SomeDude15

    Thread Starter New Member

    Apr 5, 2015
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    This is the label on it. _57.JPG

    These are being converted from used servers to be used to power HAM radios
     
  4. SomeDude15

    Thread Starter New Member

    Apr 5, 2015
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    I do not have any other pictures that are clearer.
     
  5. ScottWang

    Moderator

    Aug 23, 2012
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    I saw some cases used the switching power to modified it became a adjustable power, I think you need to open the case and check the main IC number, and find the datasheet, maybe could find out the method.
     
  6. LDC3

    Active Member

    Apr 27, 2013
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    Ooops :oops:, I meant the numbers on the little black square that spans the positive and negative connectors.
     
  7. OBW0549

    Well-Known Member

    Mar 2, 2015
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    I don't see anything in picture 2 that would be increasing the output voltage; all I see is the addition of a pair of screw terminals for output connection, and what I'm guessing is some sort of overvoltage protective device like a TVS.

    There's probably a trimpot somewhere inside the enclosed part of the device that adjusts the output voltage, and my guess would be that was what was done to unit #2 to raise the voltage.
     
  8. SomeDude15

    Thread Starter New Member

    Apr 5, 2015
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    I found the trimpot but it will only adjust to 12.7
     
  9. SomeDude15

    Thread Starter New Member

    Apr 5, 2015
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    I wish I had another picture. I was hoping somebody could make out what it is and what it's doing.
     
  10. ScottWang

    Moderator

    Aug 23, 2012
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    You need to find out the datasheet of IC and modify the resistor, the manufacturer will set up and limited the range of adjustable voltage to be adjust.
     
  11. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    12.7 V is 5% plus some margin, and almost certainly is the intended max. adjustment range of the design. A straight-on view of the end of the supply might show more clues inside. Also, most high-current supplies have remote-sense inputs (probably on some of the narrow fingers). You can manipulate the output of the supply by injecting a small current into the remote sense. If you're very lucky, the main control loop is designed such that this extra current can be derived from the main output voltage so that the supply regulates at the new. higher output voltage.

    If these are used and out of warranty, rip off the lid and lets see inside.

    ak
     
  12. SomeDude15

    Thread Starter New Member

    Apr 5, 2015
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    The female bullets are where I connect my devices to be powered which I added.

    IMG_0815.jpg IMG_0816.jpg IMG_0818.JPG
     
  13. SomeDude15

    Thread Starter New Member

    Apr 5, 2015
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  14. wayneh

    Expert

    Sep 9, 2010
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    Or, just run the radios off the 12V it supplies.
     
  15. SomeDude15

    Thread Starter New Member

    Apr 5, 2015
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    Where's the challenge and fun in that?
     
  16. Reloadron

    Active Member

    Jan 15, 2015
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    HP PSU.png Looks like they added a resistor on the connector (output) which I would guess is a sense pin. Look at the connector in the second image. Look at the left side and there looks to be a small resistor in there. My guess is like many supplies there are sense leads used and that resistor figures into things.

    Ron
     
  17. SomeDude15

    Thread Starter New Member

    Apr 5, 2015
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    What that thing is and does is what I was trying to figure out
     
  18. Reloadron

    Active Member

    Jan 15, 2015
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    It looks to be a resistor used to "trick" the sense lead for the - supply side. My guess is a low value resistor. What are the colors on it? The band colors? This is a typical way to "trick" the supply into sensing a lower output voltage than there is and outputting a higher voltage.

    Ron
     
  19. SomeDude15

    Thread Starter New Member

    Apr 5, 2015
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    That's what it is doing.
     
  20. Reloadron

    Active Member

    Jan 15, 2015
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    Just add a like resistor to the second supply you want to modify. That should do it. If you are unsure of the resistor value just lift one leg of the resistor on the modified supply and measure it.

    Ron
     
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