Converting an ATX power supply to a benchtop power supply

Discussion in 'General Electronics Chat' started by thakid87, Jun 4, 2010.

  1. thakid87

    Thread Starter Active Member

    May 23, 2009
    Hello all.

    I am sure you see many questions related to these, but I have a few I'd like to ask before I dive into this project.

    I have a 400W ATX power supply that I would like to convert to a benchtop power supply. The ATX supplies the following voltages:

    5V @ 28A (Red)
    12V1 @ 14A (Yellow)
    3.3V @ 30A (Orange)
    12V2 @ 15A (Yellow/Black)
    -12V @ 0.3A (Blue)
    +5VSB @ 2.5A (Purple)
    PG (Gray)

    I only want to use one of the 5V supplies, one of the 12V supplies, the 3.3V, the -12V and if possible add a variable voltage supply with a potentiometer.

    Does it matter which of the 5V and 12V supplies I use?

    I am going to place fuses in line with each supply, are 1 amp fuses suitable?

    Would anybody know what PG and +5VSB are?

    Thanks for all the help.
  2. beenthere

    Retired Moderator

    Apr 20, 2004
  3. SgtWookie


    Jul 17, 2007
    The WikiHow article isn't bad, but it mentions the use of electrical tape.

    I don't use "electrical tape" for anything that I expect to last for more than a week. The adhesive on "electrical tape" gets gummy, and the tape eventually unravels and falls off.

    Use heat-shrink tubing for insulation, and use nylon zip-ties to keep your wiring organized and neatly tied into bundles/harnesses.
  4. thakid87

    Thread Starter Active Member

    May 23, 2009
    Thanks for the link, beenthere. I hope this applies to all ATX PSU's. It clears up the PG and the 5VSB, but not on the two 12V lines.

    I will keep that in mind, Sgt. Wookie.

    Are the fuse sizes suitable for this?

    Is the idea of a pot smart for variable voltage? I am going to use a three position ON-OFF-ON switch to be used for Fixed-Off-Variable modes.
  5. SgtWookie


    Jul 17, 2007
    I have no information for you on the two separate 12v supplies. I would not be surprised if they were actually the same supply internally.

    You can use 1A fuses if you would like. That will tend to protect your project if you have accidentally shorted something in it.

    Not if it is to supply power. Pots are most frequently used to supply a low level signal voltage of very small amounts of current; typically <5mA.

    ATX form factor computer supplies are only designed to output fixed voltages. Attempting to change/vary their output voltages would be complex. You would need to start by generating a schematic of the supply, which will not be easy; particularly for a n00b.
  6. Bernard

    AAC Fanatic!

    Aug 7, 2008
    A pot is good only for light loads, like a bias supply for a comparator or as a control element for a V regulator. Use fuses with rating less than supply capability and greater than expected load. Article on pg2 was right on- tried it out, all colors matched- turns out that supply is good- so far.
  7. Bychon


    Mar 12, 2010
    Just a note on tape...not only is it unreliable, but there is no legal use of tape in the world of electricians (National Electrical Code). Probably because it's unreliable.
  8. someonesdad

    Senior Member

    Jul 7, 2009
    Except for marking conductors that have non-standard colors. But I agree, I would never use it on any wiring except perhaps low voltage stuff like automobiles. However, it fails and leaks over time, so it's not a good solution there either. When I do use it, the only stuff I'll use is Scotch 33+.
  9. SgtWookie


    Jul 17, 2007
    Yes, using it simply to mark wires is legal, but I don't get why, as it will sooner or later fall off.

    It's handy for attaching a wire or wires to a fish tape.

    A buddy showed me a neat trick a couple years back; used a plastic shopping bag taped to a nylon rope and a wet/dry vacuum to suck the bag/rope combo through a conduit. Sure beats using fish tape.

    I don't use "electrical tape" on autos, either. Made that mistake years ago; wound up with a gummy, unravelling mess.

    I mostly use heat shrink. If I want to wrap a harness or something similar, I'll use a special silicone tape that adheres to itself. It's much more expensive than electrical tape, but if properly applied and the ends secured in place with zip-ties or lacing tape, it won't come off of it's own accord. It is suitable for use on aircraft wiring.

    3M Scotch brand 70 tape is one such tape:
    "Rescue Tape" is a similar type product:

    I've seen kit-built and other homebuilt airplanes that people used "electrical tape" on. It just makes me cringe when I see stuff like that.
  10. coldpenguin

    Active Member

    Apr 18, 2010
    I often get things wrong, so double check yourself first.

    I think, that if you were to use the details in the following link:

    and have R2 being changed to a variable resistor AND a resistor, the value of which, when the variable resistor gave you a minimum voltage you will ever use, it might suffice.

    With the fuses, they are there primarily to reduce the effect of an accident (they cannot stop an accident, but they can reduce the length of time that accident is happening for).
    Rules for choosing should be:
    Your resistor value should be less than the amount that the rail
    Slightly more than you will be using on the project (so as not to blow when the project is working normally). So, without knowing what you are doing, 1A seems reasonable to fit these two rules.
  11. Norfindel

    AAC Fanatic!

    Mar 6, 2008
    As written there, the supplies should be separated, but those current ratings are likely peak, or something more obscure, as if you add all the power of the different lines, it goes to more than 400w. Who knows what the continuous power rating really is.