ATX Benchtop Power Supply

Need a low-accuracy, high-noise, obscene-current benchtop supply? Build this with junk from your basement!

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I had an old ATX power supply that I swapped in to a much older computer some years ago. The computer (a trusty Pentium 4) isn't much use anymore, so I grabbed the PSU as I was parting out the rest of the chassis.

This has been done many times before, and there are many great guides online (some of which I'll link to in the sidebar).

[update 1/15/2016]

Periodically, I notice in my email that someone has "liked" or "skulled" this project. Awesome, but... sadly, as mentioned in the last log update (more than half a year old, as of this intro), I pretty thoroughly killed this power supply through my own stupidity and haven't actually pulled the thing off the shelf since. I eBay'd a nifty old HP bench supply, and most of my stuff is USB powered, and... well, I just haven't needed it since. Either way, if I were to do it all over again, I'd probably start from scratch instead of doing a conversion.

In any case--if you're looking for a DIY power supply project, I suggest checking out @Stefan Lochbrunner's excellent list:

I'm pulling together pictures I took of the build process; the circuitry is super duper simple, so most of the tips I can offer are centered around hardware concerns (it's a bit of a tight fit). Hopefully you can learn from my careless mistakes and build an even better cheap power supply.

I'll probably mention this a few times, but safety is a concern with this project; switchmode power supplies operate directly on 120vac mains, and it's not always cut-and-dry where the circuit board is potentially dangerous. Worse, as compared to standard transformer-type supplies that operate on 60Hz AC, these units tend to have large capacitors on the high voltage side that can be lethal for days or weeks after the power supply is unplugged. My supply has an insulating sheet under the board that I didn't remove; in fact, I never took the main board out at all, as all of the modifications were to the metal chassis or the low voltage wiring side. My supply had also been unpowered for over a year, so there likely wasn't much left in the caps. Even so, I treated the board as if it were a live high-voltage circuit.

In other words: don't mess around with this stuff unless you know what you're doing.

  • 1 × ATX power supply Mine is rated for 575 watts max, but that's probably overkill for most bench use. Pretty much any working power supply will be fine.
  • 1 × Power resistors I didn't have the right values so rather than order anything, I just grabbed a bunch that I could find and strung them together to get the right values. Not the prettiest approach bot it worked.
  • 1 × LED with housing and appropriate resistor for 5vdc In my case, it's a green T 1-3/4 in a metal panel mount case paired with a 330 ohm 1/4 watt resistor.
  • 6 × Binding posts I used black for ground and red for everything else, but feel free to defy convention. Choose the quantity based on how many outputs you want; in my case, I did 'em all (+3.3vdc, +5vdc, +12vdc, -5vdc, and -12vdc).
  • 1 × Cool on-off switch I spent a long time scrounging through switch bins at the local surplus store before I found mine. Take your time and find the right one.

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  • 1
    Step 1

     Locate an appropriate power supply and check out its spec label. I lucked out; mine has all the info I need:

    The five voltages listed are the standard outputs for an ATX supply; in my case, I decided to pull all five out. The most important thing to note here is minimum current rating; switchmode supplies will only put out a stable voltage if you give them a minimum load. In this case, I need to hang a 1-amp load off the +5vdc and +12vdc lines, and a 0.3-amp load off the +3.3vdc line.

  • 2
    Step 2

    Time to apply Ohm's Law to determine proper resistor values. Pretty simple: V=IR, and since the resistor is connected between ground and each supply rail, V = supply voltage. They give us current, so we can calculate the three resistor values:

    3.3V rail: 11 ohms

    5V rail: 5 ohms

    12V rail: 12 ohms

    Simple enough. But remember, resistors work by converting electrical energy into heat; as such, they all have wattage limits. P=IV, so:

    3.3V rail: 1W

    5V rail: 5W

    12V rail: 12W

    Too much power through a resistor = overheating = bad.

  • 3
    Step 3

    Calculations in hand, head to your local surplus store. In the midst of getting lost in a sea of ammo cases, CB radios, unidentifiable motors, and assorted pieces of self-adhesive foam, collect some parts:

    You'll note that they didn't have any large wattage resistors, so I picked up a handful (at 10 cents apiece with a baker's dozen discount) of various sizes to build up the values and wattages I need. Also note the awesome organ-style pull switch and the handy red insulating bushings for the binding posts.

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Stefan Lochbrunner wrote 01/15/2016 at 22:23 point

Thanks for the shoutout! I'm glad you approve :)

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Dominic Robillard wrote 05/18/2015 at 15:44 point

Nice project. I built one too some time ago, but my next upgrade would be to add this: within the enclosure, and add a new post labeled "variable voltage". Its only 2A, but it would be a neat way to also have random voltages, which I find very useful for testing analog circuits, etc...

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zakqwy wrote 05/18/2015 at 15:49 point

Great idea, thanks! I haven't revisited this one in some time (well... since killing the damn thing last spring and putting it on the back of my shelf). I think v02 will use a modular adapter so I can easily swap the PSU itself when I inevitably destroy it (again). 

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davedarko wrote 01/20/2015 at 18:01 point

This is something I should have done so much earlier, I have a power supply that I want to convert, but there is no minimal load given. Let's see if their support has any thoughts, but with my recent luck with support and service I doubt that. Anyway, great instructions!

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zakqwy wrote 01/20/2015 at 18:07 point

Definitely a cheap and easy way to make a high-amperage 3.3, 5, and 12vdc supply. I still think it's worth building a breakout board with an ATX supply connector; my method was compact but a bit sketchy, to be honest. And still broken.. haha.

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davedarko wrote 01/20/2015 at 18:20 point

Well, I often use the arduinos onboard power supply and was in luck that I never needed much more, but combined with a variable regulator I can get 3.3V, 5V, 12V and anything between 0V and 12V - an obvious good choice. A breakout board would be cool indeed, but the price with boards and connectors etc. will be much higher then. I will cut off the connectors but leave the box intact, since it will be in a "rack" and everything will be drilled/routed through a front panel.

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