An ATX breakout board and how I modded it.

After a long hiatus I recently got interested again in electronics, an I'm in the process of setting up a minimal lab again. In order to get an additional bench power supply I considered converting an ATX PSU to a 'benchtop' power supply. ATX PSUs supply 5V,12V,-12V and 3.3V with more than sufficient amperage. They do not offer variable voltage nor they are current regulated, but the nominal outputs are stable and a lot of people - myself included - already have one or two ATX power supplies lying around.

At first, I considered modding directly my candidate PSU (a 500W Cooler Master unit, almost brand new). There are many tutorials about the procedure on youtube and other sites. For example, this video on the very interesting GreatScott! YT channel could be used as a starting point. There are many variations of this mod: some tutorials recommend dangerous practices, some projects do not make much sense (a Chinese bench PSU can be bough for under 50 EUR, it doesn't make a lot of sense to spend 20-30 EUR to mod an ATX with voltmeters, amperometers etc then) practically. The GreatScott! video is a minimal mod requiring very little additional components.

In the end I decided againts the procedure, the main reason being the fact that I'd like a fully reversible mod: I would like to be able to use the ATX PSU for testing motherboards from time to time, cutting the ATX connector and drilling holes goes a little bit against that.

Enter the 'ATX breakout board'. A small circuit with an ATX female connector, a switch and some binding posts. The minimum required to turn on the PSU and use the four voltages in a circuit. I believe that the idea arose first in the open source hardware community. Anyway, nowadays one can buy a ready made circuit for less than 10 EUR. Just look for ATX breakout board on ebay or Amazon. First of all, let me say that the best available design overall it's not available ready made, but only in PCB form and/or in CAD files. It has been designed by an Italian, Francesco Truzzi and is described here. The board has everything, USB ports with recharging protocol support, variable voltage support, dummy load support, the works. Too much hassle though, so I decide to buy this model on Amazon.  It sports long brass distancers and a separate ground binding post for every voltage, as well as the lowest price, over other designs, like the 'Seedstudio' board. Here's my untouched board:

There's a small switch for the POWER ON ATX line and a blue power led. Of course, ATX PSUs are always 'on' as long they're plugged on mains and the switch on the back near the three prong socket is on. The switch merely connects a specific line to a 5V 'standby' rail which is always available, usually with a couple ampere  supply, turning on the rest of the PSU. One needs only to connect the ATX male connector to the female connector on board and switch everything on. This model works both with 20 pin and 24 pin ATX connector, the rails are connected correctly and I've tested them.
In comparison with other breakout boards this model lacks a dummy load resistor. Many ATX PSUs in fact do refuse to start if no load is connected to them. Usually a 10W 10 Ohm resistor connected to the +5V or the +12V rail will solve the problem. The best thing would be to load both rails. 10W power resistors are really the mininum for this kind of job. Other boards are usually shipped with a 10W white resistor to solder on specific holes already drilled on the PCB. On the other hand more recent PSUs start without a load with no problems. My unit does so, and there are no differences in measured voltages on the outputs with or without a load. I had planned to build a completely reversible, detachable dummy load recycling a 15 year old molex to floppy power cable and some scrounged power resistors:

but for the moment, I've no need. Really, do use 10W resistors, I've tried the 5W 12Ohm part shown, and it gets hot damn quick.

Back to our breakout board. One big problem is the binding posts. They do look like standard 4mm banana sockets, but 1) they are too short for a real banana plug 2) they don't even  have holes to put wires into:

They're usable, but what the heck, I decided to change them with standard, longer posts you can find anywhere. I'll find a use for the original posts sometime. They're  secured with a couple of nuts, not soldered. One modification I could really find usable is to add a USB port for chargin/powering things up. The ideal solution is the one designed by Francesco Truzzi: add the USB connectors on the board, with support for USB negotiation and charging protocols via the TPS2513 chip. But a simple usb cable connected directly to the 5V rail with no additional components will work. I used one of the many USB extension cord I had lying around and cut it this way:

One need only to expose the GND (black) and +5V (red wires) . Now, one could safely connect them to the main 5V rail, with clip-on connectors even, but rememeber the 5V standby power with 2-3A I mentioned before. That line is always available, ideal for charging stuff!. The rail is not available on the posts of course, but is on the connector and therefore on the PCB. Look here:

I've marked the 5VSTNDBY line and one the ground pins. To connect the cable I proceeded this way: i drilled a 4mm hole on an 'empty' part of the pcb, slid the cable in, and soldered the wires directly to the connector pins, after having prepared about 2cms of shrinking tube. Then I secured
the cable to the hole with a couple small zip ties and shrunk the shrinking tube. Voilà, an USB power output. I've tried directly with a lot of devices (phones, e-readers etc) I had lying around - no problem. Even connecting an old 1.1 USB hub as a port mutiplier works, but I've the feeling that in this case the hub is limit the current output somehow.

Here is the modified board from above, with the new binding posts:

Here it is from below. Note that I left the soldering clips on, they could be used for alligator clip cables, and that I added also some faston connectors I had around (only six because I had only six around, and for a long time as well...)

Another idea I had is to recycle the power cable from an old, broken wall-wart PSU to have a power output for my Arduinos. The barrel power plug on Arduinos accepts 5V-12V. I connected two fastons on the cable this way (check the connection with a multimeter, Arduino need a tip positive connection but in my case the tip was connected to the wire strand with a minus sign all along, even if the wall war was tip positive!):

And I connected to.. the -12V rail inverting + and -. The reason is that on my PSU the 12V is a bit higher, while the -12V is spot on. -12V has fewer amps, but sufficient for any Arduino test. Of course, one could use ring or clip on connectors instead of my old fastons.

As a final touch, I looked in the kitchen. I found the cover of an old tupperware like box: it was more or less of the right size and was also conveniently convex. I sawed off the edges and drilled four holes to connect the brass posts. The fact that the cover is convex avoids the board standing on the M3 screws I used and allows for a better grip on the bench (or the ATX PSU) anyway.

I also had some adhesive rubber feet from a long gone, 20 year old computer case. I put it on the back of the PSU to help it stand vertically. Here is the finished unit.

If you want to see the unit in action, I've shot a short video:

That's it. The next step would be to add a voltage regulator, something based on the LM317 (with low amperages) or LM350 (slightly more powerful). There are some ultra-cheap chinese kits, even with a voltmeter which could do the trick. We'll see...
Of course, if you want to get the board and modify it like I did, please make sure to know what you're doing and to take the necessary precautions when working with electrical devices and/or soldering equipment.