ATX Breakout Board design overview

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ATX BB HW.jpg

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 $13.80 at Seeed Studio. Read about the design below.

Contents

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

ATX BB SCH.png

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

ATX BN JACKS.jpg

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

ATX BB PS SWITCH2.jpg

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

600

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

ATX BB loadRes.jpg

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

ATX-PCB.png

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

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

ATX BB PCB.png

Click for a full size placement image.

PartQuantityValuePackage
D1,D2,D3,D4,D5,D6,D77Yellow LEDsLED-805
F1,F2,F3,F4,F551.25 amps, 15volts1812L_PTC
J21ATX connector 90degreesCON_ATX_PWR_90DEG
ON/OFF1Push button DPDT styleSW_PUSH_BUTTON_MINI
R1, R4210K resistorR603
R211.2K resistorR603
R3, R5, R6, R743.3K resistorR603
R1819 Watt 10R resistor (optional)KH214-8
T11Thumb screw terminal 4mm blackPAD_ROUND_4.1MM
T2,T3,T4,T54Thumb screw terminal 4mm redPAD_ROUND_4.1MM

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 $13.80 at Seeed Studio.

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