NEW PROTOTYPE: ATX Breakout Board

in Prototypes by DP | 22 comments

Latest designs and full project overview are available on the ATX Breakout Board wiki.

Old ATX computer power supplies are cheap-to-free, and make great lab supplies with lots of power at several common voltages. The ATX breakout board brings all the power pins to screw terminals for easy access. For a modicum of safety, a 1.25 amp resettable polyfuse on each supply help protect your project from the full current of ATX supply.

Available for $14 at Seeed Studio.  Read about the design below.

Overview

A computer power supply ATX connector attaches to the right angle ATX jack. Thick traces route the -12, -5, 3.3, 5, and 12 volt power pins to screw terminals. Each voltage has a 1.25 amp resettable polyfuse and indicator LED attached. An ON/OFF button grounds the ‘green’ wire (pin 14) of the ATX supply to control it, just like a motherboard board would.

Hardware

Click for a full size schematic image. Schematic and PCB were designed with the freeware version of Cadsoft Eagle, download the latest project files from our Google Code project page.

Power supply connectors

The 12, 5, 3.3, -5, and -12 volt power lines are all brought to terminals. There are also two ground terminals. We used thumb screw terminals that also accept quick connecting banana plugs in the top.

ATX supplies rarely have the -5volt rail, and it’s been a completely optional part of the ATX standard for years. We didn’t populate the-5volt terminal or the second ground on the production version board to save on parts and shipping weight.

Power switch

Pin 14, the ‘green’ wire of the ATX jack, is routed to a switch that can turn the ATX supply on and off. When pin 14 is held low, the supply turns on. Many PSU to bench supply hacks jumper this pin to ground with a wire, we think a dedicated switch is a lot safer.

Mains on lights whenever the PSU is plugged into an outlet. When the power switch is on and the supply is stable, Power good lights.

Resettable fuses

Computer power supplies pack a big punch. It’s common to have 30 amps or more on the 5volt rail alone. To protect projects (and people!) from the full force of that current, each voltage has a 1.25 amp resettable polyfuse (F1-F5). If there’s a surge in the circuit the fuse will blow, but it’ll reset quickly after the short is removed.

Indicator LEDs connected to each power lines make it easy to spot if a voltage isn’t working. The current limiting resistor for each LED is tailored for uniform brightness, these values are a departure from our standard partlist.

Load resistor

There is a footprint for a 9 Watt 10ohm load resistor on the 5 volt rail. Some older ATX supplies won’t start without some sort of load.

In our experience most ATX supplies don’t require a significant load on the 5 volt rail to start. An artificial load just wastes electricity and creates unnecessary heat. In the production version we include the resistor but don’t solder it because most will never need it.

Please let us know your experience.

PCB

We used the freeware version of Cadsoft Eagle to make the schematic and PCB. Download the latest designs and firmware from the project Google Code page.

The power traces are all extra fat and routed on the top layer whenever possible. All layer changes are reinforced with multiple vias to ensure a good connection. The design should be more than adequate to handle the 1.25 amps maximum allowed by the fuses.

The board uses a mix of through hole and surface mount parts. The SMD fuses are quite large (1812), but the indicator LEDs and resistors are all 0603.

This is the second project, after Bus Pirate v3.6, to be released in our new SoB v1 standard PCB footprint. The DP10080 (100mmx80mm) Eagle maximum PCB size is used.

Partslist


Click for a full size placement image.

For part sources please see our master partlist.

Taking it further
This project started our as something to use around the workshop, but we fell in love with it and cleaned it up for release. We’ve got a few ideas for updates, but we’re already pushing the size limits of the free Cadsoft Eagle version.

  • LM317/337 regulators for adjustable positive and negative output
  • Current measurement points

We’ll post updates on the blog, and you can join the discussion in the forum.

Get one!

You can get one for $14 at Seeed Studio.

Your purchases at Seeed Studio keep the open source project coming, we sincerely appreciate your support!

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Comments

  1. Guan Yang says:

    This won’t be very relevant for projects that use this breakout board, since it will be fused at 1.2A, but I’ve found that many cheap ATX power supplies don’t provide stable 12V at high currents (5A and above) if you don’t have a nontrivial load on 5V.

