Multi-volt DC power supply ATX - Shared grounding current question

Discussion in 'General Electronics Chat' started by RogueRose, Apr 21, 2016.

  1. RogueRose

    Thread Starter Member

    Oct 10, 2014
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    4
    My PSU is rated at 650w and has 22 18g ground wires. I'm setting up connection points for the seperate voltages & ground and am wondering how many wired I need for the ground. I'm curious how current works when there are separate voltages sharing a ground.

    If I have 20A running on 12v and 15A on 5v and 10A on 3.3v, is the current capacity needed for the ground = 35A (sum total of amps of each V)? Since there were so many I thought it might have been something like amps = wattage/lowest voltage so in this case (240 + 75 + 33)/3.3

    I've also found WIDELY varying #'s for max amp carrying load from 1.3A to 16A per ft of 18g and some don't mention strands. Wiki has it at 14A. http://en.wikipedia.org/wiki/American_wire_gauge#Tables_of_AWG_wire_sizes
     
  2. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    The number of wires you need to use is not based on the capacity of the wires themselves, but what voltage drop you can accept.

    One computer chassis I was just working on looses 0.2 volts from the PSU to the backplane over a short run of four #8 wires... But that has a 100 amp load on it. (don't do any computations with those numbers, they are all from memory.)

    Obviously you want zero drop, but you can't get that short of using superconducting wires. The ATX supply regulates the voltage inside of itself, making you live with whatever drop occurs in the wiring. More wires means less of a drop.

    You can either start by adding one wire at a time until the load side sees the correct voltage, or bite the bullet early and use them all right from the start.
     
  3. RogueRose

    Thread Starter Member

    Oct 10, 2014
    189
    4

    Thank you for clearing that up! So a short 6" run shouldn't matter to much and doesn't need nearly so many wires.
     
  4. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    No it matters so much and you may need every single wire in the cable and then some.
     
  5. MaxHeadRoom

    Expert

    Jul 18, 2013
    10,563
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    Keep in mind when using PC P.S. that the common is also generally connected to Earth ground.
    Max.
     
  6. RogueRose

    Thread Starter Member

    Oct 10, 2014
    189
    4
    ??? I've seen on many "convert ATX to bench power supply" people cutting a number of cables off. Is the point that the more that is used, the less the voltage drop over the wires. If the max wattage is 650 and distance is 6 inches, then what gauge would be appropriate for that.
    My supply had 10 sets of SATA & 4 pin connectors, so a total of 20 ground wires + whatever was attached to the MB connector. When all stripped it seems about as thick as 2-4g wire.

    Also, if anyone is reading this, there are 4 screw holes on the corners of the PCB and on the bottom there are solder colored specs around the hole. IDK if thos e are needed for grounding or not. I want to screw the PCB to a wood board but don't know if it needs to have the holes connected to a grounding source (I could solder wire to all 4 corners and connect to ground)
     
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