  2. Cristian says:

    Nice and very useful!

  3. Nice design, and a great way to re-use PC power supplies! May I suggest, though, that you fit different coloured 4mm sockets for the different voltages? With all of them red, it’s very easy to mix them up and plug something into the wrong voltage. Now, that won’t matter very much if you plug a 5V device into 3.3V, but you’ve got 12V, -5V and -12V on there. Using different colours will show at a glance which one is which, especially if the user has matching colour-coded 4mm plugs!

  4. arhi says:

    ATX supply is extremely dirty with incredibly high ripple. If you already made a PCB to get the 3,+-5,-+5 and +-12V (wait, where’s -5 ?!) out why didn’t you add some capacitance on each rail? some combo of 2x10nF + 100nF + 1uF ceramic + 10uF ceramic on each rail would improve things significantly, and adding a LC filter to 5V and 3V3 rails would be additional bonus.

    On top of this, at least 2 more output should exist, one 9V 1A (simple 7809 would suffice) + one VAR voltage 1-2A (317 or something similar)

    With regards to 5V load, I found that using a resistor is really too much of a waste of energy so I power a light from 5V. It provides proper load + can be used as backlight for a PSU case. A regular car headlight lamp can be used. It will not be as bright and hot as when it’s running at 12V (nor you want it that bright/hot), it will be cooler then the 10W resistor and will be a super useful add-on.

    • asdf says:

      Love arhi’s ideas. Please implement this in a V1.5….

      • arhi says:

        asdf, you can probably solder some capacitors on this board if you fiddle a bit with it .. as for the light you can easily attach 2 wires for a light bulb instead of power resistor on board and I noticed -5V is there, only the post is missing (weird, I never seen a PSU without -5V rail, I use that rail very often in my projects that use ATX PSU)

      • asdf says:

        how would you connect the caps and LC filters to the rails?
        I might hack around w/ the Eagle schematic since it’s open source…

  5. dogsop says:

    Hurray, been watching for this since I saw it at Maker Faire.

  6. Senso says:

    Thats a pretty handy breakout, it doesn’t require modification of the PSU and if it blows up, just unplug the molex cable and go get a new one.
    One question, are there binding posts like those available at mouser?

  7. Arup says:

    Waiting for this PCB to come in drawer and a code to come in my email. :D
    Anyways, the right angled molex connector looks uncommon. I guess it’s expensive as I can see only straight connectors often.
    You could also have routed the 5VSB to a terminal. Most PSU gives 5V@2A in the rail without powering it on completely and thus saving power. It would been useful in the cases when we need only +5V (2A is enough) for some Logic or MCU breadboarding.
    Great product.

  8. torwag says:

    One idea for V1.5 could be some dedicated solder pads to get an external power switch attached. Those powersupplies have a lot of juice and depending on what you do with it you want a real big “emergency power off” button on which you can smash if your circuit starts to send out smoke signals ;)

  9. Steve says:

    Nice breakout.

    If your starting to hit the limits of the free Eagle version, doesn’t that mean its time to switch to Kicad? :)

  10. Picasso says:

    I feel a little dumb and cant find the down for the schematic and PCB on the Google Code page.

  11. byutea says:

    wow, nice… why this unthinkable by me :D

  12. oliver says:

    Maybe jpeg artifacts are the cause, but the top trace (the one with the names on it) seems to end quite close to the contact left of it.

    Also the trace above “Full”(filment) seems awefully close.

  13. oliver says:

    Actually, the unpopulated gnd to the top left is really close 12v line too, only silk seperating it?

  14. chris kearsley says:

    what amps will it handle i am after it to handle 16A as what i want too run needs to be 16a my psu has a 12v 16 amp rail

    • Howler says:

      I would like to know this as well. I am looking to use this to power a 3d printer (so I can swap out with a different power supply if needed). But I need a lot more current than 1.5A. What can the traces on the board safely handle, if I wanted to up the Amperage on the fuses?

  15. Tom Price says:

    Here’s my ATX breakout board build. Acryclic jacket by seeedstudio.

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